Question

A. The vast majority of the DNA sequence of a plant nuclear genome is composed of repetitive DNA with only a small fraction of the genome space representing protein coding gene sequences. Discuss the structure of chromatin in repeat-rich and gene-rich regions of the plant nuclear genome, including the methylation status of the DNA in these two distinct genome regions. Also, describe the conformational change in chromatin structure required to promote gene expression and explain why this conformational change is required.

Answer 1

plant genome posses large chromosomes generally than animals. The genome size also possesses wide variation among plants, but the amount of DNA within a plant species is constant. Repetitive DNA plays important role in a structural component of chromatin and also involved in gene regulation. Coming to the fine structure of chromosome it posses common feature like repeated sequences and non-repeated regions together organised into chromosome as heterochromatin and euchromatin respectively. Repeated sequences may be basically are satellite sequences i.e microsatellite and minisatellite, interspersed sequences. These sequences are highly organised part of DNA hence are deeply stained. The repeated sequences may be highly organised into tightly packed chromatin structure. but gene-rich regions generally found in euchromatic regions where the DNA has beads on string structure. here the DNA linked to histones, RNA and other proteins. The genes are active at these regions.

The methylation status of histones decides the DNA conformation either in euchromatin or heterochromatin. At repeated regions mostly associated with the di- and tri-methylation of lysine-9 of histone 3, while in non-repeated sequences has methylation of lysine 4 of histone 3 and arginine (R) residues on H3 and H4. The methylation and Acetylation of the DNA are two important factors in Histone and DNA coiling and stopping the gene expression.

The DNA needs to be relaxed rather organised to promote gene expression, The removal H1 histone from nucleosome and acetylation, methylation at specific regions makes the genes available from transcription. actively transcribing regions posses DNase 1 sensitive regions which can distract nucleosome structures and allow gene expression.