Meiosis

Question 1

How do you characterize autosomes and sex chromosomes? (3 things)

Answer 1

1. Size
2. Centromere Index: length of short arm (parm)/ total chromosome length
3. Chromosome G-bright are regions that have less adenine to thymine as compared to guanine to cytosine. These regions contain SINE and Alu sequences, and contain house keeping genes rather than tissue specific genes. It is common in actively transcribed DNA.

Question 2

What generally occurs during meiosis and its phases?

Answer 2

Meiosis 1: The DNA replicates (during S phase) and by the end of Prophase 1 we can see the sister chromatids.

Prophase 1: the pairing of replicated homologous chromosomes

Metaphase I: chromosomes are ordered in the middle of the cell

Anaphase I: Chiasmata at the chromosome ends, paired chromosomes migrate toward the poles.

Interkinesis I: formation of nuclei and 2 daughter cells.

Meiosis II: no DNA replication and end with four haploid cells.

Question 3

What is the difference between meiosis in males vs females.

Answer 3

Meiosis I is followed by Meiosis II at puberty and this finalizes with 4 spermatids.

For females, meiosis I begins before birth. Then it stops before birth and this is the diplotene phase. only 20% will survive in the Dictyotene phase until puberty. Meiosis I is completed when the 1st polar body is extruded during ovulation. Then completion of M-II is completed with the extrusion of the second polar body at fertilization.

The second polar body is haploid which is a sign that meiosis II has been completed.

One way to tell between nondisjunction in meiosis I and II is that no normal gametes are formed in meiosis I but two normal ones are formed in meiosis II.

Question 4

What is uniparental disomy?

Answer 4

It is when you inherit two copies from a parent from a disomy (this could be done from M1 or M2 nondisjunction. Then when the other gamete joins and forms a trisomy zygote that chromatid is removed. So for one pair of chromosomes, they’re both from one parent. THis could lead to the expression of AR gene mutations from only one affected parent and father to son transmission of X linked diseases.