How is the genetic code translated from mRNA into a protein?
Topic 6.4 Translation: explain how the ribosome translates mRNA codons into a polypeptide, including the roles of tRNA and the genetic code.
A focused answer to AP Biology Topic 6.4, covering codons, the genetic code, the roles of mRNA, tRNA and ribosomes, the stages of translation, and using a codon table, with a worked translation problem.
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What this topic is asking
The College Board (Topic 6.4) wants you to explain translation: how the ribosome reads mRNA codons and uses tRNA to build a polypeptide, and how the genetic code maps codons to amino acids. You should be able to use a codon table.
The genetic code and codons
The molecules of translation
The stages
Translation has three stages: initiation (the ribosome assembles at the start codon), elongation (tRNAs deliver amino acids codon by codon and peptide bonds form), and termination (a stop codon is reached and the finished polypeptide is released).
During elongation, the ribosome moves along the mRNA one codon at a time. At each step, a tRNA whose anticodon matches the next codon binds, its amino acid is joined to the growing chain by a peptide bond, and the now-empty tRNA leaves to be recharged with another amino acid. Because the code is read three bases at a time from a fixed start, the reading frame set by the AUG start codon determines how every following codon is grouped, which is why the start codon is so important and why insertions or deletions that shift the frame are so damaging (see the mutations topic).
Translation is the second half of the central dogma: DNA is transcribed into mRNA, and mRNA is translated into protein. The genetic code being nearly universal is strong evidence of common ancestry, because the same codon means the same amino acid in bacteria and in humans. It also underlies biotechnology, since a human gene can be expressed and translated correctly in a bacterial cell.
The redundancy of the code (several codons for one amino acid) has a protective effect: it means many single-base changes are silent or change only one amino acid, so the code itself buffers proteins against some mutations. This connects translation to the mutations topic, where the consequences of changing a codon depend on exactly this structure of the code.
Try this
Q1. State the start codon and what it codes for. [1 point]
- Cue. AUG; it codes for methionine and signals the start of translation.
Q2. Explain the role of tRNA in translation. [2 points]
- Cue. Each tRNA carries a specific amino acid and has an anticodon that pairs with the complementary mRNA codon, ensuring the correct amino acid is added in the right order.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2020 (style)4 marksSection II (long FRQ excerpt). (a) Describe the roles of mRNA, tRNA and the ribosome in translation. (b) Using the codon table provided, translate the mRNA sequence 5'-AUG-GCU-UAA-3' into amino acids and identify where translation stops.Show worked answer →
A 4-point describe-and-apply FRQ on translation.
(a) Describe (3 points): (1 point) mRNA carries the codons that specify the amino acid order; (1 point) tRNA carries amino acids and has an anticodon that pairs with each codon; (1 point) the ribosome holds the mRNA and tRNAs together and catalyzes the formation of peptide bonds between amino acids.
(b) Apply (1 point): AUG = methionine (start), GCU = alanine, UAA = stop, so the chain is methionine-alanine, and translation stops at UAA (a stop codon, which codes for no amino acid).
Markers reward the three molecular roles and correctly reading the codons including recognizing the stop codon.
AP 2017 (style)1 marksSection I (multiple choice). The part of a tRNA molecule that pairs with an mRNA codon is the: (A) anticodon. (B) amino acid attachment site. (C) start codon. (D) ribosomal subunit.Show worked answer →
The correct answer is (A).
The anticodon is the three-base sequence on tRNA that pairs with the complementary mRNA codon, ensuring the correct amino acid is delivered. The amino acid attaches at the other end of the tRNA (B); the start codon (C) is on the mRNA; ribosomal subunits (D) are part of the ribosome.
Related dot points
- Topic 6.3 Transcription and RNA Processing: explain how RNA polymerase transcribes a gene into mRNA and how the primary transcript is processed in eukaryotes.
A focused answer to AP Biology Topic 6.3, covering RNA polymerase, the template strand, the differences between transcription and replication, and eukaryotic RNA processing (cap, tail, splicing), with a worked transcription example.
- Topic 6.1 DNA and RNA Structure: describe the structure of DNA and RNA and explain how it suits their role in storing and transmitting genetic information.
A focused answer to AP Biology Topic 6.1, covering the double helix, antiparallel strands, complementary base pairing, the sugar-phosphate backbone, and the differences between DNA and RNA, with a worked base-pairing calculation.
- Topic 6.7 Mutations: explain the types of mutations and how they affect gene products, phenotype and the variation available to a population.
A focused answer to AP Biology Topic 6.7, covering point mutations (silent, missense, nonsense), frameshift mutations, chromosomal mutations, their effects on proteins and phenotype, and their role as the source of new variation, with a worked example.
- Topic 6.5 Regulation of Gene Expression: explain how gene expression is regulated in prokaryotes and eukaryotes, including operons and regulatory sequences.
A focused answer to AP Biology Topic 6.5, covering the lac and trp operons, promoters, regulatory sequences, transcription factors and epigenetic control, and how regulation lets cells respond to the environment, with a worked operon example.
- Topic 1.4 Properties of Biological Macromolecules: describe the properties of carbohydrates, lipids and proteins, including the directionality of their structures and how their subunits and bonding give rise to their functions.
A focused answer to AP Biology Topic 1.4, covering carbohydrates, lipids and proteins, the four levels of protein structure, saturated versus unsaturated fats, and how subunits and bonding determine properties and function.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)