ppt of Bacterial genetic system
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Transcript of ppt of Bacterial genetic system
BACTERIAL GENETIC SYSTEM: TRANSFORMATION, CONJUGATION, TRANSDUCTION
From,SaloneeM.Sc. Biotechnology(1st sem)
BACTERIAL GENOME
Fig. : Typical bacterial cell
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
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.
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.
Conjugative plasmid
F factorTransfer genetic material
Also known as fertility factor
R factorKnown as resistance factor
Provide resistance against drugs
Col factorKill other bacteria that do not have Col plasmid by secreting colicines
REPLICATION OF PLASMID:
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.
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.
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.
In bacteria recombination results from three types of gene transfer-
a) Transformation b) Conjugation
c) Transduction
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
Fig : Griffith experiment
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.
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
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.
Donor cell
Cell lysis DNA fragments released
Priplasmic space
Competence factor
nucleasesCell 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.
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
ARTIFICIAL TRANSFORMATION
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.
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.
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
F+- F- CONJUGATION:
Formation of Hfr (High frequency recombination):
Hfr and F- CONJUGATION:
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.
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.
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
Fig. Specialized transduction
a) Specialized transduction in a bacterial cell b) Fig. showing the conversion of lac-
bacterium into lac+
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