In the lytic pathway, a virus replicates straight after infection and the host cell is destroyed. The T4 bacteriophage, which infects E. coli, always follows this route (it is a virulent phage), and lambda phage follows it after induction.The stages for a DNA phage are:1. Attachment: tail fibres bind to specific receptor proteins on the bacterial cell wall.
2. Injection: the tail sheath contracts and the phage DNA is injected through the wall and membrane. The empty capsid stays outside.
3. Takeover: early viral genes are transcribed and translated by the host. Their products stop the host making its own proteins, and in T4 an enzyme breaks down the host DNA, releasing nucleotides for the virus to use.
4. Replication and synthesis: the phage DNA is copied many times, and later viral genes code for capsid proteins, tail proteins and enzymes.
5. Assembly: heads are filled with DNA, and tails and tail fibres are attached, forming complete phages.
6. Lysis: a viral enzyme, lysozyme, breaks down the peptidoglycan cell wall. The cell bursts and releases the new phages.
The whole cycle takes about 20 to 30 minutes, and each infected cell can release around 100 to 200 new phages, each able to infect another bacterium.Worked example. A single cell releases 100 phages. If each infects a new cell, which also releases 100 phages, after two cycles there are 100 × 100 = 10 000 phages.On an agar plate covered with bacteria, lytic phages produce clear areas called plaques where the cells have been lysed.
2. Injection: the tail sheath contracts and the phage DNA is injected through the wall and membrane. The empty capsid stays outside.
3. Takeover: early viral genes are transcribed and translated by the host. Their products stop the host making its own proteins, and in T4 an enzyme breaks down the host DNA, releasing nucleotides for the virus to use.
4. Replication and synthesis: the phage DNA is copied many times, and later viral genes code for capsid proteins, tail proteins and enzymes.
5. Assembly: heads are filled with DNA, and tails and tail fibres are attached, forming complete phages.
6. Lysis: a viral enzyme, lysozyme, breaks down the peptidoglycan cell wall. The cell bursts and releases the new phages.
The whole cycle takes about 20 to 30 minutes, and each infected cell can release around 100 to 200 new phages, each able to infect another bacterium.Worked example. A single cell releases 100 phages. If each infects a new cell, which also releases 100 phages, after two cycles there are 100 × 100 = 10 000 phages.On an agar plate covered with bacteria, lytic phages produce clear areas called plaques where the cells have been lysed.