The genetic code and protein synthesis
From gene to protein
- A gene is a length of DNA that codes for one polypeptide.
- The cell reads the code in two stages: transcription, then translation.
- First, let's crack the code itself.
Practice
A gene is a sequence of DNA that codes for:
A gene is a length of DNA that codes for a single polypeptide.
The genetic code
- The code is read in triplets — 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: there are 64 triplets but only 20 amino acids, so most amino acids have more than one triplet.
Practice
The genetic code is "degenerate", which means:
There are 64 possible triplets but only 20 common amino acids, so most amino acids have several triplets.
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 and non-coding introns — the introns are removed and the exons joined to make the finished mRNA.
Practice
During transcription:
Transcription makes mRNA from the DNA template strand, catalysed by RNA polymerase, in the nucleus.
Practice
Before the finished mRNA leaves the nucleus in a eukaryote:
The non-coding introns are cut out and the coding exons spliced together to form mature mRNA.
Translation (at the ribosome)
- The mRNA leaves the nucleus and attaches to a ribosome.
- It is read in codons (one triplet of mRNA bases at a time).
- tRNA molecules bring amino acids; each has an anticodon that pairs with the matching codon.
- The ribosome joins the amino acids with peptide bonds, building the polypeptide.
Practice
During translation, tRNA molecules:
Each tRNA carries an amino acid and has an anticodon that pairs with a matching mRNA codon at the ribosome.
You've got it
Key idea
- a gene codes for one polypeptide; the code is read in triplets (universal + degenerate: 64 → 20)
- transcription (nucleus): DNA template → mRNA by RNA polymerase; introns removed, exons joined
- translation (ribosome): codons read; tRNA anticodons bring amino acids
- amino acids joined by peptide bonds into the polypeptide