Short Answer TypeThe base sequence in one of the strands of DNA is TAGCATGAT.
(i) Give the base sequence of its complementary strand.
(ii) How are these base pairs held together in a DNA molecule?
(iii) Explain the base complementarity rules. Name the scientist who framed this rule.
Long Answer TypeDescribe how the lac operon operates, both in the presence and absence of an inducer in E.coli.
Short Answer Type(a) Draw a neat labelled diagram of a nucleosome.
(b) Mention what enables histones to acquire a positive charge.Watson and Crick proposed the double helix for structure of DNA. Its features are:
(i) It is made of two polynucleotide chains, where the sugar-phosphate constitutes the backbone, and the bases project inside.
(ii) The two chains have anti-parallel polarity. It means, if one chain has the polarity 5'-3', the other has 3'-5'.
(iii) The bases in two strands are paired through hydrogen bond (H-bonds) forming base pairs (bp). Adenine forms two hydrogen bonds with Thymine from opposite strand and vice-versa. Similarly, Guanine is bonded with Cytosine with three H-bonds. As a result, always a purine comes opposite to a pyrimidine. This generates approximately uniform distance between the two strands of the helix.
(iv) The two chains are coiled in a right-handed fashion. The pitch of the helix is 3.4 nm (a nanometre is one billionth of a metre, that is 10-9 m) and there are roughly 10 bp in each turn turn. Consequently, the distance between a bp in a helix is approximately equal to 0.34 nm.
(v) The plane of one base pair stacks over the other in double helix.Long Answer TypeDescribe Frederick Griffith’s experiment on Streptococcus pneumonia. Discuss the conclusion he arrived at.
Short Answer TypeFollowing are the features of genetic codes. What does each one indicate ? [2]
Stop codon; Unambiguous codon; Degenerate codon; Universal codon.
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