Question

So, I'm confused with the S-phase in interphase. Would the number of chromosomes double in number during the S-phase? Wouldn't there be 23 pairs of chromosomes (46 total) before Sphase and 43 pairs (92 chromosomes) after S-phase? Or am I confusing them with the number of chromatids? How many chromosomes would there be during Anaphase I of meiosis, and Telephase II of meiosis (before cytokenesis)?

Answer 1

Yes, the number of chromosomes would be doubled during the S-phase. In case of diploid organism, number of chromosomes are 2n=46 before S-phase and doubles to 92 after S-phase but which remain as 4n=92 chromatids as they though doubled but remain attached at centromere and get separated only at the end of M phase. That means n=23 (46/2=23; 92/4=23) in both cases in humans. Thus, 4n=92 are the total number of chromatids produced in the cell after one round of replication. One chromosome replicates to produce two sister chromatids. Thus a pair of homologous chromosomes replicates to produce a total of 4 sister and non-sister chromatids.

The replication is confined to a single round during a complete cell cycle i.e. G₁->S->G₂->M.

When the cell enters the M phase in Meiosis I, during the Anaphase I stage, there remains 2n=46 chromosomes i.e. a total of 92 chromatids attached at the centromere. Each homolog of a chromosome pair is now separated with the help of spindle fibers to two poles (keep in mind that, only homologous chromosomes are getting separated here not the sister chromatids). At the end of Meiosis I, i.e. after cytokinesis, each separated cell contains 23 chromosomes i.e. 46 chromatids. This is the reduction division.

In Telphase II of Meiosis II, the cell now has 2n=23 chromosomes i.e a total of 46 chromatids which now get separated at their centromere to give rise to 46 chromosomes (sister chromatids will be separated now and each chromatid will thereby become a chromosome itself) before cytokinesis. After cytokinesis, each cell/gamete (as meiosis occurs during gamete formation) gets n=23 chromosomes. Each gamete is haploid (n).