Protein Synthesis: Translation | A-level Biology | OCR, AQA, Edexcel

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The key points covered of this video include:

1. Initiation of Translation
2. The Process of Translation
3. Ending Translation

Initiation of Translation

The second stage of protein synthesis is tranlation and involves the transfer of genetic information from mRNA into amino acids. After the mRNA enters the cytoplasm, it associates with a ribosome binding to the small subunit at the start codon (AUG). The first tRNA molecule with the complementary anticodon (UAC) then binds to the start codon by hydrogen bonding. This first tRNA molecule carries the amino acid methionine. The large subunit of the ribosome then binds and translation can begin.

The Process of Translation

Once the ribosome is fully assembled, a second tRNA molecule with a complementary anticodon is able to bind to the next mRNA codon. This second tRNA carries the specific amino acid coded for by the mRNA codon. A peptide bond is then formed between the two amino acids, forming a dipeptide and releasing the methionine from the first tRNA. The ribosome then moves along the mRNA strand by exactly three base pairs (one codon) and the first tRNA is released. The second tRNA molecule is now in the same position in the ribosome as the first tRNA molecule started but with a dipeptide. A third tRNA with a complementary anticodon and carrying a specific amino acid is now able to bind to the next mRNA codon. This cycle repeats many times over, forming a long polypeptide. Once a ribosome has moved along the mRNA strand away from the start codon, another ribosome is able to attach at the start codon. This results in many identical polypeptides being synthesised simultaneously from the same mRNA strand.

Ending Translation

There are no tRNA molecules with complementary anticodons for the stop codons. Therefore, once a stop codon is reached, no tRNAs can bind and the synthesised polypeptide is released from the last tRNA molecule. The ribosome units separate from the mRNA strand and are quickly reused for another round of translation. Meanwhile, the polypeptide coils and folds into a specific tertiary structure, determined by the amino acid sequence. The polypeptide may also bind to other polypeptides and/or prosthetic groups to form a protein with a quaternary structure.

Summary

Translation is the process in which a polypeptide is synthesised from mRNA by ribosomes
The ribosomes bind to the mRNA and move along the strand - many ribosomes can be bound to the same strand of mRNA
Amino acids are carried to the ribosomes by specific tRNAs that have the complementary anticodons to the mRNA codons
When each tRNA is bound in the ribosome, the amino acid is transferred from the tRNA to the growing polypeptide chain
This results in the formation of a peptide bond and requires ATP
When the stop codon is reached, the ribosome is released from the mRNA and the polypeptide is released from the tRNA
The polypeptide chain folds into a protein with a unique tertiary or quaternary structure

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[shanjunchen]

There are many mRNA copies binding with srRNAs, then starting to synthesize polypeptides at the same time, right?

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