Genetic Code

Q1. What are Termination Codons?

UAA........ochre

UAG........amber

UGA........opal  are 3 or stop Codons.These Codons don't specify any amino acids  hence , termed nonsense Codons but direct ribosome to terminate protein synthesis, so termed termination Codons.

Q2. List out the properties of genetic codes .

     Properties of genetic codes are as follows:

(i) Triplet in nature.

(ii) Non-overlapping.

(iii) Commaless

(iv)Universal

(v) Degenerate

(vi) Non-ambiguous

(vii) Deciphering of Genetic Code.

Q3. What are the initiation Codons?

    AUG and GUG are the Initiationtion Codons , among them AUG is more frequently used.

Q4. What are anticodons?

   Each tRNA has a triplet nucleotide sequence which is complementary to a codon of mRNA , and termed as anticodons.

  e.g.

Codon.          3'-CUU-5'

Anticodon.    5'-GAA-3'

Q5. What is Wobble Hypothesis ?

   Genetic codes are Degenerate, means except methionine  and tryptophan  other amino acids have more than one Codons .

   e.g.,

   Alanine  amino acid has 4 Codons for it , like GCU

       GCC

       GCA

       GCG

The first two nucleotides in each are the primary determinants, but any change in the third base may no effect in the amino acid sequence in a polypeptide. Such third base in Degenerate codes termed  wobble.

    According to Crick(1966) only the first and second bases a codon have a precise pairing with the anticodon but the pairing of third bases of codon and anticodon may be wobble(non-specific).

Q6.Prove that Genetic Code is triplet.

  Francis Crick et.al., used Proflavin ( an acridine dye ) which intercalate in DNA helix of rllB gene of T⁴ bacteriophage causing either addition or deletion of nitronen base to prove that Genetic Code is triplet:

    Insertion and deletion of base in a codon sequence in a mRNA is shown below:

   (i) 5' GUA GCC UAC GGA U..3'....if C is added at 3rd code in between U-A , it turns into 5' GUA GCC UCA CGG AU...3'. The codes from the code in which new base inserted has been changed in the reading frame, at the same time the sequence of amino acids will be altered.

  (ii) 5' GUA GCC UAC GGA U..3'.  if G is deleted from the 2nd code , it turned into 5' GUA CCU ACG GAU 3' i.e. the reading frame is shifted towards left side as well as amino acids sequence in polypeptide will be altered.

(iii) If A is added in the 2nd and A is deleted from the 3rd code then the frame turns into 5' GUA GAC CUC GGA U..3'. it means if a deletion and addition are combined to produce double mutant gene the frame shift occurs only in the region  between the two mutations and the rest remains the same.

(iv) If 3 base are added or deleted , then the reading frame beyond the mutation remained the same .

    Thus the above experiment proves that Genetic Code is triplet.

(7) Prove that Genetic Code is Non-overlapping.

   According to George Gamow , codes are triplet and overlapping.

  (i) 5' AUA CGA GUC 3' is a non-overlapping sequence in  mRNA  with 9 bases and 3 codons. Here Codons don't share common bases .

  (ii) It can be turned into an overlapping base from the beginning :

    5' AUG CGA GUC 3'.  Original sequence.

When bases overlaps ,e.g. if U and G of first codon overlapping , the new Codons appears as:

        AUG

           UGC

              GCG

Hence, the reading frame turns into:

5' AUG UGC GCG..  3'.

   Evidence:

   In sickle cell anemia the mutant polypeptide has a single amino acid substitution. 

 (a) If codes are overlapping, and a change in one base pairs in DNA would affect three consecutive Codons .As a result 3 specific amino acids in the polypeptide will alter.

  (b) If code non overlaps , there will be onle one change in the amino acid sequence in the polypeptide, not three.

   This proves that Gamow's assumption was wrong.

(8) Prove that Genetic Code is Universal.

 Genetic Code is Universal, but in 1979 it appeared that a few mitochondrial codes are exceptional:

e.g. Universal codes

UGA.......Stop codon

AUA.......Isoleucine

CUA.......Leucine

 

Mammalian Mitochondrial code

UGA.... Tryptophan

AUA.... Methionine

CUA.....Leucine

Evidence:

(i) If ribosomes and mRNA of rabbit reticulocytes are mixed with aminoacyl tRNAs and other translational ingradients of E.coli then rabbit Hb is synthesized.

(ii) Poly U synthetic mRNA promotes phenylalanine incorporation in the polypeptide in cell free extracts from a variety of different organisms ranging from bacteria to mammals.

   Similarly poly C incorporates proline and poly A incorporates lysine in all extracts of different cellular range.

  Hence, it is proved that Genetic codes are universal.