CONCEPT OF GENE

CONCEPT OF GENE

Presented By:NUPUR GUPTAM.Sc Biotech I Sem

CONTENTSHistory Definition of GeneStructure of GenePseudoallelismCis-Trans TestComplementation TestT4 BacteriophageBenzer’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

YEAR SCIENTIST GENE CONCEPT

1866 G.J. MENDEL A UNIT FACTOR THAT CONTROLS SPECIFIC PHENOTYPIC TRAIT

1902 SIR A.E.GARROD ONE GENE –ONE METABOLIC BLOCK THEORY

1940 BEADLE & TATUM ONE GENE-ONE ENZYME THEORY

1957 U.M.INGRAM ONE GENE-ONE POLYPEPTIDE THEORY

1960s C.YANOFSKY & CO-WORKERS

GENE IS A UNIT OF RECOMBINATION

Early 1970s E.B.LEWIS COMPLEMENTATION 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 FEATURESo Determines the physical as well as

physiological characters.o Situated in the chromosome.o Occupies a specific position known as

Locus.o Arranged in single linear order.o Occur in functional states called Alleles.o Some have more than 2 alleles known

as Multiple Alleles.

o Some may undergo sudden change in expression called as Mutant Gene (Mutation).

o May be transferred to its homologous (Cross-over) or non-homologous counterpart (Translocation).

o Can duplicate themselves very accurately (Replication).

o Synthesizes a particular Protein.o Determines 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

HETEROZYGOTESo Both the mutant

alleles are located in same chromosome, while Wild types are present in homologous chromosome.

TRANS HETEROZYGOTES

o One mutant allele is located in one chromosome, while other one in homologous chromosome.

o Produce wild type phenotype irrespective of whether the two mutant alleles are present in same gene or different ones.

o Produce 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.

CIS-TRANS TEST

COMPLEMENTATION TESTo Production of wild type phenotype in a

trans-heterozygote for two mutant alleles is known as Complementation.

o Such a study is known as Complementation Test.

o Results 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

o Cannot be applied to dominant or co-dominant mutant alleles.

o Applicable to non-polar mutations only. Mutants ideal for test are mainly

deletion, non-sense mutants etc.o Cis-acting genes cannot show

complementation.o Mutant 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:-o Contains chromosome

of about 200kb.o Lyses cell in 20-25

mins liberating 200-300 progeny particles.

o Produce a uniform confluent growth or lawn.

o Produce clear zones or plaques.

rII LOCUS OF T4 PHAGEo Some phage produce larger plaques

with clear margins, called as rapid lysis mutants, denoted by ‘r’.

o It has 3 distinct loci called rI, rII, and rIII.o Mutants in rII locus are recognized due

to their inability to grow in E.coli strain K12(λ).

o These are conditional lethals, a property exploited by BENZER.

EXPERIMENTo Benzer isolated over 3000 independent

mutants of rII locus. o He infected the E.coli strain K12(λ) cells

with mixture of 2 rII mutants – rII mutant I & II.

o Kept for about 90-120 mins to permit phage multiplication and cell lysis.

o Cells infected with both the mutants are selected.

o Placed on E.coli strain B lawns to detect presence of phage particles and to study the complementation between them.

COMPLEMENTATION TESTo Benzer noticed 2 arbitrary groups

within rII locus and named as ‘A’ & ‘B’.

o The mutants belonging to both A & B showed complementation, whereas those belonging to either A or B failed to complement each other.

COMPLEMENTATION TEST

RECOMBINATION MAPPING

o Frequency of = 2 x No.ofplaquesonK12(λ) recombination No. of plaques on B

o Highly efficient selection system for wild type phage particles.

o Upto 108 phage particles can be plated in single petri -plate.

o The 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:142-153

Singh B.D.(2009); “Genetics”; “Multiple Alleles”; Edition II; Kalyani Publishers; Page no:274-284

Gupta P.K.(2004-05); “Fine Structure of Gene at Genetic Level”; Edition III; Rastogi Publications; Page no:141-150

Jain H.K.; “Gene Structure & Concept”; Edition V; Page no:192-200

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:127-133

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