Written for this site from the course textbook and class slides, with page references for each statement. The chapter is on pages 615–624 of the course textbook. Image credits are on slide 19.
RNA polymerase binds to DNA at the start of a gene and separates the strands as it moves along. One strand acts as the template. RNA nucleotides pair with it, and RNA polymerase links them with covalent bonds. The RNA then separates, and the double helix reforms.
Hydrogen bonds form between complementary bases. Adenine on the template pairs with uracil in RNA, as RNA has no thymine. The mRNA therefore matches the non-template strand, with U in place of T.
Transcription copies a base sequence without changing the DNA, so a gene can be transcribed many times. This matters in non-dividing body cells, such as neurons, where the same sequences must be conserved throughout life.
Not all genes in a cell are expressed at any given time. Transcription is the first stage of expression, so it is a key point for switching a gene on or off. Liver cells and neurons have the same genes, but they transcribe different ones.
The base sequence of mRNA decides the amino acid sequence of a polypeptide. The language changes here from bases to amino acids, which is why this stage is called translation.
| Molecule | Role |
|---|---|
| mRNA | Carries codons from the gene, and binds to the small subunit of a ribosome |
| Ribosome | Holds mRNA and tRNA, with two tRNAs bound to the large subunit at once; forms peptide bonds |
| tRNA | Carries a specific amino acid, and pairs with a codon using its anticodon |
A codon is three bases on mRNA, and an anticodon is three complementary bases on tRNA. Hydrogen bonds hold them together. For example, codon G C A pairs with anticodon C G U.
Watch out: degenerate does not mean that one codon can give several amino acids.
Split the sequence into codons, then look up each in turn. A U G G C U U G U U A A gives Met, Ala, Cys, then stop. A stop codon adds no amino acid.
A tRNA binds to the next codon. A peptide bond links the growing chain to the new amino acid. The ribosome moves one codon along, and the empty tRNA leaves. The cycle then repeats, adding one amino acid each time.
In sickle cell anaemia, one base in the gene for beta-globin is substituted. The codon GAG becomes GUG, so glutamic acid is replaced by valine. At low oxygen, haemoglobin forms long fibres, and red blood cells become rigid and sickle-shaped. They can block small blood vessels, and their short life leads to anaemia.
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Print this guide and complete it as you work through the slides and videos. It asks for your own explanations, so close the notes while you try each section.
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A printable worksheet covering all eleven statements, from quick recall to sequence questions and a longer answer. It has 45 marks and takes about 45 minutes, or you can do one section at a time.
Your teacher has the mark scheme.
You will need: Printed DNA template, mRNA codon table, labelled cards for tRNA and amino acids
Safety: No practical hazards.
Carry this out with your teacher's supervision and follow your school's lab rules.
Nirenberg and Matthaei cracked the first codon in 1961, using a synthetic RNA made only of uracil.
We say the genetic code is read, copied and translated. Consider where these metaphors help, and where they may mislead.
Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.
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