Self Assessment Answer # 4 for Lesson 3 by Dr Jamie Love 2002 - 2005

This question is meant to test your understanding of chromosomes - both as a definition and your understanding of how they are "created".

You were told that the cell starts with 46 chromosomes at prophase. (That's good because you would have had a hard time counting them in prophase. They'd have been all overlapping like a pile of worms especially in early prophase.) You should understand that meant there are 46 centromeres. There are also 92 chromatids but they aren’t chromosomes. Not yet!

At metaphase the cell would still have 46 chromosomes (and 92 chromatids) because it would still have 46 centromeres, one on each chromosome. [The cell would have 96 kinetochores by now, but that's not what I was asking.]

At anaphase all the centromeres split and the chromosomes break up into their chromatids BUT the kinetochores become the new centromeres so each chromatid is now a chromosome so we now have 92 chromosomes in the cell.

At telophase the cell still has 92 chromosomes. Granted, they are packaged in two separate nuclei (daughter nuclei) containing 46 chromosomes each, but the cell hasn't divided yet so there are still 92 chromosomes within the cell's membrane.

After cytokinesis each daughter cell now has 46 chromosomes. All those chromosomes will be made of a single column of DNA that was once one chromatid. After the cell undergoes S phase it will again have proper X-shaped chromosomes, formed by two chromatids each, but we won't see them until the next M phase.

Some students think, "So what? Why all this attention to counting chromosomes? It's just too difficult to bother with!"
Well, it is difficult but it is also important. Later we will be talking about chromosome numbers and you will be lost if you haven't learned the way we do the counting. So be sure you understand this counting trick. Watch the centromeres (including when they form from the kinetochores).

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