Discuss the structure of DNA . In what ways the structure of DNA differs from that of RNA?
Answer: DNA is usually a double-helix and has two strands running in opposite directions. (There are some examples of viral DNA which are single-stranded). Each chain is a polymer of subunits called nucleotides . Each strand has a backbone made up of (deoxy-ribose) sugar molecules linked together by phosphate groups. Each sugar molecule is covalently linked to one of 4 possible bases (Adenine, Guanine, Cytosine and Thymine). A and G are double-ringed larger molecules (called purines); C and T are single-ringed smaller molecules (called pyrimidines).In the double-stranded DNA, the two strands run in opposite directions and the bases pair up such that A always pairs with T and G always pairs with C. The A-T base-pair has 2 hydrogen bonds and the G-C base-pair has 3 hydrogen bonds. The G-C interaction is therefore stronger (by about 30%) than A-T, and A-T rich regions of DNA are more prone to thermal fluctuations. RNA molecules are also polynucleotides with a sugar-phosphate backbone and four kinds of bases. The main differences between RNA and DNA are: • RNA molecules are single-stranded • The sugar in RNA is a ribose sugar (as opposed to deoxy-ribose) and has an –OH at the 2' C position highlighted in red in the figure below (DNA sugars have –H at that position) • Thymine in DNA is replaced by Uracil in RNA. T has a methyl (-CH3) group instead of the H atom shown in red in U.