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In this article, we propose to discuss the regulation of gene expression in eukaryotes with diagram.
Operons are also functional in eukaryotes. Likewise, they are of two types, inducible and repressible. Gene regulation of eukaryotes is slightly complex as compared to that of prokaryotes.
1. Different structural genes connected to a metabolic pathway do not often lie adjacent to one another. They are generally found well spaced, on the same or different chromosomes.
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2. Each structural gene seems to have its own promoter gene.
3. Eukaryotes possess sensor genes which pick up information of any change in the intracellular environment and presence or absence of hormones, vitamins, metal ions, chemicals, pathogens, etc.
4. Eukaryotes have integrator genes for coordinated functioning of structural genes present in different parts of genetic material.
5. Eukaryotes seem to have specific genes that enhance or slow down the expression of certain genes. They are called enhancer genes and silencer genes respectively.
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6. Eukaryotes structural gene has two types of interspersed regions, exons and introns (Fig. 8.6). Exons are coding sequences of a gene which on transcription form parts of mRNA that code for different regions of protein. Introns are non-coding sequences of the gene which are also called spacer DNA or intervening sequences (IVS). They do not form parts of mRNA.
The whole gene is transcribed as a heterogeneous nuclear RNA (hn RNA). The non-coding or intronic regions of RNA are removed by means of enzymes nucleases. The exonic regions of RNA are joined together to produce a single chain RNA required for functioning as translational template. The phenomenon of removal of non-coding regions and fusion of coding parts of RNA is called splicing.
7. The freshly formed mRNA undergoes several changes at 5′ and 3′ ends. It receives a cap at the 5′ end and poly A tail at the 3′ end. The tailored mRNA is then transported across the nuclear envelope for translation through ribosomes and tRNAs.
Operon-Operator Model:
It is also known as gene battery model. It was proposed by Britten & Davidson in 1969. There are four types of genes namely sensor gene, integrator gene, receptor gene and producer gene. The integrator gene is like a regulator gene of prokaryotes which synthesizes an activator RNA for activating the receptor site.
The activity of integrator gene is controlled by the sensor gene. The producer gene analogous to structural gene of prokaryotes. Receptor gene is a link between integrator & producer gene. The receptor gene gets signal from integrator gene. As a result it activates the producer gene.
Importance of Gene Regulation:
1. There are two types of gene action -constitutive and regulated. The constitutive gene action occurs in those systems which operate all the times and the cell cannot live without them, e.g., glycolysis. It does not require repression. Therefore, regulator and operator genes are not associated with it.
2. In regulated gene action all the genes required for a multistep reaction can be switched on or off simultaneously.
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3. The genes are switched on or off in response to particular chemicals whether required for metabolism or are formed at the end of a metabolic pathway.
4. Gene regulation is required for growth, division and differentiation of cells. It brings about morphogenesis.
