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