GENE-RECON,CISTRON,MUTON,EXON AND INTRONE

The factors of Mendel were called genes by Johansen 1909, who did not
know their exact nature and structure.
Gene Concept
Sutton introduced the gene concept which was elaborated by the studies of Morgan, Bridges and Miller.
The important features of the gene concept are:
i. Genes are transmitted from parents to offsprings and are responsible for the physical and physiological characteristics of the organism which are present inside the nucleus of the cell.
ii. The genes are present on the chromosome.
iii. Since the number of genes far exceeds the number of chromosomes, several genes are located on each chromosome. In man about 40,000 genes are present in 23 pairs of chromosomes.
iv. The genes are present at a specific position on the chromosome called locus.
v. Genes are arranged on the chromosomes in a linear order like beads on a string.
vi. A single gene may have more than one functional state or form. These functional states are refered to as alleles.
vii. The alleles may be dominant or recessive but sometimes co-dominance or incomplete dominance may be seen.
ix. Genes may undergo sudden heritable changes called mutations, induced by chemical and physical factors.
x. Due to mutation a gene may come to possess more than two alternative states and these states of the gene are called multiple alleles.
xi. Genes undergo duplication by a phenomenon called replication.
xii. Genes are responsible for the production of proteins called enzymes by which they show their expression which brings about a change in the organism.
xiii. A gene is a particular DNA segment which contains the information to synthesize one polypeptide chain or one enzyme. The information is contained as a sequence of nucleotides which is called genetic code. The
sequence of three nucleotides that code for an aminoacid is called Codon.
Molecular structure of a gene
A gene, is made of DNA. The gene may be subdivided into different units according to Benzer such as Recon, Muton, Cistron and Operon.

Recon

It is that smallest portion of a gene which can undergo crossing over and recombination and may be as small as a single nuclecotide pair.

Muton

It is the smallest unit of a gene that can undergo mutation and can involve a pair of nucleotides.

Cistron

It is the functional unit which can synthesize one polypeptide.

Operon

It is a group of genes having an operator a structural gene and other genes in sequence which all function as a unit.

Exons and Introns

In Prokaryotes generally, the genes are continuous segments of DNA occurring collinearly without interruption. But in Eukaryotes, the genes on the DNA strand have coding regions called exons interrupted by non-coding DNA segments which do not carry genetic information called introns. This led to the concept of interrupted genes or discontinuous genes. Such genes while producing m-RNA will first form a primary transcript which will then cut off the introns to form the functional m-RNA and this is called splicing.