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Molecular Basis of Inheritance — BIOLOGY STD 12 Science — Question

Bihar BoardEnglish MediumSTD 12 ScienceBIOLOGYMolecular Basis of Inheritance3 Marks
Question
Write any four main features of the structure of double helical DNA.
✓

Answer

1. DNA is made of two polynucleotide chains which has a backbone of sugar phosphate with nitrogen bases projected inside.
2. The two chains exhibit anti-parallel polarity which meams if the one chain has the polarity in 5' to 3' end direction then other has 3' to 5' end.
3. The bases in two strands are paired through hydrogen bonds (H-bonds) forming the base pairs. Adenine pairs with Thymine by two hydrogen bonds from opposite strand and vice versa. Similarly Guanine pairs with Cytosine by 3 hydrogen bonds and vice-versa. In this way, purine always pairs with pyrimidine of the opposite strand and vice versa so that the two strands in a helix remain almost at a constant distance from each other.
4. The two chains are coiled in right handed fashion. The diameter of the DNA helix is 20Å (angstrom) i.e. both the strands in DNA are kept apart at a distance of 20Å or 2nm (nanometer). The pitch of the helix is 34Å or 3.4nm (a nanometer is 1 billionth of a metre that is $10^{-9}$m) and there are 10 base pairs in each turn therefore the distance between a base pair in a helix is approximately 3.4 Å or 0.34nm.

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