D1.2 Protein synthesisIB Biology HL: Revision notes

RNA polymerase separates the DNA strands and links RNA nucleotides complementary to the template strand (hydrogen bonds; A on DNA pairs with U in RNA). The DNA base sequence is not changed, which matters in non-dividing cells whose sequences must last the life of the cell. Transcription is the first stage of gene expression and a key point where genes are switched on or off; not all genes are expressed at any time.

mRNA binds to the small subunit of the ribosome; two tRNAs can bind to the large subunit at once. Codons pair with complementary anticodons. The ribosome moves stepwise along the mRNA, and amino acids are joined by peptide bonds.

The code is a triplet code (4² = 16 is too few for 20 amino acids; 4³ = 64), degenerate (several codons per amino acid) and universal. Deduce amino acid sequences by reading mRNA triplets in a codon table.

A point mutation can change protein structure, e.g. sickle cell: GAG → GUG, glutamic acid → valine in β-globin.

RNA polymerase adds nucleotides to the 3' end of the growing RNA, so RNA is made 5' to 3' (the template is read 3' to 5'). Ribosomes also read mRNA 5' to 3': they start near the 5' end and move towards the 3' end, so the first codon translated is nearest the 5' end.

Transcription begins at the promoter, a non-coding sequence next to the gene. Transcription factors bind to the promoter and allow RNA polymerase to bind and start transcription.

Non-coding sequences do not code for polypeptides but have roles: regulators of gene expression (e.g. promoters), introns, telomeres (protect chromosome ends), and genes for rRNA and tRNA (transcribed but not translated).

In eukaryotes the primary transcript (pre-mRNA) is modified in the nucleus:

• introns are removed and exons spliced together to form mature mRNA;
• a 5' cap and a 3' polyA tail are added, which stabilise the mRNA.

Alternative splicing: splicing different combinations of exons lets one gene code for several different polypeptides.

Initiation: the small subunit attaches to the 5' end of the mRNA and moves along to the start codon (AUG); the initiator tRNA (methionine) pairs with it; the large subunit attaches; another tRNA then binds next to it.

Elongation sites on the large subunit:

• A site — incoming tRNA with the next amino acid;
• P site — tRNA holding the growing chain;
• E site — empty tRNA leaves.

After each peptide bond the ribosome moves one codon: A → P → E.

Many polypeptides must be modified before they function. Insulin is made in two stages: pre-proinsulin (110 amino acids) loses its signal sequence in the RER to give proinsulin; then a central section including the C-peptide is removed, leaving A and B chains joined by disulfide bonds (51 amino acids).

Proteasomes break down unneeded or damaged proteins and the amino acids are recycled. A functional proteome needs constant protein breakdown and synthesis.