D1.2 Protein synthesisIB Biology SL: Revision notes

Transcription is the synthesis of RNA using a DNA template. RNA polymerase separates the two DNA strands at a gene and links RNA nucleotides into a strand. Each RNA nucleotide pairs with the template strand by complementary base pairing, held by hydrogen bonds: adenine (DNA) with uracil (RNA), thymine with adenine, cytosine with guanine. The RNA then separates from the template.

A single DNA strand can be transcribed many times without its base sequence changing: the RNA separates and the DNA strands re-pair. In somatic cells that do not divide (such as neurons and cardiac muscle cells), these sequences must be conserved for the life of the cell.

Transcription is the first stage of gene expression. Not all genes in a cell are expressed at any given time, and transcription is a key stage at which a gene is switched on or off — every cell has the β-globin gene, but only red blood cell precursors transcribe it.

Translation is the synthesis of a polypeptide from mRNA: the base sequence of mRNA is turned into an amino acid sequence.

• mRNA binds to the small subunit of the ribosome.
• tRNA molecules each carry a specific amino acid and have an anticodon.
• Each codon (three mRNA bases) pairs with a complementary anticodon by hydrogen bonding.
• Two tRNAs can bind to the large subunit at the same time.

The code is a triplet code: with four bases, a doublet code gives only 4² = 16 combinations, too few for 20 amino acids; triplets give 4³ = 64.

• Degeneracy — more than one codon can code for the same amino acid (61 codons, 20 amino acids).
• Universality — the same codons code for the same amino acids in almost all organisms, which is why a jellyfish gene works in a mouse.

Three codons are stop codons; AUG is the start codon (methionine).

To deduce an amino acid sequence, split the mRNA into triplets from the start codon and look each one up in the table of mRNA codons; stop at a stop codon (it adds no amino acid).

During elongation the ribosome moves stepwise along the mRNA, one codon at a time. Each time, the amino acid on the incoming tRNA is linked to the growing chain by a peptide bond. A chain of n amino acids has n − 1 peptide bonds.

A point mutation (base substitution) changes one codon. Possible effects: no change (degeneracy), one amino acid changed, or a premature stop codon.

Sickle cell anaemia: template CTC → CAC, so the mRNA codon GAG → GUG and the sixth amino acid of β-globin is valine instead of glutamic acid. The hydrophobic valine makes haemoglobin molecules stick together at low oxygen, forming fibres that sickle red blood cells, which block capillaries and carry less oxygen.