Cue Column | Note Column

What is the shape of DNA? | Right-handed double helix, two antiparallel polynucleotide strands (one 5'→3', other 3'→5'), held by H-bonds between complementary bases.

What are the H-bond numbers? | A=T: 2 hydrogen bonds. G≡C: 3 hydrogen bonds. GC-rich DNA is more thermally stable (higher melting temperature).

State Chargaff's rules. | A = T; G = C. Therefore: purines = pyrimidines; A+G = T+C. Applies ONLY to double-stranded DNA.

What are the grooves? | Major groove (wider, ~22 Å) — protein binding site. Minor groove (narrower). Transcription factors bind major groove.

Who provided X-ray data? | Rosalind Franklin (Photo 51) and Maurice Wilkins provided X-ray crystallography data used by Watson and Crick to build the model (1953).

Summary:

The double helix is right-handed, antiparallel, with A=T (2 H-bonds) and G≡C (3 H-bonds). Chargaff's rules are the mathematical consequence of complementary base pairing. Know Rosalind Franklin's contribution for NEET historical questions.