The interaction between DNA and histone proteins (forming nucleosomes) plays a key role in the regulation of gene expression in eukaryotes and is a potential target mechanism in drug discovery. You are testing a drug which blocks the activity of histone acetyltransferases (HATS) in cancer cells. In general, what do you expect to happen with regards to chromatin structure and gene expression in cells treated with the drug? Please select the most correct answer.

A. The chromatin near cis-regulatory sequences will be more closed and there will be less transcription.
B. The chromatin near cis-regulatory sequences will be more open and there will be more transcription.
C. The chromatin near cis-regulatory sequences will be more open and there will be less transcription.
D. The chromatin near cis-regulatory sequences will be more closed and there will be more transcription.
E. There should be no effect on chromatin structure or gene transcription. HATs only function in the packaging of DNA prior to mitosis.

A. The chromatin near cis-regulatory sequences will be more closed and there will be less transcription.

Explanation:

In the presence of histones, the cis-regulatory sequences of DNA like promoter, enhancers etc. are not exposed. The function of the histone acetyltransferases (HATS) is to cause chromosome decondensation i.e. removal of histones from the DNA so that transcription of the DNA could occur. Histone acetyltransferases (HATS) cause acetylation of lysine amino acid of the histone proteins. Acetyl group is negatively charged so the acetylation of histone proteins leads to the removal of their positive charge which ultimately leads to the decrease in the interaction between N terminal of histones and negatively charged phosphate group of the DNA molecule. As soon as histones are removed from the DNA where cis-regulatory sequences are located, the DNA becomes accessible for transcription.

But here a drug has been added which blocks the activity of histone acetyltransferases (HATS) in cancer cells. So it is quite evident that in these cells, histones will not get removed from the cis-regulatory sequences of DNA so the DNA will be more closer or tightly packed as a result of which  less transcription will occur.

a tetrad is the foursome during meiosis made by two homologous chromosomes that have each already replicated into a pair of sister chromatids.