BIO101 Study Guide

Unit 8: Cellular Reproduction: Meiosis

8a. Identify the different types of daughter cells produced by meiotic cell division

• What is the difference between the gamete and somatic cell?
• How do diploid and haploid cells differ from each other?

Organisms that reproduce sexually must create cells that have half of their chromosomes. A gamete is a cell formed by meiosis that is non-identical to the original parent cell. We call the cells that contain all of the chromosomes somatic cells, which we also call body cells. Some body cells are in sex organs that reproduce gametes. Somatic cells are diploid; gametes are haploid.

Review The Cellular Basis of Reproduction for an overview of the different cells meiosis produces.

8b. Diagram and label the phases of meiosis given a number of chromosomes or chromosome pairs

• What happens during each phase of meiosis?

Meiosis proceeds in essentially the same way in any eukaryotic cell that undergoes meiosis. The main difference is the number of chromosomes involved. The number of chromosomes depends on the species. The variable, $N$, represents the number of different kinds of chromosomes. We also call it the haploid number. This is because a haploid cell contains just one of each type of chromosome. A diploid cell is characterized as $2N$, because a diploid cell has two of each type of chromosome (one from each sexual parent).

Meiosis allows one diploid cell to become four haploid cells. Each haploid cell is not only genetically different from the original diploid cell, but each haploid cell is also genetically different from the other three haploid cells produced. The stages of meiosis are illustrated below for a species with $N=2$. The original, diploid cell in this example ($2N$) therefore has $2\times 2=4$ overall chromosomes. Each of the four cells produced has $N=2$ overall chromosomes (they are haploid). Whatever the value of $N$, during metaphase of meiosis I, $N$ pairs of homologous, replicated chromosomes line up, and during metaphase of meiosis II, $N$ individual, replicated chromosomes line up.

Review Meiosis. You should be able to draw what the stages look like for any other value of $N$.

8c. Compare and contrast mitosis and meiosis

• How are mitosis and meiosis similar?
• How are mitosis and meiosis different?

Mitosis and meiosis are two alternative processes that can be involved in eukaryotic cell division. Here are key similarities:

• Mitosis and meiosis are both processes that divide the nucleus of a cell.

• Both processes occur in phases including prophase, prometaphase, metaphase, anaphase, and telophase.

• Meiosis II is essentially identical to mitosis, but meiosis II occurs in each of the two cells previously produced in meiosis I.

Here are key differences:

• Mitosis produces two cells that are genetically identical to the parent cell; meiosis produces four cells that are genetically distinct from each other and from the parent cell.

• Mitosis is used to produce duplicate cells for the purpose of growth of a multicellular organism or replacement of lost cells; meiosis is used to produce haploid cells out of a diploid cell for the purpose of sexual reproduction.

• Mitosis involves only one round of division; meiosis involves two rounds of division (meiosis I and II).

• In mitosis, chromosomes act individually, and homologous chromosomes do not synapse; in meiosis, homologous chromosomes go through synapsis or come close together, and each homologous pair acts throughout meiosis I as a unit.

• Mitosis does not feature crossing over; meiosis I features crossing over.

• During the metaphase of mitosis, individual, replicated chromosomes line up midway between poles (without pairing of homologs); During the metaphase of meiosis I, homologous pairs of chromosomes line up as tetrads midway between poles.

• During anaphase of mitosis, sister chromatids separate; in anaphase of meiosis I, homologs separate.

• Mitosis maintains the ploidy; meiosis cuts the ploidy in half.

Review Meiosis and Mitosis, Meiosis, and Sexual Reproduction to compare and contrast these important processes.

8d. Explain the role of meiosis

• What is the purpose of meiosis?
• How do aspects that make meiosis different from mitosis allow meiosis to fulfill that purpose?

While there are several similarities between mitosis and meiosis, there are some critically important differences that allow the two processes to serve different purposes.

The life cycle of any sexual species features fertilization, which is the fusion of unicellular gametes (one male gamete and one female gamete) to produce a unicellular zygote. The unicellular zygote that fertilization produces carries chromosomes from both gametes. Therefore, the ploidy of the zygote is double the ploidy of the gametes.

If fertilization were the only process occurring each generation, the ploidy would double each generation (tetraploid, then octaploid, etc.), and the zygote would not be able to contain the DNA.

To prevent the ploidy from doubling each generation, a separate process is needed to cut the ploidy in half. That process is meiosis.

Specifically, the reduction of ploidy occurs in meiosis I, when homologs separate and go to distinct daughter cells. Since a diploid cell that undergoes meiosis will produce haploid cells (gametes), when these haploid gametes fuse (in fertilization), the zygote will be diploid. By alternating meiosis and fertilization each generation, the ploidy simply goes back and forth between haploidy and diploidy (rather than continually increasing).

Another important purpose of meiosis is to drastically increase the genetic variability of the gametes produced. This increase in genetic variability comes in the forms of crossing over (during prophase of meiosis I) and independent assortment (during metaphase of meiosis I).

Review Mitosis, Meiosis, and Sexual Reproduction. Crossing over and independent assortment produce new genetic combinations, and separation of homologs reduces the ploidy.

Unit 8 Vocabulary

You should be familiar with these terms as you prepare for the final exam.

• 2N
• anaphase
• chromosome
• crossing-over
• diploid
• eukaryotic
• fertilization
• gamete
• genetic variability
• haploid
• homolog
• homologous
• independent assortment
• meiosis
• meiosis I
• meiosis II
• metaphase
• mitosis
• N
• nucleus
• ploidy
• prophase
• synapsis
• telophase