Gene Expression · GeneLab

Transcription of genetic information

A gene is a stretch of DNA with a genetic effect, but proteins are made on ribosomes in the cytoplasm. First the DNA's information is copied onto RNA; this is called transcription.

RNA polymerase binds the promoter upstream of the gene, opens a short stretch of the double helix and, using one strand as the template, links ribonucleotides one by one by complementary base pairing (A–U, T–A, G–C, C–G). The mRNA grows from its 5′ end towards its 3′ end. RNA has no T; uracil (U) takes its place.

Let the polymerase bind, then choose by hand the ribonucleotide that pairs with each base of the template strand; then try switching the template strand and removing the promoter.

Translation and gene mutation

Every three adjacent bases on mRNA specify one amino acid; this triplet is called a codon. The ribosome moves along the mRNA, reading one codon at a time; at one end of each tRNA an anticodon pairs with the codon, and the other end carries the matching amino acid. The ribosome links the amino acids into a chain and stops at a stop codon; this is translation. Many ribosomes often sit on one mRNA at the same time, so a little mRNA can make a lot of protein quickly.

This mRNA is the beginning of the β-globin gene. Translate it once, then click a base in the sequence on the right to change it, or insert or delete a base, and translate again to compare the outcomes of gene mutations.

Cracking the genetic code

How can four bases specify twenty amino acids? If two bases specified one amino acid there would be only 4² = 16 combinations, too few; three bases give 4³ = 64, which is enough.

In 1961 Nirenberg and Matthaei added synthetic mRNA to a DNA-free cell extract and looked for which labelled amino acid ended up in the peptide. In the same year Crick inserted and deleted bases in a phage gene and showed that a codon is made of three bases, read continuously from a fixed starting point, without overlaps or gaps.

Choose a synthetic mRNA, then add it, incubate, filter and measure the radioactivity; then use the frameshift test on the right to compare inserting 1, 2 and 3 bases.

The central dogma and epigenetics

Crick's central dogma: genetic information can flow from DNA to DNA (replication), from DNA to RNA (transcription) and on to protein (translation). Later it was found that some viruses can make DNA using RNA as a template (reverse transcription) or copy their own RNA (RNA replication). Many drugs work by blocking one of these steps.

Genes control traits in two ways: by controlling the making of enzymes and so controlling metabolism, as with the wrinkled peas in PeaLab; or by directly controlling the structure of proteins, as in sickle-cell anaemia. Even when a gene's base sequence is unchanged, its expression can change heritably; for example DNA methylation of the promoter silences the gene. This is epigenetics. Different cells expressing different genes is the basis of cell differentiation; see StemLab.

On the right, switch the genetic system, add drugs, then drag the methylation level and watch the mouse's coat colour.

In-class testing

Look back over the four topics and answer the multiple-choice questions below.