Prof. Dr. Sohan Raj Tater
Regulation of gene expression is as important as gene function. DNA codes for protein, which perform actions like making hair or carrying oxygen in our blood. Imagine if we grow hair in our mouths, teeth on our heads. In order to avoid such misshapes, all genes are regulated. Contained with in our DNA sequence are elements known as promoters and repressors that allow individual cells to control which genes are expressed. This is how the individual cells in our bodies know which genes to use. These are genes specific to each tissue (muscle, brain, lever etc.) in our bodies as well as "house keeping genes" which are present in all cells. Expressing genes in the appropriate tissue at the appropriate time is very important. Cancer, defined as any uncontrolled growth is ultimately a result of misregulation of gene expression.
Genes may be expressed in specific locations (e.g. tissue specific genes) at specific times (e.g. embryo-specific genes) or in response to environmental stimuli (e.g. light-responsive genes). Viruses are powerful Genetic Engineering tools because of their ability to target specific tissues and express specific genes in those tissues. One of the reasons that herpes virus is potentially useful for treating brain diseases because it infects brain cells and express specific genes in brain cells. The challenge is to disable the herpes causing genes while introducing functional beneficial genes.[30]
The evidence accumulated in genetics makes it virtually certain that not all the genes present in cell are active in directing the specific processes of protein synthesis. Gene action can be switched on or off in response to the cell's stage of development and external environment. In multi cellular organisms, moreover, each cell has a complete copy of organisms genetic instructions though different kinds of cells come to express different parts of genome. That is to say that a skin cell, a nerve cell, and a bone metrow cell from the same person all contain the same genes, the differences in structure and function among these cells result from the selective expression and repression of certain genes. [31]
Doctoral Thesis, JVBU