Conjugation

Question 1

Conjugation

Answer 1

Donor bacterial cell transfers plasmid DNA to a recipient bacterial cell only when both the cells are in physical contact through a specialised pilus called a sex pilus.

Question 2

Transformation

Answer 2

A recipient bacterial cell takes up cell-free/naked DNA from the environment (DNA released during lysis of another bacterial cell).

Question 3

Transduction

Answer 3

the transfer of DNA from a donor bacterial cell to recipient bacterial cell that is mediated by a bacteriophage

Question 4

What is a plasmid

Answer 4

DNA molecules that exists separately from the chromosome and contain an origin of replication – replicate independently

Question 5

Features of plasmids

Answer 5

Vary in size (<2000 bp to > 100 000 bp)
Plasmids occur as families where members will have similar sequences
Exist either as circular ds DNA (common) or linear ds DNA (rare)
Specific copy number: Low copy (1 – 15) vs High copy (> 50)
Faithfully transmitted to daughter bacterial cells
Encode non-essential but beneficial products

Question 6

Types of plasmids

Answer 6

1. Fertility (F) plasmids can direct conjugation
2. Resistance (R) plasmids: genes conferring resistance to antibacterial agents.
3. Col plasmids: genes for colicins, bacteriocins in pneumococcal defense
4. Metabolic (Degradative) plasmids: genes for metabolism of unusual molecules
5. Virulence plasmids: confer pathogenicity on the host bacterium
6. Cryptic: no obvious phenotype

Question 7

Lederberg and Tatum’s experiment

Answer 7

Showed that genetic recombination can occur in bacteria by mixing two auxotrophic E. coli strains, which produced some offspring that could grow without supplements—evidence that DNA was exchanged.

Question 8

Bernard Davis’s experiment

Answer 8

Used a U-tube with a filter that prevented physical contact between the bacteria and showed that direct cell-to-cell contact (conjugation) is required for this genetic exchange.

Question 9

New and improved conjugation

Answer 9

Luca Cavalli-Sforza discovered a derivative of an F+ strain.
Crossing F- strains with this new F+ strain produced 1000 times as many recombinants as did a normal F+strain.
Cavalli-Sforza designated this derivative an “Hfr” strain to indicate a high frequency of recombination.

Question 10

Two methods of integration

Answer 10

Plasmids that can integrate into chromosomes are episomes.
Two methods of integration:
- Recombination between shared sequences on plasmid and on the chromosome.
- Recombination via insertion elements shared between chromosome and plasmid.
Bacteria with integrated plasmid that can transfer their chromosome are called Hfr strains.

Question 11

Instegration

Answer 11

The process begins with the association of plasmid and bacterial insertion sequences. Once recognition is achieved, it results in the integration of the F plasmid in the bacterial chromosome.

The F plasmid is intact, with all its DNA sequences, such as the origin of transfer (marked by the O in the diagram), that directs the transfer of genetic material between bacterial cells.

This integration process is reversible, which means that the F plasmid can be excised and exist independently in the cytoplasm again.