This presentation is regarding Genetic code, its properties and exceptions to genetic code

1. By
K.Sudeepthi
BAM-14-15

2. The set of all the codons that specify the 20 amino
acids is termed as Genetic code , Genetic language
or Coding dictionary
Words in dictionary are in the form of codons

3. CODON
The number and the sequences of bases in
mRNA specifying an amino acid is known as
codon
ANTICODON
Set of bases in tRNA that base pairs with codon
of a mRNA is known as anticodon

5. In 1954 Gamow suggested that each codon
should have 3 nucleotides to impart specificity
to each of the amino acid for a specific codon
1 Nucleotide- 4 combinations
2 Nucleotides 16 combinations
3 Nucleotides- 64 combinations ( Most suited
for 20 amino acids)

8.  The first major break through came in 1961
when Marshall Nirenberg , Heinrich
Matthaei and S. Ochoa did crucial experiment
to decipher the genetic code.
 They demonstrated that synthetic RNA
molecules could be used as artificial mRNAs
to direct invitro protein synthesis(cell free
protein system)

9. The enzyme required for the process is
Polynucleotide phosphorylase
The enzyme polynucleotidephosphorylase
polymerizes ribonucleoside diphosphates
(NDPs) into RNA
It does not use a template, the order of
nucleotides is random
The nucleotide sequence is controlled by the
relative abundance of NDPs

10. 1. 1.Synthesis and translation of homopolymer
RNA & ratio polymer RNA followed by
amino acid analysis
2. 2.Synthesis and translation of di, tri and tetra
nucleotide repetitive RNA polymers
followed by amino acid analysis
3. 3.Triplet RNA-tRNA binding assay and
amino acid analysis

11. • Assignment of "codons" to their respective
amino acids was achieved by in vitro
biochemistry
•
• Marshall Nirenberg and Heinrich Matthaei
demonstrated polyuridilic acid[ poly U=(U)n ]
produced polyphenylalanine in a cell-free
solution from E. coli

12. • Similarly the other codons assignments and
their results
• Poly-A gave polylysine
• Poly-C gave polyproline
• Poly-G gave polyglycine
• But what of others ? ? ?

13. Ochoa and others continued the work with
random sequences of known nucleotide
composition such as 50% U and 50% G
copolymer will contain 12.5%(½ x ½ x ½) of
each of the eight possible codons:
UUU,UUG,UGU,GUU,UGG,GUG,GGU,GGG.

14. UUU-Phenlyalanine
UUG-Leucine
UGU-Cysteine
GUU-Valine
UGG-Tryptophan
GUG-Valine
GGU-Glycine
GGG-Glycine

15. More definitive data were later obtained by
H.G.Khorana using invitro systems
The ultimate ‘cracking ’’ of the code occured
when trinucleotides were found to function as
‘mini-mRNAs’

18. Starting codons
AUG codon is the start or initiation codon, i.e.,
the polypeptide chain starts either with
methionine (eukaryotes) or N-
formylmethionine (prokaryotes).
Methionyl or N-formylmethionyl-tRNA
specifically binds to the initiation site of mRNA
containing the AUG initiation codon.
In rare cases, GUG also serves as the initiation
codon, e.g., bacterial protein synthesis.

19. TERMINATION CODONS
 Three codons UAG, UAA and UGA are the
chain stop or termination codons.
 They do not code for any of the amino acids.
 These codons are not read by any tRNA
molecules (via their anticodons), but are read by
some specific proteins, called release factors
(e.g., RF-1, RF-2, RF-3 in prokaryotes and RF in
eukaryotes).
 These codons are also called nonsense codons,
since they do not specify any amino acid.


20.  Code is Triplet
 Code is Non-Overlapping
 Code is Non Ambigous
 Code is Degenerate
 Code is Universal
 Code is Commaless


21. As singlet and doublet codons are not adequate
to code for 20 amino acids it was mentioned out
that triplet code is the minimum required.
In a triplet code of 64 codons , there is an
excess of 44 codons and therefore more than
one codons are present for the same amino acid

22. The first strong evidence that genetic code is
triplet resulted in studies from genetic anlaysis
of proflavin induced mutations in the rII locus
of phage T4, carried out by by F.H.C Crick and
his colleagues in 1961

23. A non –overlapping code means that same base is
not used for two different codons of the same
message
Gammow suggested codons were overlapping but
it was disproved by Crick’s experiment

26. Ambiguity denotes that a single codon may
code for more than one amino acid
A particular codon will always code for the
same amino acid. Except when same codon in
the nucleus and mitochondria may code for
different amino acids
For eg: AUG codon in prokaryotes code for
formylmethionine at other positions code for
Methionine

27. The occurrence of more than one codon per
amino acid is called Degeneracy
Only 2 amino acids methionine(AUG) and
Tryptophan (UGG) are encoded by single codon
each

28. Leucine , Serine and Arginine are encoded by
six codons each
In all the cases of degeneracy (except Leu,
Ser,& Arg ) first two bases are common in
codons for a single amino acids

30. The degeneracy arises because of the 2 reasons
1. 1.Presence of more than one variety of
tRNA each with different anticodon specific
to one amino acid
1. 2.Ability of anticodon to base pair with more
than one codon(on mRNA ) as explained by
Wobble hypothesis proposed by Crick in
1966.

31. The degeneracy in the genetic is not at random
instead it is highly ordered
Multiple codon specifying amino acid differ by
only one day, the third or 3’ base of the codon
The degeneracy is primarily of two types
1.Partial degeneracy
2.Complete degeneracy

32.  The base at the 5’ end of the anticodon is not
as spatially confined as the other two ,
allowing it to form hydrogen bonds with any
of several bases located at the 3’ end of a
codon .
 (This is devised by Francis Crick in1966)

33. • The first two bases of the codon make normal
H-bond pairs with the 2nd and 3rd bases of the
anticodon
• At the remaining position, less stringent rules
apply and non-canonical pairing may occur
• The rules: first base U can recognize A or G,
first base G can recognize U or C, and first
base I can recognize U, C or A (I comes from
deamination of A)
• Advantage of wobble: dissociation of tRNA
from mRNA is faster and protein synthesis too

34. Base in 5’ Anticodon Base in 3’ CODO
 G G U or C
 C G
 A U
 U A or G
 I A, U, or C

36.  Restriction of bases movements:
 1 ) . In the three-dimensional structure of tRNA , the
three anticodon bases—as well as the two following (3’)
bases in the anticodon loop—all point in roughly the
same direction (figure 15-3) .
 2 ) . The first (5’) anticodon base is at the end of the
stack and is perhaps less restricted in its movements
than the other two anticodon bases—hence , wobble in
the third (3’) position of codon .
 3 ) . Not only does the third (3’) anticodon base
appear in the middle of the stack , but the adjacent base
is always a bulky modified purine residue .

37. The
adjacent
base is
always a
bulky
modified
purine
residue.

38. The ribose-ribose
distances for the wobble
pairs are close to those
of A:U or G:C base pairs
Figure 15-2 Wobble base pairing

39. A codon codes for the same amino acid in all
the organisms
Nirenberg has also stated that the genetic code
may have developed 3 billion years ago with the
first bacteria, and it has changed very little
throughout the evolution of living organisms.

40. A commaless code means there is no
punctuation between any two codons
One amino acid is coded the second amino acid
will be automatically coded by next three
codons

41. UGA which is a terminator codon codes for the
amino acid Tryptophan
In Yeast mitochondria ,CUA specifies threonine
instead of Leucine
In mammalian mitochondria AUA specifies for
methionine instead of Isoleucine

42. In 1986 a different genetic code was identified in ciliate
protozoan(Mycoplasma capricolum) that UGA code for
Tryptophan
In Tetrahymena thermophila UAA codes for Glutamine
Some mitochondrial genes have no stop codons in such
cases U or UA terminate signal by addition of A/AA
In some animal genes UGA codon encodes 21st amino acid
selenocystine
In Archae and eubacteria UAG code for 22nd amino acid
Pyrrolysine

43.  Mutations can be well explained using the
genetic code.
A) Point Mutations
1) Silent
2) Misense
3)Nonsense
B) Frame shift mutations

44.  Single nucleotide change-A to G, same amino
acid is incorporated. Mutation goes unnoticed.

45. Single nucleotide change A to C – different amino acid
incorporated. Loss of functional capacity of protein.

46.  Single nucleotide change from C to T, stop
codon is generated (In m RNA represented
by UAG), premature termination of chain,
may be incompatible with life.

48. Insertion or removal of a base/bases can alter
the reading frame with the resultant
incorporation of different amino acids