6.1 DNA and RNA Structure

Nucleic acids are polymers of nucleotides: a 5-carbon sugar, a phosphate, and a nitrogenous base. DNA uses deoxyribose and bases A, T, G, C. RNA uses ribose and A, U, G, C.

Purines (A, G) have two rings; pyrimidines (C, T, U) have one. A purine always pairs with a pyrimidine, keeping the helix width constant.

DNA strands are antiparallel: one runs 5′→3′, the other 3′→5′. A pairs with T (2 hydrogen bonds), G with C (3 hydrogen bonds), so GC-rich DNA is harder to separate.

Prokaryotes usually have one circular chromosome plus plasmids; eukaryotes have multiple linear chromosomes wrapped around histones.

Chargaff's rule: %A = %T and %G = %C.

Worked example: DNA is 30% A. Find %G. → T = 30%, so A + T = 60%; G + C = 40%; G = 20%.