DNA, the genetic code and gene expression — Edexcel International AS Level Biology
The structure of DNA and RNA, semi-conservative replication, the genetic code, transcription, translation and gene mutations.
Revision notes
7 short notes, in the order of the specification. Each one in short:
A mononucleotide is a pentose sugar (deoxyribose or ribose) joined to a phosphate group and a base. Condensation reactions link mononucleotides by phosphodiester bonds into polynucleotides. DNA is two antiparallel polynucleotide strands held in a double helix by hydrogen bonds between complementary bases: A with T, and C with G.
DNA replication is semi-conservative: each new DNA molecule contains one original strand and one newly made strand. The double helix unwinds and its hydrogen bonds break, each strand acts as a template, and DNA polymerase joins free nucleotides to the template by complementary base pairing, forming phosphodiester bonds.
Meselson and Stahl grew bacteria in heavy nitrogen (¹⁵N), then transferred them to light nitrogen (¹⁴N) and separated their DNA by density in a centrifuge. After one generation all the DNA was of intermediate density, and after two generations half was intermediate and half light, which supports semi-conservative replication.
A gene is a sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide chain. The genetic code is a triplet code, in which three bases code for one amino acid; it is non-overlapping, so each base is read once, and degenerate, so most amino acids have more than one triplet.
Transcription is the first stage of protein synthesis. In the nucleus, RNA polymerase attaches to a gene, breaks hydrogen bonds between the two DNA strands and builds a single strand of messenger RNA. Free RNA nucleotides pair with the bases of the template (antisense) strand and are joined by phosphodiester bonds.
Translation is the synthesis of a polypeptide at a ribosome. Messenger RNA codons are matched by the anticodons of transfer RNA molecules, each carrying a specific amino acid, so amino acids are joined by peptide bonds in the order the mRNA specifies. Translation starts at a start codon and finishes at a stop codon.
A gene mutation is a change in the base sequence of DNA, arising from errors in DNA replication. The mutations are substitution, insertion or deletion of bases. Some lead to cancer or genetic disorders, but many have no observable effect, for example because the genetic code is degenerate.
Exam questions
8 exam-style questions (27 marks), each with its mark scheme.
Answer the questionsFlashcards
23 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of membranes, proteins, dna and gene expression on one page, so you can see where this subtopic fits.
Open the mind mapPrint it
Free PDFs to print or save.
Quick check questions
Name the bond that joins nucleotides in one DNA strand.
Show answer
A phosphodiester bond, formed by a condensation reaction.
Which bases pair together in DNA?
Show answer
Adenine with thymine (two hydrogen bonds), cytosine with guanine (three hydrogen bonds).
What does semi-conservative mean?
Show answer
Each new DNA molecule has one original strand and one newly made strand.
What is the role of RNA polymerase in transcription?
Show answer
It breaks the hydrogen bonds, and joins RNA nucleotides complementary to the template strand by phosphodiester bonds.
What is an anticodon?
Show answer
Three bases on a tRNA molecule that are complementary to a codon on mRNA.
Frequently asked questions
What is the difference between DNA and RNA?
DNA is a double helix made of two polynucleotide strands, with deoxyribose and the bases A, T, C and G. RNA is a single polynucleotide strand with ribose, and uracil instead of thymine. DNA stores genetic information in the nucleus; mRNA, tRNA and rRNA are involved in protein synthesis.
What does semi-conservative replication mean?
In semi-conservative replication each new DNA molecule contains one original strand and one new strand. The helix unwinds, each strand acts as a template, and DNA polymerase joins complementary nucleotides to it. The Meselson and Stahl experiment, using heavy and light nitrogen, gave evidence for it.
What is the genetic code and why is it degenerate?
The genetic code is the set of rules by which base triplets code for amino acids. It is a non-overlapping triplet code. It is degenerate because there are 64 possible triplets but only 20 amino acids, so most amino acids are coded for by more than one triplet.
What is the difference between transcription and translation?
Transcription is making an mRNA copy of a gene from the DNA template strand using RNA polymerase, in the nucleus. Translation is making a polypeptide at a ribosome, using the sequence of codons on the mRNA, with tRNA anticodons bringing specific amino acids.
How can a gene mutation change a protein?
A substitution can change one codon and so one amino acid, or leave it unchanged because the code is degenerate. An insertion or deletion causes a frameshift that changes every codon after it. A change in primary structure may alter the tertiary structure and so the protein's function.
Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026