(1) Gene expression in prokaryotes : Gene expression refers to the molecular mechainism by which a gene expresses a phenotype by synthesizing a protein or an enzyme. Which determines the character. The gene contains the blue print or the information for the protein or an enzyme.
The category includes mechanism involved in the rapid turn-on and turn-off gene expression in response to environmental changes. Regulatory mechanism of this type are very important in microorganisms, because of the frequent exposure of these organisms to sudden changes in environment.
Gene concept can be studied by operon model. Operon are segment of genetic material which function as regulated unit that can be switched on and switched off, which was given by French scientists. Jacob and Monod (1961) working at Pasteur institute. They were studying lactose utilisation in mutants of E.coli. An operon consists of one to several structural genes (three in lac operon and five in tryptophan operon of Escherichia coli, nine in histidine operon of Salmonella typhimurium), an operator gene a promoter gene a regulator gene, a repressor and inducer or corepressor. Operons are of two types, inducible and repressible.
(i) Inducible operon system / lac operon system : An inducible operon system is that regulated genetic material which remains switched off normally but becomes operational in the presence of an inducer. It occurs in catabolic pathways. The components are :–
# (a) Structural genes : They are genes, which produce mRNAs for forming polypeptides/proteins/enzymes. Lac operon of Escherichia coli has three structural genes-Z (produces enzyme -galactosidase for splitting lactose/galactoside in to glucose and galactose) Y (produces enzyme galactoside permease required in entry of lactose/galactoside) and A (produces enzyme galactoside acetylase/transacetylase without any function in E.coli). The three structural genes of lac operon produce a single polycistronic mRNA. The three enzymes are, however, produced in different concentration.
# (b) Operator gene : It gives passage to RNA polymerase when the structural genes are to express themselves. Normally, it is covered by a repressor. Operator gene of lac operon is small, made of 27 base pairs.
# (c) Promoter gene : It is recognition centre / initiation point for RNA polymerase of the operon.
# (d) Regulator gene (i Gene) : It produces a repressor that binds to operator gene for keeping it nonfunctional (preventing RNA polymerase to pass from promoter to structural genes).
# (e) Repressor : It is a small protein formed by regulator gene. Which binds to operator gene and blocks passage of RNA polymerase towards structural enzymes. Repressor has two allosteric sites, one for attaching to operator gene and second for binding to inducer. Repressor of lac operon has a molecular weight of 160,000 and 4 subunit of 40,000 each.
# (f) Inducer : It is a chemical which attaches to repressor, changes the shape of operator binding site so that repressor no more remain attached to operator.
Lactose/galctoside is inducer of lac operon. As soon as the operator gene becomes free, RNA polymerase is recognised by promoter gene. cAMP is required, RNA polymerase passes over the operator gene and then reaches the area of structural genes. Here it catalyses transcription of mRNAs.
(1) Gene expression in prokaryotes : Gene expression refers to the molecular mechainism by which a gene expresses a phenotype by synthesizing a protein or an enzyme. Which determines the character. The gene contains the blue print or the information for the protein or an enzyme.
The category includes mechanism involved in the rapid turn-on and turn-off gene expression in response to environmental changes. Regulatory mechanism of this type are very important in microorganisms, because of the frequent exposure of these organisms to sudden changes in environment.
Gene concept can be studied by operon model. Operon are segment of genetic material which function as regulated unit that can be switched on and switched off, which was given by French scientists. Jacob and Monod (1961) working at Pasteur institute. They were studying lactose utilisation in mutants of E.coli. An operon consists of one to several structural genes (three in lac operon and five in tryptophan operon of Escherichia coli, nine in histidine operon of Salmonella typhimurium), an operator gene a promoter gene a regulator gene, a repressor and inducer or corepressor. Operons are of two types, inducible and repressible.
(i) Inducible operon system / lac operon system : An inducible operon system is that regulated genetic material which remains switched off normally but becomes operational in the presence of an inducer. It occurs in catabolic pathways. The components are :–
# (a) Structural genes : They are genes, which produce mRNAs for forming polypeptides/proteins/enzymes. Lac operon of Escherichia coli has three structural genes-Z (produces enzyme -galactosidase for splitting lactose/galactoside in to glucose and galactose) Y (produces enzyme galactoside permease required in entry of lactose/galactoside) and A (produces enzyme galactoside acetylase/transacetylase without any function in E.coli). The three structural genes of lac operon produce a single polycistronic mRNA. The three enzymes are, however, produced in different concentration.
# (b) Operator gene : It gives passage to RNA polymerase when the structural genes are to express themselves. Normally, it is covered by a repressor. Operator gene of lac operon is small, made of 27 base pairs.
# (c) Promoter gene : It is recognition centre / initiation point for RNA polymerase of the operon.
# (d) Regulator gene (i Gene) : It produces a repressor that binds to operator gene for keeping it nonfunctional (preventing RNA polymerase to pass from promoter to structural genes).
# (e) Repressor : It is a small protein formed by regulator gene. Which binds to operator gene and blocks passage of RNA polymerase towards structural enzymes. Repressor has two allosteric sites, one for attaching to operator gene and second for binding to inducer. Repressor of lac operon has a molecular weight of 160,000 and 4 subunit of 40,000 each.
# (f) Inducer : It is a chemical which attaches to repressor, changes the shape of operator binding site so that repressor no more remain attached to operator.
Lactose/galctoside is inducer of lac operon. As soon as the operator gene becomes free, RNA polymerase is recognised by promoter gene. cAMP is required, RNA polymerase passes over the operator gene and then reaches the area of structural genes. Here it catalyses transcription of mRNAs.