A short yet comprehensive presentation on bacterial genetics, an important microbiology topic for BDS 2nd, MBBS 2nd and MD/MS /MDS 1st . Made using CP Baveja's Textbook of Microbiology. Meant as an introduction and overview with stress on some key areas. Topics covered: Basic Principles, Synthesis of Protein, Extra Chromosomal Genetic Material, Bacterial Variation , Gene Transfer, Genetic Mechanisms of Drug Resistance, Genetic Engineering, DNA Probes, Polymerase Chain Reaction, Genetically Modified Organisms and Gene Therapy.

1. Bacterial Genetics
Dr Aaron Sarwal
MDS 1st Year (Cons)

2. Index
1. Basic Principles
2. Synthesis Of Polypeptides a. Transcription b. Translation
3. Extrachromosomal Genetic Elements
4. Bacterial Variation a. Phenotypic b. Genotypic
5. Gene Transfer a. Transformation b. Transduction c. Lysogenic
Conversion
d. Conjugation
6. Genetic Mechanisms Of Drug Resistance
7. Genetic engineering
8. DNA Probes
9. Polymerase Chain Reaction (PCR) a. Principle b. Procedure c. Application d. Types of PCR
10. Genetically Modified Organisms
11. Gene Therapy

3. Basic Principles
 DNA composed of two strands of complementary nucleotides wound
together in the form of a double helix. (Watson and Crick)
 Bacterial nucleus contains a circular chromosome of dsDNA about
1000um long when straightened.
 Structure of DNA:
 Backbone of de-oxyribose and phosphate groups.
 4 nitrogenous bases
 2 purines: Adenine (A), Guanine (G)
 2 pyrimidines: Thymine (T), Cytosine (C)
 Strands held together by hydrogen bonds between NB on opposite strands.
 Thus, a molecule of DNA has equal no of A as T and G as C. Also
A+T:G+C is constant for a species.
 During replication strands separate at one end and each stand acts as a
template.

4. Basic Principles
RNA is structurally similar to
DNA except:
Ribose sugar instead of
deoxyribose
Uracil replaces thymine
3 types of RNA
Messenger RNA (mRNA)
Ribosomal RNA (rRNA)
Transfer RNA (tRNA)

5. Basic Principles
TERM DEFINITION
GENE A segment of DNA that specifies for a particular
polypeptide is called a gene.
CODON Genetic information is stored in the DNA as a
code. Codon consists of a sequence of 3 NB ie
triplet code. One codon codes one amino acid.
One amino acid maybe coded for by more than
one codon. Eg CGU – arginine. But AGA, CGC,
CGG, CGA, AGG also arginine
NON-SENSE
CODON
UAA, UGA and UAG do not code for any amino
acid. Stop codons. Terminate synthesis of
Polypeptide.

6. Synthesis Of Polypeptides
Genetic information in DNA is transcribed on to the
RNA and then translated as the particular
polypeptide.
Several stretches of DNA don't appear to function
as codons, occurs between the coding sequences of
Gene. called as INTRONS.
Coded are called as EXONS
In transcription introns are excised form RNA before
translated by ribosomal proteins.

7. Synthesis Of Polypeptides
A. TRANSCRIPTION
• RNA Polymerase attaches
itself to the beginning of a
gene on the DNA and
synthesises m-RNA using
one of the strands in DNA as
a template.
• Since DNA acts as template
bases on m-RNA are
complementary to that DNA
B. TRANSLATION
• mRNA passes into
• mRNA +tRNA come
together on the surface of
the rRNA.
• Codon recognised by anti
codon of amino acid
tRNA.
• Ribosome moves along
mRNA until the whole
molecule has been
translated.

8. Extra Chromosomal Genetic Elements
Bacteria posses extra
chromosomal genetic DNA
called Plasmids (free
cytoplasmic state) or
Episomes (integrated state).
Not essential for survival of
bacteria
But makes the bacteria
resistant to antibiotics, and
helps them survive. Also
enables them to produce
toxins.

9. Plasmids
Plasmids are circular
DNA molecules present
in the cytoplasm of the
Bacteria
Capable of
Autonomous replication
Can transfer genes from
one bacterial cell to
other

10. One bacteria may have more than one Plasmid.
Plasmids seen in blue colour.

11. Plasmids
May encode genetic information for many properties:
o Resistance to Antibiotics
o Bacteriocins production
o Enterotoxin production
o Enhanced pathogenicity
o Reduced Sensitivity to mutagens
o Degrade complex organic molecules

12. Potentials of Plasmids
Transfers the sex and drug resistance with the help of restriction end nucleases

13. Bacterial Variation
Phenotype
is the physical
expression in a
environment. Change
according to
environment.
Genotype
Sum total of Gene
make up the genetic
apparatus of cell
established as
Genotype

14. Phenotypic Variation Genotypic Variation

15. Phenotypic Variation
Appearance differs in different situations.
Eg Typhoid bacilli flagellated normally but if
grown in Phenol agar don't grow flagella So
flagella are lost physical variation
Lactose fermentation in E. coli dependent on
Beta Galactosidase
 When lactose present - test is positive
 When lactose is absent - test turns negative

16. Genotypic Variation
Mutations
Genotypic by transfer of genes
Transformation
Transduction
(Lysogenic conversion)
Conjugation

17. Mutations
Mutation is a random, undirected, heritable variation
Caused by alteration in the nucleotide sequence at
some point of DNA which can occur due to
 Addition
 Deletion
 Substitution
(…of one or more bases)

18. Mutations
Bacteria undergo mutations
at 10-2 to 10 -10 per
bacterium per division.
Mutagenic agents
U V rays
Alkylating agents
Arcidine Dyes

19. Mutations
All genes are susceptible for mutations, but all mutations
are not expressed
Lethal mutations are harmful destroy the vital functions
Conditional Lethal mutant may survive under certain
conditions example is temperature sensitive (ts) mutant. Ts
mutant lives at 35°C (permissive temp) but dies at 39°C
(restrictive temp).

20. Mutations
1. Point mutations
• BP Substitution
• Frame Shift
2. Multisite mutations
• Gain
• Loss
• Duplication

21. Point Mutation: Base Pair Substitution
Transition
 Most frequently occurring
type of point mutation.
 Pyrimidine by pyrimidine
replacement
 Purine by purine
replacement
 AT replaced by GC
Transversion
 Pyrimidine by purine
replacement and vice versa
 CG changes to GC

22. Point Mutation: Frame Shift Mutation
Deletion Insertion

23. Multisite Mutations
 Large number of base pairs are altered in DNA. 4 types:
 Addition or Gain
 Deletion or Loss
 Duplication
 Inversion

24. Other Mutations
 Missense mutation - Triplet code is altered so as to specify an
amino acid different from that normally located at particular
position in the protein.
 Nonsense mutation - Deletion of nucleotide within a gene
may cause premature polypeptide chain termination by stop
codon.
 Suppressor Mutation - reversal of mutant phenotype by
another mutation at a point on the DNA strand distant from
that of original mutation.

25. GeneTransfer
Transformation
Transduction
Lysogenic conversion
Conjugation

26. The Three Bacterial Sexual Processes
Transformation: naked DNA is taken up from the
environment by bacterial cells.
Transduction: use of a Bacteriophages (bacterial
virus) to transfer DNA between cells.
Conjugation: direct transfer of DNA from one
bacterial cell to another.

27. The Three Bacterial Sexual Processes

28. Transformation

29. Transduction
Transduction is defined
as transfer of portion of
DNA from one bacteria to
another by
bacteriophages.
When the Phage particle
infects another bacteria
DNA transfer is effected
and the recipient cell
acquires new characters
coded by donor DNA.
Generalized involves any
segment of DNA
Restricted specific
bacteriophages transduce
only a particular genetic trait

30. Lysogenic Conversion
a. Virulent/Lytic Cycle
After a large number of
progeny are built up
inside the host, the
bacterium ruptures and
phages are released
b. Temperate/Non-lytic Cycle
 Characterized by integration of the
bacteriophage nucleic acid into the
host bacterium's genome
 The genetic material of the
bacteriophage, called a prophage, can
be transmitted to daughter cells at each
subsequent cell division, and at later
events (such as UV radiation or the
presence of certain chemicals) can
release it, causing proliferation of new
phages via the lytic cycle.

31. Lysogenicity creates new characters
Eg - Lysogenic conversion in Diphtheria bacilli
which acquires toxigenicity by lysogenization
with phage beta
Elimination of phage from toxigenic strain
renders it nontoxigenic

32. Conjugation
 A process by which a donor cell or male cell makes contact
with another cell, the recipient or female cell.
 DNA is directly transferred.
 Plasmids carry genetic information necessary for conjugation
to occur.
 Only cells that contain such plasmids can act as donor. Cells
lacking the corresponding plasmid act as recipient.
 Requires direct contact between donor and recipient

33. HFR (High Frequency Transfer) Cell and F`
Cell

34. Sexduction

35. Colicinogenic ( Col ) Factor
 Coli form Bacteria produce Colicins
 Colicins are lethal to other
Enterobacteriaceae
 Pyocins produce by Pseudomonas
 Diptherocins produced by
C.diptheria
 Plasmid transmits col factor leads at
the time of DNA replication during
conjugation.

36. Resistance Transfer Factor RTF
 Plasmids – helps to spread multiple drug resistance
 Discovered in 1959 Japan
 Infections caused due to Shigella spread resistance to
following Antibiotics
 Sulphonamides
 Streptomycin
 Chloramphenicol,
 Tetracycline
 Shigella + E.coli excreted in the stool are resistant to
several drugs in vivo and vitro
 Plasmid mediated –transmitted by Conjugation
 Episomes spread the resistance

37. Genetic Mechanisms of Drug Resistance
 Bacteria acquire drug resistance through several mechanisms
:
1. Mutations
2. Genetic transfer
Transformation,
Transduction
Conjugation
3. Biochemical Mechanisms
Decreasing permeability of drugs,
Attaining alternative pathways
Produce enzymes and inactivate drugs

38. Transposable Genetic Elements
 Structurally / Genetically – Discrete sequence of DNA – Move
around in a cut and paste manner between Chromosomal and
Extra chromosomal DNA molecules within cells.
 Called as Transposons -Jumping Genes
 Genetic transfer due to Transposition
 Small Transposons 1 – 2 Kb
 Not self replicating and depend on Plasmid or Chromosome
for replication.
 A chunk of DNA is added by Transposons

39. Transposons and R factor
 R forms may have evolved as a collection of Transposons
 Each carrying Genes that confers resistance to one or several
antibiotics
 Seen in Plasmids,
Microorganisms
Animals

40. Genetic Engineering
Genetic Engineering Was
Born from Genetic
Recombination
Genetic engineering
involves changing the
genetic material in an
organism to alter its traits
or products
A recombinant DNA
molecule contains DNA
fragments spliced
together from 2 or more
organisms

41. Modern Applications Of Genetic
Engineering
Pharmaceutical production
Insulin, interferon, hormones,
vaccines etc.
Genetically engineered plants
Animal gene alterations
Gene probes
DNA fingerprinting
The human genome initiative

42. DNA Probes
 These are radioactive labelled
copies of single stranded
DNA
 Contains 20 -25 nucleotides
 Helps in detection of
homologous DNA by
hybridization.
 Helps diagnosis of Infectious
Diseases
 Minute quantities of DNA can
be detected.

43. PCR: Polymerase Chain Reaction
 Rapid, automatic
amplification of specific DNA
sequences
 PCR consists of several cycles
of sequential DNA replication
where the products of first
cycle becomes the template
for the Next
 It makes available abundant
quantities of specific DNA
sequences starting

44. PCR: Applications
 Rapid analysis (one day)
 Versatile tool useful in infectious, genetic or neoplastic
diseases, in forsensic investigations and in the examination of
phylogenic relationships in evolution.
 It has been applied to clinical laboratory for diagnosis of
various infectious agents.
 A specific DNA sequence of a particular infectious agent is
amplified wsith the specific primers.

45. PCR: Types
 PCR
 Rt-PCR
 Multiplex PCR
 Real Time PCR

46. GMO: Genetically Modified Organisms
 The process of artificially introducing foreign DNA into
organisms is called transfection.
 The recombinant animals produced in this way are known as
transgenic or genetically modified organisms.
 Foreign DNA has been introduced into microbes, plants and
animals through recombinant DNA technology
 Transgenic organisms are available for various
biotechnological applications.

47. Gene Therapy
Ex vivo gene therapy
 Normal gene cloned in vectors
which are infectious but otherwise
relatively harmless eg adenoviruses.
 Tissues removed from the patient
are incubated with these genetically
modified viruses to transfect them
with the normal gene.
 The transfected cells are then
reintroduces into the patient by
transfusion.
In vivo gene therapy
 Cloning in vector is the same.
 The incubation step is absent.
 Virus/ naked DNA directly
introduced into the blood stream.

48. References
 CONTENT MATTER:
 A Textbook of Microbiology (4th Edition) Dr. CP Baweja
 A Textbook of Microbiology (for dental students) Dr DR Arora, Dr Brij Bala
 IMAGES:
 Google Image Search
 Wikipedia
 Reddit
 Video:
 Virtual Cell Company

49. “
”
THANK YOU!
Dr Aaron Sarwal
MDS 1st (Cons)