Gene regulation, genome and fingerprints

A cell does not make every protein all the time. In bacteria, genes that work together are often switched on and off as one unit, an operon. The best known is the lac operon, described by François Jacob and Jacques Monod. It controls the use of the sugar lactose.

Its parts:

• The i gene (the regulator gene, not 'inducer') makes a repressor protein. It is always switched on.
• Three structural genes: z makes β-galactosidase, which splits lactose into glucose and galactose; y makes permease, which lets lactose into the cell; a makes transacetylase.
• An operator next to them, where the repressor can bind, and a promoter.

How it works:

• No lactose: the repressor binds the operator and blocks RNA polymerase. The operon is off. Why make enzymes for a sugar that is not there?
• Lactose present: lactose acts as the inducer. It binds the repressor and inactivates it, so the repressor falls off the operator, and the genes are transcribed.

Because the repressor switches the operon off, this is called negative regulation.

The Human Genome Project (HGP), launched in 1990 and completed in 2003, set out to read the full sequence of human DNA, about 3 × 10⁹ base pairs. DNA was cut into fragments, copied in bacteria and yeast using vectors called BAC and YAC, and sequenced with automated machines based on the Sanger method. Two approaches were used: finding the genes that are expressed (Expressed Sequence Tags, ESTs), and sequencing everything and then working out what each part does (sequence annotation). Some findings:

• The genome has 3164.7 million base pairs and about 30,000 genes.
• 99.9% of the bases are the same in all people.
• Less than 2% of the genome codes for proteins; much of the rest is repeated sequences.
• About 1.4 million places where single bases differ between people (SNPs) were found.

DNA fingerprinting, developed by Alec Jeffreys, uses the other 0.1%. Some short DNA sequences are repeated many times in a row, and the number of repeats varies from person to person: these are VNTRs (variable number tandem repeats), or minisatellites. The steps: isolate DNA; cut it with restriction enzymes; separate the fragments by gel electrophoresis; transfer them to a membrane (Southern blotting); hybridise with a labelled VNTR probe; and detect the bands by autoradiography. The band pattern is unique to each person, except identical twins, and is half inherited from each parent. It is used in forensic science and to settle questions of parentage.