International A Level Biology / Microbiology
Lytic and latent infections
Edexcel International A Level Biology (YBI11), chapter 17: Microbiology, specification point 6.5. Free lesson.
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A lytic infection makes new viruses straight away and destroys the host cell. During latency, viral genetic material stays in the host cell without making new viruses, but it can later become active and enter the lytic cycle.
How the lytic cycle makes new phages
The lytic cycle of λ phage in E. coli ends when the host cell bursts open. The phage uses the cell’s resources to make many new phages, so the infection destroys the bacterium. Attachment and injection: the phage tail binds to specific receptor proteins on E. coli. The phage DNA enters the cell, but its protein capsid stays outside. Replication: host enzymes and nucleotides copy the phage DNA. Host ribosomes make phage proteins, including capsid proteins. Assembly: the new DNA is packed into phage heads, and tails are added. Lysis: an enzyme made from a phage gene breaks…
What latency means
Latency is when viral genetic material remains inside a host cell but is not being used to make new viruses. When viral DNA is inserted into the host’s DNA, it can be copied whenever the host cell divides. The virus can later become active and enter the lytic cycle. During latency, the viral genes are not expressed, meaning they are not used to make viral parts. No new virus particles are made until the virus becomes active.
How λ phage becomes latent
The latent route of λ phage is called lysogeny. After the phage injects its DNA into E. coli, the DNA joins the bacterial chromosome instead of taking over the cell. The joined phage DNA is called a prophage. The phage genes for making new phages are switched off, so the bacterium carries on as normal. When the bacterium divides by binary fission, it copies its chromosome and the prophage. Both daughter cells inherit the prophage. A trigger such as ultraviolet light damaging the bacterial DNA can activate the prophage. It is cut out of the chromosome and the phage enters the lytic…
How HIV becomes latent
HIV infects T helper cells, a type of white blood cell. HIV carries RNA, so it must first make DNA before its genetic material can be inserted into the host DNA. HIV enters the cell and releases its RNA and enzymes into the cytoplasm. Reverse transcriptase uses the viral RNA as a template to make DNA. Integrase inserts the viral DNA into the host DNA. The inserted HIV DNA is called a provirus. While latent, the provirus is not expressed, so no new HIV is made. When activated, it is transcribed, viral proteins are made, and new HIV particles are assembled and released.
Comparing the two routes
A virus does not always become latent first. Some viruses go straight into the lytic cycle, while λ phage can follow either route. New virus particles are made only in the lytic cycle, not during latency. In the lytic cycle, phage DNA is copied at once, new phages are assembled, and the cell lyses to release them. In latency, phage DNA is inserted into the bacterial chromosome and copied as the cell divides. No new phages are made until activation. If a drug blocks viral assembly, viral nucleic acid and capsid proteins may still be made, but they cannot be assembled into particles. No…
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