Protein synthesis and the genetic code (2.6, 2.7, 2.8)
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The genetic code is the set of rules by which the information encoded within DNA or RNA sequences is translated into proteins by living cells.
This code specifies how sequences of nucleotides correspond to specific amino acids, which are the building blocks of proteins.
The genetic code is written in triplet codes, which are sequences of three nucleotides and referred to as codons.
Each DNA codon corresponds to a specific amino acid, or to a ‘start’ or ‘stop’ command during protein synthesis.
For example, the codon AUG codes for the amino acid methionine, which also serves as the start codon.
The genetic code is non-overlapping, meaning that each nucleotide is part of only one codon.
This linear and non-overlapping reading frame is critical. A shift in the reading frame is known as a frameshift mutation. This can lead to the placement of an incorrect amino acid in a polypeptide chain or the incorrect ‘reading’ of a stop codon, resulting in a truncated or non-functional protein.
The genetic code is degenerate, which means that multiple codons can code for the same amino acid.
For example, the amino acid leucine is coded by six different codons (UUA, UUG, CUU, CUC, CUA, CUG).
This redundancy is beneficial because it provides a ‘buffer’ against mutations. A mutation in one nucleotide of a codon might not change the amino acid it codes for, thereby minimising the impact on the protein’s structure and function.
Methionine and tryptophan are the only amino acids that are coded by a single codon (AUG and UGG, respectively).
The genetic code is universal, meaning that the same codons specify the same amino acids across almost all living organisms.
The universality of the genetic code suggests a common evolutionary origin and allows for the possibility of genetic engineering, where genes from one organism can be expressed in another (e.g., using bacterial cells to produce human insulin).
A gene is a sequence of DNA bases that contains instructions for:
- the amino acid sequence in a polypeptide chain
- or a functional RNA, such as ribosomal RNA (rRNA) or transfer RNA (tRNA).
The location of a gene on a DNA molecule is fixed and called a locus.
Different versions of the same gene, alleles, are found at the same locus on homologous chromosomes.
DNA contains the ‘instructions’ for making polypeptides, but is contained in the nucleus of cells. The ribosomes, which assemble polypeptides, are in the cytoplasm.
Protein synthesis, therefore, happens in two stages:
- Transcription – messenger RNA (mRNA) is made from the DNA template. The mRNA takes the instructions from the nucleus to the ribosomes.
- Translation – ribosomes join amino acids together in the order specified by the sequence of bases in the mRNA, forming a polypeptide.
To start transcription, the DNA template of a gene must be exposed.
At the start codon of a gene, the enzyme DNA helicase unzips the DNA double helix, breaking the hydrogen bonds between complementary base pairs.
The sense strand of DNA runs 5’ – 3’ and codes for proteins; the antisense strand is complementary to it.
During transcription, the DNA sequence of a gene is copied into messenger RNA (mRNA):
- To copy the base sequence held on the sense strand, the mRNA is complementary to the antisense strand, which acts as a template.
- Free RNA nucleotides match up with their complementary base pairs on the exposed antisense strand.
- RNA polymerase joins the RNA nucleotides in condensation reactions, forming phosphodiester bonds.
- Transcription stops at the end of a gene, and the mRNA molecule leaves the nucleus through a nuclear pore.
During translation, ribosomes join amino acids together in the order specified by the sequence of bases in the mRNA codons, forming a polypeptide and determining how it will fold and function.
Each codon specifies a particular amino acid, which is delivered to the ribosome by a transfer RNA (tRNA) molecule.
ATP provides energy for an enzyme to join amino acids together by peptide bonds.
Transfer RNA, tRNA is made of RNA nucleotides. It is a small, single-stranded molecule that folds into a clover leaf shape due to complementary base pairing within its single strand.
Each tRNA has an anticodon region that pairs with a complementary codon on the mRNA.
At the other end of the molecule from the anticodon, tRNA molecules have an amino acid attachment site. Each tRNA can only bind to one specific amino acid which is determined by its anti-codon.
Translation is a multistep process involving mRNA, ribosomes, tRNA and amino acids:
- A ribosome binds to the start codon of an mRNA molecule.
- A tRNA molecule with the complementary start anticodon binds to the mRNA and the ribosome, bringing with it an amino acid.
- A tRNA molecule with an anticodon complementary to the next codon along on the mRNA molecule and an amino acid binds next.
- An enzyme catalyses the formation of a peptide bond, joining the two amino acids together, requiring ATP for energy.
- The ribosome moves along the length of the mRNA molecule, repeating the above process until reaching a stop codon. There is no complementary tRNA anticodon for the stop codon, so the polypeptide chain is released.
Multiple ribosomes can simultaneously synthesise multiple polypeptide chains from a single mRNA molecule by following each other along the mRNA.
