Lytic cycle

Key takeaways

Lytic cycle belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Lytic cycle to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Lytic cycle from memory before moving on to harder problems.

Reference excerpt

The lytic cycle ( LIT-ik) is one of the two cycles of viral reproduction (referring to bacterial viruses or bacteriophages), the other being the lysogenic cycle. The lytic cycle results in the destruction of the infected cell and its membrane. Bacteriophages that can only go through the lytic cycle are called virulent phages (in contrast to temperate phages). In the lytic cycle, the viral DNA exists as a separate free floating molecule within the bacterial cell, and replicates separately from the host bacterial DNA, whereas in the lysogenic cycle, the viral DNA is integrated into the host genome. This is the key difference between the lytic and lysogenic cycles. However, in both cases the virus/phage replicates using the host DNA machinery.

Description The lytic cycle is often separated into six stages: attachment, penetration, transcription, biosynthesis, maturation, and lysis.

Attachment – the phage attaches itself to the surface of the host cell in order to inject its DNA into the cell Penetration – the phage injects its DNA into the host cell by penetrating through the cell membrane Transcription – the host cell's DNA is degraded and the cell's metabolism is directed to initiate phage biosynthesis Biosynthesis – the phage DNA replicates inside the cell, synthesizing new phage DNA and proteins Maturation – the replicated material assembles into fully formed viral phages (each made up of a head, a tail and tail fibers) Lysis - cell wall or membrane ruptures, disintegrating it and releasing the virus in the process. The verb lyse, refers to the process of lysis.

Attachment and penetration To infect a host cell, the virus must first inject its own nucleic acid into the cell through the plasma membrane and (if present) the cell wall. The virus does so by either attaching to a receptor on the cell's surface or by simple mechanical force. The binding is due to electrostatic interactions and is influenced by pH and the presence of ions. The virus then releases its genetic material (either single- or double-stranded RNA or DNA) into the cell. In some viruses this genetic material is circular and mimics a bacterial plasmid. At this stage the cell becomes infected and can also be targeted by the immune system. It is mostly aided by receptors on the surface of the cell. The sequence of events that occur during initiation of bacteriophage infection, from adsorption (attachment) through DNA ejection from the virion into the host cell (penetration), was reviewed by Molineux.

Transcription and biosynthesis During the transcription and biosynthesis stages, the virus hijacks the cell's replication and translation mechanisms, using them to make more viruses. The virus's nucleic acid uses the host cell's metabolic machinery to make large amounts of viral components. In DNA viruses, the DNA transcribes itself into messenger RNA (mRNA) molecules that are then used to direct the cell's ribosomes. One of the first polypeptides to be translated destroys the host's DNA. In retroviruses (which inject an RNA strand), the enzyme reverse transcriptase transcribes the viral RNA into DNA, which is then transcribed again into RNA. Once the viral DNA has taken control it induces the host cell's machinery to synthesize viral DNA and proteins and begins to multiply. The biosynthesis is (e.g. T4) regulated in three phases of mRNA production followed by a phase of protein production.

Early phase Enzymes modify the host's transcriptional process by RNA polymerase. Amongst other modifications, virus T4 changes the sigma factor of the host by producing an anti-sigma factor so that the host promotors are not recognized any more but now recognize T4 middle proteins. For protein synthesis Shine-Dalgarno subsequence GAGG dominates an early genes translation. Middle phase Virus nucleic acid (DNA or RNA depending on virus type). Late phase Structural proteins including those for the head and the tail.

Maturation and lysis About 25 minutes after initial infection, approximately 200 new virions (viral bodies) are formed. Once enough virions have matured and accumulated, specialized viral proteins are used to dissolve the cells' walls. The cell bursts (i.e. it undergoes lysis) due to high internal osmotic pressure that can no longer be constrained by the cell wall. This releases progeny virions into the surrounding environment, where they can go on to infect other cells and another lytic cycle begins.

Gene regulation biochemistry There are three classes of genes in the phage genome that regulate whether the lytic or lysogenic cycles will emerge. The first class is the immediate early genes, the second is the delayed early genes and the third is the late genes. The following refers to the well-studied temperate phage lambda of E. coli.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Lytic cycle

Start with the simplest possible case. Write down what Lytic cycle claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Lytic cycle before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Lytic cycle ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Lytic cycle

In research: Lytic cycle appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Lytic cycle in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Lytic cycle is common in secondary-school and first-year university syllabi. It links to neighbouring topics Bacteriophages, Virology, so understanding it makes those chapters shorter.
In everyday life: Look for Lytic cycle outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Lytic cycle in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Lytic cycle means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Lytic cycle out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Lytic cycle in simple terms?

The lytic cycle ( LIT-ik) is one of the two cycles of viral reproduction (referring to bacterial viruses or bacteriophages), the other being the lysogenic cycle. The lytic cycle results in the destruction of the infected cell and its membrane.

Why does Lytic cycle matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Lytic cycle?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Lytic cycle.