Lecture 18 - Bacteriophages

Question 1

Describe the morphology of phage, how is its structure related to attacking bacteria

Answer 1

The phage is a complex structure that has a icosahedron head (DNA inside) stabilised with a mid section sheath with tail fiber ‘legs’. It injects its DNA through its sheath using its tail fibers to that attach to specific receptors such as pili.

Question 2

Describe the difference between the lytic and lysogenic cycle of bacteriophages

Answer 2

The lytic life cycle occurs when environment is active for replication where viruses replicate within the cell and lyse open. Viral genes are fully expressed in the lytic lifecycle. In contrast, the lysogenic lifecycle Phage DNA is inserted into the host and most functions are switched off.

Question 3

List three types of phages

Answer 3

Tailed phages - dsDNA(P1,T4,T2)
Filamentous phages - ssDNA(fd,M13)
Icosahedral phages - ssDNA (fX174), ssRNA (MS2), dsDNA (PRD1)

Question 4

Describe how some bacteria can become ‘phage resistant’

Answer 4

There are two types of phage resistance:

Naturally occurring
Absorption interference - stops the bacteriophage from entering the host cell
DNA injection inhibition - prevents the bacteria DNA from injecting into the host cell
Restriction/modification (R/M)
Abortive infection (Abi) - interfering with phage DNA replication, RNA transcription, phage development and morphogensis

Genetically engineered
Per (phage encoded resistance)
Anitsense RNA

Question 5

What are the biotechnical applications of phages?

Answer 5

Model system of molecular biology
Cloning and expression 
Phage display systems 
Phage typing
Phage therapy:
phages as natural, self- replicating antibiotics

Question 6

List the steps of the lytic lifecycle

Answer 6

1. Adsorption
2. Injection of nucleic acid
3. Transcription and translation
4. Replication of DNA
5. Maturation and release

Question 7

Describe the steps of the Lytic and Lysogetic lifecyle of the phage

Answer 7

Lytic lifecycle

1. Phage infects cell
2. Phage DNA circularizes remaining separate from host of DNA
3. Phage DNA replications and phage proteins are made
4. Cell lyses, releasing phage

Lysogetic lifecycle

1. Phage infects cell
2. Phage DNA becomes incorporated into host
3. Cell divides and prophage is passed down
4. Under stressful conditions phage is excised
5. Replication