DNA, genes and chromosomes explained

DNA is a polymer made of repeating units called nucleotides. Each nucleotide has a sugar, a phosphate and one of four bases: A, T, C or G. Two strands twist into a double helix, held together by pairs of bases: A always pairs with T, and C with G (complementary base pairing). So if one strand reads ATCG, the other reads TAGC.

The order of bases is the code. Each group of three bases codes for one amino acid. The cell copies the gene's code, carries it to a ribosome, and the ribosome joins the amino acids in order to build a protein. Proteins do most of the work in cells: enzymes, hormones, and structural proteins like collagen.

You inherit one copy of each chromosome from each parent, so you have two copies of each gene, which may be different alleles. A dominant allele is expressed with one copy; a recessive allele only with two. Genetic crosses show the chances of each outcome: try our Punnett square calculator and guide to Punnett squares.

A mutation is a change in the DNA sequence. Most have no effect; some change the protein, which can be harmful, and a few are beneficial. Mutations are the original source of the variation that natural selection acts on.

• Mixing up gene and allele: the gene is for a feature, the allele is the version.
• Pairing A with G, or C with T.
• Saying DNA is a protein (it's a nucleic acid; genes code for proteins).

See how DNA is copied when cells divide in our guide to mitosis and meiosis.

The Human Genome Project, completed in 2003, worked out the sequence of the human genome. Knowing where genes are helps scientists find genes linked to inherited disorders, understand how to treat them, and trace human migration patterns from the past. Genome sequencing is now much faster and cheaper, and is used in medicine and research.

1. Write the complementary strand for the DNA sequence A G T C.
2. How many chromosomes are in a normal human body cell?
3. What is the difference between a gene and an allele?

Answers: 1. T C A G. 2. 46 (23 pairs). 3. A gene is a section of DNA that codes for a characteristic (via a protein); alleles are different versions of the same gene.

• Nucleotide: the repeating unit of DNA (a sugar, a phosphate and a base).
• Base: A, T, C or G.
• Double helix: the twisted, two-stranded shape of DNA.
• Genotype: the alleles an organism has. Phenotype: the characteristics that result.
• Homozygous: two identical alleles. Heterozygous: two different alleles.
• Mutation: a change in the DNA sequence.