DNA & Protein Synthesis IGCSE Biology 0610

DNA Structure and Protein Synthesis Overview

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

Key concept

DNA is a double helix of two polynucleotide chains linked by complementary base pairs (A-T, C-G). A gene is a sequence of bases that codes for the amino acid sequence of a specific protein. At Supplement level, students describe how the base sequence is transcribed into mRNA and translated into a protein at ribosomes.

DNA structure and protein synthesis connect molecular biology to genetics. Core students need to know that DNA carries the genetic code as a base sequence, while Supplement students must describe the steps from gene to protein. Accurate base-pairing rules are essential.

The structure of DNA

DNA (deoxyribonucleic acid) is a large molecule made of two polynucleotide strands twisted into a double helix. Each nucleotide consists of a deoxyribose sugar, a phosphate group and one of four bases: adenine (A), thymine (T), cytosine (C) or guanine (G).

The two strands are held together by complementary base pairing: A always pairs with T, and C always pairs with G. These pairs are joined by hydrogen bonds. The sugar-phosphate groups form the backbone of each strand.

How DNA codes for proteins

The sequence of bases along one strand of DNA forms the genetic code. Each group of three bases (a triplet or codon) codes for one amino acid. The order of triplets determines the order of amino acids in the protein, and the specific sequence of amino acids determines the type of protein produced and its function.

Since there are 4 bases, there are 4 x 4 x 4 = 64 possible triplet combinations, which is more than enough to code for the 20 amino acids used in proteins. Some amino acids are coded for by more than one triplet.

Transcription and translation (Supplement)

At Supplement level, students should outline the two main stages of protein synthesis:

Transcription occurs in the nucleus. One strand of DNA is used as a template to build a complementary strand of mRNA (messenger RNA). In RNA, uracil (U) replaces thymine (T), so the base pairing is A-U and C-G. The mRNA molecule then leaves the nucleus through a nuclear pore.

Translation occurs at ribosomes in the cytoplasm. The ribosome reads the mRNA codons three bases at a time. Each codon specifies an amino acid, which is brought to the ribosome by tRNA. Amino acids are joined together by peptide bonds to form a polypeptide chain, which folds into a functional protein.

Mutations and their effects

A mutation is a change in the base sequence of DNA. A single base substitution changes one triplet, which may code for a different amino acid, altering the protein produced. This can change the shape and function of the protein, for example altering an enzyme's active site.

Not all mutations have harmful effects. Some are neutral (the changed triplet still codes for the same amino acid) and some may be beneficial, providing variation on which natural selection can act. Sickle cell anaemia is caused by a single base mutation in the haemoglobin gene.

Try a focused question (4 marks)

Describe how the base sequence of a gene leads to the production of a specific protein.

Show the answer and marking guidance

The gene is a sequence of bases on a DNA strand in the nucleus.
The DNA base sequence is transcribed into mRNA, which leaves the nucleus.
At a ribosome, the mRNA is translated: each triplet of bases codes for a specific amino acid.
Amino acids are joined in sequence to form a polypeptide/protein with a specific shape and function.

Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.

Common mix-ups to avoid

Check these points against your own answer before moving on.

Pairing A with C or T with G (the correct pairs are A-T and C-G in DNA, A-U in RNA).
Saying DNA leaves the nucleus (mRNA leaves; DNA stays in the nucleus).
Forgetting that three bases code for one amino acid (not one base per amino acid).
Confusing transcription (DNA to mRNA) with translation (mRNA to protein).