Transcription and translationAQA A-Level Biology: Revision notes
Section 1
Transcription
Transcription is the production of mRNA from a DNA gene. It takes place in the nucleus of eukaryotic cells.
- RNA polymerase attaches to the DNA at the start of a gene. The two strands separate as the hydrogen bonds between bases break.
- One strand is the template strand. Free RNA nucleotides pair with the exposed bases by complementary base pairing: A with U, C with G, G with C, T with A.
- RNA polymerase joins the adjacent RNA nucleotides by phosphodiester bonds.
- At the end of the gene the RNA is released and the DNA strands rejoin.
RNA polymerase joins nucleotides in transcription; DNA polymerase joins DNA nucleotides in replication.
Section 2
Prokaryotes and eukaryotes
In prokaryotes, transcription results directly in mRNA. It can be translated at once, even before transcription finishes.
In eukaryotes, transcription results in pre-mRNA, which contains introns and exons. Pre-mRNA is spliced: the introns are removed and the exons are joined to form mRNA. The mRNA then leaves the nucleus through a nuclear pore.
Worked example: a pre-mRNA is 2400 bases and the mRNA is 900 bases. The introns total 2400 − 900 = 1500 bases.
Section 3
Translation
Translation is the production of a polypeptide from the sequence of codons on mRNA. It takes place at ribosomes in the cytoplasm.
- mRNA binds to a ribosome, which holds two codons at a time.
- A tRNA carrying an amino acid binds, by its anticodon, to the complementary codon by hydrogen bonds.
- A second tRNA binds at the next codon. The ribosome helps a peptide bond form between the two amino acids.
- The first tRNA leaves, the ribosome moves along by one codon, and the process repeats.
- At a stop codon, translation ends and the polypeptide is released.
Section 4
The role of ATP
ATP supplies energy at two points in translation: to attach each amino acid to its tRNA, and to form the peptide bonds between amino acids.
The hydrogen bonds between codon and anticodon form spontaneously and do not need ATP.
In a translation question say what the ATP is for: attaching amino acids to tRNA and forming peptide bonds.
Section 5
Interpreting experiments on nucleic acids
Experiments on translation often use a cell-free system containing ribosomes, tRNA, amino acids and ATP, to which synthetic mRNA is added.
- Repeating UUU gives only phenylalanine: the base sequence of mRNA determines the amino acid sequence.
- Leaving out ATP or ribosomes stops polypeptide production: both are needed.
- A repeating AC unit read in threes gives two alternating codons (ACA, CAC) and two alternating amino acids: this supports a triplet code.
For each result, say what was changed, what happened, and what that shows.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Transcription and translation
- A human liver cell transcribes the gene for albumin, a protein made in large amounts by the liver. The albumin gene contains several introns.Describe what happens to the product of transcription of the albumin gene before it can be translated.2 marks
- At a ribosome in the cytoplasm of a cell, a polypeptide is being made by translation of a molecule of mRNA. Transfer RNA molecules are bringing amino acids to the ribosome.A tRNA molecule with the anticodon UAC arrives at the ribosome. Explain which mRNA codon it binds to and what happens to the amino acid it carries.2 marks
- Bacteria and human cells both transcribe their genes to make RNA. In bacteria, ribosomes can begin translating an mRNA molecule while it is still being made. In human cells, translation does not begin until the RNA has left the nucleus.Explain why a bacterial RNA can be translated while it is still being made, whereas human RNA must first leave the nucleus.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).