1. Peter John M.Phil, PhD Atta-ur-Rahman School of Applied Biosciences (ASAB) National University of Sciences & Technology (NUST)

2. Using The Genetic Code

3. Using the Genetic Code-1
Wobble Base paring
Structure of tRNA
Codon Anticodon Pairing
Modified Bases

4. Using the Genetic Code-2
tRNA Charging
Aminoacyle synthetases
Structure, function of synthetases
Proof Reading Mechanism

5. Using the Genetic Code-3
Suppressor tRNAs
Mechanism of Suppression
Frame Shifting
Programmed Frame Shifting

6. 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"

7. 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

8. 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.

9. Stop Codon Suppressor Amber Suppressor (UGA)
Ochre Suppressor (UAA, UAG)

10. 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

11. 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%)

12. Suppressor tRNA Suppressor tRNA have 2 consequence
(i) It recognize new codon
(ii) some time prevent tRNA to respond

14. Mechanism of suppression

15. 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

16. 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

17. Ribosome Influence Translation Accuracy

18. 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.

19. 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

20. Frame shifting

21. Programmed Frame shifting
Some times frame shifting is important for the expression of certain genes, this is called programmed frame shifting
Delay in ribosome can also cause frame shifting

22. 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.

23. By Passing

24. Thanks