Genetic code slide

Genetic code is a dictionary that corresponds with sequence of nucleotides and sequence of amino acids.

Genetic code is a set of rules by which information encoded in genetic material(DNA or RNA sequences) is translated into proteins by living cells.

Term given By ″ Goerge Gamow ʺ

1. GENETICCODE
Dr. M.Sankareswaran
Assistant professor, Department of Microbiology.

2. What is Genetic code???
•Genetic code is a dictionary that corresponds with
sequence of nucleotides and sequence of aminoacids.
•Genetic code is a set of rules by which information
encoded in genetic material(DNA or RNAsequences) is
translated into proteins by livingcells.
•Term given By ″ Goerge Gamowʺ

3. DISCOVERY
James Watson &Crick
•Tounderstandhowproteinsare
encodedbeganafterthestructureof
DNAwasdiscoveredbyJames
WatsonandFrancisCrick.
•George Gamow postulated that a three-
letter code must be employed to encode
the 20 standard amino acids used by
living cells to buildproteins.

4. Introduction of geneticcode
The letters A,G,T and C correspond tothe
nucleotides found in DNA. They are
organized intocodon.
The collection of codons is called Genetics
code.
For 20 amino acids there should be 20codons.
Each codon should have 3 nucleotides toimpart
specificity to each of the amino acid for a
specificcodon.
1nucleotide –4combinations
2nucleotide-16combinations
3nucleotide-64 combinations( most suitedfor 20
amino acids)

8. Codon and itstype
•Genetic code is a Dictionary consistsof
“Genetic words” calledCODONS.
•Each codon consists of three bases(triplet)
•There are 64codons.
•61 codons code for 20 amino acids foundin
protein.
•3 codons do not code for any aminoacid.

9. Type ofcodon
SenseCodons
SignalCodons
•Startcodons
•Stopcodons
Sense codon:-The codon that code foramino
acid are called sensecodon.
Signal codon:-Those codons that codefor
signal during proteinsynthesis are
called signalcodons.
For Example:-AUG, UAA, UAG&
UGA
There are Two types of signalcodons
TerminatingCodon
InitiatingCodon.

10. “TerminatingCodons”
UAA, UAG & UGA are termination codonsor
nonsense codons & are often referred to as amber, ochre &
opalcodons.
“Initiatingcodon”
AUG is the initiation codon. It codes for the first amino
acid in all proteins.
At the starting point it codes for methionine in
eukaryotes & formyl methionine inprokaryotes.

12. ANTICODON
•The base sequence of t RNA which pairs withcodon of
mRNA during translation is calledanticodon.

13. Different between codonand
anticodon
•Codon could be present in both DNA & RNA, but
anticodon is always present in RNA &never in
DNA.
•Codons are written in 5 to 3 direction whereas
anticodons are usually written in 3 to 5direction.
•Anticodon of some tRNA molecules have topair
with more than onecodon.

14. Characteristic of the geneticcode
1.Tripletcode
2.Commaless
3.Nonoverlapping code
4.The codingdictionary
5.Degeneratecode
6.Universality ofcode
7.Non ambiguous code
8.Chain initioncode
9.Chain terminationcodons

15. Triplet nature
A triplet code could make a
genetic code for 64 different
combinations (4 X 4 X 4) genetic code
and provide plenty of information in
the DNA molecule to specify the
placement of all 20 amino acids.
When experiments were
performed to crack the genetic code it
was found to be a code that was
triplet. These three letter codes of
nucleotides (AUG, AAA, etc.) are
called codons.

16. “Genetic code istriplet”
•The genetic code is triplet. There are 64codons.

17. Degeneracy
•More than one codonmay specify the same
amino acid; this is called degeneracy of the code.
•For example, except for tryptophan and
methionine, which have a single codoneach, all
other 18 amino acids have more than one codon.
•Thus, nine amino acids, namely phenylalanine,
tyrosine, histidine, glutamine, asparagine, lysine,
aspartic acid, glutamicacid and cysteine, have 2
codonseach.
•Isoleucinehas 3 codons

19. Non-overlapping
•One base cannot participate in the formation of more than one
codon.
•This means that the code isnon-overlapping.

20. Commaless
The genetic code is commaless, which means that no codonis
reserved for punctuations. It means that after one amino acid is
coded, the second amino acid will be automatically, coded by the
next three letters.

21. “Universal
ity”
•The genetic code isuniversal.
•AUG is the codon for methionine in mitochondria. The same
codon (AUG) codes for isoleucine in cytoplasm. With some
exceptions noted the genetic code isuniversal.

22. The code haspolarity
•The genetic code has polarity, that is, the code is
always read in a fixed direction, i.e., in the 5′ → 3′
direction.
•The code has a definite direction for reading of message
which is referred to aspolarity.
•Reading of message from left to right & right to left will
specify for different aminoacids.
•For Example UUG stands for leucine, & from right to left it is
GUU which stands for valine.

23. Chain Initiation Codons
•In most organisms, AUG codonis the start or initiation codon, i.e., the polypeptide chain
starts either with methionine(eukaryotes) or N-formylmethionine(prokaryotes).
•In rare cases, GUG codes for valinealso serves as the initiation codon, e.g., bacterial protein
synthesis.
Chain Termination Codons
•The three codonsare the chain stop or termination codon.
•The amber stop codon, ( UAG)
•The ochre stop codon, (UAA)
•The opal stop codon, ( UGA)
•These codonsare not read by any tRNAmolecules (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 codonsare also called nonsense codons, since they do not specify any amino acid

24. Wobblehypothesis

25. Wobblehypothesis
A=T/U
G=C
mRNA-Codon
tRNA-Antocodon

26. Wobblehypothesis
A=T/U
G=C
mRNA-Codon
tRNA-Anticodon
TRNA ANTICODON M RNA CODON
A U (WC)
C G (WC)
G C (WC) / U (W)
U A (WC) / G (W)
I A C U (W)

28. Wobblehypothesis

29. Wobblehypothesis

30. Wobblehypothesis
Crick (1966) proposed the ‘wobble hypothesis’ to explain
the degeneracy of the genetic code. According to this
hypothesis, only the first two bases of the codonhave a
precise pairing with the bases of the anticodonwhile the
third one may wobble (non specific).

32. Wobble hypothesisexplains
degeneracy
•Wobble hypothesis explains the degeneracyof the
genetic code, i.e, existence of multiple codons for
a single amino acid. Althoughthere are 61 codons
for amino acids, the number of tRNA is far less
(around 40)which is due to wobbling.

33. Biological significance of
degeneracy of the geneticcode
•If the code were not degenerate, 20 codons
would designated amino acids and 44would
lead to chaintermination.
•The probability of mutating to chain
termination would therefore be muchhigher
with a non degeneratecode.

34. FLOW OF GENETICINFORMATION

35. CLINICSIGNIFICANCE
•Mutation can be well explained using thegenetic
code.
•A) PointMutations
1)Silent
2)Misense
3)Nonsense
•B) Frame shiftmutations

36. SilentMutations
•Single nucleotide change-A to G, sameamino
acid is incorporated. Mutation goesunnoticed.

37. Missensemutations
•Single nucleotide change A to C-differentamino
acid incorporated. Loss of functional capacity of
protein.

38. Non sense Mutation
•Single nucleotide change from C to T, stop codon is
generated (In m RNA represented by UAG),
premature termination of chain, may be incompatible
withlife.

39. Frame shiftMutation
•Insertion or removal of a bases can alter the reading
frame with the resultant incorporation of different
aminoacids.