Gene expression — transcription and translation
مُقدَّم| English | المعنى |
|---|---|
| splicing | splicing |
| central dogma | central dogma |
| pre-mRNA | pre-mRNA |
| codon/ˈkəʊdɒn/ | A sequence of three nucleotides in mRNA that specifies an amino acid or a translation stop signal. |
| degeneracy/dɪˈdʒenərəsi/ | degeneracy |
| alternative splicing | alternative splicing |
| coding strand | coding strand |
| template strand/ˈtempleɪt strænd/ | template strand |
| β-thalassaemia | β-thalassaemia |
A recipe, not a machine
- A gene is a recipe. Between DNA and protein stand RNA polymerase, RNA processing, splicing and the ribosome.
- Many disease mutations break those middle steps, not the recipe itself.
From DNA to protein
- The central dogma 中心法则 in a human cell: RNA polymerase II transcribes DNA into pre-mRNA 前体mRNA at about 20 nt/s. A 5′ cap and a poly(A) tail are added.
- Splicing 剪接 removes introns and joins exons; the mature mRNA leaves the nucleus and the ribosome translates it at about 3 amino acids/s.
Put the steps from gene to protein in order.
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Transcription, processing (cap, tail, splicing), export, then translation. Each step is a place where regulation, or disease, can act.
The genetic code
- 4 bases read in threes give $4^{3} = 64$ codons 密码子: 61 name amino acids and 3 (UAA, UAG, UGA) mean stop. AUG starts, with methionine.
- Two bases would give only $4^{2} = 16$, fewer than 20 amino acids. The spare codons are spent on degeneracy 简并性, mostly at the third base, so many third-base changes are silent.
Why are codons triplets?
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Two bases cannot name 20 amino acids; three give 64 with spare codons spent on third-base degeneracy, which tolerates damage.
Worked example: timing one insulin
- Preproinsulin is 110 amino acids: $110/3 \approx 37$ s on the ribosome. Its 0.5 kb mRNA took $500/20 = 25$ s to transcribe.
- About a minute of machinery per molecule. So secretory cells make insulin ahead and store it; a signal triggers release, not synthesis.
A protein of 450 amino acids is made at 3 amino acids per second. How long does one ribosome take, in seconds?
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450 / 3 = 150 s, two and a half minutes. Many ribosomes on one mRNA (a polysome) make copies in parallel.
One gene, several proteins
- Exons can be included or left out, so alternative splicing 可变剪接 makes several proteins from one gene.
- The mRNA matches the coding strand 编码链 (with U for T); the polymerase reads the other, template strand 模板链.
The coding strand reads 5′-ATG GCT TAA-3′. What is the mRNA?
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The mRNA matches the coding strand, with U in place of T. It codes Met-Ala, then stop.
Worked example: the splice that breaks
- A (fictional) family has anaemia, but sequencing finds no change in any codon. One base inside a β-globin intron has created a new splice site.
- The spliceosome follows the new site, keeps part of the intron, shifts the reading frame and meets an early stop. This is β-thalassaemia β地中海贫血 without a single amino-acid change.
A mutation creates a new splice site inside an intron. What usually happens?
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The spliceosome follows consensus sequences; a new site diverts it, changing what counts as exon, and a frameshift follows.
Three common slips
- "Introns are junk." They carry regulatory and splicing signals (unit 1.5).
- Template or coding? The mRNA matches the coding strand, not the template.
- "One gene, one protein." Alternative splicing breaks that rule.
Because introns are spliced out, a mutation inside an intron can never cause disease.
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Introns carry splice signals and regulatory elements. A single intron base can create a splice site and cause β-thalassaemia.
Match each codon to its meaning.
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AUG starts every protein; UAA, UAG and UGA stop it; four codons share alanine because only the third base differs.
Recap
- Pol II at about 20 nt/s, then cap, poly(A) and splicing, then the ribosome at about 3 amino acids/s.
- $4^{3} = 64$ codons for 20 amino acids, with degeneracy mostly at the third base.
- A splice-site mutation can break a gene without touching its protein code.