Gene Structure

Genetic information passes from one generation to another by the DNA replication. To understand it better we should know how genetic information is organized. The basic unit of genetic information is a gene. Now, gene has been defined in many ways which include that initially geneticists considered it to be the entity responsible for conferring traits on the organism and the entity that could undergo recombination. Recombination involves exchange of DNA from one source (e.g. virus, bacterium) with that from another and is responsible for generating much of the genetic variability found in viruses and living organisms.

After the discovery and characterization of DNA, the gene was defined more precisely as a linear sequence of nucleotides with fixed start and end points.

Initially it was thought that one gene is specifically code for one enzyme, the one gene-one enzyme hypothesis. This was next modified to the one gene-one polypeptide hypothesis.

Historically, a segment of DNA that encodes a single polypeptide was termed a cistron; this term is still sometimes used. However, not all genes encode proteins; some code instead for rRNA and tRNA. It is now clear and known that some eukaryotic genes encode more than one protein. Thus a gene might be defined as a polynucleotide sequence that codes for a functional product (i.e. a polypeptide, tRNA or rRNA).

The nucleotide sequences of protein coding gene are distinct from RNA-coding genes and non coding regions because when transcribed, the resulting mRNA can be “red” in discrete sequences of sets of three nucleotides, each set being a codon. Each codon code for single amino acid.

Each codon is red from one way to produce a single product. So, code is not overlapping and there is a single starting point with one reading frame or way in which nucleotide are grouped into codon.

Genes of prokaryotic cells and viruses is quite different from the eukaryotic cells because of the arrangement in circular way. In prokaryotic cells where coding information is continuous. However, in eukaryotic cells coding information (exone) is separated by non-coding information / sequence of DNA (introns).