Download Dr. Peter John M.Phil, PhD Assistant Professor

Peter John
M.Phil, PhD
Atta-ur-Rahman School of Applied Biosciences (ASAB)
National University of Sciences & Technology (NUST)
Using The Genetic Code
Using the Genetic Code-1
 Wobble Base paring
 Structure of tRNA
 Codon Anticodon Pairing
 Modified Bases
Using the Genetic Code-2
 tRNA Charging
 Aminoacyle synthetases
 Structure, function of synthetases
 Proof Reading Mechanism
Using the Genetic Code-3
 Suppressor tRNAs
 Mechanism of Suppression
 Frame Shifting
 Programmed Frame Shifting
Suppressor mutation
 A suppressor is a second mutation that restores a
function lost by the primary mutation.
 A suppressor mutation that occurs within the same
gene is called an "intragenic suppressor", and a
suppressor mutation that occurs in a different gene
is called an "intergenic suppressor"
Suppressor mutation
 A mutation that overcome the affect of other
mutation is called “suppressor”
 Suppressor tRNA restore the function of mutant
protein, it may be non-sense/missense
 In wild type cell mutation is recognized by release
factor which terminate protein synthesis.
 If different AA is inserted than the original one then
active protein is alerted
Suppressor tRNAs
 A suppressor tRNA typically has a mutation in the
anticodon that changes the codons to which it
responds.
 When the new anticodon corresponds to a
termination codon, an amino acid is inserted and the
polypeptide chain is extended beyond the
termination codon.
 This results in nonsense suppression at a site of
nonsense mutation or in read through at a natural
termination codon.
Stop Codon Suppressor
 Amber Suppressor (UGA)
 Ochre Suppressor (UAA, UAG)
Suppressor Compete With Wild Type
 Non Sense suppressor tRNA compete with release
factor that recognize stop codon
 A missense suppressor tRNA compete with tRNA
that respond properly to its new codon
 The effectiveness of a suppressor depends upon its
affinity & its concentration in the cell
Suppressor Compete With Wild Type
 Non sense suppressor may also suppress stop codon
causing read through & elongation of protein, which
can be deleterious
 Amber
suppressor (UAG) are active (10-50%)
depending upon the environment
 Ochre suppressor (UAA, UAG) below 10%
 UAG is misread by Trp-tRNA (1-3%)
Suppressor tRNA
 Suppressor tRNA have 2 consequence
 (i) It recognize new codon
 (ii) some time prevent tRNA to respond
Mechanism of suppression
Ribosome Influence Translation Accuracy
 Two stages when error can occur in protein synthesis
 (i) Charging a tRNA with correct AA
 (ii) Codon/anti codon binding
 Ribosome & tRNA also influence protein binding
Codon & anticodon
Ribosome Influence Translation Accuracy
 Two model that ribosome discriminate b/w correct &
incorrect tRNA
 Direct recognition model: in proper/normal
binding conformational change occur in aminoacyle
tRNA, which ribosome can recognize
 Kinetic Proof Reading Model: An incorrect
aminoacyle tRNA can pas through some stages
before its degradation
Ribosome Influence Translation Accuracy
change in codon meaning
 Suppression involve recognition of a codon by
mutant tRNA that usually respond to different codon
 Re definition of the meaning of a codon occurs when
an aminoacyle tRNA is modified.
 The reading frame can be changed by framshifting or
bypassing, both of which depending upon the
properties of the mRNA.
Frame shifting
 Frame shifting occur in 2 ways
 (i) some mutant tRNA recognize 4 bp codon instead
of 3 bp
 (ii) slippery seq allow tRNA to move base up/down
mRNA in the A site
Frame shifting
Programmed Frame shifting
 Some times frame shifting is important for the
expression of certain genes,
programmed frame shifting
this
is
called
 Delay in ribosome can also cause frame shifting
By Passing
 By Pass can occur due to
 (i) Ribosomal delay
 (ii) Starvation in Amino Acid
 ribosome miss one stop codon & termination occur at
other, when identical (or synonymous) codons at
either end of the sequence that is skipped.
 They are sometimes referred to as the "take-off" and
"landing" sites.
By Passing
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