AQA A-Level Biology: Protein Synthesis (Transcription and Translation)

Protein synthesis is how the code stored in DNA is turned into a working protein. It happens in two stages: transcription, which copies a gene into mRNA in the nucleus, and translation, which reads that mRNA to build a polypeptide at a ribosome. This guide works through both, building on genes and the genetic code.

Transcription

Transcription is the production of mRNA from a gene, and in eukaryotes it happens in the nucleus.

First, the enzyme DNA helicase breaks the hydrogen bonds between the two DNA strands, unwinding the double helix over the gene. Only one of the strands, the template strand, is used. Free RNA nucleotides line up against the template by complementary base pairing, with uracil pairing with adenine in place of thymine. The enzyme RNA polymerase then joins these RNA nucleotides together, forming phosphodiester bonds by condensation reactions.

The molecule produced in a eukaryote is not finished mRNA but pre-mRNA.

Splicing

The pre-mRNA still contains the non-coding introns. In a process called splicing, carried out by enzymes in the nucleus, the introns are removed and the remaining exons are joined together to make the mature mRNA. The mRNA then leaves the nucleus through a nuclear pore to reach a ribosome.

This is one clear difference between eukaryotes and prokaryotes. Prokaryotic genes contain no introns, so no splicing is needed and mRNA is produced directly. Prokaryotic transcription also happens in the cytoplasm, because there is no nucleus.

Translation

Translation is the production of a polypeptide from the codons on the mRNA, and it takes place at a ribosome.

The mRNA attaches to a ribosome, which has room for two codons at a time, and the ribosome finds the start codon. A molecule of tRNA, carrying a specific amino acid, arrives so that its anticodon pairs with the complementary codon on the mRNA, held by hydrogen bonds. A second tRNA binds at the next codon, bringing its own amino acid alongside the first. The ribosome then catalyses the formation of a peptide bond between the two amino acids, a condensation reaction that uses energy from the hydrolysis of ATP.

The first tRNA is released, now empty, to collect another copy of its amino acid, and the ribosome moves along to the next codon. Amino acid by amino acid, the polypeptide grows, until the ribosome reaches a stop codon and the finished chain is released.

The roles of the key molecules

It helps to be clear about what each component does.

• The mRNA carries the coded sequence of codons from the DNA to the ribosome.
• Each tRNA brings a specific amino acid, matched to its anticodon, and pairs that anticodon with the mRNA codon. There are around 60 types of tRNA for the 20 amino acids, a number that reflects the degenerate code.
• The ribosome holds the mRNA and two tRNAs in place, catalyses the peptide bond, and moves along the mRNA one codon at a time.
• ATP is hydrolysed to release the energy needed to attach amino acids to tRNA and to form the peptide bonds.

Working between the sequences

Because the pairing rules are fixed, you can move between the DNA, mRNA and amino acid sequences. The mRNA codons are complementary to the triplets on the template DNA strand, remembering that uracil replaces thymine, and the tRNA anticodons are in turn complementary to the mRNA codons. A genetic code table then converts each mRNA codon into its amino acid. For example, a DNA template triplet of TGC gives an mRNA codon of ACG, which is read by a tRNA anticodon of UGC.

A change in the DNA sequence can change the protein that is made. That is the subject of the gene mutations guide.