Genetics

1. BACTERIAL GENETIC
SYSTEM: TRANSFORMATION,
CONJUGATION,
TRANSDUCTION
From,
Salonee
M.Sc. Biotechnology(1st
sem)

2. BACTERIAL GENOME
Fig. : Typical bacterial cell

3.  Circular, double stranded.
 The nucleoid is
 The region in a prokaryotic cell
that contains the genome.
 It lack a cell membrane and
contains dsDNA fibrils
 Basic histone proteins absent.
 low molecular weight polyamines
and magnesium ions present that
perform the function similar to the
histones.
 Bacterial genome contains one
chromosomal DNA and many
plasmids.
Fig. : Bacterial chromosome and
plasmid

4. BACTERIAL PLASMID:
The term plasmid was coined by J. Lederberg.
‘Plasmids are replicons that are extracellular genetic elements
in bacteria (also present in some yeast and fungi)’.
 These are
 self replicating
 contain small no of genes
 can incorporate themselves into the bacterial chromosome.

5. Classification of Plasmid:
a) Conjugative plasmids:
 Large plasmids whose replication is stringently controlled.
 Code for functions that promote transfer of the plasmid
from the donor to the recipient bacterium.
 Have genes for the synthesis of pili that help in the process
of conjugation. Eg- F factor of E.coli.
b) Non conjugative / cryptic plasmid:
Do not have any function.

6. Conjugative plasmid
F factor
•Transfer genetic
material
•Also known as fertility
factor
R factor
•Known as
resistance factor
•Provide resistance
against drugs
Col factor
•Kill other bacteria
that do not have Col
plasmid by secreting
colicines

7. REPLICATION OF PLASMID:

8. DELBRUCK and LURIA FLUCTUATION TEST:
 MAX DELBRUCK and SALVADOR
LURIA in 1943 proved that bacteria
has stable hereditary genetic
mechanism by FLUCTUATION
TEST
18 hr old bacterial culture
.5 ml plated on plates having phages
Some of the bacterial cells survived
and they and their descendents
were resistant to the phage.

9. OBSERVATION
 Great fluctuations found in the no. of phage resistant bacterial
cells
 Very less no. of cells were found to be survived in phages.
CONCLUSION:
 Mutation has occurred in the bacterial cell before its contact
with the phage and so its descendents were also found to be
resistant to the phage.
 It was not the result of contact with the phage since the
frequency was not very high.
 It was a result of mutation.

10. RECOMBINATION IN BACTERIA
 Genetic recombination refers to the exchange of the genes
between two DNA molecules to form new recombinations of
genes on a chromosome.
 Leads to genetic diversity.
 Much beneficial than mutation as
 Do not destroy genes function
 Bring together new combination of genes that lead to
valuable functions.

11. In bacteria recombination results from three types of
gene transfer-
a) Transformation
b) Conjugation
c) Transduction

12. a) TRANSFORMATION:
 Transfer of cell free or naked DNA from one cell to another in
solution.
 In 1928, Frederick Griffith found that one of the pathogenic
pneumococci now called streptococcus pneumoniae could be
mysteriously transformed into another form.
 Transformation may be
Natural Artificial

13. Fig : Griffith experiment

14. Griffith’s experiment:
 Griffith called the genetic information which could be
passed from one bacterium to another the transforming
principle.
 After 16 yrs., i.e, in 1944 Oswald Avery, Collin
MacLeod and Maclyn revisited griffiths experiment and
proved that the transforming principle was DNA and not
protein or carbohydrate.

15. Natural transformation
 It has been observed in both
gram positive
(streptococcus pneumoniae,
Bacillus subtilis) as well as
gram negative
(Haemophilus influenza)
a b
c
Fig. : (a) Streptococcus pneumoniae, (b)
Haemophilus influenza, (c) Bacillus subtilis

16. Mechanism of DNA transfer in bacteria by
transformation:
dsDNA attaches to membrane bound dsDNA binding protein
One of the two strands of the transforming DNA passes into the cell while the
other strand is degraded by a nuclease (deoxyribonuclease)
Single stranded exogenotes are unstable and will usually be degraded unless
they are integrated into the endogenote. By the process of homologous
recombination the transforming DNA integrates the bacterial chromosome.

17. Donor cell
Cell lysis DNA fragments released
Priplasmic space
Competence factor
nucleases
Cell membrane
Bacterial chromosome
Competence factor which is
highly positively charged
attract the negatively charged
donor DNA.
Single strand of the DNA is
prevented by the attack of
nucleases by specific
proteins.
Nuclease degrade one
strand of the exogenote
and allow only one strand
to pass through the cell
membrane. This reaction is
ATP dependent.

18. Rec A protein mediated
ss invasion
Degradation of
displaced strand
Bacterial DNA with
integrated new fragment
Division of bacteria
Progeny cells
with the
transformed
DNA
Fig. : Integration of single stranded DNA into the chromosome by
recombination

19. ARTIFICIAL TRANSFORMATION

20. Factors affecting transformation:
 Molecular size of DNA, molecular weight of DNA. DNA with
3 00 000 to 8 million daltons have shown successful
transformation.
 Transformation increases with increase in DNA concentration.
 It occur in the cells in late logarithmic phase of growth.

21. Significance of transformation:
 By this process a non virulent bacteria can be transformed into
virulent form.
 Transformation is usually used in laboratories for mapping of
chromosome of the bacteria.
 The frequency of transformation of two genes at the same time
is an indication of the distance between these genes on the
chromosome.

22. CONJUGATION:
 In conjugation there is transfer of the genome from one
bacterial cell to the another via pili (the cytoplasmic
bridge)
 Donor cell have F plasmid, i.e, are F+
 Recipient cell are F-
Fig. Pilus formation

23. F+
- F-
CONJUGATION:

24. Formation of Hfr (High frequency
recombination):

25. Hfr and F-
CONJUGATION:

26.  Most recombinants from mating
between Hfr and F-
cells fail to
inherit the entire set of F plasmid
genes and are phenotypically F-
.
 The integrated F factor can leave
the chromosome to form the F’
plasmid.

27.  This conjugation can be used to
map the relative positions of
genes in the bacterial
chromosome.
 One such exp was conducted by
E. WOLLMAN and F. JACOB
called the interrupted exp.

28. TRANSDUCTION
 It is the process of transferring bacterial DNA from one cell
to another by bacteriophage.
 Bacteriophage or phage are bacterial viruses.
 It is of two types
 Generalized transduction
 Specialized transduction
Fig. Generalized transduction

29. Fig. Specialized transduction
a) Specialized transduction in a bacterial cell b) Fig. showing the conversion of lac-
bacterium
into lac+

30. REFERENCES
 BOOK REFERENCES
 Life sciences: Fundamentals and practice by Pranav Kumar and Usha
Mina
 Genetics: By P. K Gupta
 A text of microbiology: By Pelzar
● WEB REFERENCES
 www.microbeonline.com
 www.studyblue.com
 www.biologicalexceptions.blogspot.com