Q.Describe the structure of DNA in brief. What do you mean by Genetic Code? OR What is Linkage? What do you mean by Sex Linked Inheritance? Give example.

Watson and Crick's double-helix model describes DNA's physical structure; the genetic code describes how that structure's base sequence is translated into protein.

Structure of DNA (Watson-Crick model, 1953):

• DNA consists of two polynucleotide chains coiled around a common axis in a right-handed double helix.
• The two strands are antiparallel (one runs 5' to 3', the other 3' to 5').
• The sugar-deoxyribose and phosphate groups form the backbone on the outside of the helix; the nitrogenous bases project inward.
• Bases of the two strands pair through hydrogen bonds according to the complementary base-pairing rule: Adenine pairs with Thymine (2 hydrogen bonds), and Guanine pairs with Cytosine (3 hydrogen bonds) — this is why Chargaff's rule holds (A=T, G=C).
• The two chains are coiled such that one full turn of the helix contains about 10 base pairs, with a diameter of about 20 angstrom.

Genetic Code: The genetic code is the set of correspondence rules between the sequence of nucleotide triplets (codons) in mRNA and the sequence of amino acids in the polypeptide synthesised.

• It is a triplet code — three consecutive nucleotides (a codon) specify one amino acid.
• It is degenerate — most amino acids are coded by more than one codon.
• It is unambiguous — each codon specifies only one amino acid. …