Q.1 What would you mutate so that the lac operon always remains ON constitutivel
ID: 261107 • Letter: Q
Question
Q.1
What would you mutate so that the lac operon always remains ON constitutively in medium containing no lactose or glucose? Explain
(a) why the operon will always be on; (b) How will you turn the operon off without mutating any DNA sequence, gene or protein? Explain. 5 pts
Q.2
Suppose, you synthesized RNA in vitro using polynucleotide phosphorylase, 3 parts ADP and one part GDP. After the reaction, you isolated all RNA molecules and translated them in a cell-free translation system. Now, answer the following questions. (A) How many different amino acids do you expect in the polypeptides synthesized in the cell-free system?Name these amino acids. Use the codon dictionary available in the textbook. (B) Which amino acid would show highest frequency of incorporation in the polypeptides synthesized in the cell-free system? Show your calculations for full credit. 5 pts
Explanation / Answer
1) FOr constitutive expression of lac operon, operator gene should be mutated. So that in the presence of glucose or in absence of lactose, repressor protein will not be able to bind the operator and thus RNA polymerase will be able to bind promoter site resulting in constitutive expression of lac operon.
a) A mutation in Lac operator does not allow the binding of a normal repressor molecule with operator.
b) The expression of Lac operon is triggered by the presence of inducer, lactose. When an inducer is present, it binds allosterically to the repressor protein, thereby inactivating the operator binding site of the Lac repressor protein.
Therefore, to turn OFF the operon without mutation, Inducer or lactose has to be removed, enabling the repressor to bind to the operator site.
2) Given 3ADP:1GDP
There will more triplets for AAA, lesser numbers of AAC, ACA, CAA, few numbers of ACC, CAC, CCA and very few CCC triplets.
a) Polypeptide will contain lysine (AAA), asparagine (AAC), Threonine (ACA, ACC), glutamine (CAA), histidine(CAC), proline (CCA, CCC)
b) As ADP is present in higher amount so the amino acid lysine (AAA) will show highest frequency of incorporation in the polypeptides synthesized i.e., 0.4%
Possible combinations Probablity of occurence of triplets Possible triplets Final percentage 3A (3/4)3 = 0.4% AAA 0.4 2A:1C (3/4)2 (1/4) = 0.14% AAC, ACA, CAA 3*0.14 = 0.4 1A:2C (3/4) (1/4)2 = 0.04 ACC, CAC, CCA 3* 0.04 = 0.12 3C (1/4)3 = 0.01 CCC 0.01Related Questions
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