Genetic code in genetics

1. PRESENTATION TOPIC:
GENETIC CODE
BY: AKSHITA MENGI
ROLLNO. 12

2. INTRODUCTION :
• Central dogma of molecular biology describes the two step process by which
information in genes flows into proteins. It describes that genetic
information from DNA is transferred to RNA by process of transcription.
• Then RNA is responsible for formation of proteins by translation.
• Nucleotides present in mRNA i.e. A,U,C,G are arranged or organized in
certain codons which will have information of formation of protein .
• As the language of nucleotide sequence on mRNA is translated to language
of amino acid sequence so translation requires genetic code through which
information contained in nucleic acid is expressed in sequence of amino
acids.
• The collection of codons is Genetic code.

3. GENETIC CODE :DEFINITION
• It is set of three nitrogenous bases in m RNA , which provide genetic information
for amino acid during synthesis of polypeptide chain.
• It defines how triplet codons specify which amino acid will be added next during
protein synthesis .
• Term given by scientist George Gamow
• For 20 amino acids there should be at least 20 codons.
• Why only triplet ?
• If one nucleotide acts as codon , there will be 4 combinations possible i.e.
A,U,G,C which is not sufficient for 20 amino acids.
• If two nucleotide acts as codon then also only 16 combinations will be formed
.(AA/AU/AC/AG/UU/UG/CC/CA/CU/CG/GG/GA/GU/GC).
• If three nucleotides acts as codon there will be 4*4*4 = 64 combinations are
formed . George Gamow postulated 3 letter codon must be employed to encode
20 standard amino acid used by living cell for protein synthesis.
• Because we are having 64 codons and 20 amino acid so for every amino acid we
have more than one codon except for methionine and tryptophan.

5. • TERMINATION CODONS : Also called chain termination codon or nonsense codon or stop
codons.
• Out of 64 codons , three donot code for any amino acid and terminates translation .
• These are UAA – ORCHE CODON
UAG – AMBER CODON
UGA – OPAL CODON
INITIATION CODON : Also called as start codon. AUG is initiation codon. It is first codon of m
RNA transcript that is translated and codes for first amino acid in all proteins .It codes for
methionine in eukaryotes ,formyl methionine in prokaryotes . The most common start
codon is AUG. In few m RNA GUG AND UUG also act as initiation codon as AUG codon is
unavailable ( in archea & bacteria ).
In e.coli AUG and UUG is read as formyl methionine if it is used as start codon, when used
within coding region it is read as methionine.
Similarly GUG as formyl methionine if act as start codon . GUG as valine if read in coding region
.

6. ANTICODONS
The base sequence of t RNA which pairs with codon of m RNA during translation is anticodon
• Codon written in 5’ – 3’ direction ; anticodon written in 3’ – 5’end .
• Codons defines which anticodon to come next with an amino acid to create the protein
strand .
• Anticodons helps in bringing a particular amino acid at its proper position during
translation .
• Anticodons of some t –RNA molecules have to pair with more than one codon .
BASE NUMBER 123
CODON 5’- AUG – 3’
ANTICDON 3’- UAC - 5’
BASE NO. 3 2 1

7. CHARACTERISTICS OF GENETIC CODE
• TRIPLET CODE : Genetic code is a triplet code consisting of 64 codons .Codon is a set of 3 letter
combination of nucleotide .
COMMA LESS : The gene is transcribed and translated from a fixed starting point to fixed point .
Punctuations are not present between the codons . No comma , no punctuation .A comma less
code base can be written as :
UUU CUC GUA UCC ACC bases
Phe Leu Val Ser Thr Amino acids In such a a case any mutation involving a deletion of a base (
C) would result in drastic change in genetic message .
UUU UGC UAU CCA CC BASES
Phe Ser Tyr Pro Amino acids
NON OVERLAPPING : Genetic code is non overlapping , any single ribonucleotide at specific location in
m RNA is part of only one triplet codon . Each letter is read only once . Mutational changes in one
letter would affect only one word .

8. • UNIVERSAL : It means that the same code is used for all life forms . This is not strictly true .
There is few context dependent codons also In this context the genetic code is nearly
universal .
• Most organisms. Exceptions.
UGA ---- Stop Trp ---- Mycloplasma , spiroplasma
CUG ----- Leu Thr ---- Mytochondria in yeasts
UAA ,UAG--- Stop Gln ----- Paramecium , Acetabularia
UNAMBIGOUS : The genetic code is non ambiguous means one codon cannot specify more
than one amino acid . EG UUU codes for phenyl alanine , it cannot code for any other
amino acid . However ambigunity at initiation have been encountered . AUG encodes for
methionine both at internal sites and at initiator position. Methionine is modified into N
formyl methionine in prokaryotes. In prokaryotes , alternative initiator codon may used
GUG is common. It specify valine at internal site but always N-formyl methionine at interior
position .

9. • POLARITY OF GENETIC CODE : The code has definite direction for reading of message which
is referred to as polarity .
• 5’----3’ end .
• Reading of message from left to right and right to left will specify for different amino acids .
• Eg UUG = leucine
• GUU = Valine
DEGENERATE : A given amino acid can be specified by more than one triplet codon. A
collection of codons which specify the same amino acids is called codon family. The different
codons for a given amino acid are said to be synonymous codons . Degeneracy is redundancy
of genetic code . Term was given by Bernfield and NIRENBERG .
Eg UUU and UCC are synonymous for phenylalanine .
UCU,UCC ,UCA,UCG , AGU AND AGCC are synonymous for serine .
Exception : Tryptophan & Methionine .

10. • The degeneracy of genetic code is advantageous to organism . When two or
more triplets specify the same amino acid , it is usually the third letter of the
codon which differs . Arginine is specified by 6 codons . The first two letters
are identical in 4 of these codons . Similarly the first two letters are same in
remaining two codons . If the CGU codon for arginine mutates to CGA the
latter codon also specifies arginine . In fact any base change in third position
of any of four codon will specify arginine .
• Thus degeneracy of the code is a protective device against the effects of
mutations .
• Degeneracy permits a number of silent mutations in which there are codon
changes but no amino acid changes. Thus the protein retains its normal
amino acid sequence , and the codon mutation does not result in phenotypic
changes .

11. WOBBLING PHENOMENON
• According to wobble hypothesis the 3rd base of a codon is not very
important and the specificity of a codon is particularly determined by first
two bases . It explains the degeneracy of codon.
• Eg Alanine is encoded by GCU
GCC ALANINE
GCA
GCC
HOW THIS HAPPENS ? First two bases in codon in m rna (5’-3’)base pair
traditionally with 2nd and 3rd base of anticodon in t RNA( 3’-5’). But non
traditional base pairing is observed between the third base of codon and 1st
base of anticodon . The reduced specificity between 3rd base of codon and
complementary nucleotide in anticodon is responsible for wobbling.
WOBBLE POSITION

12. • For example G at wobble position of anticodon can pair with C or U .
• U at wobble position can pair with A or G .
• I means inosine is a base present in t RNA . I at wobble position can pair
with U C or A .
• Wobble concept explains that some t RNA molecules can pair with more
than one codon .This means that we dont need 61 different t RNA
molecules only half as many are required .
• Proline has 4 codons CCU,CCC , CCA , CCG . The first three codons can
be recognized by single t RNA having inosine at first place.

13. THANK YOU