Mitosis versus Meiosis

Sections

Full-Length Text

KEY DIFFERENCES BETWEEN MITOSIS AND MEIOSIS

1. Parent cell types

• Mitosis: diploid somatic cell
• Meiosis: diploid germ line cell (reproductive cell precursor)

2. Tetrad formation (Meiosis only)

• Prophase I

3. Crossing over (Meiosis only)

• Chiasmata: site of genetic recombination, occurs in prophase I

4. Synaptonemal complex (Meiosis only)

• Zipper-like protein structure that holds homologues together: specific to meiosis I

5. Kinetochore orientation

• Mitosis: sister chromatid kinetochores face opposite poles
• Meiosis I: sister chromatid kinetochores face the same pole
• Meiosis II: sister chromatid kinetochores face opposite poles

6. Genetic variability (Meiosis only)

• Crossing over (genetic recombination)
• Random fertilization
• Independent assortment: each tetrad positions itself on the metaphase plate independently of other tetrads
• Meiosis produces four genetically distinct haploid daughter cells
• Mitosis produces two genetically identical diploid daughter cells

Full-Length Text

• Here we will learn the key differences between meiosis and mitosis.

• Start a table.

• Denote the six key differences, now, which we'll learn through this tutorial:
  • Parent cell types
  • Tetrad formation
  • Crossing over
  • Synaptonemal complex
  • Kinetochore orientation
  • Genetic variability

Let's begin with the parent cells: the first key difference between the two processes.

• For mitosis, draw a somatic cell with two sets of 23 chromosomes.
  • Indicate that it is diploid (2n), (46 chromosomes).
  • Write that somatic cells comprise most of the body's cells.

• For meiosis, draw a germ line cell with two sets of 23 chromosomes.
  • Indicate that it is also diploid (2n), (46 chromosomes).
  • Write that germ line cells are reproductive cell precursors, which, as we'll see, are haploid.

Now, let's illustrate the distribution of genetic material in each process.

• Indicate that DNA replicates in the S-phase of mitosis and meiosis.
  • This step is identical in both processes.

• Draw our somatic parent cell in prophase of mitosis.

• Show that each condensed chromosome pairs with its duplicate at the centromere.

• Now, draw our germ line cell in prophase I of meiosis.

• Show that the homologous chromosomes pair with each other as a tetrad, which marks the second key difference between mitosis and meiosis: we truncate the chromosomes for reasons we'll show in a moment.
  • Paternal and maternal chromosomes are homologous when they contain the same genes in the same order, but contain varying allelic combinations.

• Now, fill the truncated segment of the chromosomes with a segment of the opposite homologue.

• Label this as the chiasma (plural: chiasmata), which is the site of genetic recombination; indicate that this particular mode of genetic recombination is called crossing-over, which marks the third key difference between mitosis and meiosis.

This brings us to our fourth major difference: the synaptonemal complex. Let's take a closer look at the tetrad to illustrate this structure.

• Draw a portion of the tetrad, particularly the region in which paternal and maternal homologues join, as two rows of chromatin loops.

• Indicate that a zipper-like protein structure joins the homologues.

• Write that the synaptonemal complex, which is specific to meiosis I, functions in synapsis; it holds homologues together and keeps them closely aligned until anaphase I.

Now, let's draw metaphase of mitosis.

• Show that sister chromatids line up along the metaphase plate.

• Specifically, indicate that the kinetochores in each sister chromatid pair face opposite poles.
  • Thus, when microtubules depolymerize, they pull the chromatids within each pair to opposite sides of the dividing cell.

Now, draw metaphase of meiosis I to illustrate the fifth key difference between these processes.

• Show that the tetrad aligns on the metaphase plate.

Let's take a closer look at this tetrad.

• Draw the tetrad and include the chiasma.

• Now, draw kinetochores on the outward facing sides of the homologous pairs.
  • In meiosis, the kinetochores of sister chromatids are adjacent to one another.

• Now draw spindle fibers that connect to the kinetochores.

• Write that the kinetochores of sister chromatids face the same spindle pole.
  • This is different from metaphase of mitosis, in which sister chromatid kinetochores face opposite spindle poles.

• Next, illustrate that when the microtubules depolymerize in anaphase I, they pull homologous chromosomes apart.

• Finally, show that after anaphase I, telophase I and cytokinesis, the resulting two daughter cells are haploid.

• Indicate that anaphase, telophase and cytokinesis in mitosis produce two diploid daughter cells.
  • These daughter cells are genetically identical.

• Illustrate that meiosis II begins almost immediately after the first cytokinesis, and produces four haploid daughter cells.

• Finally, indicate that each of these daughter cells is genetically distinct from each other and from their parent cell.

This brings us to the final key difference between mitosis and meiosis: genetically distinct daughter cells.

• Show that there are three facets to genetic variability in meiotic division.
  • Crossing over (genetic recombination), which we've already learned.
  • Random fertilization; a single male gamete must fuse with a single female gamete, which multiplies the genetic variability generated by independent assortment.
  • Independent assortment.

• Write that independent assortment refers to the fact that each tetrad positions itself on the metaphase plate independently of the other tetrads, which produces about 8 million possible combinations of chromosomes in haploid gametes!

Let's visualize how, now.

Start with two cells in metaphase I:

• In the first cell, draw representative paternal chromosomes on one side of the metaphase plate and maternal chromosomes on the other.

• In the second cell, draw representative paternal and maternal chromosomes on opposite sides of the metaphase plate.

Now, below each of these possibilities, draw two cells to represent metaphase II.

• Draw the alignment of chromosomes on the metaphase plate in each of these cells.
  • Note that they are each distinct from each other.

• Finally, draw two more cells below each of the cells in metaphase II.
  • These are the final haploid daughter cells.

• Fill in the chromosomes.

• Combined with crossing over during metaphase I and random fertilization, meiosis produces genetic variation within a given population.
  • This marks the most significant distinction between mitotic and meiotic cell division.

References:

1. Campbell, N. A. & Reece, J. B. Biology, 7th ed. (Pearson Benjamin Cummings, 2005).

2. Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K. & Walter, P. Molecular Biology of the Cell, 5th ed. (Garland Science, 2008).

3. Alberts, B., Bray, D., Hopkin, K., Johnson, A., Lewis, J., Raff, M., Roberts, K. & Walter, P. Essential Cell Biology, 3rd ed. (Garland Science, 2010).

4. Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Scott, M. P., Bretscher, A., Ploegh, H. & Matsudaira, P. Molecular Cell Biology, 6th ed. (W. H. Freeman and Company, 2008).

5. Marieb, E. N. & Hoehn, K. Human Anatomy & Physiology, 10th ed. (Pearson, 2016).

6. Polinsky, K. R. Tumor Suppressor Genes. (Nova Publishers, 2007).

7. Coleman, W. B. & Tsongalis, G. J. The Molecular Basis of Human Cancer. (Springer Science & Business Media, 2001).