DNA and protein synthesis (3.4.2)
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The genome is the complete set of DNA in a cell or organism, including all of its genes. It contains the instructions for building and maintaining that organism.
The proteome is the full set of proteins that a cell or organism can produce.
The genome is mostly constant, while the proteome varies between cells and over time, depending on which genes are expressed.
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.
During transcription, the DNA sequence of a gene is copied into pre-messenger RNA (pre-mRNA):
- At the start codon of a gene, an enzyme unzips the DNA double helix.
- Free RNA nucleotides match up with their complementary base pairs on the exposed DNA strand.
- RNA polymerase joins the RNA nucleotides in condensation reactions, forming phosphodiester bonds.
- DNA rejoins behind RNA polymerase to limit the number of exposed bases.
- Transcription stops at a terminator sequence, and the pre-mRNA molecule leaves the nucleus through a nuclear pore.
In eukaryotes, the RNA molecule created during transcription contains both introns and exons. This is pre-mRNA. The sequences of introns are removed, and exons are joined together in a process called splicing which forms mature mRNA.
Most prokaryotic DNA does not contain introns, so transcription directly results in mRNA, which can be used to synthesise polypeptides.
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.
