Loading....
Coupon Accepted Successfully!

Biochemistry

Open Flashcards

Purines, Pyrimidines and Nucleic Acid Metabolism

Question
13 out of 59
 

Which is true about histone acetylation (AIIMS May 2010)



A Increased euchromatin formation

B Increased heterochromatin formation

C Methylation of DNA

D DNA replication

Ans. A Increased euchromatin formation

a. In histone acetylation and deacetylation, the histones are acetylated and deacetylated on lysine residues in the N-terminal tail and on the surface of the nucleosome core as part of gene regulation.

b. Typically, these reactions are catalyzed by enzymes with "histone acetyltransferase" (HAT) or "histone deacetylase" (HDAC) activity. The source of the acetyl group in histone acetylation is Acetyl-Coenzyme A, and in histone deacetylation the acetyl group is transferred to Coenzyme A.

c. Acetylated histones and nucleosomes represent a type of epigenetic tag within chromatin. Acetylation brings in a negative charge, acting to neutralize the positive charge on the histones and decreases the interaction of the N termini of histones with the negatively charged phosphate groups of DNA.

d. As a consequence, the condensed chromatin is transformed into a more relaxed structure which is associated with greater levels of gene transcription.

e. This relaxation can be reversed by HDAC activity. Relaxed, transcriptionally active DNA is referred to as euchromatin. More condensed (tightly packed) DNA is referred to as heterochromatin. Condensation can be brought about by processes including deacetylation and methylation; the action of methylation is indirect and has no effect upon charge.

f. This charge neutralization model has been challenged by recent studies, according to which transcriptionally active genes are correlated with rapid turnover of histone acetylation.

g. This requires that the HATs and HDACs must act continuously on the affected histone tail. Methylation at a specific lysine residue (K4) is involved in targeting histone tails for continuous acetylation and deacetylation.

h. Eukaryotic DNA is packaged into a nucleoprotein structure known as chromatin, which is comprised of DNA, histones, and nonhistone proteins.

i. Chromatin structure is highly dynamic, and can shift from a transcriptionally inactive state to an active form in response to intra- and extracellular signals. A major factor in chromatin architecture is the covalent modification of histones through the addition of chemical moieties, such as acetyl, methyl, ubiquitin, and phosphate groups.

j. The acetylation of the amino-terminal tails of histones is a process that is highly conserved in eukaryotes, and was one of the earliest histone modifications characterized. Since its identification in 1964, a large body of evidence has accumulated demonstrating that histone acetylation plays an important role in transcription.

k. Despite our ever-growing understanding of the nuclear processes involved in nucleosome acetylation, however, the exact biochemical mechanisms underlying the downstream effects of histone acetylation have yet to be fully elucidated.

Purines, Pyrimidines and Nucleic Acid Metabolism Flashcard List

59 flashcards
1)
2)
3)
4)
5)
6)
7)
8)
9)
10)
11)
12)
13)
14)
15)
16)
17)
18)
19)
20)
21)
22)
23)
24)
25)
26)
27)
28)
29)
30)
31)
32)
33)
34)
35)
36)
37)
38)
39)
40)
41)
42)
43)
44)
45)
46)
47)
48)
49)
50)
51)
52)
53)
54)
55)
56)
57)
58)
59)