Question

4a.How does chromatin structure play a key role in gene transcription? Use terms such as histone acetyltransferase and ATPdependent, chromatin remodeling , histone remodeling, histone code in explanation

4b. What is negative supercoiling and how does it affect gene expression?

Answer 1

Answer part 4a)

The chromatin structure is a complex and compact strucutre which contains both, loosely packed transcription matter (euchromatin) and tightly packed non-transcription matter (heterochromain). The euchromatin is bound to alkaline histone proteins in a set of four core histone proteins in order to increase complexity. The loosening and tightening of this euchromatin wound over the histone protiens occurs by histone deacetylase and histone acetylation enzymes which is an ATP-dependent reaction. Also, stress-indcued histone remodelling such as methylation, acetylation, phosphorylation etc also play a crucial role in controlling the gene expression. Importantly, the histone code remains a crucial molecular signature for each individual and degree of histone modification remains a fingerprint for each individual. The nature of histone modification and the extent of histone acetylation/methylation/phosphorylation is the key feature of stress/stimulus- induced changes which has direct role in modulating gene expression.

Answer part 4b)

The chromatin is a complex structure. The DNA is collected in the form of genes which are more compactly arranged as euchromatin on the histone proteins. These histone protiens then arrange in 3-dimensional arrangement to form a solenoid like structure. These structures then become more wounded over each other to form coiled structures. The coiling of these coils in a specific manner (right handed coiling) and is termed as positive supercoiling. On the other hand, the left-handed supercoiling of DNA strands upon itself when the DNA unwounds is termed as negative supercoiling. The linear and rotational tension produced in the DNA strands during coiling/uncoiling is maintained by enzyme topoisomerase.

The topoisomerase helps in uncoiling of the DNA during replication and transcription and the negative supercoiling further avails DNA for replication and transcription. Structural complexity of rotated DNA is released by binding of topoisomerase and maintained by topoisomerase and it promotes replication and transcription.