Presentation on theme: "CONCEPT OF GENE Presented By: NUPUR GUPTA M.Sc Biotech I Sem."— Presentation transcript:
CONCEPT OF GENE Presented By: NUPUR GUPTA M.Sc Biotech I Sem
CONTENTS History Definition of Gene Structure of Gene Pseudoallelism Cis-Trans Test Complementation Test T4 Bacteriophage Benzer’s Experiment on rII Locus
HISTORY Term GENE was introduced by JOHANSSEN in 1909 based on Mendelian Factors. Gene Concept was given by SUTTON. Studied & Elaborated by MORGAN, BRIDGES, and MULLER.
SUMMARY OF EVOLUTION OF GENE CONCEPT YEARSCIENTISTGENE CONCEPT 1866G.J. MENDELA UNIT FACTOR THAT CONTROLS SPECIFIC PHENOTYPIC TRAIT 1902SIR A.E.GARRODONE GENE –ONE METABOLIC BLOCK THEORY 1940BEADLE & TATUMONE GENE-ONE ENZYME THEORY 1957U.M.INGRAMONE GENE-ONE POLYPEPTIDE THEORY 1960sC.YANOFSKY & CO-WORKERS GENE IS A UNIT OF RECOMBINATION Early 1970sE.B.LEWISCOMPLEMENTATION TEST IN DROSOPHILA
CLASSICAL DEFINITION OF GENE Gene is the Unit of Function (one gene specifies one character), Recombination, and Mutation.
MORDERN DEFINITION OF GENE Gene is the Unit of Genetic Information, i.e., the sequence of DNA that specifies one polypeptide. Includes coding as well as non-coding regulatory sequences.
ESSENTIAL FEATURES oDetermines the physical as well as physiological characters. oSituated in the chromosome. oOccupies a specific position known as Locus. oArranged in single linear order. oOccur in functional states called Alleles. oSome have more than 2 alleles known as Multiple Alleles.
oSome may undergo sudden change in expression called as Mutant Gene (Mutation). oMay be transferred to its homologous (Cross-over) or non- homologous counterpart (Translocation). oCan duplicate themselves very accurately (Replication). oSynthesizes a particular Protein. oDetermines the sequence of amino acid in the polypeptide chain (The Genetic Code).
SOME TERMS RELATED TO GENE BENZA has coined new terms to denote the relationship between DNA molecule and genetic phenomenon.
RECON - It is the smallest unit of DNA capable of undergoing Crossing Over and Recombination. MUTON - It is the smallest unit of DNA which can undergo Mutation. CISTRON - It is the unit of Function. It is the Gene in real sense capable of synthesizing a Polypeptide chain of an Enzyme. COMPLON - It is the unit of Complementation.
ALLELES WITHIN GENE SHOWING RECOMBINATION AND MUTATION SITES A REGION SHOWING TWO CISTRONS
It is the phenomenon shown by Pseudoalleles. Term Pseudoalleles was given by MORGAN (1928) and LEWIS (1948). These are located almost at same place on linkage map, interpreted as closely linked and functionally related genes. Referred as any two or more mutations which are allelic (similar) in function but not in structure. Cluster is called as Pseudoallelic series or Complex Loci. PSEUDOALLELISM
CIS-TRANS TEST CIS HETEROZYGOTES oBoth the mutant alleles are located in same chromosome, while Wild types are present in homologous chromosome. TRANS HETEROZYGOTES oOne mutant allele is located in one chromosome, while other one in homologous chromosome.
oProduce wild type phenotype irrespective of whether the two mutant alleles are present in same gene or different ones. oProduce mutant phenotype if the two alleles are located in same gene and wild type phenotype if in two different genes. So, by comparing the phenotypes produced in cis and trans heterozygote, we can find if mutant alleles are present in same or two different genes.
COMPLEMENTATION TEST oProduction of wild type phenotype in a trans-heterozygote for two mutant alleles is known as Complementation. oSuch a study is known as Complementation Test. oResults are highly precise, reliable and permit an operational demarcation of the gene.
INTRAGENIC COMPLEMENTATION Complementation shown by mutant allele within the gene. Their active products are multimers of homologous polypeptides, which may be either a homomultimer or a heteromultimer. Can be inactive or partially active.
LIMITATIONS oCannot be applied to dominant or co- dominant mutant alleles. oApplicable to non-polar mutations only. Mutants ideal for test are mainly deletion, non-sense mutants etc. oCis-acting genes cannot show complementation. oMutant alleles located in same gene may show complementation.
STUDY ON rII LOCUS OF T4 BACTERIOPHAGE A Great Contribution to recombination mapping by SEYMOUR BENZER in 1962.
STRUCTURE OF T4 BACTERIOPHAGE
FEATURES:- oContains chromosome of about 200kb. oLyses cell in mins liberating progeny particles. oProduce a uniform confluent growth or lawn. oProduce clear zones or plaques.
rII LOCUS OF T4 PHAGE oSome phage produce larger plaques with clear margins, called as rapid lysis mutants, denoted by ‘r’. oIt has 3 distinct loci called rI, rII, and rIII. oMutants in rII locus are recognized due to their inability to grow in E.coli strain K12( λ). oThese are conditional lethals, a property exploited by BENZER.
EXPERIMENT oBenzer isolated over 3000 independent mutants of rII locus. oHe infected the E.coli strain K12( λ ) cells with mixture of 2 rII mutants – rII mutant I & II. oKept for about mins to permit phage multiplication and cell lysis. oCells infected with both the mutants are selected. oPlaced on E.coli strain B lawns to detect presence of phage particles and to study the complementation between them.
COMPLEMENTATION TEST oBenzer noticed 2 arbitrary groups within rII locus and named as ‘A’ & ‘B’. oThe mutants belonging to both A & B showed complementation, whereas those belonging to either A or B failed to complement each other.
RECOMBINATION MAPPING o Frequency of = 2 x No.ofplaquesonK12( λ) recombination No. of plaques on B oHighly efficient selection system for wild type phage particles. oUpto 10 8 phage particles can be plated in single petri -plate. oThe number of plaques on K12( λ) represents the number of wild type phage particles present in the lysate.
REFERENCES Gupta P.K.(2009); “Genetics”; “Recombination and Resolution of Gene”; Edition IV; Rastogi Publications; Page no: Singh B.D.(2009); “Genetics”; “Multiple Alleles”; Edition II; Kalyani Publishers; Page no: Gupta P.K.( ); “Fine Structure of Gene at Genetic Level”; Edition III; Rastogi Publications; Page no: Jain H.K.; “Gene Structure & Concept”; Edition V; Page no: Lewin Benjamin; “What is a Gene? A Genetic View”; Wiley Eastern Publications; Page no:3-20 Verma P.S. and Agarwal V.K.(2006); “Fine Structure of Gene”; S.Chand Publication; Page no: