Translation

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Translation

Translation is the second step in protein synthesis. The steps involved are initiation, elongation, and termination. The process requires a lot of metabolic energy.

Location of translation

mRNA that has been produced during transcription binds to a ribosome in the cytoplasm.
The ribosome is the site of protein synthesis.
- In eukaryotes, ribosomes can also be found attached to the rough endoplasmic reticulum. So, translation can happen there too.
In prokaryotes, both transcription and translation occur in the cytoplasm. This means both processes occur simultaneously.
- The newly formed mRNA molecule binds to a ribosome while the gene is still being transcribed.

Initiation

Translation begins when the start codon of an mRNA molecule binds to the rRNA of a ribosome.
- The start codon is almost always AUG.
Six bases (two codons) can fit inside the ribosome at one time.
One molecule of tRNA binds to the first codon in the ribosome.
The tRNA molecule has an anticodon that is complementary to a specific codon.
The anticodon allows the correct tRNA molecule to bind to the correct codon.

Elongation

The large ibosomal consists of three compartments:
- The A (aminoacyl) site binds incoming charged aminoacyl tRNAs.
- The P (peptidyl) site binds charged tRNAs carrying amino acids that have formed peptide bonds with the growing polypeptide chain, but have not yet dissociated from their corresponding tRNA.
- The E (exit) site releases dissociated tRNAs so that they can be recharged with free amino acids

Elongation - 2

During translation elongation, the ribosome moves along the mRNA and ensures correct binding of mRNA codons to complementary tRNA anticodons.
Elongation proceeds with charged tRNAs entering the A site and then shifting to the P site followed by the E site with each single-codon “step” of the ribosome.
- Ribosomal steps are induced by conformational changes that advance the ribosome by three bases in the 3' direction.
The energy for each step of the ribosome is donated by an elongation factor that hydrolyzes GTP.

Elongation - peptide bond

Peptide bonds form between the amino group of the amino acid attached to the A-site tRNA and the carboxyl group of the amino acid attached to the P-site tRNA.
- The formation of each peptide bond is catalyzed by peptidyl transferase.
The amino acid bound to the P-site tRNA is also linked to the growing polypeptide chain.
As the ribosome steps across the mRNA, the former P-site tRNA enters the E site, detaches from the amino acid, and is expelled.

Termination

Termination of translation occurs when a nonsense codon (UAA, UAG, or UGA) is encountered.
Upon aligning with the A site, these nonsense codons are recognized by release factors in prokaryotes and eukaryotes.
These factors instruct peptidyl transferase to add a water molecule to the carboxyl end of the P-site amino acid.
- This reaction forces the P-site amino acid to detach from its tRNA, and the newly made protein is released.
The ribosome detaches from the mRNA.

Genetic code

The genetic code is a triplet code, with each RNA codon consisting of three consecutive nucleotides that specify one amino acid.
- For example, the mRNA codon CAU specifies the amino acid histidine.
The code is degenerate; that is, some amino acids are specified by more than one codon.
- For example, CCU and CCG are both codons for proline.
The same genetic code is universal to almost all organisms on Earth, providing for the common ancestry of life on earth.