Bacteriophage - Free Printable

Actually, you did not upload an image — the text you provided is a description of a diagram illustrating the lytic and lysogenic cycles of bacteriophage (virus) infection in bacteria.

Since no actual image was uploaded, I will solve the “problem” by explaining the entire process shown in your textual description, which clearly outlines the two main pathways a bacteriophage can take after infecting a bacterial host cell: the lytic cycle and the lysogenic cycle.

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## 🧬 OVERVIEW: BACTERIOPHAGE LIFE CYCLES

Bacteriophages (or phages) are viruses that infect bacteria. After attaching to a host cell, they can follow one of two paths:

1. Lytic Cycle — leads to immediate replication and destruction of the host.
2. Lysogenic Cycle — integrates viral DNA into the host genome and replicates passively with the host cell.

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## 🔍 STEP-BY-STEP EXPLANATION OF THE DIAGRAM

1. Attachment (Adsorption)

> *“The first step in the multiplication of a virus is its attachment to a host cell; more than one virus particle can simultaneously adsorb to a single cell.”*

- The phage uses tail fibers or other surface proteins to bind specifically to receptors on the bacterial cell wall.
- Multiple phages can attach to one bacterium — this increases the chance of successful infection.

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2. Entry of Viral Chromosome

> *“Entrance of the viral chromosome into the host cell”*

- The phage injects its genetic material (DNA or RNA — here, it’s called “viral chromosome”) into the bacterial cytoplasm.
- The protective protein coat (capsid) remains outside the cell.

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3. Fork in the Road: Lytic vs. Lysogenic Pathways

At this point, the virus decides (based on environmental cues and molecular switches) whether to enter the lytic or lysogenic cycle.

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## 🟢 LYTIC CYCLE (DESTRUCTIVE PATHWAY)

> *“Lytic cycle (usually takes 15–60 min at 37°C)”*

This pathway results in rapid production of new virus particles and lysis (bursting) of the host cell.

#### Steps:
1. Multiplication of the viral chromosome
- The viral DNA hijacks the host’s machinery to replicate itself.
- Host DNA is often degraded to provide nucleotides.

2. Synthesis of viral proteins
- Viral genes direct the host ribosomes to make capsid proteins and enzymes needed for assembly.

3. Assembly of new virions
> *“The viral chromosomes are surrounded by newly synthesized protective coats.”*
- New phage particles are assembled from replicated DNA and synthesized capsids.

4. Lysis and release
> *“Release of new virus particles by lysis of the host cell wall”*
- Phage-encoded enzymes (like lysozyme) break down the bacterial cell wall.
- Hundreds of new phages burst out to infect neighboring cells.

✔ Result: Host cell dies. Rapid spread of virus.

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## 🔵 LYSOGENIC CYCLE (DORMANT/INTEGRATED PATHWAY)

> *“Release of the viral chromosome. This is generally rare, may occur only once in 10,000 divisions of a lysogenic bacterium.”*

This is a “stealth” mode where the virus hides inside the host genome.

#### Steps:
1. Integration into host genome → Prophage
> *“Prophage”*
- The viral DNA integrates into the bacterial chromosome as a prophage.
- It becomes part of the host’s DNA and is replicated along with it during cell division.

2. Lysogenic bacteria divide normally
> *“Lysogenic bacteria usually divide at the same rate as normal bacteria.”*
- The prophage is passed vertically to daughter cells without harming them.

3. Induction (rare event)
> *“Release of the viral chromosome... may occur only once in 10,000 divisions…”*
- Under stress (e.g., UV light, chemicals), the prophage can excise itself from the host genome.
- It then enters the lytic cycle, producing new viruses and lysing the cell.

✔ Result: Virus remains dormant for generations. Can reactivate later.

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## 💡 KEY DIFFERENCES

| Feature | Lytic Cycle | Lysogenic Cycle |
|------------------------|------------------------------|-------------------------------|
| Host cell fate | Lyses (dies) | Survives (initially) |
| Viral DNA | Replicates independently | Integrates as prophage |
| Speed | Fast (minutes) | Slow (generations) |
| Outcome | Immediate viral spread | Latent infection |
| Induction | Not applicable | Triggered by stress |

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## 🧪 WHY IS THIS IMPORTANT?

- Medical relevance: Some pathogenic bacteria become more dangerous when carrying prophages (e.g., *Corynebacterium diphtheriae* produces diphtheria toxin only when infected by a specific phage).
- Biotechnology: Phages are used in gene therapy, CRISPR systems, and phage therapy against antibiotic-resistant bacteria.
- Evolution: Horizontal gene transfer via lysogeny contributes to bacterial evolution and adaptation.

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## ✔ FINAL ANSWER (Summary)

The diagram illustrates the two life cycles of bacteriophages:

- In the lytic cycle, the virus immediately replicates, assembles new particles, and destroys the host cell within minutes.
- In the lysogenic cycle, the viral DNA integrates into the host genome as a prophage, replicating silently with the host for many generations until triggered to enter the lytic cycle.

This dual strategy allows bacteriophages to maximize survival and propagation under different environmental conditions.

Let me know if you’d like to dive deeper into any specific step, or if you have a follow-up question!