Final answer:
Constitutive heterochromatin in yeast and animal cells includes features like delayed replication timing, low gene expression, DNA methylation, and histone modifications which all contribute to tighter DNA packing and gene silencing.
Step-by-step explanation:
The features associated with constitutive heterochromatin in yeast and animal cells include replication timing, gene expression regulation, DNA methylation, and histone modifications. Constitutive heterochromatin is a compact form of DNA which is generally replicated later in the cell cycle and is less transcriptionally active. DNA in these regions is often heavily methylated, which contributes to the silencing of gene expression. Furthermore, histone modifications such as methylation and acetylation play a crucial role in the regulation of gene expression by altering the chromatin structure, which can either condense or relax to allow or prevent the binding of transcription factors and RNA polymerase.
Histones are the protein molecules around which DNA wraps to form nucleosomes, the fundamental subunits of chromatin. This chromatin can be modified through actions like methylation and acetylation of DNA and histone proteins—mediated by enzymes such as histone methyltransferases and acetylases. In contrast, areas rich in housekeeping genes that are frequently expressed are mostly found in a more open chromatin state, known as euchromatin. Epigenetic control, such as DNA methylation and histone modifications, regulates access to genes within the chromosome, thus managing gene expression levels in eukaryotic cells.