Viruses [Lytic and Lysogenic Cycle]

What kind of bacteriophages are there?

• Lytic bacteriophages

• Temperate bacteriophages

What are the stages of the lysogenic reproductive cycle?

1. Attachment

2. Penetration

3. Replication

4. Spontaneous induction

What happens during attachment of the lysogenic reproductive cycle?

• Attachment site on the tail fibre recognizes and attaches to specific receptors with a complementary shape on the bacterial surface

• Specific strains of bacteriophages can only absorb to specific strains of host bacteria → viral specificity (complementary site on bacterial cell wall)

What happens during penetration of the lysogenic reproductive cycle?

1. After attachment of the virus, bacteriophage releases lysozyme → Digests bacterial cell wall → Release of molecules that changes base plate conformation

2. Tail sheath contracts and thrusts hollow tube through bacterial cell wall

What occurs to the phage DNA during replication of the lysogenic replication cycle?

• Originally linear phage DNA forms a circle

• Lytic → Bacteriophage multiplies and is transcribed, leading to the production of new phage and eventual cell lysis

• Lysogenic → Integration of circular phage DNA into circular bacterial DNA

What is a prophage and how does it remain latent in the bacterial DNA for so long?

• Phage DNA inserted into bacterial DNA

• Most prophage genes are repressed by repressor proteins (products of phage genes)

  • Repressors stop transcription of all other phage genes

  • Phage genes that would otherwise direct synthesis and release of new virions are turned off

• Every time host cell’s machinery replicates the bacterial chromosome, it replicates prophage DNA which will be found in all progeny cells

What happens during spontaneous induction of the lysogenic cycle?

• Repressor protein is destroyed by increased protease activity, causing the prophage to be excised and enter the lytic cycle

• Frequency is enhanced by UV light / exposure to agents that damage DNA, which activates cellular proteases

What are the stages of the lytic reproductive cycle?

1. Replication

2. Maturation

3. Release

What happens during replication of the lytic cycle?

• Bacteriophage DNA is immediately transcribed to synthesise mRNA using host cell polymerase

• Highly virulent phages produce early proteins that completely take control from the host cell

• Enzymes encoded by the phage genome take over the bacterium’s macromolecular synthesizing machinery for own use to create many copies of phage DNA, enzymes and structural components

How does the phage create many copies of phage DNA during replication in the lytic cycle?

• Uses the host cell nucleotides and several of its own enzymes to synthesise many copies of phage DNA

• Uses bacterium’s metabolic machinery to synthesise phage enzymes and structural components

What is the eclipse period of replication in the lytic cycle?

• Complete, infective virions are not yet present

• Only separate components like DNA and proteins are present

What is early replication and late replication in the lytic cycle?

Early → Bacteriophage genome replicates and bacteriophage components begin to be produced by way of the host bacterium’s metabolic machinery

Late → Production of bacteriophage components and enzymatic progresses

What happens during maturation of the lytic cycle?

• Bacteriophage DNA and capsid are assembled into a DNA-filled head

• Head, tail and tail fibres are assembled independently and joined in a specific sequence

  • Tail fibres + tail → Head + tail

What happens during release of the lytic cycle?

• Lysozyme causes the bacterial cell wall to break down

  • Coded for by the phage gene, synthesized within the cell

• Newly-produced bacteriophages are released from the host cell

• Mature bacteriophages infect other susceptible cells in the vicinity

  • Viral multiplication cycle is repeated within those cells