The gene is a smallest unit of genetics. The term gene was coined by Johanson (1909). The gene is a linear sequence of nucleotides in DNA that determines the heredity of a specific character. The genes store and express the genetic informations that will contribute towards the phenotype, and will be passed on to the successive generations. Each gene occupies a special position in a chromosome which is termed as locus.

Structure of Gene:

Seymor Benzer (1955) subdivided gene into different units such as-

1. Cistron – It is the largest part of gene (DNA). It is the functional unit of gene. It can synthesize one polypeptide.
2. Recon – It is the small part of the gene which is a unit of recombination. It may be as small as a single nucleotide pair. It can undergo crossing over & recommbination.
3. Muton – It is smallest unit of the gene that can undergo mutation.
4. Operon – It is a group of genes having an operator gene, a structural gene and other genes in sequence which all function as a unit.
5. Replicon – It is the smallest part of DNA that replicates from a single origin of replication or any genetic element that can regulate and effect its own replication from initiation to completion.
6. Complan – It is a DNA segment that is responsible for complementation. Some enzymes are made up of two or more polypetide chains. Their active groups are complementary to each other.