Biotechnology and Bioinformatics (1+1)

1.3.5. Gene Expression in Eukaryotes

Gene regulation in eukaryotes is more complex than in bacteria and other prokaryotes. Higher eukaryotes have several thousand genes. Eukaryotes are multicellular organisms which can also undergo differentiation. Thus, the cells in the undifferentiated stage not only grow and divide, but are also destined to become part of specialized tissues such as the liver, spleen or heart in an animal and the leaf, root, stem or flower in an angiosperm. Thus regulation of gene expression in the eukaryotic cell is very complex. Most multicellular organisms contain different types of cells that serve specialized functions. The cells of an animal’s heart, blood, skin, liver, and muscles all contain the same genes. But in order to carry out their specific functions within the body, each cell must produce different protein s and respond to changing environmental stimuli, such as

glucose levels in the blood or body temperature. Such specialization is possible only with sophisticated gene regulation.

The information on the eukaryotic gene for assembling a protein is not continuous, but split. However, when messenger RNA is formed from such genes, the unwanted RNA regions are removed and the regions coding for amino acids are joined together. This process is referred as splicing . Thus, bases in the messenger RNA and amino acids in proteins are collinear even in eukaryotic cells, although the genes are split. The regions of a gene, which become part of a mRNA and code for different regions of the protein, are referred to as exon s. The regions which do not form part of RNA processing before mRNA formation are referred to as intron s. In eukaryotes, genes involved in coding for the enzymes of a particular metabolic pathway need not to be linked. Sometimes they are present even on different chromosomes. However, such genes are regulated together just as in bacterial operon s. The basic processes of induction and repression is constantly regulated by the changing environment in the cell. Thus during growth and development, small molecules such as hormone s, vitamins, metal ions, chemicals and invading pathogens can induce or repress certain genes and this would result in the production or absence of certain proteins. This ultimately leads to the operation or non-operation of metabolic pathways leading to altered cell function. This is the underlying molecular basis of growth, development, differentiation and disease brought about by the influence of the environment on gene expression.

Eukaryotes use a variety of mechanisms to ensure that each cell uses the exact proteins it needs at any given moment. In one method, eukaryotic cells use DNA sequence s called enhancers to stimulate the transcription of genes located far away from the point on the chromosome where transcription occurs. If a specific protein binds to an enhancer site on the DNA, it causes the DNA to fold so that the enhancer site is brought closer to the site where transcription occurs. This action can activate or speed up transcription in the genes surrounding the enhancer site, thereby affecting the type and quantity of proteins the cell will produce. Enhancers often exert their effects on large groups of related genes, such as the genes that produce the set of proteins that form a muscle cell.

Gene regulation can also take place after transcription has occurred by interfering with the steps that modify mRNA before it leaves the nucleus to take part in translation. This process typically involves removing exons (segments that code for specific proteins) and introns. These sections of the mRNA can be modified in more than one way, enabling a cell to synthesize different proteins depending on its needs.