Transcript TRANSLATION - Grade 12 Biology

TRANSLATION
The reading of the mRNA transcript and the creation of the polypeptide
chain.
RIbosomes:
Composed of a small subunit (40S) and a large subunit (60S)
INITIATION:
Ribosome recognizes the 5' cap on the mRNA transcript and binds to the
RNA
The subunits act as a clamp (80S) around the mRNA
http://www.mtmi.vu.lt/pfk/funkc_dariniai/nanostructures/images/0901whitesides_ribo.jpg
http://www.modares.ac.ir/elearning/Dalimi/Proto/Lectures/week2/ribosome_1%5B1%5D.gif
GENETIC CODE
There are 20 amino acids found in proteins, only 4 bases in mRNA (U C A
G)
Codons: sequences of three bases used to code for an a.a.
43=64 possible codons (some amino acids have more than one codon)
Ex. UUU UUC, UCU, UCC all code for phenylalanine (a.a.)
This redundancy helps to reduce errors
AUG: start codon (Methionine) used 99% of the time
UAA, UAG, UGA: stop codons
The mRNA transcript is read in sets of 3 nucleotides (one codon) to
determine which a.a. is next.
http://anthro.palomar.edu/biobasis/images/protein_synthesis_at_ribosomes.gif
ELONGATION of the POLYPEPTIDE CHAIN
AUG is the first codon read for every protein (start codon)
Ensures the correct reading frame (phase in which the mRNA is read) is
used by the ribosome
The large subunit of the ribosome has two sites for tRNA
A (acceptor) site
P (peptide) site
tRNA carrying the methionine enters the P 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 a.acid moves to the P
site, a tRNA brings the next a.acid to the A site and it 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.
http://sun.menloschool.org/~birchler/cells/animals/ribosome/synthesis.GIF
tRNA
tRNA delivers amino acids to the
ribosome
Looks like a cloverleaf
Has an anticodon – sequence of
three bases that recognize the
mRNA codon (complementary to)
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
Aminoacyl t-RNA synthases add
the amino acids to tRNA at the
binding site, the tRNA is now a
aminoacyl-tRNA
TERMINATION
Ribosome will reach a stop codon (UGA, UAG, UAA)
No tRNA 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.
MODIFICATIONS
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
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”.