AQA A-Level Biology: Genome Projects

Reading the entire DNA sequence of an organism, its genome, has opened up new ways to understand disease, design drugs and identify species. This short guide explains what a genome and a proteome are, why sequencing is useful, and why in complex organisms you cannot simply read off the proteome from the genome. It links the molecular detail of earlier topics to its practical applications.

Genome and proteome

Two terms are central here.

• The genome is the complete set of genes in a cell.
• The proteome is the full range of proteins that a cell can produce.

The two are connected, because genes code for proteins, but as we will see they are not simply interchangeable in complex organisms.

Genome sequencing

Genome sequencing means identifying the DNA base sequence of an organism's entire genome. This is valuable because, from the base sequence, the amino acid sequences of the proteins the organism can make can be worked out. Sequencing projects have now read the genomes of a wide range of organisms, including humans: the Human Genome Project sequenced all human DNA and identified around 20,000 genes.

Sequencing has many uses.

• Identifying genes or alleles linked to genetic diseases and cancers, so that new targeted drugs or gene therapies can be developed.
• Screening patients, allowing early prevention and personalised medicine tailored to a person's genotype.
• Identifying species and working out their evolutionary relationships.

One important application is vaccine development against pathogens. Sequencing a pathogen's genome allows its proteome to be identified, which reveals potential antigens, proteins that would trigger an immune response, that could be used in a vaccine.

Why the genome is not simply the proteome

In simple organisms such as bacteria, the genome translates fairly directly into the proteome. In complex organisms it does not, for two reasons.

• Much of the DNA is non-coding, including introns within genes, which do not code for polypeptides.
• There are regulatory genes that control the expression of other genes, for example by coding for miRNA, rather than coding for proteins themselves.

Because of this, you cannot simply read the proteome off the genome; you have to account for which sequences are actually expressed as protein.

How sequencing is changing

Sequencing technology continues to develop. Methods have become automated, which makes them faster, cheaper and able to work on a much larger scale, and they are continuously updated as the technology improves.

How this fits together

Genome projects turn the molecular biology of DNA and proteins into practical tools for medicine and biology. Reading a genome is one thing; deliberately cutting, copying and moving genes between organisms is another, and that technology is the subject of the recombinant DNA technology and PCR guide.