The genetic code and protein synthesis · ⁨遗传密码与蛋白质合成⁩

From gene 基因 to protein

• A gene is a length of DNA that codes for one polypeptide.
• The cell reads the code in two stages: transcription 转录, then translation 翻译.
• First, let's crack the code itself.

The genetic code

• The code is read in triplets 三联体 — groups of three bases.
• Each triplet codes for one amino acid, or signals start/stop.
• It is universal (nearly all life uses the same triplets) and degenerate 简并: there are 64 triplets but only 20 amino acids, so most amino acids have more than one triplet.

Protein synthesis: transcription then translation

Transcription (in the nucleus)

• The gene is copied into mRNA.
• The DNA strand copied is the template strand; RNA polymerase joins RNA nucleotides that pair with it (uracil pairs with adenine).
• In eukaryotes the first transcript has coding exons 外显子 and non-coding introns 内含子 — the introns are removed and the exons joined to make the finished mRNA.

Introns are cut out and the coding exons spliced together to make the mature mRNA

Translation (at the ribosome)

• The mRNA leaves the nucleus and attaches to a ribosome.
• It is read in codons 密码子 (one triplet of mRNA bases at a time).
• tRNA molecules bring amino acids; each has an anticodon 反密码子 that pairs with the matching codon.
• The ribosome joins the amino acids with peptide bonds, building the polypeptide.

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