protein synthesis: translation making the protein from the code
TRANSCRIPT
Protein Synthesis: Translation Making the Protein
from the Code.
Overview of Translation1) Messenger RNA moves from the
nucleus to the cytoplasm and attaches to the ribosome.
2) As the ribosome moves along the, mRNA, tRNAs with their anticodons (matching the mRNA codons) carry the proper amino acids to the ribosomes.
The first codon is the start
codon (AUG). The tRNA
with the anticodon UAC
carries the amino acid
methionine.
3) As two amino acids are brought side-by-side, they bond by a condensation reaction (dehydration synthesis).
Bond
4) The tRNA that held the first amino acid leaves.
Bond
The ribosome continues to move along the mRNA, while a new tRNA brings the next amino acid into place.
6) This continues until the “stop” codon is reached.
(The protein is then processed for use in, or out of the cell.)
stop codon
Translation Animation
A More Detailed Look at Translation
Steps in Translation• Initiation
• Elongation
• Termination
The Ribosome Binding sites
• There are three binding sites on the ribosome:
• One holds mRNA so codons are accessible.
• The other two hold tRNAs.
The binding sites that hold tRNAs are the “A” site and the “P” site.
Initiation 1) The smaller ribosome unit attaches to
the mRNA strand (at the methyl guanine MG cap.)
Many ribosomes may bond to the same mRNA. This is called a polysome.)
2) The first tRNA attaches at the start codon. Its three-base anticodon matches the mRNA codon. At the other end is the attachment point for the Amino acid.
3) A transfer RNA with an amino acids is called a charged amino acid. (An enzyme and ATP bind to the correct amino acid to the transfer RNA molecule. At that point it is ready to carry the amino acid to its correct place in the growing polypeptide chain.)
4) The larger ribosome subunit attaches to the smaller one (with the energy from GTP or guanosine triphosphate). The Ribosome is complete at this point and protein synthesis can proceed.
Elongation1) The amino acid-charged tRNA that corresponds
to the next mRNA codon binds to the A site on the ribosome.
2) A peptide bond forms between two adjacent amino acids. (condensation reaction)
3) The ribosome moves along the mRNA and the process continues. The previous tRNA leaves.
The ribosome moves along so that the next tRNA has now moved from the A to the P site. Another charges tRNA brings an amino acid to the growing protein chain.
Termination1) Elongation continues until
the “stop codon” is reached.
2) A special protein binds to the stop codon at the A site.
3) The newly-formed polypeptide is released. The tRNAs are released. The two ribosome subunits separate.
Translation Animation
Translation Animation
Where do the Proteins Go?• Proteins formed at the ribosomes, free in the
cytosol, are used within the cell.
• Proteins formed at the R.E.R. (rough endoplasmic reticulum) are transported out of the cell.
Transport and Modification
• The first few amino acids provide the direction that proteins will travel within the cell. It is known as the signal sequence.
• The polypeptide chain may be modified so that it can function properly. It may be cut into smaller segments, or a portion may be removed with special enzymes.