Fine Structure of Genes

• Gene is responsible for some phenotype.
• It must perform a function in each cell in which it is active.
• Since, genes can undergo mutation, a permanent change in the organization of the genetic material is also possible.
• Fine structure analysis of gene has resulted in the concept of ‘cistron’, a functional unit of gene; which is further sub-divisible into unit of recombination, ‘recon’ and the ‘muton’, smallest length of DNA undergoing mutations.
• Following concepts explains the fine structure of genes.

Concepts of Fine structure of genes

1. Classical concept:

• This model is also known as Bead theory.
• According to bead, gene is smallest structural unit arranged in chromosome and considered as individual by recombination and to be the smallest unit capable of mutation.
• Bead theory emphasize gene as:

a. The gene is viewed as a fundamental unit of structure, indivisible by crossing over which takes place between genes but never within them, so unit of recombination.

b. The gene is viewed as fundamental unit of mutation. Mutation result in change of total one allelic form into another i.e. there are no smaller components within it thus can change.

2. Modern Concept:

• Benzer in the 1950s defined the gene which consists of a series of nucleotides pair due to which gene is not unit of structure.
• Thus, Bead model concept was incorrect.
• According to Benzer, gene is not unit of structure, consists of several nucleotides pair capable of bearing mutation in each pair.
• Each pair can bear recombination, hence a gene consists of several sites of mutation and recombination.
• Seymour Benzer in 1955 introduced the term cistron, muton and recon.

Cistron (Unit of function)

• It is a segment of DNA having information of synthesis of particular protein or RNA.
• It is responsible for the expression of a trait.
• It can be several bp(base pairs) long.

Muton (Unit of mutation)

• It is a segment of DNA that can undergo mutation.
• It consist of few nucleotides (one to few bp long).

Recon (Unit of recombination)

• It is a segment of DNA that participates in recombination through crossing over during meiosis.
• It consists of a few to many base pairs.

Operon

• It is a combination of an operator gene, a structural gene or sequence of structural genes which act together as a unit.