Lecture 3: Bacterial Genetics

Question 1

What is vertical gene transfer (VGT)?

Answer 1

Transmission of genetic material during reproduction and replication

Question 2

What is an example of VGT?

Answer 2

Binary fission

Question 3

What is horizontal gene transfer (HGT)?

Answer 3

Movement of genetic information between organisms and the acquisition of new material conferring new properties and traits.

Question 4

What are 3 examples of HGT?

Answer 4

1. Conjugation
2. Transformation
3. Transduction

Question 5

What is binary fission and why is it needed?

Answer 5

Binary fission is required for cell replication and involves the bacterial chromosome. It is defined as the ability of prokaryotes to double in mass before dividing.

Question 6

What are FTs?

Answer 6

FTs are a family of bacterial proteins that are structure analogues of the eukaryote tubulin.

Question 7

How does binary fission and DNA replication occur?

Answer 7

1. DNA replication occurs at the same time as membrane replication, so cell elongation is observed.
2. FTs monomers move to a point mid-way between the end points of the cell.
3. FTs proteins form a ring, and it is here where site-specific recombination occurs.
4. Tiny hole in the FTs ring allows the movement of the replicated chromosome.
5. Constriction of the FTs ring leads to a septum forming, eventually leading to complete cell division.
6. FTs proteins drag membrane down to form 2 separate cells and drags the chromosomes to the central points.
7. FTs proteins are reabsorbed through the membrane and then broken down until needed again.

Question 8

What is site-specific recombination?

Answer 8

Site-specific recombination is a type of genetic recombination in which DNA strand exchange takes place between segments possessing at least a certain degree of sequence homology.

Question 9

Where is the chromosome during DNA replication?

Answer 9

Chromosome is anchored to the cell membrane.

Question 10

What experiment proved the process of transformation?

Answer 10

Griffiths Experiment; S and R strains of Streptococcus pneumoniae.

Question 11

Describe the outcomes of the transformation experiment

Answer 11

R strains (non-lethal) were able to uptake dead S strain material and be transformed into S strain (lethal). Dead S material needs living bacterial cell in order to be expressed as the transforming principle is DNA.

Question 12

What is the transforming principle in transformation?

Answer 12

RNA and Protein have no effect on transformation, but DNA is essential.

Question 13

What is transduction?

Answer 13

Transfer of DNA between bacterial cells where a bacteriophage is the transfer agent.

Question 14

What are the 2 types of transduction?

Answer 14

1. Specialised transduction

2. Generalised transduction

Question 15

What is specialised transduction?

Answer 15

Where the excision of the viral DNA from the host DNA takes genes adjacent to the site of lysogeny, and incorporates them into the viral genome. Bacteriophage transduces genes between competent bacterial cells.

Question 16

Describe the process of specialised transduction

Answer 16

1. Bacteriophage transfer only a few restricted gene (DNA fragments) from donor bacteria to recipient bacteria. Specialised transduction is carried only by temperate bacteriophage which undergoes lysogenic cycle in donor cell.
2. Genome integrated with host cell’s DNA at certain location and remains dormant and is passed from generation to generation into daughter cells during cell division.
3. Lysogenic cell is exposed to certain stimuli and causes induction of virus genome from host cell genome and begins lytic cycle.
4. On induction from donor DNA, this phage genome sometimes carries a part of bacterial DNA with it. Transfer of donor DNA fragments into new recipient cells can occur.

Question 17

What genes can be transferred during specialised transduction?

Answer 17

In specialized transduction only those restricted genes, situated on the side of integrated viral genome, have a chance to enter recipient cell.

Question 18

What is generalised transduction?

Answer 18

Any gene in a host genome can be transduced into a phage genome, not just adjacent to the site of lysogeny.

Question 19

Describe the process of generalised transduction

Answer 19

1. Bacteriophage infects donor cell and begins lytic cycle.
2. Virus hijack host cell and synthesize virus components such as genome, enzymes, capsid, head tail and tail fibres. Then viral enzyme hydrolyses host cell DNA into small fragments.
3. Donor DNA can be incorporated into the virus capsid (bacteriophage head).
4. Bacteriophage infects a new cell and transfers donor DNA into new bacteria. Since this donor DNA is not viral DNA, it does not replicate inside recipient bacteria but undergoes homologous recombination with recipient cell’s chromosomal DNA forming recombinant cell.

Question 20

What genes can be transferred during generalised transduction?

Answer 20

All the fragments of donor DNA from any region of chromosome have an equal chance to enter transducing bacteriophage.

Question 21

What is conjugation?

Answer 21

Process by which one bacterium transfers genetic material to another, of the same species, through direct contact.

Question 22

What is needed for conjugation to occur?

Answer 22

One bacterium serving as the donor of the genetic material; must possess the fertility factor. One bacterium serving as the recipient of the genetic material.

Question 23

What is the role of the fertility factor?

Answer 23

Codes for a pilus that acts as a physical bridge between the cytoplasm of the 2 cells; donor and recipient.

Question 24

How is the DNA copied for transfer during conjugation?

Answer 24

The donor cell uses its ribosomes to copy the plasmid and the copy is transferred through the pilus to the recipient cell.

Question 25

What is significant about the transfer of genetic material during conjugation?

Answer 25

Transfer is unidirectional.

Question 26

What is bacterial compatibility?

Answer 26

The ability of a bacterial plasmid to replicate in conjunction with another plasmid within the same bacterial cell.

Question 27

What determines a plasmids compatibility?

Answer 27

The origin of replication determines the plasmid’s compatibility.

Question 28

What is the issue with replication for plasmids that utilise the same replication system?

Answer 28

Plasmids that utilise the same replication system cannot co-exist in the same bacterial cell. They are said to belong to the same compatibility group. The introduction of a new origin, in the form of a second plasmid from the same compatibility group, mimics the result of replication of the resident plasmid. Thus, any further replication is prevented until after the two plasmids have been segregated to different cells to create the correct prereplication copy number.