Bacterial Genetics

Question 0

What is the bacterial chromosome?

Answer 0

A double-stranded DNA molecule that is closed in a giant loop.
Only one copy of this molecule per cell - haploid state + no nuclear membranes surrounding their DNA.

Question 1

What are the 4 ways by which bacteria exchange genetic fragments?

Answer 1

1. Transformation
2. Transduction
3. Conjugation
4. Transposon insertions

Question 2

What is necessary for a transformation to occur?

Answer 2

1. The recipient bacterium must be competent : it must have structures on its cell wall that can bind the DNA and take it up intracellularly.
2. Usually recipient competent bacteria are of the same species as the donor.
3. If there is enough homology between strands, the DNA that has been brought in can then incorporate itself into the recipient’s genome.

Question 3

When we say that transduction has occured?

Answer 3

Transduction occurs when a virus that infects bacteria, called a bacteriophage, carries a piece of bacterial DNA from one bacterium to another.

Question 4

What are the two major types of phages?

Answer 4

1. Virulent phages

2. Temperate phages

Question 5

How do virulent phages behave?

Answer 5

They infect the bacteria, reproduce, and then lyse and kill the bacteria.

Question 6

How do temperate phages behave?

Answer 6

1. They have a good temperament and do not immediately lyse the bacteria they infect.
2. Undergo adsorption and penetration like the virulent phage, but then, rather than undergoing transcription, its DNA becomes incorporated into the bacterial chromosome.
3. The DNA waits for command to activate.

Question 7

How the integrated temperate phage genome is called?

Answer 7

A prophage.

Question 8

How is a bacterium that has a prophage integrated into their chromosome called?

Answer 8

Lysogenic : because at some time the repressed prophage can become activated.

Question 9

What describes the term lysogenic immunity ?

Answer 9

The ability of an integrated bacteriophage (prophage) to block a subsequent infection by a similar phage. (Produces a repressor protein)

Question 10

What are the two types of transduction?

Answer 10

1. Generalized transduction : virulent phages.

2. Specialized transduction : temperate phages.

Question 11

What are the steps in generalized transduction?

Answer 11

1. After phage penetration into the host bacterium, the phage DNA is transcribed, replicated, and translated into capsids and enzymes.
2. At this same time the bacterial DNA is repressed and eventually destroyed.
3. Sometimes pieces of the bacterial DNA are left intact.
4. If these pieces are the same size as the phage DNA, they can ACCIDENTALLY be packed into the phage capsid head.
5. Following lysis of the cell and release of the phages, the ONE phage with bacterial DNA can then infect another bacterium.
6. It will inject the piece of bacterial DNA that is ACCIDENTALLY carrying.
7. If there is some homology between the newly injected strand and the recipient bacterial genome, the piece MAY be incorporated.

Question 12

Why will the recipient cell in generalized transduction survive?

Answer 12

Because the bacteriophage is only carrying bacterial DNA and no viral genes that encode for replication and lysis are present.

Question 13

What happens when a prophage becomes active?

Answer 13

1. The prophage DNA is spliced out of the bacterial chromosome and is then replicated, translated and packed into a capsid.
2. Sometimes, there is an error in splicing, and a piece of bacterial DNA that lies at one side of the prophage will be cut, replicated, and packaged with the phage DNA.
3. This MAY result in a transfer of that piece of bacterial DNA to another bacteria - SPECIALIZED TRANSDUCTION

Question 14

What happens in DNA conjugation?

Answer 14

Bacterial sex at its best!
DNA is transferred directly by cell-to-cell contact, resulting in an extremely efficient exchange of genetic information.
It can occur between unrelated bacteria.

Question 15

What is necessary for conjugation to occur?

Answer 15

One bacterium must have a self-transmissible plasmid, also called an F plasmid (for fertility, not the other word!).

Question 16

For what structure has the self-transmissible plasmid (F plasmid) a gene?

Answer 16

The F plasmid has a gene that encodes enzymes and proteins that form the sex penis, that is, sex pilus.

Question 17

What happens after the formation of the sex pilus?

Answer 17

1. This long protein structure protrudes from the cell surface of the donor F(+) bacterium and binds to and penetrates the cell membrane of the recipient bacterium.
2. Now, a CONJUGAL bridge has formed.
3. A nuclease breaks off one strand of the F plasmid DNA, and this strand passes through the sex pilus (conjugal bridge) to the recipient bacterium.

Question 18

What happens at the end of conjugation?

Answer 18

BOTH bacteria have double-stranded circular F plasmids.

Question 19

Rarely, the F plasmid becomes integrated with the bacterial chromosome (much like a temperate phage), forming a Hfr cell.
In what two unique mechanisms of DNA transfer can this integration result?

Answer 19

1. Normal conjugation with an F(-) cell. The entire bacterial chromosome (including the integrated F plasmid) will transfer from the Hfr cell to the recipient cell.
2. The integrated F plasmid in the Hfr cell may be excised at a different site from that of integration. This can result in an F plasmid that now also contains a segment of chromosomal DNA. These plasmids are called F’ (F prime) plasmids. This conjugation is analogous to specialized transduction.

Question 20

Why are plasmids tremendously important medically?

Answer 20

Because certain plasmids encode enzymes that degrade antibiotics (penicillinase), or generate virulence factors (such as fimbriae and exotoxins).

Question 21

What are the transposons?

Answer 21

Mobile genetic elements. (DNA pieces with legs)
Can insert themselves into a donor chromosome WITHOUT having DNA homology.
They can carry genes for antibiotic resistance and virulence factors.

Question 22

Into what DNA are transposons inserted?

Answer 22

1. Phage
2. Plasmid
3. Bacterial chromosome

Question 23

Mention 3 important features of transposons.

Answer 23

1. They do not replicate independently but are copied during their host’s DNA transcription.
2. When they leave the DNA they are incorporated in, there is frequently ABERRANT excision and the transposon can carry new DNA away to another site.
3. The clinical importance of transposons is that antibiotic resistance can spread rapidly - even between different bacterial genera.