Nucleic acids and evidence for DNA replication

What you’ll learn

• Compare DNA and RNA nucleotide structure.
• Explain complementary base pairing and semi-conservative replication.
• Use density-band evidence to distinguish replication models.

Understand the idea

A nucleotide contains a phosphate group, a pentose sugar and a nitrogenous base. DNA uses deoxyribose and the bases A, T, C and G; RNA uses ribose and U in place of T. Phosphodiester bonds join adjacent nucleotides into a sugar–phosphate backbone. DNA normally consists of two antiparallel strands, with complementary bases held by hydrogen bonds: A pairs with T and C with G. These base pairs support both a stable double helix and accurate information copying.

During replication, the strands separate and each acts as a template. Free nucleotides align by complementary base pairing, and DNA polymerase joins the new nucleotides by catalysing formation of the backbone. Each product double helix contains one parental strand and one newly made strand, hence semi-conservative replication. Base pairing specifies which nucleotide belongs at each position; the polymerase forms covalent links between successive nucleotides, rather than joining complementary bases with peptide bonds.

Meselson and Stahl used nitrogen isotopes to distinguish DNA of different densities. After cells with heavy-labelled DNA replicated once in light nitrogen, an intermediate-density band was observed, inconsistent with the original conservative prediction of separate heavy and light bands. After a second replication, intermediate and light bands appeared, distinguishing semi-conservative replication from the simple dispersive prediction. The power of the experiment lies in comparing different models’ predictions at more than one generation.

Key formulae

$$\%A=\%T,\qquad\%C=\%G\quad\text{in double-stranded DNA}$$

$$\%A+\%T+\%C+\%G=100$$

Worked example

In a double-stranded DNA sample, 24% of its bases are adenine. Find the percentages of the other three bases.

1. Complementary pairing requires thymine also to make up 24%.
2. A and T together account for 48%, leaving 52% for C and G.
3. C and G occur equally, so each makes up 26%.

Answer: T = 24%, C = 26%, G = 26%.