Question 39

[michaelgar7090]

I agree that choice C is correct, but I think the explanation is deeply flawed (or I’m way confused).

The answer says:
“The chromosome consists of two chromatids, each of which is a double helical DNA. When the chromosome duplicates, each double helix splits in half, with the original strands acting as a template for building a half-old, half-new double helix...”
Some of this seems incorrect. Before duplication, each chromosome (and we have 22 homologous pairs, and 1 pair of sex chromosomes) consists of one double helix DNA, not two.
During replication, each strand of the double helix serves as a template for replication. After replication, we have two chromatids connected by the centrosome, our familiar X; (the centrosome has also replicated, but since the centrosomes are connected we still consider this to be one chromosome with two chromatids. The number of chromosomes is determined by the number of separate centromeres, not by the number of chromatids.) After duplication we still have 23 pairs of chromosomes, but twice as much DNA content---each chromosome now has two chromatids. One strand of each chromatid is labeled, so choice C is correct.

The answer goes on: “After the DNA is duplicated, we have one cell with four chromatids. After one round of DNA duplication, we will have two identical chromosomes, each with two chromatids that themselves are each half labeled and half unlabeled.“ If we’re talking about mitosis, and talking about starting with one chromosome, this answer is incorrect. We never have one cell with four identical chromatids, even during meiosis. During mitosis, the familiar X shaped chromosome we see consists of 2 sister chromatids which are pulled apart during metaphase. Each daughter cell gets one of the two identical chromatids, (which when separated are called chromosomes); each daughter cell does NOT get one chromosome with two chromatids already there. The daughter cell will duplicate that chromatid in its own cell cycle... So we had 2n chromosomes before mitosis, and 2n after: only the amount of DNA per chromosome changes.

I'm also posting a comment on question 40...

[jellywing_2058]

The explanation has been modified and there will be a drawing added to the explanation for clarity.