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In living system, different genes give rise to different types of RNAs and proteins, but all the proteins are not needed all the time and in all the types of tissues. Thus, throughout the lifecycle of a multicellular organism, though, all cells contain same set of genes, but in different types of tissues different genes are expressed to produce various types of RNAs and proteins, depending upon the need, e.g., digestive enzymes are specifically produced in alimentary canal, growth hormone in pituitary, insulin in pancreas, testosterone hormone in testis, progesterone and estrogen hormones in ovary, etc, Similarly, in all the organisms (multicellular as well as unicellular), expression of genes also varies with the environment. Thus, gene expression is highly regulated and energy is not wasted in the expression of unwanted genes, as protein synthesis requires energy in the form of ATP, GTP, etc. (Fig. 11.1).


Fig.11.1 Various levels of gene regulation.

In addition to the regulated genes, products of some essential genes are required all the time so that they are not regulated at all and remain continuously expressed all the time, e.g., respiratory enzymes of glycolysis and Krebs cycle, ribosomal proteins, tRNAs, rRNAs, etc. Such genes are called housekeeping genes.

In case of regulated genes, in different types of cells, some mechanisms allow the expression of the desired genes. Such regulatory mechanism have most extensively been studied in prokaryotic systems, particularly in Escherichia coli. In prokaryotes, since, the DNA is not is not bounded by a nuclear envelope and, moreover, the DNA is not found to be associated with any histone and non-histone proteins (naked DNA), the regulatory mechanisms are quite different from those found in eukaryotes.