Bacterial genetics - Dr. Kozel

Question 1

Do bacteria exhibit control of gene expression?

Answer 1

Yes, it allows coordinated response of multiple genes to environmental changes.

Question 2

How do bacteria control gene expression?

Answer 2

Through an operon. An operon is under the control of a promotor or repressor DNA sequence. This sequence can activate or turn off expression of a gene or group of genes.

Question 3

What is quorum sensing?

Answer 3

Quorum sensing is the ability of a population of bacteria to be sense their environment. If there is a small population then they remain sort of inactive - as the population increase they can respond to cues created by the increasing population. One of the things this allows is the coordinated control of gene expression.

Question 4

Give examples of some of the responses that are turned on in relation to quorum sensing.

Answer 4

1. Virulence factors are turned on.
2. Biofilm may be produced such as by Pseudomonas bacteria.
3. Pathogenicity islands are triggered.
4. Toxin production such as by C. diphtheria.
5. Hemolysin production such as by E. coli.

Question 5

Give an example of an operon.

Answer 5

The Lactose operon.

Question 6

How does the Lac operon function?

Answer 6

The Lac operon is an example of negative regulation. It consists of 3 control genes.
1. Promoter in region of DNA where RNA polymerase initiates transcription.
2. Operon - acts as a traffic light for transcription of structural genes.
3. Regulatory gene - codes for repressor protein.
The three structural genes coding for enzymes that degrade lactose are located next to each other on the chromosome. In the absence of lactose, operon is repressed by binding of repressor protein to operator sequence. Lactose binds repressor protein when present and reverses repression.

Question 7

What is negative regulation?

Answer 7

Genes are constitutively turned off unless they are needed. This saves energy for the cells.

Question 8

What is the most common form of control of gene expression in bacteria?

Answer 8

Negative regulation.

Question 9

Describe some methods of bacterial mutation.

Answer 9

1. single base change - may result in a silent mutation if it results in no change to the encoded amino acid or it may change the amino acid coded and is called a missense mutation.
2. Changes in numerous bases. If may cause a frameshift mutation if it does not occur in multiples of three - this can result in a useless peptide or premature truncation of a peptide. Extensive insertions and deletions result in null mutation where the gene function is entirely destroyed.

Question 10

Is mutation an efficient means for the development of antibiotic resistance in a bacterial cell?

Answer 10

No, the primary way a cell develops antibiotic resistance is genetic exchange with another cell.

Question 11

What is the most common form of mutation?

Answer 11

Silent mutations.

Question 12

What are the 3 fundamental ways that bacteria exchange genetic material?

Answer 12

1. Transformation
2. Conjugation
3. Transduction

Question 13

Describe transformation.

Answer 13

This is the naked uptake of DNA. It requires a competent cell so therefore occurs only in certain species. The uptake of the DNA (competency) can be induced by artificial means such as electroporation or inducing changes in membrane permeability. The DNA that is taken up is integrated in the chromosome via homologous recombination.

Question 14

When do bacterial cells tend to become competent?

Answer 14

Near the end of their exponential growth phase.

Question 15

What is competence when applied to bacterial cells?

Answer 15

The ability to take up naked DNA from the environment. This usually occurs for some cells at the end of the log growth phase when they are producing a protein called competence factor.

Question 16

Describe conjugation.

Answer 16

This is the one-way transfer of DNA from a donor cell (male) to a recipient cell (female) through a sex pilus. The donor cell must have F factor - this codes for the sex pilus and allows the 2 cells to contact eachother directly and for genes to travel thru the sex pilus to be transferred to the recipient cell.

Question 17

Is F factor part of the bacterial chromosome?

Answer 17

No, it is encoded in a plasmid called the F plasmid.

Question 18

What is a plasmid?

Answer 18

A plasmid is a small genetic element that can integrate into a host chromosome.

Question 19

Describe the F plasmid.

Answer 19

The F plasmid contains all the genes necessary for it’s own transfer, the ability to make a sex pilus, and the ability to initiate DNA synthesis at the transfer origin of the plasmid. The F plasmid transfers itself.

Question 20

What is an F+ cell?

Answer 20

This is a cell that has acquired the F plasmid - it also describes its mating type. Male bacteria have the F plasmid and female bacteria do not.

Question 21

What is an F- cell?

Answer 21

This cell does not contain the F plasmid.

Question 22

What is a sex pilus?

Answer 22

This is a structure that can bind to a recipient F- cell, bringing the 2 cells into close proximity and allows the transfer of an F plasmid to the recipient.

Question 23

What is the composition of a sex pilus?

Answer 23

Repeating units of a single protein called F pilin.

Question 24

How do genes from the bacterial chromosome get transferred with the F plasmid?

Answer 24

The plasmid integrates into the bacterial chromosome at a specific point in both the plasmid and chromosome - this cells is now a high frequency cell.There is site specific cleavage at the replication origin for conjugal transfer and DNA replication. The genes adjacent to the F factor cross to the other cell then the F factor goes last. Once in the new cell the DNA is integrated via homologous recombination. This process is called High frequency transfer.

Question 25

What sorts of things do plasmids encode for?

Answer 25

1. antibiotic resistance
2. resistance to many antibacterial compounds including heavy metals, mercurials and toxic anions
3. resistance to radiation
4. virulence factors such as toxins and attachment proteins
5. various metabolic pathways
6. bacteriocins - compounds toxic for other bacteria

Question 26

What are transposons?

Answer 26

Mobile gene elements that allow for transfer of DNA within a cell either from one position to another in a genome or between different molecules of DNA such as plasmid to genome.

Question 27

Describe simple transposons.

Answer 27

These are basically insertion sequences - they contain no known genes except those needed for transposition. They are usually stretches of transposition genes with inverted repeats at the ends that are needed for recognition by transposase.

Question 28

What is transposes?

Answer 28

Enzyme that catalyzes disconnection of insertion sequence from flanking DNA, then inserts sequence in new DNA site. The transposition process does not require regions of extensive homology for insertion.

Question 29

Describe complex transposons.

Answer 29

These are transposons that include insertion sequences along with additional genes - such as those for toxin production or antibiotic resistance.

Question 30

Describe conjugal resistance plasmids.

Answer 30

These are plasmids that contain resistance transfer factor ie the genes needed for conjugal transfer. They also carry drug resistance transposons (r factor).

Question 31

What is transduction?

Answer 31

Genetic transfer mediated by bacterial viruses called bacteriophages.

Question 32

Describe the process of transduction.

Answer 32

A bacteriophage infects a bacteria and during the process of viral replication, portions of the bacterial chromosome may be packaged into the bacteriophage particle (then the particle is primarily host DNA) and is transferred to the next infected bacteria where the DNA undergoes recombination with that host chromosome. This packaging is accidental and random and is called generalized transduction.

Question 33

Describe the bacteriophage lifecycle.

Answer 33

1. The phage attaches to a host bacteria and injects its double-stranded DNA. The phage DNA circularizes and enters the lytic cycle or lysogenic cycle. In the lytic cycle, New phage DNA and proteins are synthesized and assembled into visions. The cell lyses and virions are released. In the lysogenic cycle phage DNA integrates within the bacterial chromosome by recombination becoming a prophage. The bacteria replicates and the viral DNA is replicated along with the bacterial. Occasionally the prophage may excise from the chromosome to initiate a lytic lifestyle.

Question 34

Describe specialized transduction.

Answer 34

Transduction in which only specific genes are transferred to new host cell. This is mediated by the lysogenic cycle. For example - a prophage exists in a galactose using cell (has gal gene). The phage genome excises and carries with it the adjacent gal gene, the phage matures and the cell lyses and the phage infects a new cell- one that does not carry the gal gene. The prophage and the gal gene get integrated into the host cell and this cell now has the gal gene. The genes that get transferred depend upon where the prophage inserts into the host chromosome.