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BIO101 Study Guide

Unit 8: Cellular Reproduction: Meiosis

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

  • What is the difference between the gamete and somatic cell?
  • How do diploid and haploid cells differ?
  • Why is a haploid cell unable to undergo meiosis?
  • Which types of cells can undergo mitosis?

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. The cells that contain all of the chromosomes are somatic cells, which we also call body cells. Some body cells are in sex organs that reproduce gametes. Somatic cells are diploid (contain two sets of chromosomes), whereas gametes are haploid (have one set of chromosomes).

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

  • What happens during each phase of meiosis?
  • Where in the body does meiosis take place?
  • Can prokaryotes undergo meiosis? Why or why not?
  • Is there any chance of error inherent in meiosis?

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.

Meiosis proceeds in essentially the same way in any eukaryotic cell. 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).

The stages of meiosis are illustrated below for a species with N = 2. The original diploid cell in this example (2N), therefore, has 2 × 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.

Meiosis

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8c. Compare mitosis I and meiosis II

  • What are the similarities and differences between mitosis and meiosis?
  • Why is there only one division in mitosis but two divisions in meiosis?
  • How do organisms benefit from being able to perform both mitosis and meiosis?

Mitosis and meiosis are alternative processes in eukaryotic cell division. Here are key similarities:

  • Mitosis and meiosis both divide the nucleus of a cell.
  • Both processes occur in phases that include prophase, prometaphase, metaphase, anaphase, and telophase.
  • Meiosis II is essentially identical to mitosis, but meiosis II occurs in the two cells produced previously in meiosis I.

Here are the 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 the parent cell.
  • Mitosis produces duplicate cells to help grow a multicellular organism or replace lost cells; meiosis produces haploid cells out of a diploid cell for the purpose of sexual reproduction.
  • Mitosis only involves 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 (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 the anaphase of mitosis, sister chromatids separate; during the anaphase of meiosis I, homologs separate.
  • Mitosis maintains the ploidy; meiosis cuts the ploidy in half.

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8d. Compare mitosis and meiosis in terms of their parent and daughter cells

  • How does the purpose of meiosis differ from that of mitosis?
  • What aspects of meiosis support genetic variation?
  • What would happen if there was only one division in meiosis?
  • What is the difference in ploidy between somatic cells and gametes?

While mitosis and meiosis share several similarities, some critically important differences 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 (the number of sets of chromosomes in a cell) 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 octoploid, etc.), and the zygote would not be able to contain the DNA.

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

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 (the exchange of genes between homologous chromosomes during prophase of meiosis I) and independent assortment (the random alignment of homologous chromosomes during metaphase of meiosis I).

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8e. Explain the role of meiosis in adaptation and evolution

  • What is survival of the fittest?
  • Why is genetic change necessary in today's ecosystems?
  • What types of alleles might be disadvantageous to a species?
  • How does genetic change occur in a population through meiosis?

Due to the process of crossing over (of homologous chromosomes) in Prophase I, the resulting daughter cells after meiosis are genetically unique. This creates variation and, ultimately, the survival of the fittest. Since offspring have new combinations of alleles, they increase the possibility of their survival until reproductive age and continue their species.

These small genetic changes over time create the variety of species and subspecies we see today. Those with alleles that give them an ecological advantage over others survive to mate. Those that lack them, die off taking their non-adaptable alleles with them and out of the breeding population.

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Unit 8 Vocabulary

This vocabulary list includes terms you will need to know to successfully complete the final exam.

  • crossing over
  • diploid
  • fertilization
  • gamete
  • haploid
  • haploid number
  • independent assortment
  • meiosis
  • ploidy
  • somatic cell
  • survival of the fittest
  • synapsis
  • zygote