Gene expression and mutation: the genetic code as triplets of bases, transcription of a gene into messenger RNA, translation at the ribosome using transfer RNA, codons and anticodons, the role of gene mutation (substitution, insertion and deletion) and how mutations can be silent, harmful or beneficial, and the regulation of gene expression so that different genes are switched on in different cells.

What this dot point is asking

CCEA wants you to explain that the genetic code is read in triplets of bases, describe transcription of a gene into messenger RNA and translation of mRNA into a polypeptide at the ribosome using transfer RNA, explain the role of codons and anticodons, describe gene mutations (substitution, insertion and deletion) and how they can be silent, harmful or beneficial, and explain how gene expression is regulated so different genes act in different cells.

The genetic code and transcription

A gene is too valuable to leave the nucleus, so its code is first copied into a mobile molecule, messenger RNA. In transcription:

1. The enzyme RNA polymerase binds to the gene and the DNA double helix unwinds and unzips at that point, exposing the bases.
2. One strand (the template strand) is used as a template. Free RNA nucleotides line up against it by complementary base pairing (adenine pairs with uracil in RNA, and cytosine with guanine).
3. RNA polymerase joins the nucleotides together to form a single-stranded messenger RNA (mRNA) molecule that is a complementary copy of the template.
4. The mRNA detaches and leaves the nucleus through a nuclear pore, carrying the code to a ribosome.

Translation at the ribosome

In translation the mRNA code is used to build the polypeptide:

1. The mRNA binds to a ribosome, which reads the bases in groups of three (codons).
2. A transfer RNA (tRNA) molecule carrying a specific amino acid has a triplet of bases called an anticodon that is complementary to the first codon, and it binds there by base pairing.
3. A second tRNA brings the next amino acid to the adjacent codon, and a peptide bond forms between the two amino acids.
4. The ribosome moves along the mRNA by one codon; the first tRNA leaves to collect another amino acid, and the process repeats.
5. The polypeptide grows in the order set by the codons until a stop codon is reached, when no tRNA binds and the finished polypeptide is released.

Gene mutation

A gene (point) mutation is a change in the base sequence of a gene, usually arising as a random error during DNA replication or caused by a mutagen such as ionising radiation or certain chemicals.

• A substitution replaces one base with another, changing only one codon. Because the code is degenerate, the new codon may still code for the same amino acid (a silent mutation), or it may change one amino acid.
• An insertion adds a base and a deletion removes one. Either shifts the reading frame of every codon after the point of change (a frameshift), so a completely different sequence of amino acids is coded after that point and the protein is usually non-functional.

Most mutations are harmful or silent, but occasionally a mutation is beneficial, producing a useful new allele. This new variation is the ultimate source of the genetic diversity on which natural selection acts.

Regulation of gene expression

Every body cell contains the same complete set of genes, yet cells become very different from one another. This is possible because gene expression is regulated: only the genes a particular cell needs are switched on (transcribed and translated), while the rest stay switched off. During differentiation, signals determine which genes are active, so a liver cell expresses the genes for liver enzymes while a nerve cell expresses a different set. The unused genes are still present but are not transcribed, so the two cells make different proteins and take on different structures and functions.

Examples in context

Example 1. Sickle-cell anaemia. A single base substitution in the gene for the beta-globin chain of haemoglobin changes one codon, so valine replaces glutamic acid. This one amino acid change makes the haemoglobin clump and distorts red blood cells into a sickle shape, showing how even a single-base substitution can have a serious effect when the changed amino acid is critical.

Example 2. Antibiotic resistance from mutation. A chance mutation in a bacterium can produce an enzyme that breaks down an antibiotic. In the presence of the antibiotic this resistant bacterium survives and reproduces while others die, so the new allele spreads, an example of a rare beneficial mutation supplying variation for natural selection.

Try this

Q1. State where transcription and translation each take place in a eukaryotic cell. [2 marks]

• Cue. Transcription in the nucleus; translation at a ribosome in the cytoplasm.

Q2. Explain why a deletion mutation usually has a greater effect than a substitution. [3 marks]

• Cue. A deletion shifts the reading frame, so every codon after the deletion is read differently (a frameshift) and a different sequence of amino acids is coded; a substitution changes at most one codon and may be silent.

Q3. Explain how cells with identical genes can become different cell types. [2 marks]

• Cue. Gene expression is regulated, so different genes are switched on in different cells, making different proteins.