A typical mammalian cell has 22 meters long DNA molecule whereas the nucleus in which

it is packed measure about 10-6 m. Explain in how such a long DNA molecule is packedwithin a tiny nucleus in the cell.

The long sized DNA are accomodated in small areas through packing or compaction. DNA is acidic due to presence of a large number of phosphate groups. Compaction takes place by folding and attachment of DNA with basic proteins called histones. The unit of compaction is called nucleosome. The compaction is carried out with the help of lysine and arginine rich basic proteins called histones. Five types of histone proteins are found in nucleus: H1, H2, H2B, H3 and H4. Four of them (H2A, H2B, H3 and H4) occur in pairs to produce histone octamer or core of nucleosome. Their positively charged ends lie towards the outside. They attract negatively charges strands of DNA. Linker DNA is the DNA connecting two adjacent nucleosomes. It bears H1 histone protein. Length of linker DNA is varied. The nucleosomal organization has approximately 10 nm thickness which gets further condensed and coiled to produce a solenoid of 30 nm. This solenoid structure undergoes further coiling and produce a chromatin fiber of 30-80 nm and than a chromatid of 700 nm. The chromatid undergoes a further coiling and produce a chromosome of 1400 nm.

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Actually a mammalian cell has 2.2m long DNA not 22m and this DNA is packed within this Small nucleus because of supercoiling of DNA around histone proteins.

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