Compare lytic and lysogenic cycles

Overview

Bacteriophages (viruses that infect bacteria) can reproduce through two fundamentally different strategies: the lytic cycle (immediate reproduction and host destruction) and the lysogenic cycle (genome integration and delayed reproduction). Understanding these cycles reveals how viruses balance between rapid spread and long-term survival.

The virus "chooses" based on host cell health and environmental stress. Healthy host + abundant resources → lysogenic (why rush?). Stressed host → lytic (abandon ship before it sinks!).

The Lytic Cycle: Destroy and Reproduce

Step-by-Step Process

1. Attachment (Adsorption)

• Virus binds to specific receptor proteins on bacterial cell surface
• Why specific? Each virus has tail fibers (in bacteriophages) or spike proteins that fit host receptors like a lock-and-key
• Example: T4 phage binds to OmpC protein on E. coli

2. Penetration (Injection)

• Viral genome (DNA or RNA) enters the host cell
• In bacteriophages: Tail sheath contracts like a syringe, injecting DNA while protein coat stays outside
• Why leave the coat outside? Only nucleic acid contains instructions—protein would be useless bulk

3. Biosynthesis

• Host's ribosomes, nucleotides, and amino acids are redirected to make viral components
• Host DNA is often degraded by viral nucleases for raw materials
• Two parallel assembly lines:
  • Line A: Viral DNA replication (10-200 copies)
  • Line B: Viral protein synthesis (coat proteins, tail fibers, enzymes)

4. Maturation (Assembly)

• Viral DNA packaged into protein coats → complete virions
• Self-assembly: Coat proteins spontaneously fold around DNA (like Lego pieces snapping together)
• Quality control: Defective virions (empty coats, incorrect DNA) are common (~10-30% failure rate)

5. Lysis (Release)

• Virus produces lysozyme enzyme that digests the peptidoglycan cell wall
• Cell bursts, releasing 50-200 new virions
• Timing matters: Lyse too early → few copies. Too late → virions degrade inside.

Typical values for T4 phage:

• Burst size: ~100-200 virions
• Latent period: ~25-30 minutes

Why this matters: Higher burst size × shorter latent period = faster epidemic spread.

Timeline:

• 0-5 min: Attachment and injection (no visible change)
• 5-20 min: Biosynthesis (host DNA degraded, viral DNA/proteins accumulating)
• 20-25 min: Maturation (100 virions assembled per cell)
• 25 min: Lysis—cells burst

Result: 1000 cells → 100,000 free virions in 25 minutes.

Why this step? The exponential growth potential explains why viral infections can become overwhelming so quickly—each generation increases population100-fold.

The Lysogenic Cycle: Hide and Wait

Step-by-Step Process

1. Attachment and Penetration

• Same as lytic cycle—virus injects DNA into host

2. Integration (Recombination)

• Viral DNA circularizes inside the host
• Viral integrase enzyme catalyzes recombination between viral att (attachment) site and host att site
• Viral DNA inserts into host chromosome → becomes prophage
• Why circular first? Circular DNA is more stable and allows site-specific recombination

Where:

• $att_P$ = phage attachment site (15 bp core sequence)
• $att_B$ = bacterial attachment site on chromosome
• $att_L$, $att_R$ = left and right junctions flanking integrated prophage

Key insight: The crossover creates hybrid junctions that lock the prophage in place. Reversal requires excisionase enzyme.

3. Replication as Prophage

• Host DNA polymerase copies prophage DNA along with host genome
• Every time bacterium divides: prophage DNA duplicates
• One infection → millions of copies (through host reproduction, not lysis)

4. Repression of Lytic Genes

• Prophage produces CI repressor protein that binds to viral promoters
• Blocks transcription of lytic genes (those needed for biosynthesis and lysis)
• Negative feedback loop: More CI → more repression → lysogenic state maintained

5. Induction (Switch to Lytic)

• Environmental stress (UV light, chemicals, starvation) damages host DNA
• Host RecA protein activated → cleaves CI repressor
• Lytic genes de-repressed → prophage excises → enters lytic cycle
• Why switch? If host is dying anyway, better to reproduce now than die with the ship

Silent spread:

• Lysogenic bacterium divides every 20 minutes
• After 10 generations (200 min): 1 → 1024 cells, all carrying prophage
• No cell deaths, no free virions—virus "hides" in genome

Induction trigger:

• Expose culture to UV light (mimics DNA damage from sunlight)
• Within 60 minutes: all 1024 lysogens enter lytic cycle
• Result: ~100,000 virions released simultaneously (1024 cells × ~100 virions each)

Why this step? This demonstrates the lysogenic "time bomb" strategy—one infection can generate a massive synchronized burst after many bacterial generations.

Side-by-Side Comparison

Why it's wrong: Lysogeny is temporary. Environmental stress (UV radiation, chemical damage, nutrient depletion) triggers induction, where the prophage excises from the chromosome and enters the lytic cycle. The cell then lyses and dies, releasing virions.

The correct view: Lysogenic viruses delay killing the host, not avoid it. The lysogenic phase is a waiting strategy—the virus bides its time inside the chromosome, then switches to lytic when conditions are right.

Steel-man: The confusion is understandable because during lysogeny, the host genuinely does survive and even benefits sometimes (see lysogenic conversion below). The key is recognizing lysogeny as a phase, not a permanent state.

The fix: Think "lysogenic = lytic on pause." The virus always retains the ability to lyse—it just waits for optimal timing.

Decision Factors: Lytic vs. Lysogenic

Favor Lytic:

• High host density: Abundant new hosts to infect → spread rapidly
• Stressed host: Damaged DNA, low nutrients → host likely dying anyway
• High viral DNA:protein ratio: Enough resources accumulated for immediate reproduction

Favor Lysogenic:

• Low host density: Few bacteria around → conserve by hiding in genome
• Healthy host: Thriving bacterium = reliableplication vehicle
• CII protein accumulation: This viral protein promotes CI repressor production → lysogenic pathway

Scenario A: Healthy host

1. Host cell has abundant nutrients → high ATP, amino acids
2. CII protein stable (not degraded by host proteases)
3. CII activates transcription of CI repressor gene
4. CI repressor accumulates → blocks lytic genes → lysogenic cycle

Scenario B: Stressed host

1. Host cell nutrient-deprived → low energy
2. CII protein degraded rapidly (host proteases active)
3. CII cannot activate CI repressor
4. Lytic genes expressed by default → lytic cycle

Why this step? This demonstrates how viral reproduction "senses" host condition through simple protein stability—no complex sensing needed, just biochemistry responding to environmental conditions.

Lysogenic Conversion: Viral DNA Changes Host Traits

Important Examples

1. Corynebacterium diphtheriae (Diphtheria)

• Prophage carries ==tox gene== → produces diphtheria toxin
• Non-lysogenic strain: Harmless bacterium
• Lysogenic strain: Deadly pathogen causing diphtheria disease
• Medical importance: Only toxin-producing strains are dangerous

2. Streptococcus pyogenes (Scarlet Fever)

• Prophage produces erythrogenic toxin → red rash
• Converts throat infection (strep throat) into scarlet fever

3. Vibrio cholerae (Cholera)

• CTXφ prophage carries cholera toxin genes (ctxAB)
• Toxin causes massive water loss → deadly dehydration

4. Escherichia coli O157:H7

• Prophage contributes Shiga toxin genes
• Converts harmless gut E. coli into deadly hemorrhagic strain

Evolutionary perspective: Prophages are ~10-20% of many bacterial genomes. They're not just parasites—they're gene delivery vehicles that drive bacterial evolution.

Active Recall Practice

Strategy 1 - Lytic (the smash-and-grab): The pirate virus boards the ship, steals all the supplies to build100 baby pirate viruses, then blows up the ship to release them into the ocean. Super fast, but the ship is destroyed.

Strategy 2 - Lysogenic (the stowaway): The pirate virus sneaks aboard and hides in the ship's blueprints. Now every time the ship builds a copy of itself (bacteria divide in half to make two bacteria), it accidentally copies the pirate's instructions too! One pirate becomes thousands without destroying any ships. But if the ship starts sinking (stressed bacteria), the pirate says "Time to bail!" and switches to Strategy 1—makes 100 babies and blows up the ship.

Why two strategies? If there are lots of ships in the ocean, smash-and-grab works great—plenty of targets. But if ships are rare, better to hide and wait until there are more ships around. It's like choosing between spending all your money now or saving it for later!

Mnemonics and Memory Aids

LYSOGENIC = LYing SOft, GENerating Invisibly, Cell lives

• LYing: Prophage dormant
• SOft: Silent integration
• GENerating: Through host reproduction
• Invisibly: Hidden in chromosome
• Cell lives: Host survives

Decision mnemonic: "Abundant And healthy? Stay hidden. Scarce Or Stressed? Strike now!"

• Abundant hosts + healthy = lysogenic
• Scarce hosts or stressed = lytic

Connections

• Viral Structure and Classification - prerequisite: understanding what viruses are
• Bacterial Chromosome Structure - where prophages integrate
• Gene Regulation in Prokaryotes - CI repressor operates like lac repressor
• Horizontal Gene Transfer - lysogenic conversion is a mechanism
• Pathogenic Bacteria - many pathogens depend on prophage genes
• HIV Replication Cycle - retroviruses use lysogenic strategy (provirus)
• Bacterial Growth Curves - lytic cycles affect population dynamics
• Molecular Switches and Feedback - lambda switch is a classic bistable system
• Evolution of Virulence - lytic vs. lysogenic affects virulence evolution
• CRISPR-Cas Systems - bacterial defense specifically targets phage DNA

Key Formulas Summary

Burst Size: $B = \frac{N_{\text{virions released}}}{N_{\text{infected cells}}}$

Viral Growth Rate (lytic cycle): $N(t) = N_0 \cdot B^{t/L}$ where $L$ = latent period, $B$ = burst size

Prophage Copy Number (lysogenic cycle): $N(t) = N_0 \cdot 2^{t/G}$ where $G$ = host generation time (no virion production, just vertical transmission)

#flashcards/biology

What is the lytic cycle? :: A viral reproductive strategy where the virus immediately uses the host cell's machinery to produce many viral copies, then lyses (bursts) the cell to release virions. The host cell dies.

What is the lysogenic cycle? :: A viral reproductive strategy where viral DNA integrates into the host chromosome as a prophage, replicating silently with the host. The cell survives and passes viral DNA to offspring.

Compare burst size: lytic vs. lysogenic initial infection :: Lytic produces 50-200 virions per cell immediately. Lysogenic produces 0 virions initially but generates potentially millions of prophage-carrying cells through host reproduction.

Concept Map

Hinglish (regional understanding)