From Gene to Protein
A gene is a length of DNA whose base order fixes the amino acid order of the proteins it can build, read three bases at a time. Covers the messenger copy, the ribosome, and reading a codon table.
What a learner can do afterwards
- Uses a codon table to turn a short base sequence into a run of amino acids.
- Explains why the code is read in threes rather than singly or in pairs.
- Says where in the cell each stage happens and why the message has to leave the nucleus.
1 · Read
A protein-coding gene is a length of DNA whose base order fixes the amino acid order of the proteins it can build. The code is read in three base words called codons. Two base words give only 16 combos, too few for 20 amino acids, so three base words give 64, which is plenty. AUG starts nearly every protein and three stop triplets end it.
First the gene is copied into a travelling message. In the nucleus, an enzyme called RNA polymerase reads the template strand and writes matching mRNA, with template T, A, G and C directing RNA A, U, C and G. The message then leaves the nucleus, because the building machinery waits outside it in the cytoplasm.
At a ribosome the message is read triplet by triplet, and carrier molecules called tRNA ferry the matching amino acids into a chain. Learn this mini table: AUG means start, UUU means phenylalanine, GGG means glycine, UAA means stop. So AUG UUU GGG UAA builds start, phenylalanine, glycine, then ends. Grouping starts at AUG, so the start codon sets the frame for the triplets that follow. The chain folds into a working shape, and the shape decides the job.
Decode slowly in groups of three from the start codon. Never regroup the letters, since shifted groups spell different acids. Stop at the first stop triplet.
Copy the gene in the nucleus, read it in threes at the ribosome, fold the chain.
Readings written for this topic. Videos by Khan Academy on YouTube; viewing follows platform terms.
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.