Transcript What is a Gene? - GAURAV KUMAR PAL

CONCEPT OF
GENE
Presented By:
NUPUR GUPTA
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
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 FEATURES
o 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 nonhomologous 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.
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 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.
CIS-TRANS TEST
CIS
HETEROZYGOTES
o 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
TEST
o 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 codominant 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 200300 progeny particles.
o Produce a uniform
confluent growth or
lawn.
o Produce clear zones or
plaques.
rII LOCUS OF T4 PHAGE
o 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.
EXPERIMENT
o 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
TEST
o 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
 www.wikipedia.com
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