DNA and molecular biology: study guide

Key ideas

DNA is a double helix whose base pairing sets simple numerical rules. Counting bases, lengths, codons and PCR copies are the usual calculations.

• Chargaff’s rules: in double-stranded DNA, A = T and G = C, so %A + %G = 50 %. The GC content is %G + %C.
• A base pair occupies 0.34 nm along the helix, and a full turn is 10 base pairs (3.4 nm).
• Three bases make a codon, so a coding sequence of n bases codes for about n ÷ 3 amino acids, and a polypeptide of m amino acids has m − 1 peptide bonds.
• PCR doubles the target each cycle: copies = N₀ × 2ⁿ. Restriction enzymes cut at a specific sequence, so k cuts give k + 1 linear fragments.

Key equations

• A = T and G = C (Chargaff)
• %A + %G = 50
• length = base pairs × 0.34 nm
• codons = bases ÷ 3
• copies = N₀ × 2ⁿ

Worked example

If 30 % of the bases in a DNA sample are A, then T is 30 %, and G and C are each (100 − 60) ÷ 2 = 20 %. A gene of 3000 bp is 3000 × 0.34 = 1020 nm long and has 1000 codons. With one stop codon it codes for 999 amino acids, giving 998 peptide bonds and a mass of about 999 × 110 ≈ 110 kDa.

Common mistakes

• Applying Chargaff’s rule to single-stranded DNA or RNA, where A does not have to equal T or U.
• Counting the stop codon as an amino acid.
• Using the number of bases instead of base pairs when calculating the length of the double helix.

Practice

Test yourself with practice questions on this topic: DNA and molecular biology