Genes, DNA, and how genes work

Understand the idea

Genes are the basic units of heredity: segments of DNA that carry instructions, especially for proteins. DNA is a double helix of two polynucleotide chains. Four bases write the code: adenine (A), cytosine (C), thymine (T), and guanine (G). About three billion bases are in a human; roughly 99% are the same in everyone.

William Bateson used ‘genetics’ in 1905. Wilhelm Johannsen named the ‘gene’ in 1909. A gene sits at a locus on a chromosome. The genome is the full set. Humans have 23 pairs of chromosomes. A codon is three bases that specify an amino acid. Transcription copies a gene into mRNA; translation builds the protein.

Genes control DNA and RNA work and therefore enzymes and structures such as haemoglobin, which carries oxygen. Phenotype comes from genotype plus environment. Many traits are polygenic. Mutations create alleles; natural selection and drift change how common those alleles are.

What to remember

• Gene = DNA instruction, often for a protein; locus = its place.
• DNA bases A, C, T, G; codon = three bases.
• Transcription then translation turn a gene into protein.

Recap

A gene is a DNA paragraph. Chromosomes are the books. Expression copies the paragraph into RNA and then into protein, which does the body’s work.