The genome of higher eukaryotes is very complex. Eukaryotes genome contains DNA many times as compared to prokaryotic genome. For example, drosophila has 5,000 to 10,000 genes. Human haploid genome seems to have at least 23,000 to 1,00,000 genes. In eukaryotes, most of the DNA is non functional or inactive and known as excess DNA or repetitive DNA. The diploid organism has two sets chromosomes. The genome in eukaryotes controls various function such as; growth and division of cells, differentiation and specialization of tissues such as muscles, liver, or heart in animals and parenchyma, chlorenchyma, Xylem and phloem in plants. As the eukaryotic genomes is very large, the genes expression and its regulation become very complex.
Genes regulation
In prokaryotes and eukaryotes, genes are regulated by various factors.
Following terms are used in genes regulation:
a) Exons and introns – In eukaryotes, some of the nitrogenous based do not code for amino acids. They are inserted between those segments of bases that normally code for amino acids. The coding segments of genes are called exons and non-coding segments are called introns.
b) Splicing – When the unwanted introns are removed and functional regions (exons), responsible for coding, are again joined, it is called as splicing.
c) Inducible genes and Inducer – All the genes present on the chromosome are not expressed simultaneously. The genes that remain inactive or repressed (i.e. an inducer) is present in the medium, are called inducible genes. The phenomenon of the action of these genes is called enzyme induction and substrate is called inducer.
For example, E. coli grown in a medium without lactose, does not produce enzymes required for lactose metabolism. But when the same bacteria is placed in a lactose supplemented medium, it starts producing enzyme like ß-galactosidase required for converting lactose to glucose and galactose. Therefore, since lactose is used to induce this enzyme, it is called inducer and this phenomenon is called enzyme induction.
d) Repressible genes and repression – When E. coli is supplied with certain metabolite more than required, the action of some genes, responsible for formation of some specific enzymes, can be inhibited or repressed. Repression may take place in the case even if the metabolite is being provided from outer source. As a result certain genes are repressed and do not produce enzymes. Such inactivated genes are known as repressible genes and phenomenon is called enzyme repression.
e) Co-repressor – Molecules that binds with the repressor protein to from a function repressor complex is called co-repressor.
In a tryptophan opero, tryptophan acts as a co-repressor by binding with the repressor protein to form a complex which on binding with the promoter switches it off and hence no transcription take place.
f) Constitutive genes – These refer to prokaryotic genes whose expression is not regulated. The products of these genes are produced at a constant, often low rate. Such genes are called constitutive genes and their expression is said to be constitutive. Such genes are involved in photosynthesis and respiration.
g) Structural genes – Genes that contains the information to determine the sequence of amino acid is called structural genes.
h) Regulatory genes – These genes codes for the product that regulates the level of expression of the structural genes. Although it is located at site away from the structural genes, it is called key element of operon. It forms repressor protein to make repressor complex.
i) Promoter genes – Genes that form the binding site of RNA polymerse is called promoter genes. Each genes may be regulated by a specific promoter.
j) Operator genes – Genes that operates the activity of structural genes, is called operator genes. It lies adjacent to the promoter site. Structural genes are expressed or not expressed depending upon whether the operator genes are on or off.
k) Operon genes – Genes are genetic unit consisting of an operator, a promoter and one or more structural genes whose activity is influenced by operator genes.
Operon concept
In order to study genes regulation or induction, Jacob and Monad (1961) proposed Operon concept in prokaryote (E. coli). An opron is a group of coordinately regulated genes, the products of which typically catalyze a multi-enzyme metabolic pathway and its controlling elements. Controlling elements include promoter genes, operator genes and regulatory genes.
Although there are many operons in bacterial cells, but the lactose or Lac operon discovered by Jacob Monad is classic example of all operons.
1. Structure of the lac operon – Two classes of genes are needed to named Z, Y, and A that code for three enzymes mentioned below.
• Z genes that codes for ß-galactosidase.
• Y genes that codes for galactoside permease.
• A genes that codes for thiogalactoside transacetylase.
These genes are located in a row adjacent to each other and hence they are called linked. They are known as polycistronic. Structural genes are regulated by operator genes and promoter genes.
a) Operator genes – Operator genes lies between the promoter genes and the structural genes. The operator genes act as a switch. Structural genes are expressed or not expressed depending upon whether operator genes are on or off. Single operator genes regulate all the three structural genes.
b) Promoter genes – Single promoter genes direct proper initiation of transcription. The lac Z, lac Y and lac A genes are expressed as a polycistronic message from a common promoter. The binding of DNA dependent RNA polymerase and promoter initiates the transcription of structural genes.
2. Regulatory genes – Regulatory genes are located away from structural genes. Hence regulatory genes are often not considered as part of operon. However, regulatory genes are the key element of operon. In codes for product that regulates the level of expression of structural gene. The regulatory genes constantly transcribe mRNA to produce the repressor protein.
Regulation of Lac operon expression
The absence and presence of lactose (inducer) switch on or off the transcription of mRNA and protein synthesis. This phenomenon can be described in following steps:
I. When E. coli is grown in a medium in absence of lactose, the regulator genes produce a repressor protein that bind the operator genes and block its activity. RNA polymerase can not move from promoter to structural genes. It stops the transcription of mRNA from structural genes and thus protein synthesis is switched off. Hence the enzyme is produced.
II. When the lactose is introduced in the medium, lactose binds to the repressor protein. In this way, repressor protein fails to bind to the operator genes. Then the operator genes remain active and hence switch is turned on. Operator genes induce RNA polymerase to bind to promoter mRNA corresponding to all three enzymes; Z genes code for ß-galactosidase, Y genes for galactoside permease and A genes for thiogalactosede transcetylase. With the expression of these three enzymes metabolism of lactose beings
Synthesis of enzymes is continued unless and until all the lactose molecules are consumed. When the last molecules of lactose bound to repressor is consumed, the inactive repressor becomes active and thus binds to operator site to switch off the operon as normal.
Role of repression and constitutive enzymes
When a substrate required by bacterium is supplied in excess amount from the outside, bacterium stops or inhibits the production of substance. In other way, we can say that the genes are being inactivated. These inactivated genes are thus called repressible genes and the phenomenon is called repression.
However, some of the cellular activities are functioning normally and constantly such glycolysis. The genes, that constantly expressed to take care of normal cellular activity such as glycolysis, are known as constitutive genes. The expressions of these genes are not regulated. The enzymes produced by bacterium for above function are known as constitutive enzymes. The constitutive enzymes are dehydrogenases.
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