• A gene is a molecular hereditary unit of all living
Gene carries all the information to build and maintain the
cell and pass genetic traits to the generations.
• 1940s, George Beadle and Edward Tatum
• Gene act by the production of the enzymes
and each gene is responsible for production of
the single enzyme that in turn effect a single
step in metabolic pathway.
• Diagrammatic representation of production
• This model, which is known as one-gene-oneenzyme hypothesis, provides the first
evidence of function of the gene.
• some genes were responsible for the
functions of the enzymes and each gene
apparently control one specific enzyme.
• Several elements of secondary structure
combine to produce a pattern, or motif, that is
found in numerous other proteins.
RELATIONSHIP BETWEEN GENE MUTATION AND
• To alter the protein function only the change of
Ingram, showed this 1st time in 1957 when he
study the globular protein called hemoglobin
molecule which transport oxygen in cells.
Hemoglobin consists of 4 polypeptide chains :
• Two identical alpha chains each containing 141
• Two identical beta chains each containing 146
RELATIONSHIP BETWEEN GENE
MUTATION AND ALTERED PROTEINS
• Structure: gene is indivisible by crossing over.
Crossing over always occurs between the genes
but never within them.
• Function: gene is the fundamental unit of
function. Parts of gene cannot function.
• Change: gene is also treated as a fundamental
unit of change or mutation. It changes from one
allelic form to another. There are no smaller
components within it that can be changed.
• Seymour Benzer in 1950s showed that bead
theory was not correct.
• Benzer was able to use genetic system in
which extremely small level of recombination
could be detected.
• The smallest units of mutation and
recombination are now known to be
correlated with single nucleotide pairs.
Fine structure analysis of gene
• Life cycle of bacteriophage
• Plaque morphology and rII system of phageT4.
• The concept of selection in genetic crosses
• Deletion mapping
• Destruction of bead theory
• T4 are viruses that infect the bacterium E. coli. The
infection ends with destruction (lysis) of the bacterial cell.
• They have been enormously useful in genetic studies
a. Viruses of two (or more) different genotypes can
simultaneously infect a single bacterium.
b. The DNA molecules of one of the infecting viruses can
recombine with that of another forming recombinant
c. The huge number of viruses released from a huge number
of bacterial hosts enables even rare recombination events
to be detected.
• T4 can be used to
a. detect mutations within a single gene
b. speed up the process of mapping these point
mutations by the use of deletion mutants.
Selection in genetic cross
• The procedure was to infect strain B in liquid
culture with two mutants to be tested
• Benzer eventually found some 2000 different
mutations in the rII gene. The recombination frequency
between some pairs of these was as low as 0.02.The T4
genome has 160,000 base pairs of DNA extending over
~1,600 centimorgans (cM).
• So 1 cM ≅ 100 base pairs
• So 0.02 cM represents a pair of adjacent nucleotides.
• From these data, Benzer concluded that the
– smallest unit of mutation and
– the smallest unit of recombination
• was a single base pair of DNA.
Mapping Point Mutations Within A
• Benzer was able to speed up the mapping
process by taking advantage of the discovery that
some of his mutants did not have point mutations
but deletions instead. In contrast to the
properties of T4 viruses with point mutations, T4
viruses with deletions in rII showed
a. no recombination with other rII mutants or any
other genes for that matter;
b. never back-mutated.
• Deletions can be mapped by the same procedure used for
point mutations. Simply cross pairs of deletion mutants and
see if they produce progeny that can grow on E. coli strain K.
• Here is a hypothetical example. Each of 6 strains of deletion
mutants are crossed with each of the others.
1 and 3 do not overlap
must shift 4 away from 2
6 must extend under 3
right-hand end of 4 must be
removed from over 6
left-hand end of 6 must not
overlap 5 but must continue
to overlap 2.
∴ shorten right-hand end of 5
• From the results, one can draw a map showing
the order and relative size of the deletions.
• With such a deletion map, one can now
quickly map the location of point mutations
• In this test, E. coli strain K growing in liquid
culture, was co-infected with two different rII mutants.
• each should be able to produce the gene product
missing in the other — complementation.
• Benzer coined the term cistron for these genetic units
REVISED BEAD THEORY
The nucleotide pair is the fundamental unit of
3. Benzer coined the term cistron for genetic
units of function