Topic :Genetic code

By,Pillai Aswathy viswanathPG 1 BotanySt. thomas college kozhencherry

Introduction• Watson and crick proposed

the double helical structure for DNA in 1953

• They also suggested that the genetic information which passed from generation to generation and which controlled the activities of the cell, might be stored in the form of the sequence of the bases in the DNA molecule

• There are only 4 nitrogen bases in DNA molecule namely A,T,C,G

• The linear arrangement of nitrogenous bases in DNA molecule determines the sequence of the amino acid in a protein molecule

• In order for a biological cell to make a proteins ,the genetic information from DNA has to be translated

• So first the gene encoded in DNA is transcribed into a single strand of mRNA

• This happened in the cell nucleus

• The mRNA then moves outside the nucleus to ribosome, where protein synthesis take place

• Ribosome moves along the mRNA strand, reading the three nucleotides at a same time specifies one amino acid

• The each 3 nucleotide called the codon• So the relationship between 3-base sequence

and an amino acid is known as genetic code

The Genetic Code

• There are 64 possible combination of 3 nucleotides sequences

• Out of 64 ; 61 of these code for 20 amino acid including the initiation codon methionine

• Initiation codon is AUG, which initiates protein synthesis

• The other 3 are called stop codons• UAG (amber codon), UAA (ochre codon) and UGA

(opal codon), • Termination codon , which code for no amino acid

but instead cause protein synthesis to terminate.

• From the table , that several of the triplets have the same letter but in different sequence and these code for different amino acid

• It means that sequence of letters in the triplets is most important in determining what amino acid to be coded.

Important features of the genetic codes1)The code is a triplet :-• The group of bases specifying one amino acid

is called a codon• There are 64 codons code for the 20 amino acid• Each individual “word” in the code is composed

of three nucleotide bases and it codes for a specific amino acid

• There for, it often called a triplet codon

Proof for the triplet code

Crick-Brenner experiment: • An elegant and important experiment

performed in 1961 by Francis Crick and Sydney Brenner.

• The experiment proved that the genetic code was a triplet code

• Each codon must contain 3 letters and also the sequence of 3 bases specify one amino acid

• The effect of inserting or deleting particular numbers of bases in a coding sequence was examined

• If the E.coli phage T4 is grown in a medium

containing the substance proflavin• Errors will occasionally be made during DNA

replication• Daughter molecule will formed that either have

an additional nucleotide pair or lack a nucleotide pair

• A base pair addition or deletion represent a mutation

• It upsets the phase of units by which the code is read for a specific protein

• Every amino acid down stream from the added base will be different

mRNA from original DNA Ser His Phe Asp Lys Leu

5’--AGC CAC UUA GAC AAA CUA-- 3’ 5’—AGC ACA CUU AGA CAA ACU A-- 3’ Ser Thr Leu Arg Gln Thr • Indeed ,mutant phage, called frame shift

mutants, are found among the progeny• If a proflavin-induced mutant is grow again

in a medium containing proflavin, • Some phage appear in which the wildtype

phenotype has been restored

What mutational events would result in such restoration

• A possible mechanism for this reversion might be removal of the additional pair of bases from the DNA of a base-addition mutant, but such an event would seem unlikely

• Mutation are produced randomly• So removal of a particular added base pair

that gave rise to the mutation should occur with much less frequency than the addition that produced the original mutation

• However ,the observed frequencies of mutation and reverse mutation were comparable

• A mutation resulting in removal of a base pair sufficiently close to the added base pair might also restore the reading frame

Ser His Phe Asp Lys Leu 5’--AGC CAC UUA GAC AAA CUA-- 3’ 5’—AGC ACA CUU AGA CAA ACU A-- 3’ Ser Thr Leu Arg Gln Thr C

5’—AGC ACA UUA GAC AAA CUA-- 3’ Ser Thr Phe Asp Lys Leu

• In certain cases might result in production of a biological functional protein

• Even though the amino acid sequences of the original wild type and the one formed by a mutation plus a reverse mutation were not identical

• This proved in the study of a collection of proflavin-induced mutants in the gene called rIIB of Ecoli phage T4

• If the 2 phage strains each carrying a different mutation in the same gene are crossed

• Some wild type phage progeny arise by genetic recombination

• However, when 2 randomly selected proflavin induced mutants were crossed

• Wild type phage progeny did not always arise

• That the mutant could be placed in 2 distinct classes termed (+) and (-)

• The (+) mutants were considered to have one additional pair of bases

• The (-) mutants were considered to have one lack pair• Crosses between 2 mutants in different classes ie; 1 (+) and 1 (-) having the wild type phenotype• But no such wild type phenotype were formed by

crossing mutants in the same class

(+)*(+) or (-)*(-)

• In (+) (-) double mutant arising the shifted reading frame following the (+) locus would be incorrect

• The correct reading frame will restored at the (-) locus• Between the 2 sites of mutation , the amino acid

sequence would not be the wild type• A double mutant of the type (+) (+) or (-) (-)• Addition of 2 bases pairs or lack of 2 bases pairs• each double mutation would shift the reading

frame by 2 bases• Such a shift does not yield a wild type phenotype• Because a functional protein is not made

• Clearly , double mutants of the type (+)(+)or(-)(-) would never have the wild type phenotype

• Since the genetic code cannot be a 2 letter code

• If it were a 2 letter code, the reading frame would be restored in this combination

• By construction of triply mutant recombination we can determine whether the code is triplet code

Tolerant region Intolerant region

w T ABC DEF GHI JKL MNO PQR STU VWX

+ AB1 CDE FGH IJK LMN OPQ RST UVW X

+ ABC DE2 FGH IJK LMN OPQ RST UVW X

+ ABC DEF GHI JK3 LMN OPQ RST UVW X

++ AB1 CDE 2FG HIJ KLM NOP QRS TUV WX

+++AB1 CDE 2FG HIJ 3KL MNO PQR STU VWX

• It was found that the triple mutants (+) (+) (+) and (-) (-) (-) have the wild type phenotype

• Whereas the mixed triples (+) (+) (-) and (-) (-) (+) remain mutant• So the combination of 3 mutants of the

same type can yield a wild type phage• So we concluded that the genetic code

must be a triplet codon

2)Non overlapping code

• The code is sequentially read in group of three

• A nucleotide that forms part of a tirplet is never a part of the next triplet

3) Genetic code is commaless

• Genetic code on mRNA is read continuously with out any punctuation along the reading frame

• It means there is no comma between the adjacent codons and each codon immediately follwed by the next codon

4)Genetic code is unambiguous

• Genetic code is always unambiguous in the sense that a codon always specifies the same amino acid in all organism

• It will not code for more than one amino acid

5)Genetic code is degenerate

• Ther are 64 possible codons for only 20 amino acid that are found in proteins

• It was concluded that in most cases a single amino acid code for 2 to several different codons

• This multiple system of coding is known as degenerate genetic code

• Except for methionine and tryptophan which have only one codon each

• All other amino acid have 2 or more codons

• The degeneracy of coding system provides protection to organisms against many harmful mutation

• If the amino acid has more than one codon • The first 2 bases in the codon will be the

same • Only the 3rd one is different • If the 3rd bases is altered it will not change

the amino acid• So the effect of mutation will be minimised

6)Genetic code has polarity

• Genetic code polarized with definite initiation and termination codons

• This enables a gene to specify the same protein always

• Also it causes the code to read in a fixed direction

• AUG act as the initiation codons and the 1st amino acid to be incorporated in the synthesis of all proteins is methionine

• UAG UAA and UGA act as the stop codons

7)Genetic code is universal

• The universality of the genetic code was first established by Marshall Caskey and Nirenberg 1967

• Genetic code is considered universal this means that the same code is present in all organism

• Also that a codon specifies the same amino acid in all organism

Exceptions to the standard code

• In 1960s, the genetic code was established to be ‘universal’ for all living organisms.

• The codons were found to be the same for all organisms leading to the idea of that the genetic code is universal

• However, from late 1970s, variations of universal code have been found in various genetic systems.

• The genetic code was subsequently determined for many other organism ranging from bacteria to mammals including humans

• After it was established any subsequent changes in the code would prove to be lethal

• If one codon changed then all similar codons in the entire organisms genome would have to change simultaneously

• In fact a few rare exceptions to the universal code must be found

• Most of the exceptions are found in mitochondrial genome

Exceptions

Organism Normal codon Usual meaning

New meaning

Mammalian mitochondria

AGA , AGG

AUA

UGA

Arginine

Isoleucine

Stop codon

Stop codon

Methionine

Tryptophan

Drosophila mitochondria

AGA , AGG

AUG

UGA

Arginine

Isoleucine

Stop codon

Serine

Methionine

Tryptophan

Organism Normal codon Usual meaning

New meaning

Yeast mitochondria

AUA

UGA

CUA , CUC , CUG CUU

Isoleucine

Stop codon

leucine

Methionine

Tryptophan

Threonine

Higher plant mitochondria

UGA

CGG

Stop codon

Arginine

Tryptophan

Tryptophan

Protozoan nucleiMycoplasma capricolum bacteria

UAA UAG

UGA

Stop codon

Stop codon

Glutamine

Tryptophan

Reference• Veer bala rastogi (2008).fundamental of

molecular biology .published by Ane books India

• Daniel L.Hartl,David Freifelder,Leon A.

Snyder(1987).Basic Genetics.Jones and Bartlett Publishers

• http://www.answers.com/topic/genetic-code#ixzz36hBojiS2

• Griffiths, Anthony J.F., Jeffrey H. Miller, David T. Suzuki, Richard C. Lewontin, and William M.

Gelbart. 1993. An Introduction to Genetic Analysis 5th ed. W.H. Freeman and Company.

• Gupta P.K(2007).genetics classical to modern Rastogi publications.