Transcript Protein Synthesis: Translation

Protein Synthesis:
Making the Protein
Translation
from the Code.
Overview of Translation
1) 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-byside, 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 acid is called a
charged tRNA. (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.
Elongation
1) 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.
Termination
1) 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.