Transcript 5.4 Translation

TRANSLATION

The reading of the mRNA transcript and the
creation of the polypeptide chain.

Occurs on the Ribosomes:
 Composed of a small subunit (40S) and a
large subunit (60S)
 Made up of ribosomal RNA and protein

The mRNA transcript is read by the
ribosome in sets of 3 nucleotides to
determine the amino acid sequence.

tRNA deliver the appropriate amino acid to
the ribosome where the polypeptide chain
is built.
Genetic Code

There are 20 amino acids found in proteins, but only 4
bases in the code (U C A G of mRNA)

Codons: sequences of three nucleotide bases used to code
for an amino acid. Ex. UAC codes for Tyrosine

43=64 possible codons
therefore some amino acids can have more than one codon
 Ex. UUU UUC, UCU, UCC all code for phenylalanine
 This redundancy helps to reduce errors

The Genetic Code

AUG: start
codon used 99%
of the time (codes
for Methionine)

UAA, UAG,
UGA: stop
codons
tRNA

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
tRNA delivers amino acids to the
ribosome
Looks like a cloverleaf
Has an anticodon – sequence of three
bases that recognize (are
complementary to) the mRNA codon
 ex. The GCC codon specifies the
a.a. alanine
 The anticodon on the tRNA
carrying alanine would be CGG
Opposite arm carries the amino acid
binding site
Amino acids are added by Aminoacyl
t-RNA synthases to make aminoacyltRNA
1) Initiation


The small subunit of the ribosome
recognizes the 5' cap on the mRNA
transcript and binds to the RNA.
The ribosome will position itself at
AUG (the first codon read for every
protein)
Ensures the correct reading frame
(phase in which the mRNA is read)
is used by the ribosome
 The large subunit binds and the
subunits act as a clamp (80S) around
the mRNA.

Sites on the Ribosome

The large subunit of the
ribosome has two sites that
the mRNA moves through:

A (acceptor) site: where
new amino acids enter the
ribosome

P (peptide) site: where the
polypeptide chain grows
as the amino acids are
 At a third site, the catalytic site,
combined.
the peptide bonds are made
between the amino acid in the A
site and the P site.
2) Elongation of the
Polypeptide Chain

The AUG codon will be
in the P site, tRNA
carrying the
corresponding amino acid
(methionine) enters this
site.

The next tRNA carrying
the required a.a. enters the
A site.

At the catalytic site (b/w
A and P) a peptide bond
forms between the two
amino acids.

The ribosome moves
over one codon. The
second tRNA moves
to the P site, a new
tRNA brings the next
a.a. to the A site and
the a.a. is added to the
chain.

The first tRNA is
released to be recycled
for use with another
amino acid.

The process repeats
with the polypeptide
chain trailing in the
cytoplasm.
Translation
Translation
3) Termination

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Ribosome will reach a stop codon (UGA, UAG,
UAA)
No tRNA or a.a. exists for a stop codon
A release-factor protein helps the two ribosome
subunits to fall off the mRNA and the
polypeptide chain is released.
4) Post-Translational Modifications

Polypeptide chains travel through the RER and
are modified to become functional proteins.

Some amino acids may be glycosylated (sugar
added) or phosphorylated (phosphate added) or
altered in another way.

Enzymes may cleave (cut) the chain at specific
places.
Wobble Hypothesis

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In codons the third base may differ between 2
codons that code for the same amino acid (UAU
and UAC both code for tyrosine).
If the tRNA's anticodon is AUA it can still bind
to UAC.
This flexibility allows for the correct amino acid
to be added to the polypeptide chain despite
errors in the gene sequence.
The proposal that tRNA can recognize more than
one codon by unusual base pairing is known as
the “wobble hypothesis”.