What you need to know
- 6.5(ii)The terms lyticDescribes a virus life cycle in which new viruses are made and the host cell bursts (lyses) to release them. and latencyA period when a virus stays in a host cell without making new viruses or causing symptoms..
HIV reverse transcription and T helper destruction are 6A 8. Bacterium-versus-virus structure is 6A 1. Do not turn this page into a phage-therapy protocol.
By the end, you should be able to…
- Describe the lytic pathway as replication followed by host-cell lysis.
- Define latency as viral nucleic acid remaining in the host without immediately making new virions.
- Link lambda phageA bacteriophage that infects E. coli and can follow either a lytic or a lysogenic cycle. prophagePhage DNA inserted into a bacterial chromosome during lysogeny. and HIVHuman immunodeficiency virus: an RNA retrovirus that infects T helper cells and can lead to AIDS. provirusViral DNA, such as HIV DNA, that has been inserted into a host cell’s chromosome. as two latency examples, not as identical cycles.
Build your understanding
Lytic: copy, then burst
In the lyticDescribes a virus life cycle in which new viruses are made and the host cell bursts (lyses) to release them. pathway the virus attaches, its nucleic acid enters, new genomes and capsids are made using the host’s resources, and the host cell lyses, releasing virions. lambda phageA bacteriophage that infects E. coli and can follow either a lytic or a lysogenic cycle. can finish this way in a bacterium. The host is destroyed as a cell; the particles go on to other cells.
Lytic is a reproductive strategy, not a synonym for “dangerous to humans”. TMVTobacco mosaic virus: a rod-shaped RNA virus that infects plants. does not lyse a bacterium. Name the host when you use the word.
Follow the process
Lytic: attachment to the host → viral nucleic acid enters → the host’s resources make new genomes and capsids → the host cell lyses → virions are released and can infect other cells. latencyA period when a virus stays in a host cell without making new viruses or causing symptoms. is a different path: the nucleic acid remains, for example as a λ prophagePhage DNA inserted into a bacterial chromosome during lysogeny. or an HIVHuman immunodeficiency virus: an RNA retrovirus that infects T helper cells and can lead to AIDS. provirusViral DNA, such as HIV DNA, that has been inserted into a host cell’s chromosome., without that immediate burst. Do not put a phage tail on HIV, or reverse transcriptaseAn enzyme from retroviruses, such as HIV, that makes DNA using RNA as a template. on λ phage.
Latency: the genome waits
Latency means the viral nucleic acid remains in the host cell without immediately producing new particles. In λ phage, DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. can insert as a prophage and be copied whenever the bacterium divides: that quiet state is lysogenyA stage in which phage DNA is inserted into the bacterial DNA and copied with it, without making new viruses.. An induction signal can switch the phage back to lytic replication.
HIV can insert DNA into a T helper cellA T cell that binds to an antigen-presenting cell and releases cytokines that activate B cells and T killer cells. genome as a provirus and remain latent in some cells. That is the same specification word, latency, in a different host. Do not copy the phage tail onto HIV, and do not copy reverse transcriptase onto λ phage.
Not “cured”
If no particles are being made, the infection can still be there. Latency is why a later burst, or a later fall in T helper numbers, can appear after a quiet period. 6.6 will use that idea for HIV. Here you only need the two terms and one named example of each.
Lysogeny is the phage version of latency. You may meet the word; the specification term to define is latency.
NOT: no particles being released does not mean the virus has gone. Do not draw a phage tail on HIV.
- Lambda can insert as a prophage and be copied when the bacterium divides
- An induction signal can return lambda to the lytic pathway
- HIV latency is a provirus in host DNA, not a bacterial prophage
- Lambda does not use reverse transcriptase
- Lytic describes the host cell bursting, not how dangerous a virus is to people
Two traps this diagram blocks
- Do not say an undetectable particle count means the virus has disappeared.
- Do not copy the lambda tail onto HIV or reverse transcriptase onto lambda.
From the diagram alone
Question
Distinguish the lytic pathway from latency, using lambda phage as the example. [4]
Show a model answer
In the lytic pathway, lambda DNA enters a bacterium, new phage particles are made, and the cell bursts to release them. In latency the DNA remains as a prophage and is copied when the bacterium divides, without that immediate burst. A later signal can switch it back to lysis.
| Term | What the nucleic acid is doing | Named example |
|---|---|---|
| Lytic | Replicating then lysing the host | λ phage bursting a bacterium |
| Latency | Remaining in the host without making virions | λ prophage; HIV provirus |
Make your reasoning visible
Build the explanation
X happens because Y, leading to Z. X: new phage particles appear outside the bacterium. Y: after attachment and entry, the host makes viral genomes and capsids and then lyses. Z: the cell is destroyed and the virions can infect further cells. Incomplete: “The virus gets angry and explodes.” Improved: “lambda phageA bacteriophage that infects E. coli and can follow either a lytic or a lysogenic cycle. nucleic acid enters the bacterium, new particles are assembled, and lysis releases them.” The improvement is a sequence, not a mood.
Name the host cell
“The virus is latent” is weaker than “HIVHuman immunodeficiency virus: an RNA retrovirus that infects T helper cells and can lead to AIDS. DNADeoxyribonucleic acid: a double-stranded polynucleotide that carries the genetic code. remains in a T helper cellA T cell that binds to an antigen-presenting cell and releases cytokines that activate B cells and T killer cells. cell as a provirusViral DNA, such as HIV DNA, that has been inserted into a host cell’s chromosome.”.
Lytic is a cycle, not a mood
State attachment, replication and lysis, not “it becomes angry”.
Key explanations
- Lambda phage can complete that pathway in a bacterium. Name the host rather than calling every virus lyticDescribes a virus life cycle in which new viruses are made and the host cell bursts (lyses) to release them. in a person.
- In lambda, the DNA can be a prophagePhage DNA inserted into a bacterial chromosome during lysogeny. copied at each bacterial division. That quiet state is lysogenyA stage in which phage DNA is inserted into the bacterial DNA and copied with it, without making new viruses.. The specification word is latencyA period when a virus stays in a host cell without making new viruses or causing symptoms..
Avoid this misconception
The claim: If you cannot detect particles, the virus has gone.
The correction: Latency means the nucleic acid can still be there, replicated with the host, without a current burst of virions.
Burst now, or wait?
A virus cannot copy itself. Once inside a host cell it can take over at once and burst the cell, or its nucleic acid can sit quietly in the host for a long time.
What the model shows
λ phage infecting a bacterium by the lytic pathway or by latency, and HIV latency in a T helper cell.
What to look for
When new virions appear, and what happens to the viral DNA when the host divides.
What it tells us
Lytic means replicate, then lyse. Latency means the viral nucleic acid remains in the host without making virions, until a signal ends it.
Be careful what you claim
Illustrative times and numbers; one starting cell.
Try it in the model: press Next stage and watch the phage attach.
Lytic pathway and latency
Step λ phage through a bacterium by the lytic pathway, then by latency as a prophage, and compare HIV latency as a provirus in a T helper cell. Times and numbers are illustrative.
What you see
- A host cell and a virus at each stage: λ phage and a bacterium, or HIV and a T helper cell. The strip below follows the route for the chosen pathway.
- Two graphs over time: the number of host cells holding viral nucleic acid, and the total number of new virions released.
How to use it
- Choose the virus and the pathway, then press Next stage. On the latency path, let the host divide, then send the signal that ends latency.
What it shows
- Lytic: the virus replicates inside the host cell using its resources, then the cell lyses and releases virions (6.5(ii)).
- Latency: the viral nucleic acid remains in the host cell, as a λ prophage or an HIV provirus, without immediately producing new virions (6.5(ii)).
Limits of the model
- Times and burst sizes are typical, rounded and illustrative (λ phage about 100 virions per cell; HIV much more variable). Only one virus infects one starting cell, and every cell is assumed to behave the same way.
- How HIV turns its RNA into DNA, and how it destroys T helper cells, is left to 6A 8. λ’s choice between the two paths is shown as your choice, not as the real regulatory switch.
Guided tour · step 1 of 8
- In Model, just below these steps, keep Virus and host on λ phage in a bacterium and Pathway on Lytic.
- Press Next stage until the bacterium bursts in the simulator.
The phage attaches and injects its DNA. The bacterium’s own enzymes, ribosomes, nucleotides and ATP copy the phage DNA and make capsids and tails. The cell then lyses, releasing about 100 new virions, and is destroyed.
Model
Questions
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Define the lytic pathway.
Show suggested answer
The virus replicates inside the host and the host cell bursts, releasing new virions.
Practice question
Retrieval and explanation · suggested answer
Define latency for a virus.
Show suggested answer
Viral nucleic acid remains in the host cell without immediately producing new virions.
Practice question
Retrieval and explanation · suggested answer
How can λ phage switch from latency to lytic?
Show suggested answer
Latent phage DNA sits as a prophage. An induction signal causes it to leave that quiet state, replicate and lyse the bacterium.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A diagram shows HIV RNA entering a cell and, much later, new HIV particles budding. Identify the missing latency step and one thing the diagram must not borrow from λ phage.
Show a hint
DNA in the host genome.
Explore the reasoning
Reverse transcription and insertion of viral DNA as a provirus are the latency step (detail in 6A 8). The diagram must not give HIV a head-and-tail phage capsid or a bacterial host. Budding from a T helper is not phage lysis of a bacterium, even though both can follow a latent period.
Recall this lesson
Lesson recall questions
Question 1 of 5: Compare the nucleic acid of a bacterium with that of a virus.
Bacteria vs viruses
Practice question
Retrieval and explanation · suggested answer
Compare the nucleic acid of a bacterium with that of a virus.
Lesson: 6A 1: Bacteria and Viruses
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Phage as a laboratory switch, not a medicine
6A · How Viruses ReproduceNature Education’s bacteriophage definition treats phages as viruses that infect bacteria, with lytic bursts and, in temperate phages such as lambda, a quiet integrated state. Molecular biologists used…Specification 6.5(ii)Read & use the scienceClose article
Why this matters
Nature Education’s bacteriophage definition treats phages as viruses that infect bacteria, with lytic bursts and, in temperate phages such as lambda, a quiet integrated state. Molecular biologists used that switch as a tool. Phage therapy is a separate, contested medical idea and is not 6.5(ii) recall. A latent genome can persist while few new particles are made; lysis releases particles after replication. The two outcomes cannot be inferred merely from finding viral DNA in one sample at one time.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.5(ii) understand what is meant by the terms lytic and latency
Use the science
Why is “lambda is useful in labs, therefore latency is a treatment” a category error?
Explore a possible response
Lab use exploits a genetic switch in bacteria. Latency in 6.5 is a specification term for viral nucleic acid remaining in a host, not a licensed therapy.
Explore the source
- Scitable: bacteriophage (phage)Nature Education