How is the base sequence of a gene used to build a specific polypeptide through transcription and translation?
The genetic code is universal, non-overlapping and degenerate. Transcription produces mRNA from DNA, in eukaryotes pre-mRNA is spliced to remove introns, and translation at ribosomes uses tRNA and the genetic code to assemble a polypeptide from amino acids.
An exam-focused answer to the AQA A-Level Biology 3.4.2 dot point on protein synthesis. Walks through transcription, splicing of pre-mRNA, the roles of mRNA, tRNA and the ribosome, and translation, with the properties of the genetic code.
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What this dot point is asking
AQA wants you to describe how the base sequence of a gene determines the amino acid sequence of a polypeptide, in two stages: transcription (DNA to mRNA, with splicing in eukaryotes) and translation (mRNA to polypeptide at the ribosome, using tRNA). You also need the structural differences between mRNA and tRNA and the properties of the genetic code.
The genetic code
Each amino acid is coded for by a triplet of bases. In mRNA a triplet is called a codon.
- Universal - the same codon codes for the same amino acid in nearly all organisms.
- Non-overlapping - each base is part of only one codon.
- Degenerate - most amino acids have more than one codon (there are 64 codons for about 20 amino acids).
Transcription
Transcription makes a single-stranded mRNA copy of one gene. It happens in the nucleus.
- DNA unwinds. Hydrogen bonds between the two DNA strands break over the region of the gene, exposing the bases. Only one strand, the template (antisense) strand, is transcribed.
- RNA polymerase acts. Free activated RNA nucleotides line up against the template strand by complementary base pairing. Adenine on DNA pairs with uracil on RNA (RNA has no thymine).
- mRNA forms. RNA polymerase joins adjacent RNA nucleotides, forming phosphodiester bonds, to build the mRNA molecule. Behind it, the DNA strands rejoin.
The base sequence of the resulting pre-mRNA is complementary to the template strand, and therefore (with U for T) identical to the sense strand.
Splicing (eukaryotes only)
In eukaryotes the gene contains exons (coding) and introns (non-coding). The pre-mRNA contains both. Before translation:
- Introns are removed and the exons are joined together to form mature mRNA.
- This is splicing.
The mature mRNA then leaves the nucleus through a nuclear pore. Prokaryotes have no introns, so their mRNA is translated directly and needs no splicing.
mRNA versus tRNA
| Feature | mRNA | tRNA |
|---|---|---|
| Shape | Long, single straight strand | Clover-leaf, folded with hydrogen bonds |
| Coding unit | Codon (triplet of bases) | One anticodon (triplet) |
| Function | Carries the code for a polypeptide | Carries a specific amino acid to the ribosome |
| Made from | The gene, in the nucleus | A gene; reused many times |
Translation
Translation builds the polypeptide at a ribosome in the cytoplasm.
- mRNA binds to a ribosome. The ribosome exposes two codons at a time, starting at the start codon.
- tRNA delivers an amino acid. A tRNA whose anticodon is complementary to the exposed codon binds by complementary base pairing. It carries a specific amino acid.
- A second tRNA binds to the adjacent codon, carrying its amino acid.
- A peptide bond forms between the two amino acids, catalysed by the ribosome, using ATP.
- The ribosome moves along the mRNA by one codon. The first tRNA detaches (to be reloaded with another amino acid) and the cycle repeats.
- Termination. When the ribosome reaches a stop codon, no tRNA binds, and the completed polypeptide is released.
The order of codons on the mRNA therefore determines the order of amino acids, and so the primary structure of the polypeptide, which in turn determines how it folds.
Common mistakes
Try this
Q1. A length of DNA template strand reads 3'-TAC-GCA-AAT-ATC-5'. Write the mRNA codon sequence transcribed from it. [2 marks]
- Cue. mRNA is complementary with U for T: 5'-AUG-CGU-UUA-UAG-3'.
Q2. Explain why a tRNA molecule is described as specific. [2 marks]
- Cue. It has a particular anticodon that is complementary to only one codon, and it carries the one amino acid coded for by that codon.
Q3. Describe two differences between transcription and translation. [2 marks]
- Cue. Transcription makes mRNA from a DNA template in the nucleus using RNA polymerase; translation makes a polypeptide from mRNA at a ribosome in the cytoplasm using tRNA.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2018 AQA Paper 25 marksDescribe how a polypeptide is produced by translation of an mRNA molecule.Show worked answer →
A 5-mark translation answer needs the ordered sequence with named molecules.
- mRNA binds to (attaches to) a ribosome; two codons are exposed.
- A tRNA with an anticodon complementary to the first (start) codon binds; it carries a specific amino acid.
- A second tRNA with a complementary anticodon binds to the next codon, carrying its amino acid.
- A peptide bond forms between the two amino acids (catalysed by the ribosome, using ATP).
- The ribosome moves along the mRNA one codon at a time; the process repeats; the first tRNA leaves and is reused, until a stop codon is reached and the polypeptide is released.
Markers reward correct use of "codon", "anticodon", "complementary", "peptide bond" and "stop codon" in the right order.
2020 AQA Paper 22 marksExplain why mRNA is described as a copy of one gene rather than of the whole DNA molecule.Show worked answer →
A 2-mark explain answer needs two linked points.
- Only one gene is transcribed at a time; RNA polymerase transcribes the base sequence of a single gene from its template (antisense) strand.
- mRNA is short enough to leave the nucleus through a nuclear pore, whereas the whole DNA molecule is too large and stays in the nucleus.
Related dot points
- In prokaryotic cells DNA molecules are short, circular and not associated with proteins. In the nucleus of eukaryotic cells DNA molecules are very long, linear and associated with proteins called histones. A gene is a base sequence of DNA that codes for the amino acid sequence of a polypeptide or a functional RNA. The genome is the complete set of genes in a cell and the proteome is the full range of proteins a cell can produce.
An exam-focused answer to the AQA A-Level Biology 3.4.1 dot point on DNA, genes and chromosomes. Compares prokaryotic and eukaryotic DNA, defines gene, locus, allele, genome and proteome, and explains exons, introns and the triplet code.
- Gene mutations involve a change in the base sequence of chromosomes. They can arise spontaneously during DNA replication and include base substitution and base deletion. Because the genetic code is degenerate, not all mutations result in a change to the amino acid sequence. Mutagens increase the rate of mutation, and mutations are one source of genetic diversity within a gene pool.
An exam-focused answer to the AQA A-Level Biology 3.4.3 dot point on mutation. Explains base substitution and deletion, frameshift effects, why the degenerate code buffers some mutations, the role of mutagens, and how mutation contributes to genetic diversity.
- Meiosis produces haploid daughter cells from a diploid parent cell, halving the number of chromosomes so that fertilisation restores the diploid number. Genetic variation arises from independent segregation of homologous chromosomes and from crossing over between homologous chromosomes during meiosis, and the number of possible combinations can be calculated.
An exam-focused answer to the AQA A-Level Biology 3.4.4 dot point on meiosis. Explains how two divisions halve the chromosome number, how independent segregation and crossing over generate variation, and how to calculate the number of possible chromosome combinations.
- Genetic diversity within a population, expressed as the number of different alleles in a gene pool, is acted on by natural selection. Random mutation produces new alleles, and selection results in changes in allele frequency. Directional and stabilising selection produce different effects, and selection leads to anatomical, physiological and behavioural adaptations that increase the chance of survival and reproduction.
An exam-focused answer to the AQA A-Level Biology 3.4.5 dot point on natural selection. Explains how selection changes allele frequencies, contrasts stabilising, directional and disruptive selection, and covers anatomical, physiological and behavioural adaptations.
- A species is a group of similar organisms able to reproduce to give fertile offspring. Each species is given a binomial name. Courtship behaviour helps members of a species to recognise each other and is used in classification. Phylogenetic classification arranges species into a hierarchy of groups that share a common ancestor, and the taxa from domain to species reflect evolutionary relationships.
An exam-focused answer to the AQA A-Level Biology 3.4.6 dot point on species and taxonomy. Defines species and the binomial system, explains phylogenetic classification and the taxonomic hierarchy from domain to species, and covers the role of courtship behaviour.