8.10: Why It Matters- Protists - Biology

8.10: Why It Matters- Protists - Biology

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Why discuss the organisms in Kingdom Protista?

Protists exist as a kind of “grey area” in biology. The organisms found in this kingdom are varied, and often seem like they might be more closely related to organisms in another kingdom: some protists resemble bacteria, some resemble fungi, and some resemble plants.

One group of these plant-like protists are the micro-algae. Watch this video to learn how these microscopic organisms could serve as a future fuel source:

A YouTube element has been excluded from this version of the text. You can view it online here:

Learning Outcomes

  • Identify the common characteristics of protists
  • Classify protists into unique categories
  • Describe the role that protists play in the ecosystem

Nitrogen was originally formed in the hearts of stars through the process of nuclear fusion. When ancient stars exploded, they flung nitrogen-containing gases across the Universe. When the Earth was formed, nitrogen gas was the main ingredient in its atmosphere.

Today, the Earth’s atmosphere is about 78% nitrogen, about 21% oxygen, and about 1% other gases. This is an ideal balance because too much oxygen can actually be toxic to cells. In addition, oxygen is flammable. Nitrogen, on the other hand, is inert and harmless in its gaseous form. However, nitrogen gas is not accessible to plants and animals for use in their cells.

Here we will discuss how nitrogen plays a vital role in the chemistry of life – and how it gets from the atmosphere, into living things, and back again.

Better Reporting of Scientific Studies: Why It Matters

Copyright: © 2013 PLOS Medicine Editors. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Funding: The authors are each paid a salary by the Public Library of Science, and they wrote this editorial during their salaried time.

Competing interests: The authors' individual competing interests are at PLOS is funded partly through manuscript publication charges, but the PLOS Medicine Editors are paid a fixed salary (their salary is not linked to the number of papers published in the journal).

Abbreviations: ARRIVE, Animal Research: Reporting In Vivo Experiments CONSORT, Consolidated Standards of Reporting Trials EQUATOR, Enhancing the Quality and Transparency of Health Research PRISMA, Preferred Reporting Items for Systematic Reviews and Meta-Analyses RCT, randomized controlled trial STROBE, Strengthening the Reporting of Observational Studies in Epidemiology

Provenance: Written by editorial staff not externally peer reviewed.

To coincide with the Seventh International Congress on Peer Review and Biomedical Publication to be held in Chicago from September 8 to 10, 2013 [1], PLOS Medicine is launching a new Reporting Guidelines Collection [2], an open access collection of reporting guidelines, commentary, and related research on guidelines from across PLOS journals. This collection is consistent with the goals of the Peer Review Congress: “to improve the quality and credibility of scientific peer review and publication and to help advance the efficiency, effectiveness, and equitability of the dissemination of biomedical information throughout the world” [2].

As early as 1990, Iain Chalmers, one of the founders of the Cochrane Collaboration, stated that, “Failure to publish an adequate account of a well-designed clinical trial is a form of scientific misconduct that can lead those caring for patients to make inappropriate treatment decisions.” [3]. Guidelines and checklists for reporting scientific studies are not just tick box exercises rather, they help to improve the transparency and presentation of studies and, therefore, have the potential to improve the impact and implementation of scientific research.

PLOS Medicine has a strong history of promoting policies that aim to improve study design and transparency of reporting and publishing them in an open-access venue. We published our first reporting guideline – the STROBE (Strengthening the Reporting of Observational Studies in Epidemiology) Statement [4],[5] –more than 5 years ago. While the STROBE Statement was published concurrently with several other leading medical journals, critically, PLOS Medicine was the only open access journal to publish it at that time. For reporting guidelines to be useful, it is essential that they be widely disseminated, made freely available, and without restrictions on reuse. Since we published the STROBE Statement in 2007, there has been a shift toward making reporting guidelines more freely available the EQUATOR (Enhancing the Quality and Transparency of Health Research Network, launched in June 2008, provides freely accessible links to published guidelines.

To support PLOS Medicine's aim of encouraging the highest possible standards in medical research and reporting, the journal launched “Guidelines and Guidance” in 2008, a new section within the Magazine that publishes reporting guidelines, research priorities, methodological issues, and other articles providing guidance on the conduct and reporting of research [6].

Reporting guidelines have evolved since the original CONSORT Statement was published in 1996 [7] as a minimum set of recommendations for reporting randomized controlled trials (RCT). The CONSORT Statement was updated in 2001 and 2010, and several extensions of the guidelines have been developed based on more specific study designs (e.g., CONSORT Statement for cluster-based RCTs [8]) or specific intervention types (e.g., acupuncture [9]). While RCTs provide the strongest evidence for clinical efficacy of interventions in a clinical setting and play a critical role in healthcare decision-making, they are not always feasible or ethical to conduct. Over time, reporting guidelines have been published for many other types of research that can also influence policy and practice, such as epidemiologic [4],[5], diagnostic [10], prognostic [11], and genetic risk prediction [12] studies. Similarly, extensions of the STROBE Statement have been developed as research fields emerge, such as for use by researchers conducting genetic association studies [13] or studies in molecular epidemiology [14].

An important development in evidence-based medicine has been the use of systematic reviews to synthesize the best quality research evidence relevant to a particular topic. One of the most frequently accessed and cited papers published in PLOS Medicine is the PRISMA Statement (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) [15],[16], an evidence-based, minimum set of items for reporting of systematic reviews and meta-analyses. The PRISMA Statement has been endorsed by over 170 journals and includes a 27-item checklist and a four-phase flow diagram. On the PLOS Medicine website alone, it has over 100,000 views and has been cited 1,000 times [17].

Reporting guidelines have even been developed to improve abstract reporting for RCTs and systematic reviews, as extensions of CONSORT [18] and PRISMA [19], respectively. Abstracts are the first and often only part of an article that is read. Indeed, given that 50% of biomedical research is still behind a pay wall [20], the abstract is frequently the only part of the article that readers can access. Furthermore, about 40% of abstracts for RCTs have been shown to misrepresent or “spin” study findings [21], making it all the more critical that an abstract accurately represents the research findings.

While much of the focus of reporting guidelines has thus far been on health research, the animal research community is also developing reporting standards. The ARRIVE (Animal Research: Reporting In Vivo Experiments) guidelines were published in PLOS Biology in 2010 [22] and subsequently in 11 other journals. Recent efforts by the NC3Rs (National Centre for the Replacement, Refinement and Reduction of Animals in Research) have encouraged the adoption of the ARRIVE checklist. In an Editorial published in July 2013, PLOS Medicine announced a new requirement for the ARRIVE checklist for in vivo animal studies [23].

A growing body of evidence demonstrates improvements in the quality of reporting scientific studies associated with the publication of reporting guidelines however, translation of the guidelines into practice remains a challenge. A systematic review, published by the Cochrane Group, observed that journal endorsement of the CONSORT Statement is associated with more complete reporting of trials in medical journals [24]. Other studies have reported improvements in the quality of reporting after publication of CONSORT guidelines for abstracts [25] and the PRISMA Statement [26]. In a randomized trial published in BMJ, conventional peer review plus review looking for missing items from reporting guidelines led to improvements in manuscript quality compared with conventional review [27]. However, studies also show that the quality of reporting overall remains suboptimal [24],[28], as not all journals endorse or enforce the use of reporting guidelines [29]–[31].

The EQUATOR Network website houses a comprehensive library of reporting guidelines for health research [32], of which our Collection is just a subset, as well as educational materials. The PLOS Medicine Editors strongly urge (and for specific articles types, require) authors, peer reviewers, and journal editors to use these freely available resources. Most reporting guidelines have checklists that can be submitted along with a manuscript to facilitate the peer review process by allowing editors and reviewers to quickly identify essential elements of how a study was conducted.

This new Reporting Guidelines Collection aims to highlight some of the many resources now available to facilitate the rigorous reporting of scientific studies, and to improve the presentation and evaluation of published studies. Transparency in research reporting should be integral to the dissemination of scientific research. The peer review process is a critical part of research and reporting guidelines provide a mechanism to help this process. While following reporting guidelines does not necessarily make the study better, this process does give readers the information to better judge the quality, and therefore the usefulness, of research. As online publication removes the space constraints of print, reporting should be complete and transparent, and reporting guidelines aid that process.

Simon et al., The Campbell Essential Biology Series

Campbell Essential Biology makes biology interesting and understandable for non-majors biology students. This best-selling textbook, known for its scientific accuracy, clear explanations, and intuitive illustrations, has been revised to further emphasize the relevance of biology to everyday life.

The new edition incorporates instructor feedback on what key skills to highlight in new Process of Science essays and uses striking infographic figures in conveying real data to help students see and better understand how science actually works. New author-narrated Figure Walkthrough Videos appear in each chapter and guide students through key biology concepts and processes.

New topics in Why It Matters inspire curiosity and provide real-world examples to convey why abstract concepts like cell respiration or photosynthesis matter to students. This edition’s unmatched offering of author-created media supports students in the toughest topics with 24/7 access through the enhanced Pearson eText, embedded QR codes in the print text, and Mastering Biology.

Chapters 21-29 are included in the expanded version of the text, Campbell Essential Biology with Physiology.

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Post by Esamelden Abdelnaem on April 26, 2017

Post by Esamelden Abdelnaem on April 26, 2017

Post by Esamelden Abdelnaem on April 26, 2017

Post by Esamelden Abdelnaem on April 26, 2017

Post by Sanjin Kulenovic on June 24, 2014

Post by Sanjin Kulenovic on June 24, 2014

Hi Nehlia, nice lecture, although at example 1, you made a mistake by putting energy in the left side of the respiration equation. Also, previously, while talking about ATP, you could have introduced it as an energetic molecule that it is, called Adenosine Ari Phosphate, which is made inside the cells. Hope my comments can help.

Post by Dana Nourie on August 24, 2012

I thought that chlorophyll absorbs all frequencies of light *except* for green. It reflect the green frequency, which is why we see it as green. If it absorbed green, we'd see it as other colors, not green.

Why cell biology is so important?

Have you ever been ill? Even if it was a ‘tummy bug’ it will have been your cells that were affected by the poisonous chemicals or toxins from bacteria cells in the bad food.

You may know of someone who has been ill with a disease or disorder such as meningitis, malaria, diabetes, a type of cancer, cystic fibrosis, or Alzheimer’s disease. All these diseases and disorders are caused by problems at a cell or molecular level. Physical damage such as a burn or broken bone also causes damage at cell level.

By understanding how cells work in healthy and diseased states, cell biologists working in animal, plant and medical science will be able to develop new vaccines, more effective medicines, plants with improved qualities and through increased knowledge a better understanding of how all living things live.

Eventually it will be possible to produce a ‘health forecast’ by analysing your database of genetic and cell information. Using this you will be able to take more control over your health in a preventive way.

But cell biology is not just about disease. It has greatly assisted the human fertility programme. DNA testing has been used in archaeology to provide evidence that a living person is related to a long dead ancestor.

In plant science it has been used to show that two plants that look different have the same genetic origins.

Forensic medicine uses cell biology and DNA fingerprinting to help solve murders and assaults. Neither the courts of law nor the criminals can escape the importance of cell biology.

Biotechnology uses techniques and information from cell biology to genetically modify crops to produce alternative characteristics to clone plants and animals to produce and ensure high quality food is available at lower costs to produce purer medicines and in time organs for the many people who need transplants.
Cell biology is about all this and can make an exciting career.

It is also important that everyone feels informed about how the increase in knowledge about cell biology could affect him or her and society in general. Society will have to make informed decisions about such things as growing organs for transplanting into humans and, in those areas where vitamin ‘A’ deficiency causes blindness, growing rice modified to produce the vitamin.

A basic understanding of cell biology including genetics will be as important as having some knowledge about computers and the Internet.

If you needed a kidney transplant and no donated human organ were available, would you refuse to have one from a pig specially developed to provide organs for humans?

You are a rice farmer and a parent. You know that each year more than one million children die and another 124 million are made more susceptible to measles and diarrhoea due to shortage of vitamin A. You have heard about a new strain of genetically modified rice producing vitamin A is available. Would you grow it and let your family eat it?

Biology Anatomy & Physiology - Biology bibliographies - in Harvard style

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How Bones Work

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How Muscles Work

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Choi, H. F. and Blemker, S. S.

Skeletal Muscle Fascicle Arrangements Can Be Reconstructed Using a Laplacian Vector Field Simulation

2013 - PLoS ONE

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Your Bibliography: Choi, H. and Blemker, S., 2013. Skeletal Muscle Fascicle Arrangements Can Be Reconstructed Using a Laplacian Vector Field Simulation. PLoS ONE, 8(10), p.e77576.

Boundless Anatomy and Physiology | Simple Book Publishing

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Joints and Skeletal Movement | Boundless Biology

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Invertebrate vs Vertebrate - Difference and Comparison | Diffen

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Bone Marrow Anatomy: Overview, Types of Bone Marrow, Blood Cell Formation

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Tendon | anatomy

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Injury, C., Maladies, D., Eligibility, T., Information, T., Participate?, C., Site, F., Risks, P., Info, P., Patient, R., Investigator, P., Injury, C., Maladies, D., Eligibility, T., Information, T., Participate?, C., Site, F., Risks, P., Info, P., Patient, R. and Investigator, P.

Hyaline Cartilage & Full Anatomical Knee Model

In-text: (Injury et al., 2017)

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Eversion (Joint Movement)

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Eversion (Joint Movement)

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Perimysium : Anatomy of Muscle Structure

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Korthuis, R. J.

Skeletal muscle circulation

2011 - Morgan & Claypool Life Sciences - San Rafael, Calif.

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Everything Maths and Science

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Plantar flexion: Function, anatomy, and injuries

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Bicep Brachii Muscles (biceps) is responsible for pulling and rotating

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10.2 Skeletal Muscle | Anatomy and Physiology

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Gait - Physiopedia

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Muscle-Ligament Role in Joint Tension

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Siegler, S.

Foot and ankle joint biomechanics

2007 - Journal of Biomechanics

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Fitness Defined: Concentric and Eccentric Contractions &#40and Why It Matters&#41

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System, M.

Muscular System

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System, S.

Skeletal System

In-text: (System, 2017)

Your Bibliography: System, S., 2017. Skeletal System. [online] InnerBody. Available at: <> [Accessed 16 September 2017].

Anatomical Planes

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The Radial Nerve

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Tortora, G. J., Derrickson, B. and Tortora, G. J.

Principles of anatomy & physiology

2014 - Wiley - Hoboken, N.J.

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Paraspinal Muscles: Where are they and what do they do?

In-text: (Paraspinal Muscles: Where are they and what do they do?, 2017)

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Ankle and Intertarsal Joints | Anatomy 622 Coursebook

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Basic Knee Anatomy Video

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Introduction to Bone Biology

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Osteoclasts - Everything You Need To Know - Dr. Nabil Ebraheim

In-text: (Osteoclasts - Everything You Need To Know - Dr. Nabil Ebraheim, 2017)

Seed Plants

Lesson Objectives

  • Describe the importance of the seed.
  • Explain the ways in which seeds are dispersed.
  • Define and give examples of gymnosperms.
  • Define and give examples of angiosperms.
  • Explain some uses of seed plants.

Check Your Understanding


  • anther
  • calyx
  • carpel
  • complete flowers
  • conifers
  • corolla
  • dormant
  • ginko
  • incomplete flowers
  • ovary
  • sepals
  • stamen
  • stigma

Seeds and Seed Dispersal

What is a Seed?

If you’ve ever seen a plant grow from a tiny seed, then you might realize that seeds are amazing structures. The seed allows a plant embryo to survive droughts, harsh winters, and other conditions that would kill an adult plant. The tiny plant embryo can simply stay dormant, in a resting state, and wait for the perfect environment to begin to grow. In fact, some seeds can stay dormant for hundreds of years!

Another impressive feature of the seed is that it stores food for the young plant after it sprouts. This greatly increases the chances that the tiny plant will survive. So being able to produce a seed is a very beneficial adaptation, and as a result, seed plants have been very successful. Although the seedless plants were here on Earth first, today there are many more seed plants than seedless plants.

How are Seed Plants Successful?

For a seed plant species to be successful, the seeds must be dispersed, or scattered around in various directions. If the seeds are spread out in many different areas, there is a better chance that some of the seeds will find the right conditions to grow. But how do seeds travel to places they have never been before? To aid with seed dispersal, some plants have evolved special features that help their seeds travel over long distances.

One such strategy is to allow the wind to carry the seeds. With special adaptations in the seeds, the seeds can be carried long distances by the wind. For example, you might have noticed how the "fluff" of a dandelion moves in the wind. Each piece of fluff carries a seed to a new location. If you look under the scales of pine cone, you will see tiny seeds with "wings" that allow these seeds to be carried away by the wind. Maple trees also have specialized fruits with wing-like parts that help seed dispersal, as shown in Figure below.

Maple trees have fruits with

Some flowering plants grow fleshy fruit that helps disperse their seeds. When animals eat the fruit, the seeds pass through an animal’s digestive tract unharmed. The seeds germinate after they are passed out with the animal's feces. Berries, citrus fruits, cherries, apples, and a variety of other types of fruits are all adapted to be attractive to animals, so the animals will eat them and spread the seed (Figure below).

Fleshy fruits aid in seed dispersal since animals eat the fruits and carry the seeds to a new location.

Some non-fleshy fruits are specially adapted for animals to carry them on their fur. You might have returned from a walk in the woods to find burrs stuck to your socks. These burrs are actually specialized fruits designed to carry seeds to a new location.


Plants with "naked" seeds, meaning they are not enclosed by a fruit, are called gymnosperms. Instead, the seeds of gymnosperms are usually found in cones.

There are four phyla of gymnosperms:


Conifers, members of the phylum Coniferophyta, are probably the gymnosperms that are most familiar to you. Conifers include pines, firs, spruces, cedars, and the coastal redwood trees in California that are the tallest living vascular plants.

Conifers have their reproductive structures in cones, but they are not the only plants to have that trait (Figure below). Conifer pollen cones are usually very small, while the seed cones are larger. Pollen contains gametophytes that produce the male gamete of seed plants. The pollen, which is a powder-like material, is carried by the wind to fertilize the seed cones that contain the female gamete (Figure below).

A red pine, which bears seeds in cones, is an example of a conifer.

The end of a pine tree branch bears the male cones that produce the pollen.

Conifers have many uses. They are important sources of lumber and are also used to make paper. Resins, the sticky substance you might see oozing out of a wound on a pine tree, are collected from conifers to make a variety of products, such as the solvent turpentine and the rosin used by musicians and baseball players. The sticky rosin improves the pitcher’s hold on the ball or increases the friction between the bow and the strings to help create music from a violin or other stringed instrument.


Cycads, in the phylum Cycadophyta, are also gymnosperms. They have large, finely-divided leaves and grow as short shrubs and trees in tropical regions. Like conifers, they produce cones, but the seed cones and pollen cones are always on separate plants (Figure below). One type of cycad, the Sago Palm, is a popular landscape plant. During the Age of the Dinosaurs (about 65 to 200 million years ago), cycads were the dominant plants. So you can imagine dinosaurs grazing on cycad seeds and roaming through cycad forests.

Cycads bear their pollen and seeds in cones on separate plants.


Ginkgoes, in the phylum Ginkgophyta, are unique because they are the only species left in the phylum. Many other species in the fossil record have gone extinct (Figure below). The ginkgo tree is sometimes called a "living fossil," because it is the last species from its phylum.

One reason the ginkgo tree may have survived is because it was often grown around Buddhist temples, especially in China. The ginkgo tree is also a popular landscape tree today in American cities because it can live in polluted areas better than most plants.

Ginkgoes, like cycads, has separate female and male plants. The male trees are usually preferred for landscaping because the seeds produced by the female plants smell terrible when they ripen.

Ginkgo trees are gymnosperms with broad leaves.


Gnetophytes, in the phylum Gnetophyta, are a very small and unusual group of plants. Ephedra is an important member of this group, since this desert shrub produces the ephedrine used to treat asthma and other conditions. Welwitschia produces extremely long leaves and is found in the deserts of southwestern Africa (Figure below). Overall, there are about 70 different species in this diverse phylum.

One type of gnetophyte is Welwitschia.


Angiosperms, in the phylum Anthophyta, are the most successful phylum of plants. This category also contains the largest number of individual plants (see Figure below). Angiosperms evolved the structure of the flower, so they are also called the flowering plants. Angiosperms live in a variety of different environments. A water lily, an oak tree, and a barrel cactus, although different, are all angiosperms.

Angiosperms are the flowering plants.

The Parts of a Flower

Even though flowers may look very different from each other, they do have some structures in common. Follow along in Figure below as the structures are explained below:

  • The green outside of a flower that often looks like a leaf is called the sepal (Figurebelow). All of the sepals together are called the calyx, which is usually green and protects the flower before it opens.
  • All of the petals (Figurebelow) together are called the corolla. They are bright and colorful to attract a particular pollinator, an animal that carries pollen from one flower to another.
  • The next structure is the stamen, consisting of the stalk-like filament that holds up the anther, or pollen sac. The pollen is the male gametophyte.
  • At the very center is the carpel, which is divided into three different parts: (1) the sticky stigma, where the pollen lands, (2) the tube of the style, and (3) the large bottom part, known as the ovary.

The ovary holds the ovules, the female gametophytes. When the ovules are fertilized, the ovule becomes the seed and the ovary becomes the fruit.

When flowers have all of these parts, they are known as complete flowers. Other flowers may be missing one or more of these parts and are known as incomplete flowers. Table below summarizes the parts of the flower.

A complete flower has sepals, petals, stamens, and one or more carpels.

This image shows the difference between a petal and a sepal.

The Parts of a Flower
Flower partDefinition
sepalsThe green outside of the flower.
calyxAll of the sepals together, or the outside of the flower.
corollaThe petals of a flower collectively.
stamensThe part of the flower anther that produces pollen.
filamentStalk that holds up the anther.
antherThe structure that contains pollen in a flower.
carpel“Female” part of the flower includes the stigma, style, and ovary.
stigmaThe part of the carpel where the pollen must land for fertilization to occur.
styleTube that makes up part of the carpel.
ovaryLarged bottom part of the carpel where the ovules are contained.

How Do Angiosperms Reproduce?

Flowering plants can reproduce two different ways:

  1. Self-pollination: Pollen falls on the stigma of the same flower. This way, a seed will be produced that can turn into a genetically identical plant.
  2. Cross-fertilization: Pollen from one flower travels to a stigma of a flower on another plant. Pollen travels from flower to flower by wind or by animals. Flowers that are pollinated by animals such as birds, butterflies, or bees are often colorful and provide nectar, a sugary reward, for their animal pollinators.

Why Are Angiosperms Important to Humans?

Angiosperms are important to humans in many ways, but the most significant role of angiosperms is as food. Wheat, rye, corn, and other grains are all harvested from flowering plants. Starchy foods, such as potatoes, and legumes, such as beans, are also angiosperms. And as mentioned previously, fruits are a product of angiosperms to increase seed dispersal and are also nutritious foods.

There are also many non-food uses of angiosperms that are important to society. For example, cotton and other plants are used to make cloth, and hardwood trees are used for lumber.

Lesson Summary

  • Seeds consist of a dormant plant embryo and stored food.
  • Seeds can be dispersed by wind or by animals that eat fleshy fruits.
  • Gymnosperms, seed plants without flowers, include conifers, cycads, gingkoes, and gnetophytes.
  • Angiosperms are flowering plants.
  • Seed plants provide many foods and products for humans.

Review Questions


1. How do seeds help plants adapt to their environment?

2. What are two ways that plants disperse their seeds?

3. What are some examples of gymnosperms?

4. Firs, spruces, and pines belong to what group of gymnosperms?

5. Where is the pollen stored in a flower?

6. How are plants pollinated?

Apply Concepts

7. What is the purpose of a plant developing a fruit?

8. How are gymnosperms and angiosperms different?

9. What are some uses that seed plants have for humans?

10. Why is the ginkgo tree considered a “living fossil”?

Think Critically

11. Why did angiosperms evolve the ability to produce flowers? Use the terms "adaptation" and "environment" in your explanation.

Further Reading / Supplemental Links

Points to Consider

Now that we have discussed the types of plants, we turn to plant responses.

  • Do you think plants can respond to their environment? Why or why not?
  • How might plants and fruit change colors?
  • How do you think trees know when it’s time to lose their leaves?


Although the process of meiosis is related to the more general cell division process of mitosis, it differs in two important respects:

usually occurs between identical sister chromatids and does not result in genetic changes

Meiosis begins with a diploid cell, which contains two copies of each chromosome, termed homologs. First, the cell undergoes DNA replication, so each homolog now consists of two identical sister chromatids. Then each set of homologs pair with each other and exchange genetic information by homologous recombination often leading to physical connections (crossovers) between the homologs. In the first meiotic division, the homologs are segregated to separate daughter cells by the spindle apparatus. The cells then proceed to a second division without an intervening round of DNA replication. The sister chromatids are segregated to separate daughter cells to produce a total of four haploid cells. Female animals employ a slight variation on this pattern and produce one large ovum and two small polar bodies. Because of recombination, an individual chromatid can consist of a new combination of maternal and paternal genetic information, resulting in offspring that are genetically distinct from either parent. Furthermore, an individual gamete can include an assortment of maternal, paternal, and recombinant chromatids. This genetic diversity resulting from sexual reproduction contributes to the variation in traits upon which natural selection can act.

Meiosis uses many of the same mechanisms as mitosis, the type of cell division used by eukaryotes to divide one cell into two identical daughter cells. In some plants, fungi, and protists meiosis results in the formation of spores: haploid cells that can divide vegetatively without undergoing fertilization. Some eukaryotes, like bdelloid rotifers, do not have the ability to carry out meiosis and have acquired the ability to reproduce by parthenogenesis.

Meiosis does not occur in archaea or bacteria, which generally reproduce asexually via binary fission. However, a "sexual" process known as horizontal gene transfer involves the transfer of DNA from one bacterium or archaeon to another and recombination of these DNA molecules of different parental origin.

Meiosis was discovered and described for the first time in sea urchin eggs in 1876 by the German biologist Oscar Hertwig. It was described again in 1883, at the level of chromosomes, by the Belgian zoologist Edouard Van Beneden, in Ascaris roundworm eggs. The significance of meiosis for reproduction and inheritance, however, was described only in 1890 by German biologist August Weismann, who noted that two cell divisions were necessary to transform one diploid cell into four haploid cells if the number of chromosomes had to be maintained. In 1911, the American geneticist Thomas Hunt Morgan detected crossovers in meiosis in the fruit fly Drosophila melanogaster, which helped to establish that genetic traits are transmitted on chromosomes.

The term "meiosis" is derived from the Greek word μείωσις , meaning 'lessening'. It was introduced to biology by J.B. Farmer and J.E.S. Moore in 1905, using the idiosyncratic rendering "maiosis":

We propose to apply the terms Maiosis or Maiotic phase to cover the whole series of nuclear changes included in the two divisions that were designated as Heterotype and Homotype by Flemming. [8]

The spelling was changed to "meiosis" by Koernicke (1905) and by Pantel and De Sinety (1906) to follow the usual conventions for transliterating Greek. [9]

Meiosis is divided into meiosis I and meiosis II which are further divided into Karyokinesis I and Cytokinesis I and Karyokinesis II and Cytokinesis II respectively. The preparatory steps that lead up to meiosis are identical in pattern and name to interphase of the mitotic cell cycle. [10] Interphase is divided into three phases:

    : In this very active phase, the cell synthesizes its vast array of proteins, including the enzymes and structural proteins it will need for growth. In G1, each of the chromosomes consists of a single linear molecule of DNA. : The genetic material is replicated each of the cell's chromosomes duplicates to become two identical sister chromatids attached at a centromere. This replication does not change the ploidy of the cell since the centromere number remains the same. The identical sister chromatids have not yet condensed into the densely packaged chromosomes visible with the light microscope. This will take place during prophase I in meiosis. : G2 phase as seen before mitosis is not present in meiosis. Meiotic prophase corresponds most closely to the G2 phase of the mitotic cell cycle.

Interphase is followed by meiosis I and then meiosis II. Meiosis I separates replicated homologous chromosomes, each still made up of two sister chromatids, into two daughter cells, thus reducing the chromosome number by half. During meiosis II, sister chromatids decouple and the resultant daughter chromosomes are segregated into four daughter cells. For diploid organisms, the daughter cells resulting from meiosis are haploid and contain only one copy of each chromosome. In some species, cells enter a resting phase known as interkinesis between meiosis I and meiosis II.

Meiosis I and II are each divided into prophase, metaphase, anaphase, and telophase stages, similar in purpose to their analogous subphases in the mitotic cell cycle. Therefore, meiosis includes the stages of meiosis I (prophase I, metaphase I, anaphase I, telophase I) and meiosis II (prophase II, metaphase II, anaphase II, telophase II).

During meiosis, specific genes are more highly transcribed. [11] [12] In addition to strong meiotic stage-specific expression of mRNA, there are also pervasive translational controls (e.g. selective usage of preformed mRNA), regulating the ultimate meiotic stage-specific protein expression of genes during meiosis. [13] Thus, both transcriptional and translational controls determine the broad restructuring of meiotic cells needed to carry out meiosis.

Meiosis I Edit

Meiosis I segregates homologous chromosomes, which are joined as tetrads (2n, 4c), producing two haploid cells (n chromosomes, 23 in humans) which each contain chromatid pairs (1n, 2c). Because the ploidy is reduced from diploid to haploid, meiosis I is referred to as a reductional division. Meiosis II is an equational division analogous to mitosis, in which the sister chromatids are segregated, creating four haploid daughter cells (1n, 1c). [14]

Prophase I Edit

Prophase I is by far the longest phase of meiosis (lasting 13 out of 14 days in mice [15] ). During prophase I, homologous maternal and paternal chromosomes pair, synapse, and exchange genetic information (by homologous recombination), forming at least one crossover per chromosome. [16] These crossovers become visible as chiasmata (plural singular chiasma). [17] This process facilitates stable pairing between homologous chromosomes and hence enables accurate segregation of the chromosomes at the first meiotic division. The paired and replicated chromosomes are called bivalents (two chromosomes) or tetrads (four chromatids), with one chromosome coming from each parent. Prophase I is divided into a series of substages which are named according to the appearance of chromosomes.

Leptotene Edit

The first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin threads". [18] : 27 In this stage of prophase I, individual chromosomes—each consisting of two replicated sister chromatids—become "individualized" to form visible strands within the nucleus. [18] : 27 [19] : 353 The chromosomes each form a linear array of loops mediated by cohesin, and the lateral elements of the synaptonemal complex assemble forming an "axial element" from which the loops emanate. [20] Recombination is initiated in this stage by the enzyme SPO11 which creates programmed double strand breaks (around 300 per meiosis in mice). [21] This process generates single stranded DNA filaments coated by RAD51 and DMC1 which invade the homologous chromosomes, forming inter-axis bridges, and resulting in the pairing/co-alignment of homologues (to a distance of

Zygotene Edit

Leptotene is followed by the zygotene stage, also known as zygonema, from Greek words meaning "paired threads", [18] : 27 which in some organisms is also called the bouquet stage because of the way the telomeres cluster at one end of the nucleus. [23] In this stage the homologous chromosomes become much more closely (

100 nm) and stably paired (a process called synapsis) mediated by the installation of the transverse and central elements of the synaptonemal complex. [20] Synapsis is thought to occur in a zipper-like fashion starting from a recombination nodule. The paired chromosomes are called bivalent or tetrad chromosomes.

Pachytene Edit

The pachytene stage ( / ˈ p æ k ɪ t iː n / PAK -i-teen), also known as pachynema, from Greek words meaning "thick threads". [18] : 27 is the stage at which all autosomal chromosomes have synapsed. In this stage homologous recombination, including chromosomal crossover (crossing over), is completed through the repair of the double strand breaks formed in leptotene. [20] Most breaks are repaired without forming crossovers resulting in gene conversion. [24] However, a subset of breaks (at least one per chromosome) form crossovers between non-sister (homologous) chromosomes resulting in the exchange of genetic information. [25] Sex chromosomes, however, are not wholly identical, and only exchange information over a small region of homology called the pseudoautosomal region. [26] The exchange of information between the homologous chromatids results in a recombination of information each chromosome has the complete set of information it had before, and there are no gaps formed as a result of the process. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through an ordinary light microscope, and chiasmata are not visible until the next stage.

Diplotene Edit

During the diplotene stage, also known as diplonema, from Greek words meaning "two threads", [18] : 30 the synaptonemal complex disassembles and homologous chromosomes separate from one another a little. However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing-over occurred. The chiasmata remain on the chromosomes until they are severed at the transition to anaphase I to allow homologous chromosomes to move to opposite poles of the cell.

In human fetal oogenesis, all developing oocytes develop to this stage and are arrested in prophase I before birth. [27] This suspended state is referred to as the dictyotene stage or dictyate. It lasts until meiosis is resumed to prepare the oocyte for ovulation, which happens at puberty or even later.

Diakinesis Edit

Chromosomes condense further during the diakinesis stage, from Greek words meaning "moving through". [18] : 30 This is the first point in meiosis where the four parts of the tetrads are actually visible. Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible. Other than this observation, the rest of the stage closely resembles prometaphase of mitosis the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form.

Meiotic spindle formation Edit

Unlike mitotic cells, human and mouse oocytes do not have centrosomes to produce the meiotic spindle. In mice, approximately 80 MicroTubule Organizing Centers (MTOCs) form a sphere in the ooplasm and begin to nucleate microtubules that reach out towards chromosomes, attaching to the chromosomes at the kinetochore. Over time the MTOCs merge until two poles have formed, generating a barrel shaped spindle. [28] In human oocytes spindle microtubule nucleation begins on the chromosomes, forming an aster that eventually expands to surround the chromosomes. [29] Chromosomes then slide along the microtubules towards the equator of the spindle, at which point the chromosome kinetochores form end-on attachments to microtubules. [30]

Metaphase I Edit

Homologous pairs move together along the metaphase plate: As kinetochore microtubules from both spindle poles attach to their respective kinetochores, the paired homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes. This attachment is referred to as a bipolar attachment. The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along the metaphase plate, with respect to the orientation of the other bivalents along the same equatorial line. [17] The protein complex cohesin holds sister chromatids together from the time of their replication until anaphase. In mitosis, the force of kinetochore microtubules pulling in opposite directions creates tension. The cell senses this tension and does not progress with anaphase until all the chromosomes are properly bi-oriented. In meiosis, establishing tension ordinarily requires at least one crossover per chromosome pair in addition to cohesin between sister chromatids (see Chromosome segregation).

Anaphase I Edit

Kinetochore microtubules shorten, pulling homologous chromosomes (which each consist of a pair of sister chromatids) to opposite poles. Nonkinetochore microtubules lengthen, pushing the centrosomes farther apart. The cell elongates in preparation for division down the center. [17] Unlike in mitosis, only the cohesin from the chromosome arms is degraded while the cohesin surrounding the centromere remains protected by a protein named Shugoshin (Japanese for "guardian spirit"), what prevents the sister chromatids from separating. [31] This allows the sister chromatids to remain together while homologs are segregated.

Telophase I Edit

The first meiotic division effectively ends when the chromosomes arrive at the poles. Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells. However, cytokinesis does not fully complete resulting in "cytoplasmic bridges" which enable the cytoplasm to be shared between daughter cells until the end of meiosis II. [32] Sister chromatids remain attached during telophase I.

Cells may enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage.

Meiosis II Edit

Meiosis II is the second meiotic division, and usually involves equational segregation, or separation of sister chromatids. Mechanically, the process is similar to mitosis, though its genetic results are fundamentally different. The end result is production of four haploid cells (n chromosomes, 23 in humans) from the two haploid cells (with n chromosomes, each consisting of two sister chromatids) produced in meiosis I. The four main steps of meiosis II are: prophase II, metaphase II, anaphase II, and telophase II.

In prophase II, we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrosomes move to the polar regions and arrange spindle fibers for the second meiotic division.

In metaphase II, the centromeres contain two kinetochores that attach to spindle fibers from the centrosomes at opposite poles. The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate. [33]

This is followed by anaphase II, in which the remaining centromeric cohesin, not protected by Shugoshin anymore, is cleaved, allowing the sister chromatids to segregate. The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles. [31]

The process ends with telophase II, which is similar to telophase I, and is marked by decondensation and lengthening of the chromosomes and the disassembly of the spindle. Nuclear envelopes re-form and cleavage or cell plate formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes.

Meiosis is now complete and ends up with four new daughter cells.

The origin and function of meiosis are currently not well understood scientifically, and would provide fundamental insight into the evolution of sexual reproduction in eukaryotes. There is no current consensus among biologists on the questions of how sex in eukaryotes arose in evolution, what basic function sexual reproduction serves, and why it is maintained, given the basic two-fold cost of sex. It is clear that it evolved over 1.2 billion years ago, and that almost all species which are descendants of the original sexually reproducing species are still sexual reproducers, including plants, fungi, and animals.

Meiosis is a key event of the sexual cycle in eukaryotes. It is the stage of the life cycle when a cell gives rise to haploid cells (gametes) each having half as many chromosomes as the parental cell. Two such haploid gametes, ordinarily arising from different individual organisms, fuse by the process of fertilization, thus completing the sexual cycle.

Meiosis is ubiquitous among eukaryotes. It occurs in single-celled organisms such as yeast, as well as in multicellular organisms, such as humans. Eukaryotes arose from prokaryotes more than 2.2 billion years ago [34] and the earliest eukaryotes were likely single-celled organisms. To understand sex in eukaryotes, it is necessary to understand (1) how meiosis arose in single celled eukaryotes, and (2) the function of meiosis.

The new combinations of DNA created during meiosis are a significant source of genetic variation alongside mutation, resulting in new combinations of alleles, which may be beneficial. Meiosis generates gamete genetic diversity in two ways: (1) Law of Independent Assortment. The independent orientation of homologous chromosome pairs along the metaphase plate during metaphase I and orientation of sister chromatids in metaphase II, this is the subsequent separation of homologs and sister chromatids during anaphase I and II, it allows a random and independent distribution of chromosomes to each daughter cell (and ultimately to gametes) [35] and (2) Crossing Over. The physical exchange of homologous chromosomal regions by homologous recombination during prophase I results in new combinations of genetic information within chromosomes. [36]

Prophase I arrest Edit

Female mammals and birds are born possessing all the oocytes needed for future ovulations, and these oocytes are arrested at the prophase I stage of meiosis. [37] In humans, as an example, oocytes are formed between three and four months of gestation within the fetus and are therefore present at birth. During this prophase I arrested stage (dictyate), which may last for decades, four copies of the genome are present in the oocytes. The arrest of ooctyes at the four genome copy stage was proposed to provide the informational redundancy needed to repair damage in the DNA of the germline. [37] The repair process used appears to involve homologous recombinational repair [37] [38] Prophase I arrested oocytes have a high capability for efficient repair of DNA damages, particularly exogenously induced double-strand breaks. [38] DNA repair capability appears to be a key quality control mechanism in the female germ line and a critical determinant of fertility. [38]

In life cycles Edit

Meiosis occurs in eukaryotic life cycles involving sexual reproduction, consisting of the constant cyclical process of meiosis and fertilization. This takes place alongside normal mitotic cell division. In multicellular organisms, there is an intermediary step between the diploid and haploid transition where the organism grows. At certain stages of the life cycle, germ cells produce gametes. Somatic cells make up the body of the organism and are not involved in gamete production.

Cycling meiosis and fertilization events produces a series of transitions back and forth between alternating haploid and diploid states. The organism phase of the life cycle can occur either during the diploid state (diplontic life cycle), during the haploid state (haplontic life cycle), or both (haplodiplontic life cycle, in which there are two distinct organism phases, one during the haploid state and the other during the diploid state). In this sense there are three types of life cycles that utilize sexual reproduction, differentiated by the location of the organism phase(s). [ citation needed ]

In the diplontic life cycle (with pre-gametic meiosis), of which humans are a part, the organism is diploid, grown from a diploid cell called the zygote. The organism's diploid germ-line stem cells undergo meiosis to create haploid gametes (the spermatozoa for males and ova for females), which fertilize to form the zygote. The diploid zygote undergoes repeated cellular division by mitosis to grow into the organism.

In the haplontic life cycle (with post-zygotic meiosis), the organism is haploid instead, spawned by the proliferation and differentiation of a single haploid cell called the gamete. Two organisms of opposing sex contribute their haploid gametes to form a diploid zygote. The zygote undergoes meiosis immediately, creating four haploid cells. These cells undergo mitosis to create the organism. Many fungi and many protozoa utilize the haplontic life cycle. [ citation needed ]

Finally, in the haplodiplontic life cycle (with sporic or intermediate meiosis), the living organism alternates between haploid and diploid states. Consequently, this cycle is also known as the alternation of generations. The diploid organism's germ-line cells undergo meiosis to produce spores. The spores proliferate by mitosis, growing into a haploid organism. The haploid organism's gamete then combines with another haploid organism's gamete, creating the zygote. The zygote undergoes repeated mitosis and differentiation to become a diploid organism again. The haplodiplontic life cycle can be considered a fusion of the diplontic and haplontic life cycles. [39] [ citation needed ]

In plants and animals Edit

Meiosis occurs in all animals and plants. The end result, the production of gametes with half the number of chromosomes as the parent cell, is the same, but the detailed process is different. In animals, meiosis produces gametes directly. In land plants and some algae, there is an alternation of generations such that meiosis in the diploid sporophyte generation produces haploid spores. These spores multiply by mitosis, developing into the haploid gametophyte generation, which then gives rise to gametes directly (i.e. without further meiosis). In both animals and plants, the final stage is for the gametes to fuse, restoring the original number of chromosomes. [40]

In mammals Edit

In females, meiosis occurs in cells known as oocytes (singular: oocyte). Each primary oocyte divides twice in meiosis, unequally in each case. The first division produces a daughter cell, and a much smaller polar body which may or may not undergo a second division. In meiosis II, division of the daughter cell produces a second polar body, and a single haploid cell, which enlarges to become an ovum. Therefore, in females each primary oocyte that undergoes meiosis results in one mature ovum and one or two polar bodies.

Note that there are pauses during meiosis in females. Maturing oocytes are arrested in prophase I of meiosis I and lie dormant within a protective shell of somatic cells called the follicle. At the beginning of each menstrual cycle, FSH secretion from the anterior pituitary stimulates a few follicles to mature in a process known as folliculogenesis. During this process, the maturing oocytes resume meiosis and continue until metaphase II of meiosis II, where they are again arrested just before ovulation. If these oocytes are fertilized by sperm, they will resume and complete meiosis. During folliculogenesis in humans, usually one follicle becomes dominant while the others undergo atresia. The process of meiosis in females occurs during oogenesis, and differs from the typical meiosis in that it features a long period of meiotic arrest known as the dictyate stage and lacks the assistance of centrosomes. [41] [42]

In males, meiosis occurs during spermatogenesis in the seminiferous tubules of the testicles. Meiosis during spermatogenesis is specific to a type of cell called spermatocytes, which will later mature to become spermatozoa. Meiosis of primordial germ cells happens at the time of puberty, much later than in females. Tissues of the male testis suppress meiosis by degrading retinoic acid, proposed to be a stimulator of meiosis. This is overcome at puberty when cells within seminiferous tubules called Sertoli cells start making their own retinoic acid. Sensitivity to retinoic acid is also adjusted by proteins called nanos and DAZL. [43] [44] Genetic loss-of-function studies on retinoic acid-generating enzymes have shown that retinoic acid is required postnatally to stimulate spermatogonia differentiation which results several days later in spermatocytes undergoing meiosis, however retinoic acid is not required during the time when meiosis initiates. [45]

In female mammals, meiosis begins immediately after primordial germ cells migrate to the ovary in the embryo. Some studies suggest that retinoic acid derived from the primitive kidney (mesonephros) stimulates meiosis in embryonic ovarian oogonia and that tissues of the embryonic male testis suppress meiosis by degrading retinoic acid. [46] However, genetic loss-of-function studies on retinoic acid-generating enzymes have shown that retinoic acid is not required for initiation of either female meiosis which occurs during embryogenesis [47] or male meiosis which initiates postnatally. [45]

Flagellates Edit

While the majority of eukaryotes have a two-divisional meiosis (though sometimes achiasmatic), a very rare form, one-divisional meiosis, occurs in some flagellates (parabasalids and oxymonads) from the gut of the wood-feeding cockroach Cryptocercus. [48]

Recombination among the 23 pairs of human chromosomes is responsible for redistributing not just the actual chromosomes, but also pieces of each of them. There is also an estimated 1.6-fold more recombination in females relative to males. In addition, average, female recombination is higher at the centromeres and male recombination is higher at the telomeres. On average, 1 million bp (1 Mb) correspond to 1 cMorgan (cm = 1% recombination frequency). [49] The frequency of cross-overs remain uncertain. In yeast, mouse and human, it has been estimated that ≥200 double-strand breaks (DSBs) are formed per meiotic cell. However, only a subset of DSBs (

5–30% depending on the organism), go on to produce crossovers, [50] which would result in only 1-2 cross-overs per human chromosome.

Nondisjunction Edit

The normal separation of chromosomes in meiosis I or sister chromatids in meiosis II is termed disjunction. When the segregation is not normal, it is called nondisjunction. This results in the production of gametes which have either too many or too few of a particular chromosome, and is a common mechanism for trisomy or monosomy. Nondisjunction can occur in the meiosis I or meiosis II, phases of cellular reproduction, or during mitosis.

Most monosomic and trisomic human embryos are not viable, but some aneuploidies can be tolerated, such as trisomy for the smallest chromosome, chromosome 21. Phenotypes of these aneuploidies range from severe developmental disorders to asymptomatic. Medical conditions include but are not limited to:

    – trisomy of chromosome 21 – trisomy of chromosome 13 – trisomy of chromosome 18 – extra X chromosomes in males – i.e. XXY, XXXY, XXXXY, etc. – lacking of one X chromosome in females – i.e. X0 – an extra X chromosome in females – an extra Y chromosome in males.

The probability of nondisjunction in human oocytes increases with increasing maternal age, [51] presumably due to loss of cohesin over time. [52]

In order to understand meiosis, a comparison to mitosis is helpful. The table below shows the differences between meiosis and mitosis. [53]

Meiosis Mitosis
End result Normally four cells, each with half the number of chromosomes as the parent Two cells, having the same number of chromosomes as the parent
Function Production of gametes (sex cells) in sexually reproducing eukaryotes with diplont life cycle Cellular reproduction, growth, repair, asexual reproduction
Where does it happen? Almost all eukaryotes (animals, plants, fungi, and protists) [54] [48]
In gonads, before gametes (in diplontic life cycles)
After zygotes (in haplontic)
Before spores (in haplodiplontic)
All proliferating cells in all eukaryotes
Steps Prophase I, Metaphase I, Anaphase I, Telophase I,
Prophase II, Metaphase II, Anaphase II, Telophase II
Prophase, Prometaphase, Metaphase, Anaphase, Telophase
Genetically same as parent? No Yes
Crossing over happens? Yes, normally occurs between each pair of homologous chromosomes Very rarely
Pairing of homologous chromosomes? Yes No
Cytokinesis Occurs in Telophase I and Telophase II Occurs in Telophase
Centromeres split Does not occur in Anaphase I, but occurs in Anaphase II Occurs in Anaphase

How a cell proceeds to meiotic division in meiotic cell division is not well known. Maturation promoting factor (MPF) seemingly have role in frog Oocyte meiosis. In the fungus S. pombe. there is a role of MeiRNA binding protein for entry to meiotic cell division. [55]

It has been suggested that Yeast CEP1 gene product, that binds centromeric region CDE1, may play a role in chromosome pairing during meiosis-I. [56]

Meiotic recombination is mediated through double stranded break, which is catalyzed by Spo11 protein. Also Mre11, Sae2 and Exo1 play role in breakage and recombination. After the breakage happen, recombination take place which is typically homologous. The recombination may go through either a double Holliday junction (dHJ) pathway or synthesis-dependent strand annealing (SDSA). (The second one gives to noncrossover product). [57]

Seemingly there are checkpoints for meiotic cell division too. In S. pombe, Rad proteins, S. pombe Mek1 (with FHA kinase domain), Cdc25, Cdc2 and unknown factor is thought to form a checkpoint. [58]

In vertebrate oogenesis, maintained by cytostatic factor (CSF) has role in switching into meiosis-II. [56]

Discrimination resources

If you have questions about policies or concerns about discrimination in your workplace, the human resource department is often a good place to start. To learn more about discrimination in housing and employment, or to file a complaint, visit:

1 Pascoe, E. A. & Richman, L. S. (2009). Perceived discrimination and health: A meta-analytic review. Psychological Bulletin 135(4): 531-554. Doi: 10.1037/a0016059

2 Kinderman, P., Schwannauer, M., Pontin, E., & Tai, S. (2013). Psychological processes mediate the impacts of familial risk, social circumstances and life events on mental health. PLoS One 8(10), e76564.

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