How many of these strains of E. coli would have no B-galactosidase activity? a ,

Question

How many of these strains of E. coli would have no B-galactosidase activity? a , o", z..Y (no glucose, no lactose) b) r', o'.z', yY (no glucose, high lactose) c, o", Z., Y. (high glucose, no lactose) d) f',o',2, v'(high glucose, high lactose) e) ,o',z,Y' (no glucose, no lactose) f ,o,z,Y (high glucose, high lactose) 8)o,Z,Y- (high glucose, high lactose) h) ,o zY (no glucose, no lactose) i) oz,Y (no glucose, high lactose) i) '.. o. z..Y (high glucose, no lactose) k) ,o,z,Y (high glucose, high lactose) I) ,o,z,Y (no glucose, no lactose) m),o,Y (no glucose, high lactose) n) ,o,z,' (high glucose, no lactose) o) ,o,Z,Y (high glucose, high lactose) p) I, ", z..Y (no glucose, no lactose) q) ,o' z,Y Ino glucose, high lactose) r) , o+, zY' (high glucose, no lactose s) ,o+,z',Y (high glucose, high lactose) .A. three O B. four O C. six O D. eight

Explanation / Answer

Answer is option (d) - 8

A strategy bacteria employ to control expression of genes is by grouping functionally related genes
together. This ensures that they can be co-regulated. Such a group of contiguous coordinately controlled genes are called Operons. All the necessary genes of an operon are transcribed together
to produce one mRNA called a polycistronic message.
The message starts from a single promoter and can be controlled together by controlling that promoter. The mRNA has many Ribosome binding sites so the protein can be synthesized individually from the polycistron.

Lac Y- galactosidase permease is part of he operon and allows the inducer lactose to enter the cell.

Beta - galactosidase (LacZ) has two functions= it Cleaves galactoside bonds between Glu and Gal in lactose molecule and also Isomerizes lactose to allolactose that acts as an inducer.

Lac repressor (product of lacI gene) tetramer binds to operator region next to promoter of the operon. Thus ensuring Lac operon is repressed in absence of lactose. When lactose is present, it binds to the repressor causing a conformation change to occur and activating the operon.

Lac O - operator site is the specific sequence on the DNA for binding of Repressor protein tetramer.

Lac operon is kept inactive in presence of glucose. Glucose is also preferred to lactose, so if both are present glucose it utilised first and then lactose. (Diauxic growth curve)

If both glucose and lactose is present then after glucose is utilised, the lactose is taken in by the bacteria and the operon is activated later on.

option (a) no enzyme activity as there is no lactose to induce the operon.

option (c) High glucose inhibits the operon by catabolite repression.

option (e) (f) In both cases Lac Z gene is mutated so no activity will be seen

option (g) Lac Y is mutated to without the permease gene, the lactose cannot enter the cell to induce the operon. Mutant LacY retain the capacity to synthesize beta - galactosidase but it cannot be induced by adding lactose to the medium.

option (p) (q) (r) (s) - Is mutant is a mutated super repressor protein that is insensitive to the inducer. It binds to the operator and doesnot leave. So the operon is not activated.

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