microbial
GENETICS
DR NAZIA KHAN
1. Define the terms gene, genome,
genotype and phenotype with
clinical examples
2. Describe the prokaryotic
genome
3. Describe the various process of
gene transfer from bacteria to
other with clinical examples
4. Describe plasmids and their
clinical importnce
GENETICS
• The science of genetics defines and analyzes
heredity, or constancy and change in the vast
array of physiologic functions that form the
properties of organisms.
TERMINOLOGY IN GENETICS
• GENE-segment of DNA that carries
information for a specific
biochemical or physiologic
properties.
It is the Unit of heredity
• LOCUS-Large number of genes
constitute locus
• GENOME-All genes taken together
within an organism comprise that
organism’s genome. The size of gene
and an entire genome is usually
expressed in the number of base
pairs(bp) present.
Eg; kilobases ,megabases.
GENOTYPE
a)Alteration in the sequence of
DNA within a gene or in the
organization of genes
b)Not Influenced by
environmental changes
c)Are stable
d)Heritable
Genotypic variations are due
to mutations or by other
methods of genetic transfer
.
PHENOTYPE
a)Collective structural and
physiologic properties of a
cell or an organism
b)Influenced by environmental
changes
c)Are temporary
d)Not heritable
Eg of phenotypic variation:
typhoid bacilli
Prokaryotic genome
The genome is organized into discrete
elements known as chromosomes.
• The set of genes in a given
chromosome is arranged in a linear
fashion, but the no. of genes per
chromosome is variable.
• Bacterial cell contains a
single(haploid)chromosome.
Bacterial chromosome:
• ds,closed circular, naked
macromolecule folded and twisted
i:e super coiled.
• A few bacteria (eg, Brucella
species)have genomes consisting of
two circular DNA molecules
• With few exceptions, bacterial genes
are haploid
NONCHROMOSOMAL ELEMENTS OF PROKARYOTIC GENOME
PLASMIDS
• Small extra chromosomal piece of genetic
material that can replicate autonomously
and can maintain in cytoplasm of bacterium
for many generations.
• Not essential for normal life
• Confers-drug
resistance,
bacteriocin
production, toxigenecity
• Not as stable as chromosome and may be
lost during replication
• Important vectors for genetic engineering
• TYPES:CONJUGATIVE & NON CONJUGATIVE
TRANSPOSABLE ELEMENTS
• These are pieces of DNA that move from
one genetic element to another, from
plasmid to chromosome or vice versa.
• Like plasmids they do not exist as free
entities but they must either be
incorporated into a plasmid or
chromosome.
• Also known as jumping genes
• TYPES:INSERTION SEQUENCE
TRANSPOSONS
EPISOME: If plasmid integrates into host cell genome.
F-PLASMID: Confers maleness to host cell
GENE EXCHANGE BETWEEN BACTERIA
• The DNA composition of organisms can be remarkably fluid.
• Three broad mechanisms mediate efficient movement of DNA between cells—
1. Conjugation: only one strand of DNA is transferred .The recipient completes the
structure of ds DNA by synthesizing the strand that complements the strand
acquired from the donor
2. Transduction: donor DNA is carried in a phage coat and is transferred into the
recipient by the mechanism used for phage infection
3. Transformation. the direct uptake of donor DNA by the recipient cell, may be
natural or forced.
• Forced transformation is induced in the laboratory, after treatment with high salt
and temperature shock, many bacteria are rendered competent for the
assimilation of extracellular plasmids.
• The capacity to force bacteria to incorporate extracellular plasmids by
transformation is fundamental to genetic engineering.
Conjugation
 Through Plasmids
 involves cell to cell contact, and requires
mobilization of the donor(male) bacterium’s
chromosome to recipient by mating or
contact.
 Donor has F plasmid(F⁺ ), recipient lacks
it(F⁻ )
 F+ cells grow special tubes called “sex pilli”
from their bodies
 F plasmid/sex factor/ fertility factor is
conjugative plasmid which encodes for sex
pilus
 Transfer of DNA by conjugation is very
common among GRAM NEGATIVE
BACTERIA, but is rare in gram positive
bacteria.
Hfr CONJUGATION
 free plasmid= F plasmid can
only transfer itself. This isn’t
all that is useful for genetics.
 sometimes F plasmid can
become incorporated into the
bacterial chromosome, by a
crossover between the F
plasmid and the chromosome.
The resulting bacterial cell is
called an “Hfr”, which stands
for “High frequency of
recombination”.
 Hfr bacteria conjugate just like
F+ do, but they drag a copy of
the entire chromosome into
the F- cell.
.
F’(F PRIME) Hfr ↔ F+
 When F factor reverts from integrated to free
state, a piece of the bacterial chromosome
can become incorporated into the F plasmid.
This is called an F’ (F-prime) plasmid.
 Conjugation with an F’ (or a regular F
plasmid) is much faster and more efficient
than with an Hfr, because only a very small
piece of DNA is transferred.
 Since the F’ carries a bacterial gene, this
allele can be rapidly moved into a large
number of other strains.
 When a F’ cell mates with a recipient cell, it
also transfers host genes which are
incorporated in F plasmid along with some F
plasmid DNA.
 This process of transfer of host genes through
F plasmid resembles transduction and is
known as SEXDUCTION.
.
RESISTANCE PLASMID/R PLASMID/R FACTOR:
 It consists of two parts: (R factor=RTF + r determinant).
- RESISTANCE TRANSFER FACTOR(RTF):responsible for conjugal transfer.
-RESISTANCE DETERMINANTS (r) :codes for resistance against various drugs.
 Responsible for MDR bacteria
 Resistance is plasmid mediated , transferable by conjugation
 This is k/a Transferable or infectious drug resistance( e.g E.coli, Kleb, Proteus,
Vibrio)
 Apart from drug resistance , enterotoxin and hemolysin production in some
E.coli are shown to be transmitted by RTF
TRANSDUCTION:
 most common mechanism for gene exchange
 Mediated by BACTERIOPHAGES
 In their life cycle these viruses integrate their
DNA into the bacterial cell’s chromosome.
 When the production of viral products is
completed, viral DNA is excised from the
bacterial chromosome, and packaged within
protein coats.
 The viruses are then released when the infected
bacterial cell lyses.
 In transduction, the virus not only packages its
own DNA but may also package a portion of the
donor bacterium’s DNA.
 Role of transduction: plasmid determining
penicillin resistance in staphylococci is
transferred from cell to cell by transduction
 Method of genetic engineering in the t/t of
some inborn metabolic defects.
.
TRANSFORMATION:
 It involves recipient cell uptake of free DNA
released into environment when another
bacterial cell(i:e donor) dies and undergoes
lysis.
 This DNA, which had constituted the dead
cell’s genome, exists as fragments in the
environment.
 Transformation may be natural or artificial.
 Those bacteria which are able to take up this
free DNA, i:e are able to undergo
transformation are said to be COMPETENT.
 commonly associated with the members of
the genera Haemophilus, streptococccus,
and Neisseria
 not a common phenomenon in bacteria
 artificial transformation: is carried out in the
laboratory by a variety of techniques, like
treatment of the cells with CALCIUN
CHLORIDE, which renders their membranes
more permeable to DNA
 Transformation plays a major role in the
development of antibiotic resistance
.
Genetic mechanism of drug resistance
• Either by- mutation
- methods of gene transfer
• MDR imp in TB
• Initially due to an Antibiotic bacteria die, but soon
resistant mutants appear & multiply unchecked.
• use of 2 or more drugs
• Resistant to more than one drug at a time is rare
.
Genetic Engineering:
 The deliberate modification of an organism’s genetic
information by directly changing its nucleic acid genome is
called genetic engineering and is accomplished by a
collection of methods known an RECOMBINANT DNA
TECHNOLOGY.
 isolate the genes coding for any desired protein from mo’s
or from cells of higher forms(man), & to insert them into
suitable mo’s in such a way that it can be expressed in the
formation of specific(desired) proteins
 The enzymes which recognizes and cleave specific sequences
are known as RESTRICTION ENZYMES or RESTRICTION
ENDONUCLEASES( e.g Eco R1).
 They cleave ds DNA at specific oligonucleotide sequences
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MEDICAL APPLICATIONS.OF GENETIC ENGINEERING:
Production of medically useful proteins such as
somatostatin, insulin,human growth hormone and
some interferones is of great practical importance.
Interleukin-2 and blood-clotting factor VIII have
recently been cloned.
An interesting development is the use of transgenic
corn and soyabean plants to produce monoclonal
antibodies for medical use.
Synthetic vaccines(for malaria and rabies) are also
being tried.
DNA probes can be used for diagnosis of infections
A type of genetic surgery known as SOMATIC CELL
GENE THERAPY may be possible.