Protein Synthesis

Protein Synthesis

Overview

Summary

Protein synthesisProtein synthesis1.1. DNA unwindsDNA unwinds2.2. mRNA copy is made of one of the DNA strands mRNA copy is made of one of the DNA strands

(template strand).(template strand).3.3. mRNA copy moves out of nucleus via nuclear mRNA copy moves out of nucleus via nuclear

pore into cytoplasm.pore into cytoplasm.4.4. tRNA molecules are activated or “charged” as tRNA molecules are activated or “charged” as

their complementary amino acids are attached their complementary amino acids are attached to their 3’ Acceptor site.to their 3’ Acceptor site.

Aminoacyl-tRNA = tRNA with corresponding a.a. attached to its 3’ acceptor site

5.5. The mRNA attaches to the small subunit The mRNA attaches to the small subunit of the ribosome in cytoplasm with the help of the ribosome in cytoplasm with the help of the 5’ cap on the mRNA and protein of the 5’ cap on the mRNA and protein initiation factors.initiation factors.

6.6. A tRNA bonds complementarily with the A tRNA bonds complementarily with the start codon of the mRNA (AUG) via its start codon of the mRNA (AUG) via its anticodon (UAC) at the “P” site of the anticodon (UAC) at the “P” site of the ribosome.ribosome.

7.7. A second tRNA bonds with the next three A second tRNA bonds with the next three bases of the mRNA (at the “A” site).bases of the mRNA (at the “A” site).

Translation - animation

8.8. The 2The 2ndnd amino acid joins onto the amino acid of amino acid joins onto the amino acid of the first tRNA via a peptide bond catalyzed by the first tRNA via a peptide bond catalyzed by peptidyl transferase, which is associated with peptidyl transferase, which is associated with the ribosome.the ribosome.

9.9. The ribosome moves along. The first tRNA The ribosome moves along. The first tRNA leaves the ribosome.leaves the ribosome.

10.10. A third tRNA brings a third amino acidA third tRNA brings a third amino acid11.11. Eventually a stop codon (UGA, UAG, UAA) is Eventually a stop codon (UGA, UAG, UAA) is

reached on the mRNA. The newly synthesised reached on the mRNA. The newly synthesised polypeptide leaves the ribosome with the help polypeptide leaves the ribosome with the help of a protein “Release Factor.”of a protein “Release Factor.”

mRNA attaches to small ribosomal subunit (40S)

Translation. mRNA used to make polypeptide chain (protein)

1.

•First the mRNA attaches itself to a ribosome (to the small subunit).•Six bases of the mRNA are exposed.•A complementary tRNA molecule with its attached amino acid (methionine) base pairs via its anticodon UAC with the AUG on the mRNA in the first position P.•Another tRNA base pairs with the other three mRNA bases in the ribosome at position A.•The enzyme peptidyl transferase (associated with the ribosome) forms a peptide bond between the two amino acids.•The first tRNA (without its amino acid) leaves the ribosome.

Translation 2

The ribosome moves along the mRNA to the next codon (three bases).The second tRNA molecule moves into position P.Another tRNA molecule pairs with the mRNA in position A bringing its amino acid.A growing polypeptide is formed in this way until a stop codon is reached.

End of Translation

A stop codon (UGA, UAG, UAA) on the mRNA is reached and this signals the ribosome to leave the mRNA. A newly synthesised protein is now complete!

Wobble Hypothesis – a tRNA can recognize more than one codon where the 3rd nucleotide differs

Rough endoplasmic reticulum

One ribosome from the RER

A ribosome