Here's a breakdown:
1. Initiation Complex Formation:
- Before elongation begins, the ribosome binds to the mRNA and the first tRNA carrying methionine (Met) is positioned in the P site of the ribosome. This complex is called the initiation complex.
2. Codon Recognition:
- The next codon on the mRNA enters the A site of the ribosome.
- A tRNA carrying the amino acid matching that codon enters the A site, guided by the complementary base pairing between codon and anticodon.
3. Peptide Bond Formation:
- The existing polypeptide chain in the P site is transferred to the newly arrived amino acid in the A site, forming a peptide bond.
- This reaction is catalyzed by peptidyl transferase, an enzyme activity of the large ribosomal subunit.
4. Translocation:
- The ribosome moves one codon down the mRNA, shifting the tRNA carrying the growing polypeptide chain from the A site to the P site.
- The empty tRNA from the P site exits through the E site.
5. Repeat:
- The process of codon recognition, peptide bond formation, and translocation repeats until a stop codon is encountered on the mRNA.
Key Players:
* Ribosomes: The molecular machines responsible for protein synthesis.
* mRNA: The messenger RNA molecule that carries the genetic code from DNA to the ribosome.
* tRNA: Transfer RNA molecules that bring specific amino acids to the ribosome.
* Amino acids: The building blocks of proteins.
* Peptidyl transferase: The enzyme that catalyzes the formation of peptide bonds.
In essence, elongation is a cyclical process of:
1. Reading the mRNA code
2. Adding the correct amino acid to the growing polypeptide chain
3. Moving the ribosome along the mRNA
This continues until the entire mRNA sequence is translated and the polypeptide chain is complete.