Answer any two questions (restrict each answer within 200 words):
Describe the structure of a mature human sperm with the help of a well-labelled diagram.

Explanation

The outline of Watson and Crick model of DNA is as follows
(i) DNA molecule consists of two helically twisted strands connected together by base pairs, which align themselves in antiparallel or in opposite direction.
(ii) The two strands are interwined in a clockwise direction, i.e. in the form of a right handed helix and haveantiparallel arrangement.
(iii) Each strand consists of a backbone made up of alternating deoxyribose sugar and phosphate. The phosphate joins the two sugars through a phosphodiester bond.
(iv) The nitrogenous bases are stacked inside the helix and pair with the base of the opposite strand through hydrogen bonds (H-bonds).
(v) There is a specificity in base pairing. Adenine (A) always pairs up with thymine (T) by two H-bonds and guanine (G) always pairs up with cytosine (C) by three H-bonds. This pairing of purines with pyrimidines leads to a uniform width of DNA throughout its length (20 Å).

(vi) The twisting of strand results into major (deep) and minor ishallow) groove. A complete turn measures 34 Å with a distance of 3.4 A between two successive base pairs.