Amino Acids, Proteins and Porphyrins
The protein rich in basic amino acids, which functions in the packaging of DNA in chromosomes, is:
|C||Hyaluronic acid binding protein.|
a. Histones are “basic proteins”, they have unusually higher concentration of basic amino acids
b. There are five classes — H1, H2A, H2B, H3 and H4
c. H2A and H2B → are lysine rich histones while H3 and H4 are arginine rich histones
d. Histones are the most abundant chromatin protein; H1 is least tightly bound to chromatin and are easily removed with a salt solution
e. Ds DNA wraps twice around a histone octamer formed by H2A, H2B, H3 and H4→called Nucleosome
f. Function of Nucleosome (= Histone + DNA) → is to condense DNA (i.e. Pack DNA) It also stabilizes
h. “The assembly of nucleosome is probably mediated by the anionic nuclear protein, nucleoplasmin.
i. Histones, which are strongly cationic, can bind non-specifically to strong anionic DNA by
j. forming salt bridges”.
k. The super-packing of nucleosomes in nuclei is seemingly dependent upon the interaction of the H1 histones with adjacent nucleosomes.
l. These four core histones (i.e H2A, H2B, H3 and H4) are subject to five types of covalent modification:
i. Acetylation - Acetylation of histones H2 and H4 is associated activation or inactivation of gene
transcription; Acetylation of core histones is associated with chromsomal assembly during
ii. Methylation:- Methylation of histones is correlated with activation and repression of gene transcription.
iii. Phosphorylation — Phosphorylation of histones H1 is associated with condensation of chromosomes during the replication cycle.
iv. ADP-Ribosylation- is associated with DNA repair
v. Covalent linkage (H2A only to ubiquitin (The nuclear protein)