Transcript Translation - St. Robert CHS

Translation
tRNA
acceptor site of
amino acid
tyrosine
Anticodon arm
Anticodon: recognizes the codon of the mRNA.
The recognition is facilitated by complementing base
pairs.
• Every tRNA carries only
one specific amino acid
• The third base in each
codon may differ b/w two
codons that code for the
same a.a.
– E.g, AAU and AAC both
code for tyrosine.
• If the tRNA’s anticodon is
UUA , it can still bind to
the codon AAC.
• This flexibility makes
it possible for the
correct a.a. to be
added in the growing
polypeptide chain,
even when errors
were made in the
gene sequence of
mRNA
• Aminoacyl-tRNA- is a tRNA molecule
with its corresponding amino acid to its
acceptor site at the 3’ end
• Aminoacyl-synthetase
– is the enzyme that adds the appropriate
amino acid to each tRNA
– there are at least 20 of them
– each is specific for a particular amino acid
and tRNA
Ribosome
• rRNA is produced from a
DNA template in the
nucleolus of nucleus
• rRNA is packaged with a
variety of proteins into
ribosomal units, one
larger than the other
• Subunits move separately
through nuclear envelope
pores into cytoplasm
where they combine to
form ribosomes
 The ribosome brings
the mRNA strand, the
aminoacyl-tRNA, and
the enzymes involved
in building
polypeptides together
• They facilitate
complementary base pairing
b/w tRNA anticodons and
mRNA codon.
• Contains:
• -one mRNA site
• -two tRNA binding sites:
• P site-holds the tRNA attached
to the growing polypeptide
• A site-holds the tRNA carrying
the next amino acid to be
added to polypeptide chain
The Steps in Translation
Polypeptide Chain Initiation
• mRNA attaches to the
ribosome
• The start codon (AUG) is the
first codon that is recognized
by the ribosomes
• AUG ensures that the correct
reading frame is used by the
ribosomes
• AUG codes for methionine ,
therefore every protein initially
starts with the amino acid
methionine
• Met enters the P site of the
ribosome which signals the
process to begin
Polypeptide Chain Elongation
• The next tRNA with
the appropriate
anticodon attaches to
the codon on the
mRNA, (carrying the
required amino acid)
enters the A site.
• tRNA at the P site is released
from its amino acid and the
mRNA codon by enzyme
action
• Triplet at A site moves to P site
bringing next triplet to A site
• Amino acid at P site attaches
to amino acid at A site (by a
peptide bond)
• tRNA that have been released
are recycled by aminoacyltRNA synthesize adding a
correct amino acid to them
• Above process
repeats itself until a
STOP signal comes
to the A site.
Polypeptide Chain Termination
• Stop signal does not
bring any amino acid
in as it moves to the P
site and is released.
• Stop Codons: UAG,
UGA, and UAA
• A protein which is known
as a release factor
recognizes that the
ribosome has slowed
down and aids in the
release of the polypeptide
chain from the ribosome
• The ribosome subunits
fall off the mRNA
• Translation is over!
At this point, modifications to the
polypeptide may include:
• sugars added to various amino acids
through glycosylation
• Phosphates may be added to various
amino acids through phosphorylation
• Enzymes may cleave the chain at various
points