FEATURES OF GENETIC CODE AND NON SENSE CODONS

by mrs. Anita jain Pgt biology

k.v. no.1 hindan ghaziabad

GENETIC CODEThe relationship between the sequence of amino acids in polypeptide chain and nucleotide sequence of mRNA or DNA is called genetic code.

Or

The genetic representation of codon by which the information in RNA is decoded in a polypeptide is called genetic code.

Or

The order in which bases (nucleotides) are arranged in RNA deciding the order in which amino acids are arranged in proteins. (i.e. it is the relationship between nucleotide bases and the amino acids).

It was hypothesised by George Gamow that triplet code is operative as DNA contains only four bases while the no. of Amino acids are 20. So it generates (4)=64 triplet codons.

Har Gobind Khurana synthesised homopolymers &copolymers i.e. RNA molecules with specific combinations of bases.

Marshal Nirenberg helped in deciphering the code by making a cell free system for protein synthesis.

For this work , Har Gobind khurana shared Nobel prize in 1968 with Nirenberg & Holley.

CODON Sequence of 3 nitrogenous bases (triplet) on MRNA

coding for a particular amino acid is known as codon.

Or A triplet of bases present on MRNA coding for one

amoinoacid is called codon.

E.g. If MRNA has a triplet code AUG, for which corresponding aminoacid in the protein would be methionine. Similarly UUU code for phenylalanine . Thus codons for all 20 aminoacids of proteins were found. Many amino acid have more than one codon.

Anti-codon

A triplet of bases present on ERNA & that corresponds with codon of MRNA is called anti-codon.

e.g. UAC (ERNA anticodon)

THE SALEINT FEATURES OF GENETIC CODE1) The genetic code are triplet . Thus 4 bases (AUGC) when

arranged in the form of triplet code can generate 43 or 64 codons. Of these codons do not code for aminoacid but function as Stop/ terminator codons/ Non-sense codons.

(UAA, UAG & UGA), so there are 61 codons which code for 20 aminoacids.

2) One codon codes for only one aminoacid hence it is ‘unambiguous’ & ‘specific’.

3) Some amino acids are coded by more than one codon hence the code is ‘degenerate’

E.G. methionine is the amino acid that is coded by only one codon i.e. AUG.

4) AUG have dual functions as it codes for methionine & also acts as initiation/ starf codon. It is the first amino acid in the polypeptide chain.

5) The codons are real in a continuous fashion in 5’ 3’ direction i.e. without punctuation.

6) The code is non-overlapping i.e. same letter cannot for 2 different codons.

7) The code is nearly universal i.e. the codon codes for same amino acid in any organism from bacteria to human beings./ (i.e. in all organism).

e.g. UUU would code for phenylalanine , in all organisms.

Comma less

Non sense codon Non ambiguous

(UAA, UAG, UGA)

universal

Initiation Codon

(AUG)

Degenerate

Linear

Non Overlapping

Triplet

Genetic code

U

C

A

G

UUU phe UUC pheUUA leuUUG leu

UCU serUCC serUCA serUCG ser

UAU tyrUAC tyrUAA stopUAG stop

UGU cysUGC cysUGA stopUGG trp

CUU leu CUC leuCUA leuCUG leu

CCU proCCC proCCA proCCG pro

CAU hisCAC hisCAA gin CAG gin

CGU argCGC argCGA argCGG arg

AUU ileAUC ile AUA ile AUG met

ACU thrACC thrACA thrACG thr

AAU asn AAC asnAAA lysAAG lys

AGU serAGC serAGA argAGG arg

GUU val GUC valGUA valGUG val

GCU ala GCC alaGCA ala GCG ala

GAU aspGAC asp GAA glu GAG glu

GGU gly GGC glyGGA glyGGG gly

U C A G

First position

Second position third position

UCAGUCAGUCAGUCAG

RECAPITULATION1.Define genetic code , codon , & anti-codon.2.What are initiating codons?3.Mention the features of genetic code.4.What are terminating codons?5.Why are terminating codons also called as non sense

codons?6.Following is the DNA sequence representing a part of

the gene TAC CCC CAC CAG TTA TAT ATA CGG GGG CAT CAT ATG

From it derive (i) The RNA transcript.(ii)The spliced MRNA (assuming that all codons containing

ac represent the intron.(iii)The no. of amino acid it can code for.

5. Complete the following table using alphabets (a to k)

DNA triplet 3’ 5’

MRNA codon 5’ 3’

Anticodon Aminoacid

UAC

UGG tryptoplan

AAG

GAC

6. Read the sequence of the nuleotides in the given segment of MRNA and the respective amino acid sequence in the polypeptide chain

met phe met pro val ser X A U G U U U A U G C C U G U U U C U U A A

MRNA

(i) Provide the triplet of bases (codon) for (a) valine (b) proline

(ii) Write the nucleotide sequence of the DNA strand from which this MRNA was transcribed.

(iii) What does the last codon of this RNA stand for?

7. AUG GUC GGG UUU CUU AUC UAA

Given above is a sketch MRNA in 5’ – 3’ direction.

Answer the following:

a) Make the DNA strand on which this RNA got synthesised.

b) How many amino acid will be coded on this MRNA?

c) What are their anticodons for different amino acids?

d) What happens if last ‘A’ is replaced by ‘C’? Will the chain terminate or not? Why?

8. A MRNA strand has a series of codons of which 3 are given (i)AUG (ii)UUU (iii)UAG

a) What will be the DNA codes for above codons?

b) Which amino acid will be coded?

c) What will be their anti codon?

9. If one nucleotide (encircled) gets missed in the given series then how many amino acid will be changed?

AUG AU CAA AAG UAGU

10. The sequence of the nucleotide on template strand of DNA is GTG CAT TCA GCA TGA ATG TAC.

a) Write the nucleotide sequence on coding strand.b) Write the MRNA sequence formed on basis of

template strand.c) Give the anticodons of all amino acid. Which will

be coded?

11. Given below is strand of MRNA: AUG GAC CCC CUG AUA UUU UCA UGAa) Write the base sequence of one DNA strand from

which it has been transcribed.b) Upon translation how many amino acids will be

present in resulting peptide?

Thank you