How is the information in a gene used to build a polypeptide through transcription and translation?
Describe the processes of transcription and translation and the roles of mRNA, tRNA and ribosomes in protein synthesis
A focused answer to the H2 Biology Molecular Genetics outcome on protein synthesis. Transcription of DNA into mRNA, RNA processing in eukaryotes, and translation at the ribosome with tRNA, including initiation, elongation and termination.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
SEAB wants you to describe how a gene's information is expressed as a protein through transcription (DNA to mRNA) and translation (mRNA to polypeptide), to set out the roles of mRNA, tRNA and the ribosome, and (for eukaryotes) to know that the primary transcript is processed. This is the central dogma in action and depends on DNA structure and the genetic code.
The answer
Transcription: DNA to mRNA
RNA polymerase binds the promoter of a gene and unwinds the DNA. Using one strand (the template strand) it joins free RNA nucleotides by complementary base pairing, with uracil pairing to adenine in place of thymine, building a complementary messenger RNA. It moves along the gene until it reaches a terminator, then releases the mRNA.
In eukaryotes the primary transcript (pre-mRNA) is processed in the nucleus before translation: non-coding introns are removed and coding exons are joined together by splicing, a cap is added to the start, and a poly-A tail to the end. The mature mRNA then leaves the nucleus.
Translation: mRNA to polypeptide
Translation occurs at the ribosome and uses transfer RNA (tRNA). Each tRNA has an anticodon and carries the specific amino acid for the codon it matches.
- Initiation. The small ribosomal subunit binds the mRNA and finds the start codon AUG; the first tRNA (carrying methionine) binds by its complementary anticodon; the large subunit joins.
- Elongation. The ribosome holds two codons. A tRNA with the complementary anticodon brings the next amino acid; a peptide bond forms; the ribosome moves one codon along; the empty tRNA leaves. This repeats.
- Termination. At a stop codon no tRNA matches; a release factor releases the finished polypeptide and the ribosome dissociates.
The roles, summarised
mRNA carries the message as codons; tRNA carries amino acids and matches them to codons by its anticodon; the ribosome holds everything together and catalyses peptide bond formation.
Examples in context
Example 1. One mRNA, many ribosomes. Several ribosomes can translate the same mRNA at once, forming a polysome, so many copies of a protein are made quickly from one transcript. This is one reason a cell can produce large amounts of a needed protein rapidly.
Example 2. The effect of a stop codon mutation. If a mutation creates a premature stop codon, translation ends early and a shortened, usually non-functional protein results. This shows how directly the mRNA codon sequence controls the polypeptide and links transcription and translation to the consequences of mutation.
Try this
Q1. State the enzyme that catalyses transcription. [1 mark]
- Cue. RNA polymerase.
Q2. Explain the role of the ribosome in translation. [2 marks]
- Cue. It binds the mRNA and tRNAs, holds adjacent codons so that complementary tRNAs can bind, and catalyses the formation of peptide bonds between successive amino acids.
Q3. Explain why splicing is necessary in a eukaryotic cell but not in a prokaryotic cell. [2 marks]
- Cue. Eukaryotic genes contain non-coding introns that must be removed so that only the coding exons are translated; prokaryotic genes generally lack introns, so no splicing is needed.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original6 marksDescribe the process of translation, including the roles of mRNA, tRNA and the ribosome.Show worked answer →
Examiners want the ordered stages with the role of each molecule.
Initiation: the small ribosomal subunit binds the mRNA and moves to the start codon (AUG). A tRNA with the complementary anticodon, carrying methionine, binds; the large ribosomal subunit then joins.
Elongation: the ribosome holds two codons at once. A tRNA whose anticodon is complementary to the next codon brings its specific amino acid. A peptide bond forms between the adjacent amino acids, catalysed at the ribosome. The ribosome moves one codon along the mRNA, the now-empty tRNA leaves, and the cycle repeats.
Termination: when the ribosome reaches a stop codon, no tRNA matches, and a release factor causes the completed polypeptide to be released and the ribosome to dissociate.
Throughout, mRNA carries the codon sequence (the message), tRNA carries amino acids and matches them to codons by its anticodon, and the ribosome holds the components together and catalyses peptide bond formation.
Markers reward the three stages in order, the correct role of each of the three molecules, and the codon-anticodon matching that ensures the right amino acid sequence.
Original4 marksDescribe what happens during transcription and explain one way the primary transcript is processed before translation in a eukaryotic cell.Show worked answer →
The answer needs transcription itself and one processing step.
In transcription, RNA polymerase binds the gene's promoter and unwinds the DNA. Using one strand (the template strand) as a template, it joins free RNA nucleotides by complementary base pairing (with uracil pairing to adenine) to build a complementary mRNA molecule, moving along the gene until it reaches a terminator.
In eukaryotes the primary transcript (pre-mRNA) is processed before leaving the nucleus. One processing step is splicing, in which the non-coding introns are removed and the coding exons are joined together. (Adding a cap to the start and a poly-A tail to the end are also acceptable processing steps.)
Markers reward the role of RNA polymerase, template-directed synthesis with correct base pairing, and a correct description of one processing step such as splicing.
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