Describe the structure of DNA with a neat diagram.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Open in two lines. DNA (deoxyribonucleic acid) is the genetic material of cells. Watson and Crick (1953) showed that it is a double helix of two polynucleotide strands.
2 · Draw the double helix. [Diagram: two antiparallel strands as a right-handed helix, 5′ and 3′ ends marked, A=T and G≡C pairs, major and minor grooves, 3.4 nm per turn, 2 nm width]
3 · Building blocks.
Nucleotide = base + deoxyribose + phosphate.
Purines A, G; pyrimidines C, T.
Nucleotides joined by 3′,5′-phosphodiester bonds.
4 · Features of the helix.
Right-handed; two antiparallel strands.
Backbone outside, bases inside.
A=T (2 H-bonds), G≡C (3 H-bonds); the strands are complementary.
About 10 bp per turn (10.5 in newer books), 3.4 nm pitch, 2 nm wide; major and minor grooves.
5 · Chargaff's rule. A = T and G = C; purines = pyrimidines.
6 · Significance. Stores genetic information. Complementary strands allow exact copying (replication) and act as a template for RNA (transcription).
7 · Nutrition link. Folate and vitamin B12 are needed for DNA synthesis; their lack causes megaloblastic anaemia.
8 · Close in one line. Sum up DNA as a stable, self-copying double helix that carries the genetic code.
Scoring tip: The labelled diagram, with 5′ and 3′ ends and the numbers, carries most of the marks.
Write-up practice
The check looks for key words only. It does not grade your answer and never writes it for you.