bacterial genetics

1. BACTERIAL GENETICS
Lectures of Dr. Oruba khalid

2. Key Words
• Genetics
• Bacterial genetics
• Mutation & its types
–
• Bacteriophage
• Mechanisms of gene
transfer
– Transformation
– Transduction
– Lysogenic conversion
– Conjugation

3. Bacterial Genetics
• Genetics is the study of heredity and variation.
The unit of heredity is gene, which is a segment of
DNA specifying for a particular polypeptide.
Introns - non coding sequences on a gene.
Exons - coding sequences on a gene translated into
gene products.
• Bacterial genetics is used as a model to understand DNA
replication, genetic characters, their changes & transfer
to next generations.

4. Nucleic Acids
• DNA ( deoxy ribonucleic acid ) : stores information
for protein synthesis.
• RNA ( ribonucleic acid ) : transcription & translation
of information for protein synthesis.
• Central Dogma : DNA RNA Protein

5. Structure Of DNA
• Proposed by Watson & Crick.
• Double helix model.
• Composed of 2 chains of polypeptides, each chain
has a backbone of deoxyribose sugar and phosphate
residues arranged alternately.
• 4 nitrogenous bases: Adenine (A) Purine
Guanine (G)
Thymine(T) Pyrimidine
Cytosine (C)

6. Double helical structure of
DNA by Watson & Crick

7. DNA
• DNA is copied by DNA polymerase
– In the 5  3 direction
– Initiated by an RNA primer
– Leading strand synthesized continuously
– Lagging strand synthesized discontinuously
– Okazaki fragments
– RNA primers are removed and Okazaki fragments
joined by a DNA polymerase and DNA ligase

8. DNA
Figure 8.6

9.  Replication of this DNA molecule always starts at a
certain point (the origin of replication) and it is “semi-
conservative” meaning that one strand in each of the
two resulting double strands is conserved .

10. DNA Replication:
-The identical duplication process of DNA is
termed semi-conservative because the double
strand of DNA is opened up during replication and
each strand serves as the matrix for synthesis of
a complementary strand. Thus each of the two
new double strands “conserves” one old strand.
-The doubling of each DNA molecule begins at a
given starting point called origin of replication.
This process continues throughout the entire
cycle.

12. DNA
• DNA replication is semiconservative
Figure 8.7

13. Translation
• mRNA is translated
in codons (3
nucleotides)
• Translation of mRNA
begins at the start
codon: AUG
• Translation ends at
a STOP codon: UAA,
UAG, UGA
Figure 8.2

14. The Genetic Code
Figure 8.9

15. Structure Of RNA
• Structurally similar to DNA, except for 2
major differences:
– ribose sugar
– uracil in place of thymine.
• 3 types of RNA
– m RNA (messenger RNA)
– t RNA ( transfer RNA )
– r RNA ( ribosomal RNA )

16. Genetic Information In Bacteria
Chromosome Carries properties like virulence,
pathogenicity & resistance
Plasmid Extrachromosomal genetic
material in the cytoplasm
Replicate independently
Bacteriophage Virus infecting bacteria

17. PLASMIDS
• Circular DNA molecules
• Important vectors in genetic engineering
• EPISOME
– Plasmid DNA integrated with chromosomal DNA.
• Types of plasmids
– R plasmid (drug resistance): RTF + r determinant
– F plasmid (maleness )

18. • Types of plasmids
1. Sex factor plasmids: the cell that possess this plasmid is called F+, male or the
donor cell, while the one that do not possess it is called; F-, or the recipient
cell.
2. R-plasmids (resistance plasmids) responsible for resistance to drug
3. Col- plasmids: responsible for production of bacteriocins.
4. Heavy metal ion resistant plasmids: responsible for resistance to heavy metal
ions that the bacteria may get exposed in the envirnment
5. Plasmids of catabolic activity: responsible for degredation of highly complex
compounds, such as: hydrocarbons
6. Virulence plasmids: reponsible for production of certain virulence factors such
as: toxin , hemolysin, adhesive factors,… etc
Genetic Engineering

19. Mechanisms Of Genetic Variations
• Mutation
• Transfer or exchange of genetic material
1. Transformation
2. Transduction
3. Conjugation
4. Lysogenic conversion
5. Transposition

20. • Genotype: is the gatalogue of gene arranged on the DNA molecule.
• Phenotype: the collection of characters as result of the expression of
these genes.
• Mutation: hereitiditary changes occur in the genotype which may or may
not lead to phenotypic change.

21. • The physical or the chemical agent that leads to mutation is
called mutagen, and the bacteria produced with such
genotypic change are called mutants.
• Mutation occur either spontaneously or by induction.
• Induced mutation occur as a result of the effect of one of the
followings:

22. • A-Physical: such as U.V. light, X-ray…etc.
• B-chemical:such as 5-bromouracil, nitrous
acid, hydroxylamine, nitrogen mustard,
• acridines, nitrosoguanidine… etc.
• C-Biological: such as transposons.

23. • Types of mutations:
1. Deletion mutation: this result in the loss of a piece of
DNA.
• A B C D E F G______________A B C D F G
•2. Inversion mutation:this result in recombining the
cut piece in revese order.
•A B C D E F G_____________A B C E D F G

24. • 3. Insertion mutation: a totally new base sequence is
synthesized and inserted in the DNA molecule.
• A B C D E F G_____________A B C K D E F G
• 4. Substitution mutation: a newly different base sequence is
synthesized instead of the lost one
• A B C D E F G______________A B C N E F G
• 5. Duplication mutation: an insertion of a base sequence
similar to a sequence already existed.
• A B C D E F G______________A B C D B E F G

25. Bacterial genetics
• Experiments by Nature of the genetic material
 Griffith (1928); working on S- forms and R-forms of
the strep. Pneumonia
 Avery (1944); mixing DNA extract of the S-forms
with and without Dnase, then mixing it with R-
forms
 Hershy and Chase (1952), used radioactive isotops
on bacteriophages (S25-for head protein ) and (P32-
for nucleic acids)

26. Gene transfer:
• Recombination ; is the reassortment of nucleotide sequences
within the DNA molecule
• Recombination may occur between
Donor Recepient
Chromosomal
DNA
Plasmid DNA
Viral DNA
Plasmid DNA
Chromosomal DNA
Plasmid DNA
Chromosomal DNA
Chromosomal DNA

27. Sometimes rearrangement occurs within the DNA
molecule itself without an external DNA
Any recombination or spontaneous rearrangement
leads to what is called genotypic change, this may or
may not leads to phenotypic change
The fate of the transferred DNA depends on:
 Its capability to be taken by the host cells
 Its stability within the host chromosome

28. • There are three mechanisms for gene transfer in
bacteria
Conjugation
Transformation Transduction

29. Transformation (Griffith, 1928)
Transfer of genetic information by free DNA. i.e. by direct
uptake of donor DNA by the recipient DNA.
Live noncapsulated (R) pneumococci + heat killed
capsulated (S) pneumococci
Injected into mice
Death of mice
• Live capsulated pneumococcus isolated from the blood
of mice.

31. 3. Transduction
It is the transfer of DNA from a donor to a receptor with
the help of transport bacteriophages.
Bacteriophages
Infection of another bacterium
Transfer of host bacterial DNA to the new bacterium
Acquisition of new characteristics coded by the donor DNA.

32. • Bacteriophage replication occur in 5 stages:
1. Adsorption: where the virus is adsorped on specific receptor on the bacterial
cell wall.
2. Penetration: the virus make a hole in the bacterial cell wall using its core and
its DNA is passed through this hollow core to inside the bacteria
3. Replication : the viral nucleic acid controls the bacterial cell activity and
mechanisms for its benefit and synthesize several numbers of the viral parts.
4. Maturation : during this stage, each head of a virus will be surround one part
of the viral DNA, then the other parts will be joined to each other forming
several numbers of viruses.
5. Release : finally the virus release a lysozyme that leads to cell lysis and the
release of the viruses, then each virus will infect other bacteria, and so on
until the lysis of the whole colony.

34. Bacteriophages
Definition:-
Bacteriophages are viruses that infect bacteria . They are
therefore obligate cell parasites. They possess only one type of
nucleic acid, either DNA or RNA, have no enzymatic systems
for energy supply and are unable to synthesize proteins on
their own.
Morphology:-
Similar to the viruses that infect animals and
vary widely in appearance.

35. Transduction
• Transfer of bacterial
genes via viruses
– Donor to recipient
– Virus: Bacteriophages
• Types
– Generalized
– Specialized
• Replication Cycle
– Lytic
– Lysogenic

36. Composition:
Phages are made up of protein and nucleic acid. The
proteins form the head, tail, and other morphological
elements, the function of which is to protect the phage
genome.
The nucleic acid in most phages is DNA, which occurs
as a double stranded DNA .

37. • Sometimes, the recombination of the viral DNA with the
chromosomal DNA may lead to a drastic change in the
character of the bacteria, such as becoming a powerful toxin
producer (e.g. Clostridium botulinum,, Corynebacterium
diphtheriae… etc).
• The bacteria are called Lysogenic cell, and the
process is called Lysogenic conversion

39. 2. Conjugation
It is the transfer of DNA from a donor to a receptor in a
conjugation process involving cell-to-cell contact.
Conjugation is made possible by two genetic elements:
the conjugative plasmids and the conjugative pilli .
Conjugation is seen frequently in Gram-negative rods
(Enterobacteriaceae), in which the phenomenon has been
most thoroughly researched, and enterococci

41. Transposon (Jumping Genes, Barbara McClintock)
DNA segment that can move
between chromosome & plasmids
Transposons are not self replicative, they depend on chromosomal or
plasmid DNA for replication
Insertion of transposon into a functional
gene would destroy the function of the
gene (internal mutagenic agents)
Plasmid
Chromosome
Transposon

42. • Genetic Engineering – a combination of
methods which allows to conduct artificial
recombination of DNA and produce chimerical
molecules, non-typical for nature

43. • Steps in
1. DNA from any desired source is cleaved into fragments by restriction
endonuclease.
2. The fragments are spliced into vector such as plasmid or viral genome.
3. By transformation, the vector is introduced into a host cell in which it can
replicate.
4. - After replication of the vector & amplification of the original DNA fragment,
vector is isolated.
5. The inserted fragment is cleaved back out & purified.
•see fig.

44. Recombinant DNA

45. • Application of of genetic engineering
(molecular cloning):-
a. Gene structure mapping function e.g. globin.
b. Biosynthesis e.g. insulin, GH (growth hormone), interferon.
c. Control of genetic disease.
d. Using PCR in detection & identification of specific organism e.g. detection of HIV
by PCR.
e. Gene therapy.
•
• NOTE: the field of genetic engineering (molecular cloning) involves the
introduction of new genes into the cells.

46. • References:
• 1- Jawetz, Melnick, & Adelberg’s.( 2013).
Medical Microbiology (Twenty-Sixth Edition).
• 2- Kenneth Todar. (2008).Todar’s Online
Textbook of Bacteriology ,University of
Wisconsin.
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