(3)

In the absence of glucose, if lactose is provided in the growth medium of the bacteria, the lactose gets transported into the cells by the action of enzyme permease encoded by lac y gene. A very low level of expression of lac operon has to be present in the cell all the time (otherwise lactose cannot enter the cells).

(2)

Frederick Griffith - Transformation
Francois Jacob and Jacque Monod - lac operon
Har Gobind Khorana - Genetic code
Meselson and Stahl - Semi-conservative mode of DNA replication

(2)

Lactose is the substrate for the enzyme beta-galactosidase and it regulates switching on and off of the Lac operon. Therefore, it acts as inducer.

(3)

The repressor protein synthesised from regulatory gene (i gene) binds to the operator region of the operon and prevents RNA polymerase from transcribing the operon.

(4)

Gene ‘$\mathrm{a}$’ → Transacetylase
Gene ‘$\mathrm{y}$’ → Permease
Gene ‘$\mathrm{i}$’ → Repressor protein
Gene ‘$\mathrm{z}$’ → $\mathrm{\beta }$-galactosidase

(3)

In lac operon, lactose is an inducer. It binds with repressor and inactivates it.

(4)

(4)

Alec Jeffreys (1984) invented the DNA fingerprinting technique. Alfred Hershey and Martha Chase proved that DNA is genetic material using ${\mathrm{T}}_2$ bacteriophages in an experiment. Matthew Meselson and F. Stahl proved semiconservative mode of replication in bacterium E. coli.

(3)

Operon concept is for prokaryotes that consist of operator gene, promoter gene, regulator gene and structural gene. Structural, operator and regulator genes are also present in eukaryotic gene expression along with enhancer gene but enhancer gene is present only in eukaryotic gene expression. It changes the rate of transcription of structural genes.

(1)

In Lac operon, lactose is an inducer. It binds with suppressor and inactivates it. It allows RNA polymerase access to the promoter and transcription proceeds.

(4)

The control of expression of lac operon is negative (as it is turned off normally) and inducible. Inducible operon is an operon which remains switched off normally but becomes operational in the presence of an inducer (lactose, actually allolactose a metabolite of lactose, in case of lac operon). The inducible operon generally functions in catabolic pathways. In the presence of an inducer, the repressor has a higher affinity for the inducer than for the operator gene. When lactose is added, a few lactose molecules are carried into the cell by the enzyme lactose permease as small amount of this enzyme is present in the cell even when the operon is not working. These few lactose molecules are converted into allolactose molecules which act as an inducer and bind to the repressor (a product of regulator gene). The repressor-inducer complex fails to join with the operator gene, thus it is turned on.