The term gene was introduced by Johanssen in 1909.Johanssen has defined gene as an elementary unit of inheritance which can be assigned to a particular trait.presently, a gene is defined as a unit of inheritance composed of a segment of DNA or chromosome situated at a specific locus ( gene locus ) which carries coded information associated with  specific function and can undergo crossing over as  well as mutation.

What is the structural gene 

From their work on Neurospore auxotrophs, Beadle and Tatum ( 1948,)proposed one _ gene one_ enzyme hypothesis. Yanofsky et al ( 1965) observed that certain enzymes could be composed of more than one polypeptide. They replaced one_ gene one_ enzyme hypothesis with one gene one_ polypeptide hypothesis. Further,  a gene cannot only synthesize a polypeptide but also ribosomal or transfer RNA. Gene is a  segment of DNA consisting of a stretch of base sequences that codes for one polypeptide, one transfer RNA ( tRNA) or one ribosomal RNA ( rRNA) molecule. Currently such a gene is called structural gene.The genetic system also contains  a number of regulatory genes that control the functioning of structure genes. 

   It was actually Benzer, who on the basis of his work coined the terms cistron ( jnit of function), recon ( unit of recombination) and muton ( unit of mutation).

Cistron was defined as a unit  the elements ( alleles) of which exhibit cis _ trans phenomena. Some consider the term cistron to  be almost equal to the term gene, representing lengths of DNA on chromosome which codes for a polypeptide chain or t_ RNA or r^ RNA. But precisely speaking, term cistron is not a synonym for gene.It is difficult to use the term cistron in practice, since it cannot be used unless a cistrans test has actually been conducted. 

Recon was defined as the smallest unit if recombination. It represents the length of DNA within to mutation. Since a mutation can take place by single base replacement,  a single nucleotide pair is the ultimate limit of muton. Thus one recon can have many mutons.

 From the above discussion, it is clear that cistron, recon and muton are the units, in the descending order of size and since gene is not synonymous to cistron, it will be at the top of this hierarchy. Thus a gene can consist of several  cistrons, a cistron of several recons and a recon if several mutons.

Cis_ trans test and implementation will not be discussed here as these are beyond the scope of this book.

What are the 12 types of Genes:

1): Constitutive  Genes or Non_ Regulated Genes:( House Keeping Genes). They are those genes, which are constantly expressing themselves in a cell because their products are required for the normal cellular activities e.g., genes for glycolysis, ATP_ ase. They do not require induction or repression.

2): Non_ constitutive Genes  or Regulated Genes: ( Luxury Genes or smart Genes). These genes  are not always expressing themselves  in a cell. They are  switched on or off according to the requirement of cellular activities, e.g., gene for nitrate reductase in plants, lactose system in Escherichia coli. Non_ constitutive genes are of further two types, inducible and repressible: 

(i) Inducible Genes: The genes are switched on in response to the presence of a chemical substance or inducer which is required for the functioning of the product of gene activity, e.g. nitrate for nitrate reductase.

(ii) Repressible Genes: They are those genes which continue to express themselves till a chemical ( often an end product) inhibits or represses their activity. Inhibition by an end product is known as feed back repression.

3): Multigenes: ( Multiiple Gene Family) .It is a group of similar or nearly similar genes for meeting requirement of time and tissues specific products, e.g. globin gene family ( Ė,ó,ß, ý on chromosome 11). æ and ó on chromosome 16).

4): Repeated Genes: The genes occur in multiple copies, e.g. histone genes, tRNA genes,  rRNA genes, actin genes. 

5): Single Copy Genes: The genes are present in single copies ( occasionally  2_ 3 times). They form 60_ 70% of the functional genes. 

6): Pseudogenes: They are genes, which have homology to functional genes but are unable to product functional products due to intervening nonsense codons, insertions, deletions and inactivation of promoter regions,  e.g., several of snRNA genes .

7): Processed Genes: They are Eukaryotic genes, which lack  introns. Processed genes have been formed probably due to reverse transcription or retroviruses. Processed genes are generally  nonfunctional as they lack promoters. 

8): Split Genes:Discovered in 1977 by many workers but credit is given to Sharp and Roberts ( 1977). Split genes are those genes,  which possess extra or nonessential regions interspersed with essential or coding parts. The nonessential parts are called introns, spacer DNA or intervening  sequences ( IVS). Essential or coding parts are called exons .Transcribed intronic regions are removed before RNA passes out into cytoplasm .Split genes are characteristic of euaryotes. However, certain Eukaryotic genes are completely exonic or non  split e.g., histone genes,  interferon genes.  Split genes have also been recorded in procaryotes, thymidylate synthesis gene and ribonucleotide reductase gene in T4. A gene that produces calcitonin in thyroid forms a neuropeptide in hypothanlamus removing an exon. Adenovirus has also a mechanism to produce 15_ 20 different proteins from a single transcriptional unit by differential splicing.

9): Transposons: ( Jumping Genes) .They are segments of DNA that can jump or move from one place in the genome  to another. Transposons were first discovered by Mc Clintock ( 1951) in case of Maize when she found that a segment of DNA moved into gen coding for pigmented kernels and produced light coloured kernels. Transposons possess repetitive  DNA, either similar or inverted, at their ends, some 5, 7 or 9_ nucleotide long. Enzyme  transposase ( encoded by transposon) separates the segment from its original  by cleaving  the repetitive sequences at its ends. Because transposons  make mutations when they move, the frequency of transposition must be low  enough that the chromosome does  not become disabled by mutations. For most transposons, there is little or no apparent sequence specificity for the target. Other transposons preferentially  insert at certain sequences called hotspots. 

By transposition, a segment of DNA can be transferred  from one molecule to another molecule that has no genetic  homology with either the transposable elements or with the donor DNA. In this sense it differs from recombination. 

   There are many types of transposons. In human beings the most common types of transposons belong to Alu family ( having a site for cutting by restriction enzyme  Alu I). The number of nucleotides per transposon is about 300 with about  300,000 copies in the genome . Passage of transposons from one place to another brings about reshuffling of nucleotide sequences  in genes. Reshuffling in introns often changes expression of genes  e.g., proto_ oncogenes oncogenes. Now genes may develop by exon shuffling. Other changes caused by transposons are mutations   through insertions, deletions and translocations. 

10): Overiapping Genes: In ø × 174 genes  B,E and K overlap other genes. 

11): Structural Genes: Structural genes are those genes,  which have encoded information for the synthesis of chemical substances required for cellular machinery. The chemical substances may be :

(a)  Polypeptides for the formation of structural proteins 

(b) Polypeptides for the synthesis of enzymes. 

(c) Transport proteins  like haemoglobin of erythrocytes,  lipid_ transporting proteins,  carrier proteins of cell membranes, etc..

(d)  Proteinaceous hormones. 

(e) Antibodies, antigens,  certain toxins, blood coagulation factors, etc.

(f) Non__ translated RNAs like  tRNAs, rRNA  .Broadly speaking, structural Genes either produce mRNAs for synthesis of polypeptides,  several proteins ( polycistronic mRNAs) or noncoding RNAs.

12): Regulatory  Genes: Regulatory genes do not transcribe RNAs and therefore, produce no chemicals. They are meant for controlling the functions of structural genes. The important regulatory genes are promoters,  terminators, operators and repressor producing  or reguiator genes. Repressor does not take part in cellular activity. Instead, it regulates the activity of other genes. Therefore,  repressor producing gene is of intermediate nature.

Gene Functions:

1): Genes  are unit of inheritance. 

2): They control the phenotype.

3): They control the metabolism.

4): Different linkages are produced due to crossing over.

5): Genes  undergo mutations and change their expression. 

6): New genes and consequently new traits develop due to reshuffling of exons and introns.

7): Genes change their expression due to position effect and transposons. 

8): Differentiation or formation of different types  of cells,  tissues  and organs  in various  parts  of the body  is controlled  by expression of certain  genes and nonexpression of others.

9): Development  of different stages in the life history is controlled by genes. 


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