Unit 8 Study Guide: Cellular Reproduction: Meiosis

8a. Given a number of chromosomes or chromosome pairs, diagram and label the phases of meiosis.

The way in which meiosis proceeds is essentially the same in any eukaryotic cell that undergoes meiosis. The principle difference is the number of chromosomes involved. That number of chromosomes depends on the species. The number of different kinds of chromosomes is represented by the variable, N, which is known as 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 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 2×2=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.


As you study this figure and the accompanying text in this section, you should be able to draw what the stages look like for any other value of N.

 

8b. Compare and contrast mitosis and meiosis.

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

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

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 synapse, and each homologous pair acts throughout meiosis I as a unit.
  • Mitosis does not feature crossing over; meiosis I features crossing over.
  • In metaphase of mitosis, individual, replicated chromosomes line up midway between poles (without pairing of homologs); in metaphase of meiosis I, homologous pairs of replicated chromosomes line up midway between poles.
  • In anaphase of mitosis, sister chromatids separate; in anaphase of meiosis I, homologs separate.
  • Mitosis maintains the ploidy; meiosis cuts the ploidy in half.

As you review meiosis by studying this section, you should also view these lectures to compare and contrast these important processes.

 

8c. Explain the role of meiosis.

Although there are several similarities between mitosis and meiosis, there are crucially important differences, allowing the two processes to serve separate purposes.

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

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 produced by fertilization carries the 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, then 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 those 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. That increase in genetic variability comes in the forms of crossing over (during prophase of meiosis I) and independent assortment (during metaphase of meiosis I). As you study the phases of meiosis, appreciate that crossing over and independent assortment produce new genetic combinations, and separation of homologs reduces the ploidy.

 

Unit 8 Vocabulary

This vocabulary list includes terms that might help you with the review items above and some terms you should be familiar with to be successful in completing the final exam for the course.

Try to think of the reason why each term is included.

  • 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
  • Prometaphase
  • Prophase
  • Synapse
  • Telophase
  • Zygote
Last modified: Wednesday, July 17, 2019, 5:49 PM