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Gene:

The term gene was coined by Johanssen (1909) for mendelian/actor or particulate elementary unit of inheritance which takes part in expression of a trait. Morgan (1911) called gene as the smallest segment of a chromosome which determines a trait as well as undergo crossing over. Beadle & Tatum (1948) proposed one gene-one enzyme hypothesis.

Yanofsky etal{ 1965) proposed one gene/cistron one polypeptide hypothesis. Berzer, distinguished three structural units of DNA-cistron (unit of function, determines synthesis of polypeptide), recon (unit of recombination or crossing over) and muton (unit of DNA that can undergo mutation). A muton can be as small as a nitrogen base. Cistron is commonly used as a synonym of gene.

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A gene can be structural or regulatory in nature. Regulatory gene is the one which controls the functioning of structural genes, e.g. regulator genes are repressor, promoter & operator types.

Structural gene in the one ‘which produces a biochemical molecule required for cellular machinery-t RNA, r RNA, m RNA. m RNA is involved in synthesis of structural proteins, enzymes, transport proteins, proteinaceous hormones, toxins, antigens etc. A structural gene normally codes for only a single polypeptide but a few produce POLYPROTEINS which are later broken to produce individual proteins.

There is colinearity in the arrangement of nitrogen bases (in triplets) of a structural gene and amino acid sequence of polypeptide synthesized by it. A change in even a single nitrogen base may change the amino acid specified by its triplet. Eucaryotes possess split genes where the coding bases are not continuous but are interrupted by non-coding sequences. The two are respectively called exons and introns.

Intron portions are also called spacer DNA or intervening sequences (IVS). Functional eucaryotic m RNA is formed by SPLICING-removal of intron portions and fusion of coding parts or exon portions. Split genes are also known from prokaryotes, e.g. thymidylate synthetase gene, ribonucleotide reductase gene of T4. A few eucaryotic genes are without introns.

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They are called exonic genes (= nonsplit genes), e.g. histone genes and interferon genes. Certain eucaryotic exonic gens are called processed genes. Processed genes lack both introns and promoters. They are, therefore, nonfunctional. Processed genes are believed to have been formed by reverse transcription. In some viruses, the genetic material has overlapping functions with a segment of genetic material participating in two or more genes (overlapping genes), e.g. D within E and B within A in ะค 174.

In a cell, hardly 10% of the genes are active, (90% being inactive). Different genes are active in different types of tissues/cells. Constitutive or house keeping genes are those genes which are always in action because their products are always in action because their products are always required for cellular activities, e.g. ATPase, enzymes of glycolysis. Nonconstitutive genes are those genes which can be switched on or off as per requirements. They are of two kinds, inducible (remain repressed but are switched on in the presence of an inducer chemical) and repressible (remain active till switched off by a chemical).

60-70% of functional genes are a single copy genes as they are represented only once in the whole genome. The genes having more than one copy in the same genome are called repeated genes, e.g. histone gene, t RNA genes, r RNA genes, actin genes. Multigenes or multiple gene family is a group of nearly similar products, e.g. globin gene family Pseudogenes are nonfunctional genes. They are unable to produce functional products due to inactivation of promotoer region, presence of intervening nonsense codons, insertions or deletions, e.g. several sn RNA genes.

Transposons or jumping genes are those DNA segments which can pass from one place to another in the genome. At their ends, transposons have similar or inverted repetitive DNA sequences.

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The sequences can be cleared by an enzyme ‘transposase’. Most common human transposon is Alu-family (so called as has a site for restriction endonuclease Alu-I). Transposons may take strong promoters to new sites or cause reshuffling of gene segments which lead to change in gene expression, e.g. protooncogene -> oncogene.

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