Nucleotides, DNA and RNA Structure

Revision notes

The nucleotide building block

A nucleotide contains a pentose sugar, a phosphate group and a nitrogenous base. DNA uses deoxyribose; RNA uses ribose. Adenine and guanine are purines with two rings; cytosine, thymine and uracil are pyrimidines with one ring. DNA normally uses A, T, C and G; RNA normally uses A, U, C and G.

Forming a polynucleotide

Condensation links nucleotides through phosphodiester bonds between sugar and phosphate groups, producing a sugar–phosphate backbone. Water is released in the simplified condensation model. The base projects from the sugar. A nucleic-acid strand has direction, described as 5′ to 3′. The covalent backbone is not held together by hydrogen bonds between adjacent bases.

An RNA nucleotide is formed from ribose, phosphate and an appropriate base; DNA nucleotides contain deoxyribose instead. Uracil replaces thymine in the usual RNA base set. RNA is usually single-stranded but can fold back and form complementary internal regions.

The DNA double helix

Two antiparallel polynucleotide strands associate through complementary base pairing: A with T and C with G. Hydrogen bonds between bases hold the paired strands together, with two in an A–T pair and three in a C–G pair. A purine pairs with a pyrimidine, helping maintain a consistent helix width. The backbones remain on the outside; bases point inward.

Antiparallel means one strand runs 5′→3′ while the paired strand runs 3′→5′. The base sequence stores information; the number of hydrogen bonds does not itself encode an amino-acid sequence. Complementarity permits a template relationship between strands.

Read and construct a sequence

When asked for a complementary sequence, pair each base and pay attention to direction. DNA pairing uses T; an RNA complement uses U opposite A. If both sequences must be written 5′→3′, the complementary strand must be reversed after pairing. This is a common source of errors in otherwise correct answers.

Worked application and data skills

For DNA 5′-ATGC-3′, the paired DNA strand is 3′-TACG-5′. Written in its own 5′→3′ direction it is 5′-GCAT-3′. In a double-stranded DNA sample with 30% A, T is also 30%; the remaining 40% is shared equally between G and C, giving 20% each. This percentage rule is not automatic for a single RNA strand.

Common misconceptions

Phosphodiester bonds form the backbone; hydrogen bonds connect complementary DNA bases. DNA strands are antiparallel. RNA uses uracil rather than thymine in its usual base set.

Examination tips

Label sugar, phosphate and base, and mark strand directions before complementing sequences.

Check your understanding

Five original questions. Use Tab to move between controls, arrow keys within a radio group, and Enter or Space on buttons. Feedback appears after Check Answer. Try Again clears the question. Attempts are not saved.

Sources and scope

Scope: Unit 1 B4.1. Original BTECRevision notes, schematics and questions. Independent resource; not endorsed by Pearson.

Official Pearson specification. Scientific review references are listed in the accompanying QA report.