Dr. Karthikeyan Pethusamy MD DNB (Biochemistry) explains the genetic code for the undergraduate students. Don't miss the YouTube video attached. The video is made with the same power point file.

1. Genetic Code
Dr. Karthikeyan Pethusamy

2. Size of a codon
One nucleotide code→ 41 amino acids
Two nucleotide code→ 42 =16 amino acids
Three nucleotide code→ 43 = 64 amino acids
Four nucleotide code → 44 = 256 amino acids

3. Assume 4 nucleotides code for an amino acid. What is
the possible number of amino acids encoded?
A. 16
B. 64
C. 4
D. 256

4. Cracking of genetic code

5. Memorise all ??? No 

6. Initiation and termination codons
• Initiation codons:
AUG → N-formyl Methionine (prokaryotes)
Methionine (Eukaryotes)
• Termination codons/Stop/Nonsense:
»UAA (ochre)
»UGA (opal)
»UAG (amber)

7. Genetic code

8. Unambiguous
• Degeneracy does not suggest that the code
is flawed: although an amino acid may have
two or more codons.
• Each codon specifies only one amino acid.

9. Properties of genetic code
• Degenerate
• Unambiguous
• Universal
• Nonoverlapping
• Not punctuated
• Colinear

10. Degeneracy
• An amino acid may be specified by more than one codon, so the
code is described as degenerate.
• Except Trp and Met all have more than one
• Synonymous codons: all codons that code for a particular amino
acid

11. Wobble hypothesis
• The degeneracy also arises from imprecision (wobble) in the
pairing of the third base of the codon with the first base of the
anticodon.

12. Wobbling

13. Types of point mutation

14. Universality
• All organism have same genetic code.
• Exceptions (hence nearly universal)

15. Modified amino acids
• Selenocysteine: UGA, a special insertion
element in mRNA (a stem loop) in UTR
• Others by post-translational modifications

16. Multiple codons coding for the same amino acid is known
as ………
a. Degeneracy
b. Transcription
c. Mutation
d. Frame shift mutation

17. All of the following are synonymous codon
pair, EXCEPT
A. CAU & CAC
B. AUU & AUC
C. AUG & AUA
D. AAU & AAC

18. Anticodon 5’-I-G-C-3’ can recognize all of the
following codons EXCEPT:
A. GCA
B. GCC
C. GCG
D. GCU

19. The codon that codes for the 21st amino
acid is:
A. AUG
B. UAA
C. UAG
D. UGA