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Questions
Describe the structure of a nucleosome.
With the help of a suitable diagram describe the structure of a nucleosome
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Solution 1
In eukaryotes, the packaging of DNA helix is very complex, as the amount of DNA is very high. When we analyze the structure of eukaryotic chromosome, we find that it is composed of DNA and proteins. These proteins, known as histones, are basic in nature and are positively charged. Thus, they can easily interact with the negatively charged DNA molecule. The histone proteins first organise to make a unit of eight molecules, which is known as histone octamer. The negatively charged DNA molecule gets wrapped around this positively charged histone octamer. This complex structure is called nucleosome. In a typical nucleosome, around 200 base pairs of DNA are present.
Solution 2
The nucleosomes in chromatin are seen as ‘beads-on-string’. Histones are proteins that are rich in the basic amino acid residues like lysines and arginines which carry the positive charge in their side chains. Eight molecules of histones (two each of H2A, H2B, H3 and H4) get organized to form histone octamer. DNA is negatively charged and it is wrapped around the positively charged histone octamer to form the nucleosome. Around the octamer, DNA molecule is wrapped as 1 and 3/4th turn. The DNA is called core DNA (146 bp) and small segments of DNA linking octamer are called linker DNA (54 bp). Each nucleosome contains 200 bp of DNA helix.
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