10 - Bacterial Genetics

Question 1

advantages of bacteria for genetics

Answer 1

• easily cultured
• short generation time
• haploid

Question 2

E.coli

Answer 2

• double stranded circular DNA molecule

- DNa coils up in nucleoid

Question 3

Bacteriophages of E.coli

Answer 3

viruses that infect bacteria

Question 4

Bacteriophage life cycle

Answer 4

Lytic bacteriophage multiply and then lyse the bacterial cell releasing progeny bacteriophage particles.
Temperate/lysogenic bacteriophage can integrate into the bacterial chromosome and remain dormant, replicating along with the bacterial DNA

Question 5

Describe the lytic cycle

Answer 5

1. A phage attaches to a bacterial cell and injects it’s DNA into the cell
2. All the DNA in the cell is hydrolysed
3. The cell’s metabolic machienery js used to produce clones of the phage
4. The phage then causes the bacterial cell to burst (usung lysozymes), releasing all the phages

Question 6

Describe the temperate phage life cycle

Answer 6

1. A phage will adhere to the bacterial cell and inject it’s DNA into the cell
2. The phage DNA circularises
3. The phage DNA integrates within the main bacterial chromosome, becoming a prophage
4. The bacterial cell reproduces normally and copies the prophage DNA along with the bacterial DNA and transmits it to daughter cells
5. Occasionally, the prophage may exit the bacterial chromosome, causing the cell to enter the lytic cycle

Question 7

How can gene transfer and recombination take place

Answer 7

1 Transformation
2 Transduction
3 Conjugation

Question 8

Transformation

Answer 8

the process by which bacteria will uptake of foreign DNA from the surrounding environment and incorporate it into its own genome

Question 9

transduction

Answer 9

Transfer of bacterial genes from one bacteria to another by a bacteriophage

Question 10

2 types of transduction

Answer 10

1. generalized

2. specialized

Question 11

generalised transduction

Answer 11

occurs only with virulent phage

Question 12

Describe the process of generalised transduction

Answer 12

1. A bacteriophage infects a cell, causing the cell to enter the lytic stage
2. When the phages are being packaged, some of the hydrolysed bacterial DNA gets packaged into a phage
3. The bacterial DNA will then be injected into another bacterial cell when it is infected by that phage
4. Crossing over then occurs between the original bacterial DNA and the new host cell’s DNA, forming a recombinant

Question 13

Specialised transduction

Answer 13

occurs only with temperate phage

Question 14

Describe the process of specialised transduction

Answer 14

1. A prophage within bacterial DNA exists the bacterial chromosome incorrectly, bringing some adjoining bacterial DNA with it
2. The cell then enters the lyric phage and part of the bacterial DNA will be packaged into the phage capsid along with the phage DNA
3. The phage will then infect a new host cell and insert its DNA
4. crossing over occurs between the plasmid and the host cell’s chromosome, resulting in the dna from the original bacterial cell being incorporated into the new cell’s genome

Question 15

conjugation

Answer 15

The ability to form sex pili and to transfer DNA by conjugation is determined by a plasmid called an F (for fertility) factor.

Question 16

Describe the process of conjugation from an F+ cell to an F- cell

Answer 16

1. An F+ forms a mating bridge with an F- cell
2. The F+ cell then replicates it’s F plasmid and transfers it the F- negative cell
3. The mating bridge then breaks and the new plasmid circularises, which results in the formation of a new F+ recombinant cell

Question 17

Describe the process of conjugation between an Hfr cell to an F- cell

Answer 17

1. In an Hfr cell, the F plasmid is incorporated into the main chromosome
2. The Hfr cell then for,s a mating bridge with the F- cell
3. The Hfr cell then replicates its DNA and starts transferring it to the F- cell
4. The mating bridge breaks before all the chromosome goes through
5. Crossing over occurs between the Hfr cell’s DNA and the F- chromosome result in recombinance. However the F- cell is still F- as it has not received the F gene
6. The linear DNA is then dissolved by enzymes

Question 18

At what level is bacterial gene expression regulated?

Answer 18

The transcription level

Question 19

Describe the process of negative gene regulation with repressible enzymes, using tryptophan as an example

Answer 19

1. When tryptophan (co repressor) is present, it binds to a repressor protein changing its shape
2. The repressor protein can then bind to the operator (with in the promoter) and blocks transcription
3. If tryptophan is not present, then the repressor protein can not bind to the promotor, allowing transcription to occur

Question 20

Describe The process of negative gene regulation with inducible enzymes, using lactose as an example

Answer 20

1. When lactose is absent, the repressor is active and can bind to the operator, preventing transcription
2. When lactose is present, lactose binds to the repressor, this changes the shape of the repressor, causing it to become inactive
3. The activator can no longer bind to the operator which means that transcription can go ahead

Question 21

What is the difference between positive and negative gene regulation?

Answer 21

Positive regulation involves the binding of an activator to the operator, which turns on transcription.
Where as negative regulation involves the binding of a repressor protein which prevents transcription

Question 22

Describe how cAMP causes positive gene regulation

Answer 22

• when glucose is scare, there is a high level of cAMP in the cell. The cAMP then binds to a cAMP receptor protein (CRP) and activates it. The activated protein then binds the CRP binding site and promotes transcription
• when glucose is abundant, there is a low level of cAMP in the cell. The CRP remains inactive so it does not bind to the CRP binding site. This prevents transcription.