Gene Regulation in Eukaryotes

In eukaryotes, the genes that code for the enzymes needed to catalyse the various steps of a particular metabolic pathway may not be adjacent or even be in different chromosomes. However, these genes are regulated by operon model as in bacteria. Both the inducible and the repressible systems of the operon model work in the eukaryotic cells but through a complex network of regulatory genes. Environment changes in the cells as growth and development proceed, and the gene expression is suitably regulated to cope with the new environment. Hormones, vitamins, minerals, chemicals and pathogens formed or entering the cells may induce or repress certain genes. This would produce certain proteins and stops the formation of some proteins, thereby initiating new or terminating existing metabolic pathways. This ultimately brought about by the influence of environment on gene expression forms the molecular basis of growth, development, differentiation and disease.

The following points are notable about gene regulation in eukaryotes:

• Various structural genes involved in a particular metabolic pathway are often not located adjacent to one another. They usually occur far apart on the same or even different chromosomes.

• The structural gene has coding and non-coding segments called exons and introns respectively.

• Each structural gene appears to have its own promoter gene.

• There are sensor genes to pick up changes in the intracellular environment such as presence or lack of a substance (hormone, vitamin, chemical, pathogen, etc.).

• There are integrator genes to coordinate functioning of structural genes located in different parts of DNA.

• There are enhancer and silencer genes, which accelerate or retard transcription.

In eukaryotes, the gene regulation is also thought to depend on histone and nonhistone proteins associated with the DNA. The histones have a general, non-specific control that does not distinguish between individual genes. They, in their normal form, inhibit transcription. Modification of histones makes large blocks of genes available for RNA transcription by loosening the tightly packed inactive chromatin. The specific nonhistone proteins then determine which of the available genes are to be actually transcribed. For this, the nonhistone recognizes a specific DNA sequence and combines with the DNA or its histones to open up the chromatin sufficiently for the RNA polymerase enzymes to reach the DNA. The appearance of specific nonhistone regulator proteins at certain times might be controlled by factors, such as hormones or other chemical signals, acting on the cell.