How many chromosomes and chromatids are present at/in

Question

Somatic cells of a species who sex-determination system is similar to that of humans, contain 46 chromosomes. How many chromosomes and chromatids are present at/in: (6 marks)

a) anaphase of mitosis? 

b) prophase of meiosis 1?

c) anaphase 2 of meiosis

d) an oogonial cell prior to S phase?

e) a secondary oocyte arrested prior to fertilization?

f)  a sperm cell?

Answer 1

This was always a challenging topic for some of my students back at University. Lets first make a few things clear. There are 2 sister chromatids in a single duplicated chromosome. Those sister chromatids are not considered chromosomes until they are separated in anaphase. Please note that anaphase and telophase are different from cytokinesis

a.) 96 chromosomes and 0 chromatids. As mentioned above, when a duplicated chromosome separates, each chromatid is a chromosome.

b.) 46 chromosomes each composed of 2 sister chromatids. 96 chromatids

c.) Anaphase II separates sister chromatids and not homologous chromosomes so 46 chromosomes and 0 chromatids after the separation.

d.) Oogonium are diploid so before S phase there are 46 chromosomes and 0 chromatids.

e.) Secondary Oocytes have 23 chromosomes and 0 chromatids

f.) Sperm have 23 chromosomes and 0 chromatids