Genes and loci

A gene is a sequence of DNA bases that codes for a product. That product may be a polypeptide or a functional RNA molecule.

For a gene that codes for a polypeptide, the order of its DNA bases specifies the order of amino acids in the polypeptide. Other genes code for functional RNA: RNA that has a function of its own rather than coding for a polypeptide. Two examples are:

• Ribosomal RNA, which forms part of ribosomes.
• Transfer RNA, which carries amino acids during polypeptide synthesis.

A gene occupies a fixed position on a particular DNA molecule. This position is its locus. The locus identifies where the gene is; it does not describe the gene’s base sequence or the product it codes for.

The genetic code

A polypeptide is a chain of amino acids. The genetic code relates the sequence of bases in DNA to the sequence of amino acids in a polypeptide. A group of three successive DNA bases is called a triplet; each triplet that specifies an amino acid identifies which amino acid is added at that position in the chain.

Triplets are read without overlap

The code is non-overlapping: successive triplets are read as separate groups. Each base belongs to only one triplet; it is not shared with the triplet next to it.

For example, a sequence on the DNA coding strand can be grouped as $$\mathrm{GCT\ |\ GCC\ |\ GAA}$$. The first two triplets specify alanine and the third specifies glutamic acid. The corresponding part of the polypeptide therefore has the sequence alanine–alanine–glutamic acid.

Degenerate and universal code

The code is degenerate because more than one triplet can specify the same amino acid. In the example, both $$\mathrm{GCT}$$ and $$\mathrm{GCC}$$ specify alanine. This does not mean that a single triplet specifies several different amino acids.

The code is described as universal because the same triplets specify the same amino acids in almost all organisms. There are exceptions, so universal does not mean that the code is identical in every organism.

Eukaryotic non-coding DNA and gene structure

Much of the DNA in a eukaryotic nucleus does not code for polypeptides. This non-coding DNA occurs both between genes and within genes.

Non-coding DNA between genes

Some non-coding DNA between genes consists of repeated base sequences: sequences of DNA bases that occur multiple times. These repeats do not code for polypeptides.

Exons and introns within genes

Within a eukaryotic protein-coding gene, exons contain the information that codes for the amino acid sequence of a polypeptide. In a gene with introns, the exons are separated by one or more non-coding sequences called introns.

A protein-coding gene can therefore contain both sequences that provide information for a polypeptide and non-coding sequences within the gene.