1.

The average length of a DNA double helix in a typical mammalian cell is approximately 2.2 metres and the dimension of the nucleus is about 10–6 m. (a) How is it possible that long DNA polymers are packed within a very small nucleus? (b) Differentiate between euchromatin and heterochromatin. (c) Mention the role of non-histone chromosomal protein.

Answer»

(a) The DNA is associated with two types of proteins—basic proteins (histone and protamine) and acidic non-histone chromosomal (NHC) proteins. The negatively charged DNA molecule wraps around the positively charged histone proteins to form a structure called nucleosome. The nucleosome core is made up of four types of histone proteins—H2A, H2B, H3 and H4— occurring in pairs. 200 bp of DNA helix wraps around the nucleosome by 1¾ turns, plugged by H1 histone protein. Repeating units of nucleosomes form the chromatin in nucleus, which is a thread-like structure. The chromatin is packed to form a solenoid structure of 30 nm diameter. Further supercoiling forms a looped structure called the chromatin fibre.

(b)

S. No.EuchromatinHeterochromatin
(i)Regions of chromatin which are loosely packed during interphase are called euchromatin. Regions of chromatin which are densely packed during cell division are called heterochromatin.
(ii)When chromosomes are stained with Feulgen reaction (specific for DNA), these appear as lightly stained chromatin.When chromosomes are stained with Feulgen reaction, these appear as intensely stained chromatin. 
(iii)Euchromatin contains active genes.Heterochromatin contains inactive genes.
(iv)They do not contain repetitive DNA sequences. They are enriched with highly repetitive tandemly arranged DNA sequences.

(c) The packaging of chromatin at higher level requires the presence of non-histone chromosomal protein.


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