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Molecular Basis of Inheritance

Gene Expression

We now know that genes encode proteins and proteins control the functions of a cell. Are all the genes in a cell expressed at the same time? Also, are all genes expressed all the time? No! This will not only lead to wastage of cellular energy but also affect the balance within a cell. This is why gene expression is regulated. How exactly are genes regulated? Let’s find out.

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Regulation Of Gene Expression

Protein synthesis begins at transcription, ends at translation and involves multiple steps. Therefore, regulation of gene expression can happen at any of these steps. In eukaryotes, gene regulation occurs at any of the following steps:

  • Transcriptional level i.e. during the formation of the primary transcript.
  • Processing level i.e. at the stage of splicing.
  • During transport of mRNA from the nucleus to the cytoplasm.
  • Translational level.

Gene Expression

A great example of coordinated gene regulation is the development and differentiation of embryo into adult organisms. Metabolic, physiological and environmental conditions govern the regulation of gene expression. For example, E. coli uses lactose as a source of energy.

To do so, it synthesizes an enzyme called beta-galactosidase which hydrolyzes lactose into galactose and glucose. However, if there is no lactose around to be used as an energy source, the E. coli does not need to synthesize beta-galactosidase.

Prokaryotic Gene Regulation

In prokaryotes, the main site for regulation of gene expression is transcription initiation. Within a transcription unit, the activity of RNA polymerase at the promoter is regulated by ‘accessory proteins’. These proteins affect the ability of RNA polymerase to recognize start sites. These proteins can act both positively (activators) or negatively (repressors).

In prokaryotic DNA, the accessibility of the promoter depends on the interaction of proteins with sequences called operators. In most operons, the operator is adjacent to the promoter elements. Moreover, in most cases, the operator has a repressor protein bound to it. Therefore, each operon has its own, specific operator and repressor. Let’s understand this better using lac operon as an example.

The Lac Operon

Here, ‘lac’ refers to lactose. Francois Jacob and Jacque Monod were the first to elucidate the lac operon – a transcriptionally regulated system. Lac operon consists of a polycistronic structural gene regulated by a common promoter and regulatory genes. Such arrangements are common in bacteria and are called operons. Other examples include trp operon, val operon, his operon etc.

The lac operon has the following parts:

  • One regulatory gene – The i gene where ‘i’ is derived from ‘inhibitor’. This gene codes for the repressor of the lac operon.
  • Three structural genes –
    1. The gene that codes for the enzyme beta-galactosidase that hydrolyzes lactose to glucose and galactose.
    2. The gene codes for the enzyme permease that increases the permeability of the cell to beta-galactosides.
    3. The a gene codes for transacetylase.

Lactose metabolism requires gene products of all three genes mentioned above. Lactose, the substrate for the enzyme beta-galactosidase, regulates the switching on and off of the operon. Therefore, lactose is the inducer. Let’s understand how lactose switches the operon on or off.

In the absence of lactose, the gene synthesizes the repressor which then binds to the operator region of the operon. This prevents RNA polymerase from transcribing the genes (z, y, a) on the operon. Therefore, if there is no lactose, the operon does not synthesize genes for its utilization. The action of the repressor on the lac operon is negative regulation.

In the presence of lactose, the repressor interacts with lactose and gets inactivated. Thus, RNA polymerase is free and can transcribe the genes in the operon. Therefore, if lactose is present, the operon synthesizes the genes for its utilization. Therefore, essentially, the presence of the substrate i.e. lactose regulates the synthesis of enzymes for its utilization.

Solved Example For You

Question: Match the columns –

Gene of lac operon Produces
(i) a gene (a) beta-galactosidase
(ii)gene (b) permease
(iii)gene (c) transacetylase
(iv)gene (d) repressor

Solution: The answers are: i → c, ii → d, iii → a, iv → b.

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