Genes

A gene is a sequence of DNA bases that codes for either:

• A polypeptide (protein), or
• A functional RNA molecule (e.g. tRNA or rRNA)

The key AQA definition: a gene is a sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide.

Each gene occupies a specific position on a chromosome called a locus (plural: loci). The sequence of bases in a gene determines the sequence of amino acids in a polypeptide, which in turn determines the protein's 3D shape and function.

The Genetic Code

The genetic code has several key properties that AQA loves to ask about:

Triplet code - each amino acid is coded for by a sequence of 3 bases called a codon (on mRNA) or a triplet (on DNA). There are 4 bases, so 4³ = 64 possible codons, but only 20 amino acids - this means most amino acids are coded for by more than one codon.

Degenerate - most amino acids are coded for by more than one codon (e.g. Leu is coded by UUA, UUG, CUU, CUC, CUA, CUG). This provides a buffer against mutation - a base change in the third position of a codon often still codes for the same amino acid (called a synonymous/silent mutation).

Non-overlapping - each base is read only once, as part of one codon. The code is read sequentially without overlap.

Universal - with very few exceptions, the same codons code for the same amino acids in virtually all organisms. This is powerful evidence for a common ancestor of all life.

Start and stop codons - AUG (methionine) is the universal start codon. UAA, UAG and UGA are stop codons - they don't code for any amino acid but signal the end of translation.

Chromosomes

In eukaryotes, DNA is found in the nucleus, wound around proteins called histones to form chromatin. The DNA + histone complex is called a nucleosome. Chromatin can be tightly coiled (heterochromatin - transcriptionally inactive) or loosely coiled (euchromatin - transcriptionally active).

Human cells contain 46 chromosomes arranged in 23 homologous pairs. Each member of a homologous pair:

• Is the same length
• Has the same genes at the same loci
• But may carry different alleles of those genes

This is exactly what Figure 2 in your paper showed - the karyotype with chromosomes arranged in homologous pairs.

Alleles are different versions of the same gene. They arise through mutation and occupy the same locus on homologous chromosomes. See also DNA replication and Protein synthesis for how the code is read.

In prokaryotes, DNA is:

• Circular (not linear)
• Not associated with histone proteins
• Found free in the cytoplasm (no nucleus)
• Much smaller
• Often accompanied by small circular DNA molecules called plasmids