Meiosis Part II

Sections

Full-Length Text

HUMAN CELLS

1. Somatic cells: majority of the body's cells

• 46 chromosomes – diploid (2n)

2. Reproductive cells: sperm and egg cells (gametes)

• 23 chromosomes – haploid (1n)
• Diploid germ line cells: precursors for reproductive cells; undergo meiosis

FERTILIZATION

• 1 egg and 1 sperm fuse to form zygote (2n)
• Followed by repeated cycles of mitosis to produce multicellular organism (2n)

INTERPHASE

• Parent cell (2n): two sets of 23 chromosomes
• Homologous chromosomes: contain the same genes in the same order, each from a different parent (contain different alleles)
• S-phase: each set of 23 chromosomes duplicates (92 chromosomes total), sister chromatids pair at the centromere

PROPHASE I

• >90% of meiosis
• Chromosomes condense
• Tetrad forms via synapsis: each gene aligns with its homologue (4 chromatids)
• Synaptonemal complex: zipper-like structure holds chromosomes together until crossing over occurs
• Crossing over: paternal chromosome crosses over to maternal and vice versa
• Chiasma (site of crossing over) holds tetrad together after synaptonemal complex disassembles

Other features of this phase:

• Nuclear envelope fragments
• Nucleolus disperses
• Centrosomes move to opposite poles
• Microtubules form spindle & attach kinetochores of homologous chromosomes

METAPHASE I

• Tetrads align on metaphase plate
• Sister chromatids face same pole
• Homologous chromosomes face opposite poles

ANAPHASE I

• Homologous chromosomes separate

TELOPHASE I AND CYTOKINESIS

• Two haploid daughter cells: 1 tetrad in each

PROPHASE II

• Each cell has one duplicated set of 23 chromosomes

METAPHASE II

• Sister chromatids line up on metaphase plate and face opposite poles

ANAPHASE II

• Sister chromatids separate

TELOPHASE II

• Nuclear envelope reforms
• Nucleolus reappears
• Mitotic spindles depolymerize
• Cleavage furrow

CYTOKINESIS

• 4 haploid daughter cells
• Daughter cells genetically distinct from each other and parent cells
• Each develops into reproductive cell (egg or sperm cells)

CLINICAL CORRELATION
Down's Syndrome (Trisomy 21): aneuploid gametes

• Nondisjunction: chromosome 21 fails to separate properly during meiosis I
• 2 daughter cells with extra chromosome 21 copy
• 2 daughter cells missing chromosome 21
• Trisomy 21: gamete with extra chromosome fuses with normal gamete during fertilization = zygote with 3 copies of chromosome 21

Full-Length Text

• Here we will finish learning meiosis.

Pick up with metaphase I.

• Draw a cell with a centrosome (and two centrioles) at each pole.

• Show that microtubules extend from them in all directions.
  • Microtubules bind kinetochores and help "stretch" the cell before it divides into two daughters.

• Indicate that the tetrad lines up on a region called the metaphase plate.

• Now, draw the tetrad, and show that each homologue faces the opposite spindle pole.

• Next, illustrate that microtubule spindle fibers attach to the kinetochores of each homologue.

• To clarify this point, write that:
  • Sister chromatids (their kinetochores) face the same pole.
  • Whereas homologous chromosomes (their kinetochores) face opposite poles.
  • This is unlike in metaphase of mitosis in which sister chromatid kinetochores face opposite poles.

Next, let's draw anaphase I.

• Draw an elongated cell.

• Again, draw centrosomes at opposite poles of the cell.

• Show that the spindle pulls homologous chromosomes apart, disrupting the chiasmata that are generated by crossing over.

Now, let's draw telophase I and cytokinesis in a single diagram.

• Indicate these final stages produce two daughter cells that are each haploid.

• (There are two cells that each have one set of 23 chromosomes that is, itself duplicated: 1 x 23 x 2 = 46).

• Here, draw a cell with duplicated centrosomes and a prominent cleavage furrow.

• Draw a homologue on either side of the cleavage furrow.

This brings us to meiosis II.

Begin with prophase II, which initiates upon cytokinesis, before the chromosomes even decondense.

• Indicate that we begin with 2 cells, which are each haploid, and contain a duplicated number of chromosomes (46).

• Now, draw the two haploid cells.

• Draw duplicated centrosomes and centrioles in each cell.

• Show that the spindle reforms in each cell as well.
  • Note that meiosis II resembles mitosis more closely (than meiosis I).

Now, let's draw metaphase II.

• Draw another pair of cells.

• Show the centrosomes at opposite poles.

• Indicate that the sister chromatids align on the metaphase plate.

• Now, let's note some key characteristics about the sister chromatids in this phase.
  • Write that sister chromatids (their kinetochores) face opposite poles.

• Pause to consider: How does this differ from metaphase I?
  • In metaphase I, sister chromatids face the same pole.

• How does this differ from mitosis?
  • It doesn't. Except that the sister chromatids are not genetically identical due to crossing over.

Next, let's draw anaphase II.

• Again, draw two cells with a centrosome at each pole.

• Now, indicate that the spindle pulls sister chromatids to opposite poles of the cell.

Now, draw telophase II.

• To illustrate telophase II, draw two cells, each with a cleavage furrow.

• Draw a sister chromatid on each side of each cleavage furrow.
  • They completely decondense by the end of this phase.

• Finally, show that the nuclear envelope begins to reform, the nucleolus reappears and the spindle depolymerizes.

Finally, draw cytokinesis, which is the division of cytoplasm.

• Indicate that this stage produces four daughter cells that are each haploid.
  • (There are four cells that each have one set of 23 chromosomes).

• Now, draw four individual cells to illustrate cytokinesis.

• Illustrate that the nuclear envelope reforms in each of these cells.

• Show that each cell contains one centrosome with a pair of centrioles.

• Finally, draw a chromatin fiber in each of these four cells.

• Write that each of these daughter cells genetically differ from each other as well as their parent cell.

Now, return to our first diagram, in which we tracked the formation and fertilization of gametes.

• Now, indicate that the four haploid daughter cells we have just drawn develop into reproductive cells: egg or sperm cells, depending on the gender of the individual.

• As a clinical correlation, write that Down's Syndrome (Trisomy 21) results from aneuploid gametes.
  • Specifically, there is nondisjunction: chromosome 21 fails to separate properly during meiosis I.
  • Denote that this produces two daughter cells with an extra copy of chromosome 21, and two daughter cells without chromosome 21.
  • Trisomy of chromosome 21 (a zygote with 3 copies of chromosome 21) occurs when a gamete with an extra chromosome fuses with a normal gamete during fertilization, which produces a zygote with three copies of chromosome 21 (trisomy 21).

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).