Nucleus and Chromatin Organisation Questions and Answers

Answer: C

Lysine and arginine give the histones a strong positive charge that attracts the negatively charged phosphate backbone. The interaction is electrostatic and reversible rather than covalent. Chemical modification of those residues is how the grip is adjusted.

Answer: D

The association is electrostatic, between positively charged residues and the phosphate backbone, which is why it can be loosened and re-formed. A covalent bond would make the packaging impossible to undo. Reversibility is essential, since the DNA must be unwrapped to be read or copied.

Answer: A

Open chromatin gives the transcription machinery access to the DNA, while heterochromatin is packed too tightly to be read. The distinction can be seen directly in electron micrographs as light and dark regions. The dark regions correspond to the more tightly packed heterochromatin.

Answer: B

Its dense packing excludes the transcription machinery, so it is largely silent. Regions near centromeres and telomeres are permanently heterochromatic, while other regions switch state as the cell changes what it expresses. Facultative heterochromatin is the term for regions that can switch.

Answer: C

Adding an acetyl group neutralises the positive charge on lysine, weakening the grip on the DNA. Enzymes that add and remove these groups are important drug targets in cancer treatment. Several inhibitors of histone deacetylases are now in clinical use.

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. Histone tails are rich in lysine, whose positive charge binds the negatively charged phosphate backbone of the DNA. Acetylation neutralises that charge, loosening the grip and opening the chromatin to transcription factors - which is why histone acetyltransferases act as coactivators and deacetylases as repressors, and why deacetylase inhibitors are used as anticancer drugs.

Answer: A

Methyl groups added to cytosine recruit proteins that compact the chromatin and block transcription. The pattern is copied when DNA replicates, so the silenced state is inherited by daughter cells. Silencing can therefore persist through many rounds of division.

Answer: B

They alter how a sequence is read without changing a single base, which is precisely what the word epigenetic means. This is why such marks can be reversed while a mutation cannot. That reversibility is what makes epigenetic marks attractive as drug targets.

Answer: C

The first two describe the usual associations correctly. The third is wrong, since neither changes any base in the sequence, which is what distinguishes an epigenetic mark from a mutation. One changes how the text is read, the other changes the text itself.

Answer: D

Chromosomes are arrested at metaphase, stained and arranged by size and banding pattern. Large-scale abnormalities such as an extra chromosome are visible at once. Down syndrome is diagnosed this way from an extra copy of chromosome 21.