Question

Before replication, a heterochromatic region has sites with fully methylated DNA and nucleosomes with H3K9me3 histone modifications. After DNA replication, what would be the constituents of each daughter chromatid?

1) hemimetylated DNA
2) half of the H3 histones with H3K9me3 modifications
3) all of the H3 histones with H3K9me3 modifications
4) fully methylated DNA

Answer 1

Immediately after DNA replication, each sister chromatid would possess hemimethylated DNA, and half of the H3 histones would have the H3K9me3 modification as these modifications are preserved from the parental histones.

Step-by-step explanation:

After DNA replication in the S phase of interphase, each sister chromatid would contain constituents that result from the semi-conservative replication methodology. The DNA in heterochromatic regions, prior to replication, would be fully methylated and associated nucleosomes would have histone modifications such as H3K9me3.

Given this background, the constituents of each daughter chromatid immediately after replication would be:

1. Hemimethylated DNA, which results from the new strand being synthesized without methylation, and the parental strand remaining methylated.
2. Half of the histone proteins with H3K9me3 modifications; as histones are randomly distributed between the two strands during replication.
3. Fully methylated DNA would not be present immediately after replication as the new strand is yet to be methylated.