Protein synthesis

· Protein synthesis = using information in DNA to make a polypeptide.
· A gene = a sequence of nucleotides that forms part of a DNA molecule and codes for a polypeptide.
· The sequence of bases in DNA determines the sequence of amino acids in a polypeptide.
· Main stages: transcription → mRNA production; translation → polypeptide production.
· In eukaryotes, transcription occurs in the nucleus and translation occurs at ribosomes in the cytoplasm.

This diagram summarises the central idea of protein synthesis: DNA is transcribed into mRNA, then mRNA is translated into an amino acid sequence. Use it to link the terms gene, mRNA, codon, and polypeptide. Source

Genetic code

· The genetic code is based on triplets of DNA bases.
· A triplet = three DNA bases that code for one amino acid or act as a start/stop signal.
· A codon = three bases on mRNA.
· The genetic code is universal: the same codons code for the same amino acids in almost all organisms.
· Start codon = signals the beginning of translation.
· Stop codon = signals the end of translation and does not code for an amino acid.
· Several codons can code for the same amino acid, but each codon codes for only one amino acid.

Transcription

· Transcription = copying the DNA base sequence of a gene into an RNA molecule.
· The DNA strand used during transcription is the template strand / transcribed strand.
· The other DNA strand is the non-template strand / non-transcribed strand.
· RNA polymerase binds to the DNA and joins RNA nucleotides together.
· RNA bases pair with the exposed DNA template strand: A pairs with U, T pairs with A, C pairs with G, G pairs with C.
· The RNA molecule made first is called the primary transcript.
· In eukaryotes, the primary transcript must be modified before it becomes mRNA.

mRNA processing in eukaryotes

· Introns = non-coding sequences removed from the primary transcript.
· Exons = coding sequences joined together to form mature mRNA.
· This produces an mRNA molecule containing only the coding sequence needed for translation.
· The mature mRNA leaves the nucleus and moves to a ribosome in the cytoplasm.
· Common exam wording: introns are removed and exons are joined together.

This diagram shows how a eukaryotic primary transcript is processed into mature mRNA. The key exam idea is that introns are removed and exons are joined before translation. Source

Translation

· Translation = using the sequence of mRNA codons to assemble a polypeptide.
· Translation occurs at ribosomes.
· The ribosome holds the mRNA and helps match each codon with the correct tRNA.
· tRNA carries a specific amino acid.
· Each tRNA has an anticodon that is complementary to an mRNA codon.
· Codon–anticodon complementary base pairing ensures the correct amino acid is added.
· Amino acids are joined by peptide bonds to form a growing polypeptide chain.
· Translation continues until a stop codon is reached.
· The final amino acid sequence determines the primary structure of the protein.

This diagram shows how mRNA codons are read by a ribosome and matched with tRNA anticodons. Each tRNA brings a specific amino acid, allowing the polypeptide chain to grow in the correct order. Source

Key molecules and their roles

· DNA: contains the gene that codes for the polypeptide.
· Template strand: DNA strand used to make the RNA transcript.
· Non-template strand: DNA strand not used in transcription.
· RNA polymerase: enzyme that forms the RNA transcript during transcription.
· mRNA: carries the genetic code from DNA to the ribosome.
· Codon: three bases on mRNA coding for one amino acid or a start/stop signal.
· tRNA: carries a specific amino acid to the ribosome.
· Anticodon: three bases on tRNA complementary to an mRNA codon.
· Ribosome: site of translation; holds mRNA and tRNA in position.
· Polypeptide: chain of amino acids formed during translation.

Gene mutations

· A gene mutation = a change in the sequence of base pairs in a DNA molecule.
· A mutation may alter the mRNA codons, which may alter the amino acid sequence.
· If the amino acid sequence changes, the primary structure changes.
· A changed primary structure may alter protein folding and therefore protein function.
· Some mutations have no effect, for example if the altered codon still codes for the same amino acid.

This diagram compares the three mutation types named in the syllabus. It is useful for seeing how insertions and deletions can change the grouping of bases into triplets. Source

Types of gene mutation

· Substitution = one nucleotide/base pair is replaced by another.
· Possible effects of substitution:
· Silent mutation: same amino acid produced, no change to polypeptide.
· Missense mutation: different amino acid produced, protein may change.
· Nonsense mutation: stop codon produced early, shorter polypeptide may form.
· Deletion = one or more nucleotides/base pairs are removed.
· Insertion = one or more nucleotides/base pairs are added.
· If an insertion or deletion is not in multiples of three, it causes a frameshift.
· A frameshift changes all codons after the mutation, often causing a major change to the polypeptide.
· Mutation effects depend on where the mutation occurs and whether it changes the amino acid sequence.

Exam technique

· When converting DNA to mRNA, use complementary base pairing and remember RNA contains U instead of T.
· Identify whether the DNA strand given is the template strand or non-template strand before writing mRNA.
· Group mRNA bases into codons of three from the correct start point.
· Use the codon table to find the amino acid sequence from mRNA codons.
· For mutation questions, compare the original and mutated sequences and explain the effect on codons, amino acids, and polypeptide structure/function.