Cambridge O Level Biology · Syllabus 5090 · Inheritance
DNA
What is DNA?
Deoxyribonucleic acid, the molecule that carries genetic information in living organisms. A DNA molecule consists of two strands coiled together into a double helix. Each strand is a chain of nucleotides, and each nucleotide contains one base, represented by the letters A, T, C and G. The two strands are held together by bonds between complementary pairs of bases, with A always pairing with T and C always pairing with G, so the order of bases along one strand determines the order along the other.
This definition is part of the Inheritance chapter in Cambridge O Level Biology.
DNA in context
Inheritance is the transmission of genetic information from one generation to the next. A gene is a length of DNA that codes for a protein; an allele is one of the alternative forms that gene can take. The order of the bases A, T, C and G along a strand of DNA sets the order of amino acids in a protein, which sets the protein’s shape, which sets what the protein can do — and that is how a molecule you cannot see decides a feature you can. Because gametes carry one allele of each gene, a cross between two known genotypes has a predictable outcome: Tt × Tt is expected to give 3 tall : 1 short, and Tt × tt is expected to give 1 tall : 1 short. Those are probabilities, not guarantees. Mutation makes new alleles at random, meiosis and fertilisation shuffle them into new combinations, and selection — by the environment in natural selection, by people in artificial selection — changes how common each allele becomes over generations. That change is evolution.
Common mistakes with DNA
- “A gene is a chromosome.” A chromosome is the whole structure; a gene is a length of the DNA in it, and one chromosome carries many genes. Write “A gene is a length of DNA that codes for a protein.”
- “DNA is made of amino acids.” DNA is made of nucleotides; proteins are made of amino acids. The two are connected by the coding relationship, not by being the same material. Write “Each strand of DNA is a chain of nucleotides.”
Questions students ask about DNA
What is the difference between a gene and an allele?
A gene is a length of DNA that codes for a protein. An allele is one particular version of that gene, differing from other versions in its base sequence. Every pea plant has the gene for stem height; what differs between plants is which alleles of it they carry. Genes occupy a fixed position on a chromosome, and the two chromosomes of a homologous pair carry the same gene at that position — but not necessarily the same allele.