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Genetics Gene, Structure and Function

A gene can be defined as a segment of DNA that contains information coding for the amino acid sequence of a polypeptide chain and any other regulatory sequences which are essential for its proper expression. By proper expression, we mean for the production of a normal mRNA molecule, in to correct amount, in the correct place, and at the correct time during the cell cycle or during development. A molecular definition could be that a gene is a sequence of DNA that is essential for the production of a functional product(be it a polypeptide or an RNA molecule)

In humans(and in most eukaryotic genomes), most of the genes are discontinuous. This means that they are interrupted by one or more non-coding regions. Amount of genes in humans is approximately 25000 genes arranged on 23 chromosomes.

In cells, not all the genes are active which means that not all of the are expressed. This implies that cells are differentiated(they acquire specialised functions). Promoter regions regulate the expression of genes. These regions are present at the 5' end of each gene and they include sequences which are responsible for the initiation of transcription. Only DNA sequences that are presented by a promoter are transcribed. There are also some regulatory elements. These include enhancers, silencers and locus control regions. They are not necessarily adjacent to the gene. They may lie a significant distance away from the coding portion of a gene. Both promoters and regulatory elements can still be a site of mutation in genetic disease that can interfere with the normal expression of the gene. At the 3' end of the gene, there is an untranslated region which codes a signal for the addition of a sequence of adenosine residues.

Genes are arranged linearly along the chromosomes. Each gene had a specific, defined position called a gene locus. Most genes are spread out randomly along the chromosomes. However, some genes are organised into groups or clusters. There are 2 types:
 * 1) Operons
 * 2) Multigene Families