[SagarikaGhosh]

There are around number of 22000 protein coding genes in humans. These gens contain information that is extremely critical towards development of an organism and carrying forward a species. Looking at the number of genes one would safely assume that it is not possible for all these genes to be working at the same time. Each gene has a specific function, a specific protein that it produces and therefore specific time duration for functioning. Like in a school full of hundreds of children, a teacher is required to ensure that the students are studying properly; a proper expression regulation mechanism is also needed for genes.

Gene expression regulation is important because it decides which genes will be turned on in which cell and when. These genes when turned on are responsible for proper functioning of a cell. Gene expression regulation is mediated by regulatory elements known as cis regulatory elements and trans regulatory elements.

Cis regulatory elements are DNA sequences that are involved in regulation of gene expression by acting on DNA sequences that are present on the same side as them.

Trans regulatory elements aren’t DNA sequences, but can move through the cytoplasm via diffusion and act on any DNA sequence in the cell. trans regulatory elements are usually associated with DNA sequences coding for proteins.

Cis regulatory elements may be present upstream or downstream of the target sequence and can even be found in the transcription unit. A few examples of cis regulatory elements or cis acting genes are the core promoter in transcription unit, proximal elements, enhancers which bind to activators to ensure maximum efficiency of transcription. Unlike promoters which are present near the target sequence, enhancers are located far away. They act by causing looping of the DNA sequence bringing the transcription factors together.

Another example of a cis gene is the operator gene in lac operon. It when bound by the repressor molecule is unable to initiate transcription. However, when the inducer (lactose) binds to the repressor molecule inactivating it, normal transcription can occur.