DNA, genes and protein synthesis — Edexcel International GCSE Biology
The genome, genes and chromosomes, and (Biology only) the structure of DNA, RNA and how a gene is used to make a protein.
Revision notes
3 short notes, in the order of the specification. Each one in short:
The genome is the entire DNA of an organism. A gene is a section of a molecule of DNA that codes for a specific protein. Genes are located on chromosomes, which are long DNA molecules found in the nucleus. Each gene has a different base sequence, so each codes for a different protein, which helps decide the organism's characteristics.
The structure of DNA
Biology onlyDNA is a molecule made of two strands coiled to form a double helix. Each strand is a chain of nucleotides, and the strands are linked by paired bases. Adenine always pairs with thymine, and cytosine with guanine, so the base sequence on one strand gives the other: A T G C pairs with T A C G.
RNA and protein synthesis
Biology onlyProtein synthesis is how a gene is used to make a protein. In transcription, in the nucleus, a single strand of mRNA is made as a copy of the gene by complementary base pairing, with uracil instead of thymine. In translation, at a ribosome, tRNA anticodons pair with mRNA codons, and each codon codes for one amino acid.
Exam questions
6 exam-style questions (18 marks), each with its mark scheme.
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Quick check questions
What is a gene?
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A section of a molecule of DNA that codes for a specific protein.
What is the genome?
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The entire DNA of an organism.
Biology only Which base pairs with adenine in DNA?
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Thymine.
Biology only Which base is in RNA instead of thymine?
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Uracil (U).
Biology only Where are amino acids joined together to make a polypeptide?
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At a ribosome (translation).
Frequently asked questions
What is the difference between a gene, a chromosome and the genome?
A gene is a section of a molecule of DNA that codes for a specific protein. A chromosome is a long DNA molecule, found in the nucleus, with many genes along it. The genome is the entire DNA of an organism, including all of its genes and the rest of its DNA.
What is the structure of DNA?
Biology only DNA is made of two strands coiled to form a double helix. Each strand is a chain of nucleotides, and the two strands are linked by paired bases. There are four bases: adenine pairs with thymine, and cytosine pairs with guanine. So the sequence on one strand determines the sequence on the other.
What is the difference between transcription and translation?
Biology only Transcription happens in the nucleus: the DNA strands separate and a single strand of mRNA is made as a copy of the gene. Translation happens at a ribosome in the cytoplasm: tRNA brings amino acids, its anticodons pair with mRNA codons, and the amino acids are joined into a polypeptide that folds into a protein.
What is the difference between DNA and RNA?
Biology only DNA has two strands forming a double helix, while RNA is single stranded. DNA contains the bases adenine, thymine, cytosine and guanine. RNA has uracil instead of thymine, so in RNA adenine pairs with uracil. DNA stays in the nucleus, while mRNA carries the gene's instructions to a ribosome.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026