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Genetic Code

One Mark Questions:-

1.Give an example of a codon having dual function.

Ans:- AUG codon has dual functions. It codes for methionine (met) and also acts as an initiation codon for polypeptide synthesis.

2. How does a degenerate code different from an unambiguous one?

3. Write the specific features of the genetic code AUG.

Ans:- AUG codon has dual functions. It codes for methionine (met) and also acts as an initiation codon for polypeptide synthesis.

4. Mention the role of the codons AUG and UGA during protein synthesis.

Ans:- AUG has dual functions. It functions as initiation codon during protein synthesis and also codes for methionine. UGA does not specify any amino acid hence it functions as terminator codon.

5. Mention two functions of the codon AUG.

Ans:- AUG codon has dual functions. It codes for methionine (met) and also acts as an initiation codon for polypeptide synthesis.

Two Mark Questions:-

6. Following are the features of genetic codes. What does each one indicate? Stop codon; Unambiguous codon; Degenerate codon; Universal codon.

Ans:- (a) Stop codon : Codons that do not code for any amino acids and signal polypeptide chain termination. E.g., UAA, UAG, UGA.

(b) Unambiguous codon : Codons that specify only one amino acid and not any other. E.g., AUG codes for methionine.

(c) Degenerate codon : More than one codons codes for a single amino acid. In degenerate codons, generally the first two nitrogen bases are similar while the third one is different. E.g., UUU and UUC codes for phenylalanine.

(d) Universal codon : A codon that is applicable universally i.e., specifies the same amino acid from a virus to a tree or human being..

7. (a) Name the scientist who suggested that the genetic code should be made of a combination of three nucleotides.

(b) Explain the basis on which he arrived at this conclusion.

ANS:-(a) George Gamow suggested that the genetic code should be made up of three nucleotides.

(b) He proposed that since there are only 4 bases and if they have to code for 20 amino acids, the code should constitute a combination of bases made up of three nucleotides. Combination of 43 (4 × 4 × 4) would generate 64 codons; generating many more codons than required.

8. One of the salient features of the genetic code is that it is nearly universal from bacteria to humans. Mention two exceptions to this rule. Why are some codes said to be degenerate?

Ans:- Exceptions to the universality of genetic code are: UAA and UGA are termination codons and do not code for any amino acid. But in Paramecium and some other ciliates, these codons code for glutamine. Genetic code is non-overlapping is most organisms. But, f × 174 has 5375 nucleotides that code for 10 proteins which require more than 6000 bases. Three of its genes E, B and K overlap other genes. Nucleotide sequence at the beginning of E gene is contained within gene D. Likewise gene K overlaps with genes A and C. A similar condition is found in SV-40. More than one codons code for a single amino acid, thus are called degenerate codons. In degenerate codes, the first two nitrogen bases are similar while the third one is different, e.g., UUU and UUC are the degenerate codes that code for amino acid phenylalanine

9. Genetic codes can be universal and degenerate. Write about them, giving one example of each.

Ans:- Genetic code is universal i.e., a codon specifies the same amino acid from a virus to a tree or human being. Example : mRNA from chick oviduct introduced in Escherichia coli produces oval lumen in the bacterium exactly similar to one formed in chick. Genetic code is degenerative i.e., all other amino acids, except tryptophan and methionine, are specified by two (e.g. phenylalanine – UUU, UUC) to six (e.g., arginine–CGU, CGC, CGA, CGG, AGA, AGG) codons. They are therefore called degenerate or redundant codon. In degenerate codons, generally the first two nitrogen bases are similar while the third one is different.

10. Name the category of codons UGA belongs to. Mention another codon of the same category. Explain their role in protein synthesis.

Ans:- UGA belongs to the category of termination codons. Other codons of same category are UAG and UAA. The termination codons do not code for any amino acid and therefore terminate the process of protein synthesis.

11.(a) Name the scientist who postulated the presence of an adapter molecule that can assist in protein synthesis. Ans:- Crick postulated the presence of tRNA as an adapter molecule.

(b) Describe its structure with the help of a diagram. Mention its role in protein synthesis.

Role of tRNA in protein synthesis: tRNA is an adapter molecule meant for transferring amino acids during protein synthesis. tRNA binds to a particular amino acid at 3′ end. The charged tRNA take the same amino acid to mRNA over particular codons corresponding to their anticodons. tRNA holds peptidyl chains over the mRNAs. The initiator tRNA has the dual function of initiation of protein synthesis as well as bringing in of the first amino acid.

12. (a) Name the enzyme responsible for the transcription of tRNA and the amino acid the initiator tRNA gets linked with.

(b) Explain the role of initiator tRNA in initiation of protein synthesis.

Ans:- (a) Enzyme RNA polymerase III is responsible for the transcription of tRNA. Methionine is the amino acid with which the initiator tRNA gets linked. (b) Anticodon of initiator tRNA i.e., tRNA linked with methoinine carries this amino acid to mRNA, establishes temporary hydrogen bonds with the initiation codon (AUG) of mRNA to start the process of protein synthesis.

13. Unambiguous, universal and degenerate are some of the terms used for the genetic code. Explain the salient features of each one of them.

Ans:- Refer to question No:-6

14.(a) Name the scientist who called tRNA as an adaptor molecule.

(b) Draw a clover leaf structure of tRNA showing the following: Tyrosine attached to its amino acid site. Anticodon for this amino acid in its correct site (codon for tyrosine is UAC).

(c) What does the actual structure of tRNA look like?

Ans:- (a) Francis Crick

(b)

(c) Actual structure of tRNA is compact and looks like an inverted L.

15. Correlate the codons of mRNA strand with amino acids of a polypeptide translated.

5′ AUGACCUUUCAUUCGUGUAA 3′→ mRNAMet- r-Phe-His-Ser-Cys.→ Translated polypeptide

Infer any 3 properties of genetic code with examples from the above information.

Ans:- From the given example it is inferred that genetic code is degenerate, universal and unambiguous. Also refer to answer 6.

16:- Identify giving reasons, the salient features of genetic code by studying the following nucleotide sequence of mRNA strand and the polypeptide translated from it.

AUG UUU UCU UUU UUU UCU UAG Met – Phe – Ser – Phe – Phe – Ser

Ans:- Salient features of genetic code are:

(a). Triplet : Three adjacent nitrogen bases constitute a codon which specifies one particular amino acid in a polypeptide chain, e.g., in the given polypeptide, codon AUG specifies methionine, codon UUU specifies phenylalanine and so on.

(b). Initiation codon AUG (codes for methionine) acts as an initiation codon for polypeptide synthesis.

(c). Termination codons : Three termination codons (UAA, UAG and UGA) do not code for any aminoacid, e.g., in the given polypeptide, UAG does not code for any amino acid.

(d). Unambiguous codons : One codon specifies only one amino acid and not any other, e.g., UCU codes for serine (Ser) only and UUU codes for phenylalanine (Phe) only.

(e). Commaless : The genetic code is continuous and does not possess pauses in between the triplets.

(f). Degeneracy of Code : Since there are 64 triplet codons and only 20 amino acids, the incorporation of some amino acids must be influenced by more than one codon. e.g., amino acid Phe is specified by two codons i.e. UUU and UUC. (g) Colinearity: There is a colinearity between the sequence of nitrogen bases on DNA or mRNA and the sequence of amino acids in a polypeptide chain.

Five Mark Questions:-

16. (a) What is a genetic code ?

(b) Explain the following : Degenerate code; Unambiguous code; Initiator code.

Ans (a) The relationship between the sequence of amino acids in a polypeptide and nucleotide sequence of DNA or mRNA is called genetic code.

(b) Refer to answers 6

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