Final answer:
After DNA replication, each daughter chromatid in a heterochromatin region contains a mixture of old and new DNA strands, with newly synthesized DNA and histones modified to preserve the heterochromatic state, including continued DNA methylation and H3K9me3 histone modifications.
Step-by-step explanation:
The student asked what the constituents of each daughter chromatid would be after DNA replication in a heterochromatin region with fully methylated DNA and nucleosomes with H3K9me3 histone modifications. During the S phase of interphase, each original DNA strand serves as a template for the production of a complementary strand, resulting in two identical sister chromatids that are linked together. Each daughter chromatid would have a mixture of old (parental) and new (daughter) DNA strands due to the semi-conservative nature of DNA replication.
Regarding the specific modifications mentioned in the question, the fully methylated DNA sites and the histones with H3K9me3 modifications would undergo a process to ensure that the newly synthesized DNA and the histones associated with it are modified accordingly to maintain the heterochromatic state. This includes methylation of the new DNA strand and addition of the H3K9me3 marker to the new histones. This maintains the transcriptionally inactive state of the heterochromatin after replication.