Transcript What is a Gene? - GAURAV KUMAR PAL
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 YEAR 1866 1902 1940 1957 1960s Early 1970s SCIENTIST G.J. MENDEL SIR A.E.GARROD
BEADLE & TATUM U.M.INGRAM
C.YANOFSKY & CO-WORKERS E.B.LEWIS
GENE CONCEPT A UNIT FACTOR THAT CONTROLS SPECIFIC PHENOTYPIC TRAIT ONE GENE –ONE METABOLIC BLOCK THEORY ONE GENE-ONE ENZYME THEORY ONE GENE-ONE POLYPEPTIDE THEORY GENE IS A UNIT OF RECOMBINATION COMPLEMENTATION TEST IN
DROSOPHILA
CLASSICAL DEFINITION OF GENE
• Gene is the Unit of Function specifies one character), Recombination Mutation .
(one gene , and
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
o o o o o o Determines the physical physiological characters .
as well as Situated in the chromosome .
Occupies a specific position known as Locus .
Arranged in single linear order .
Occur in functional states called Alleles .
Some have more than 2 alleles known as Multiple Alleles .
o o o o o Some may undergo sudden change in expression called as Mutant Gene ( Mutation ).
May be transferred to its homologous ( Cross-over ) or non homologous counterpart ( Translocation ).
Can duplicate themselves accurately ( Replication ).
very Synthesizes a particular Protein .
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 synthesizing a Polypeptide chain of an Enzyme.
Gene in real sense capable of COMPLON - It is the unit of Complementation .
A REGION SHOWING TWO CISTRONS ALLELES WITHIN GENE SHOWING RECOMBINATION AND MUTATION SITES
PSEUDOALLELISM
It is the phenomenon shown by Pseudoalleles .
Term Pseudoalleles was given by and LEWIS (1948).
MORGAN (1928) 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 Loci .
Pseudoallelic series or Complex
o
CIS-TRANS TEST
CIS HETEROZYGOTES Both the mutant alleles are located in same chromosome, while Wild types are present in homologous chromosome.
o TRANS HETEROZYGOTES 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
o o o
COMPLEMENTATION TEST
Production of wild type phenotype trans-heterozygote in a for two mutant alleles is known as Complementation .
Such a study is known as Complementation Test .
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 heteromultimer .
or a Can be inactive or partially active .
LIMITATIONS
o o o o Cannot be applied to dominant or co dominant mutant alleles.
Applicable to non-polar mutations only.
Mutants ideal for test are mainly deletion , non-sense mutants etc.
Cis-acting genes complementation.
cannot show Mutant alleles located in same gene complementation.
may show
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 o o Lyses cell in 20-25 mins liberating 200 300 progeny particles .
Produce a uniform confluent growth lawn .
or Produce clear zones plaques .
or
o o o o
rII LOCUS OF T4 PHAGE
Some phage produce larger plaques with clear margins, called as rapid lysis mutants , denoted by ‘ r ’.
It has rIII .
3 distinct loci called rI , rII , and Mutants in rII locus are recognized due to their inability to grow in E.coli strain K12( λ).
These are conditional lethals , a property exploited by BENZER .
o
EXPERIMENT
Benzer isolated over 3000 independent o o o o He infected the E.coli strain K12( λ ) cells with mixture of 2 rII mutants – rII mutant I & II.
Kept for about 90-120 mins to permit phage multiplication and cell lysis .
Cells infected with both the mutants are selected.
Placed on E.coli strain B lawns to detect the complementation between them.
o o
COMPLEMENTATION TEST
Benzer noticed
2 arbitrary groups
within rII locus and named as ‘
A
’ & ‘
B
’.
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 o o o Frequency of = 2 x No.ofplaquesonK12( λ) recombination No. of plaques on B Highly efficient selection system for wild type phage particles.
Upto 10 8 phage particles single petri -plate.
can be plated in The number of plaques on K12( present in the lysate .
λ) represents the number of wild type phage particles
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 www.wikipedia.com