D1.2 Protein synthesis

Genes do their work by directing the building of proteins, and proteins do almost everything in a cell — they are enzymes, structures, pumps and signals. Protein synthesis is the two-stage process that turns the coded instructions in DNA into a functional polypeptide: transcription copies a gene into messenger RNA, and translation reads that message to assemble amino acids in the correct order. The link between the two is the genetic code, a universal dictionary of three-base codons. Hold on to the central idea — DNA → RNA → protein — and the detail of D1.2 hangs neatly off it.

Transcription: copying a gene into mRNA

Transcription is the synthesis of messenger RNA (mRNA) using a DNA template, and it takes place in the nucleus in eukaryotes. The enzyme RNA polymerase binds to the gene and separates the two DNA strands. One strand, the template (antisense) strand, is read; RNA polymerase adds complementary RNA nucleotides to build the mRNA.

Base pairing in transcription follows the usual rules with one change: RNA contains uracil (U) instead of thymine, so adenine on the template pairs with uracil in the RNA. The mRNA produced is therefore complementary to the template strand and (apart from U replacing T) identical to the other DNA strand, the sense strand. Once complete, the mRNA carries the gene’s message out of the nucleus to a ribosome.

The genetic code: codons

The information in mRNA is read in groups of three bases called codons. Each codon specifies one amino acid (or a start or stop signal). Key properties of the genetic code that the syllabus expects you to know:

• It is a triplet code: three bases code for one amino acid.
• It is universal: the same codons specify the same amino acids in almost all organisms, which is strong evidence for a common origin of life and which makes genetic engineering between species possible.
• It is degenerate: most amino acids are coded for by more than one codon, so some base changes do not alter the protein.

With four possible bases in groups of three there are 4³ = 64 possible codons, comfortably more than enough for the 20 amino acids — which is why the code can afford to be degenerate.

Translation: building the polypeptide

Translation is the synthesis of a polypeptide at a ribosome, using the sequence of codons on the mRNA. Three components work together:

• The mRNA provides the sequence of codons to be read.
• Transfer RNA (tRNA) molecules carry amino acids to the ribosome. Each tRNA has an anticodon of three bases that pairs with a complementary mRNA codon, and it carries the specific amino acid that matches that codon.
• The ribosome holds the mRNA and tRNAs in place and catalyses the formation of peptide bonds between adjacent amino acids.

The ribosome moves along the mRNA one codon at a time. For each codon a tRNA with the matching anticodon delivers its amino acid, a peptide bond joins it to the growing chain, and the empty tRNA leaves to collect another amino acid. Translation continues until a stop codon is reached, releasing the completed polypeptide, which then folds into a functional protein.

From base sequence to amino acid sequence

Putting the stages together lets you predict a polypeptide from a gene. Because complementary base pairing links each step, the sequence of bases in DNA determines the sequence of codons in mRNA, which determines the order in which tRNAs deliver amino acids, which determines the amino acid sequence of the polypeptide. The amino acid sequence in turn determines how the protein folds and therefore its function.

This chain of cause and effect explains why a change in one DNA base (a mutation) can change a codon, alter one amino acid and so change the protein — the basis of the sickle-cell example you meet in D1.3. In exam questions you may be given a DNA template strand and a codon table and asked to work out the mRNA and the corresponding amino acids; the safe route is template DNA → mRNA (remember A pairs with U) → read codons in the table.