Viral Structure, Lytic Cycle, and Lysogenic Cycle
A virus is a non-cellular infectious particle made of genetic material (DNA or RNA) inside a protein coat, and it can reproduce only inside a host cell, either by the lytic cycle, which destroys the host cell quickly, or the lysogenic cycle, in which the viral DNA is integrated into the host’s DNA and copied silently. In brief: viruses have no cytoplasm, ribosomes or metabolism of their own. They are about 20 to 300 nanometers across, much smaller than bacteria. A bacteriophage is a virus that infects bacteria, and is the classic model for the lytic and lysogenic cycles.
Viruses cause diseases such as influenza, measles, HIV/AIDS, COVID-19 and the common cold. This guide describes viral structure, the steps of viral replication, the lytic and lysogenic cycles, the special case of retroviruses such as HIV, compares the two cycles, works through a bacteriophage example and corrects common mistakes.
What is the structure of a virus?
All viruses have two basic parts, and some have a third.
- Genetic material: DNA or RNA, single-stranded or double-stranded, depending on the virus. Viral genomes are small: from a few thousand bases to a few hundred thousand, compared with several million in bacteria.
- Capsid: a protein coat made of subunits called capsomeres. It protects the genome and, in many viruses, helps to attach to the host cell. Capsids are helical, icosahedral (20-sided) or complex in shape.
- Envelope (in some viruses): a lipid membrane taken from the host cell, with viral proteins in it. Influenza virus, HIV and coronaviruses are enveloped. Enveloped viruses are generally destroyed by soap and alcohol, which dissolve lipids.
Bacteriophages, such as T4, have a head that contains DNA, a tail and tail fibers that attach to the bacterial surface. Viruses are specific: they infect only certain hosts and cell types, because proteins on their surface fit receptors on the host cell. HIV infects T helper cells, which have the CD4 receptor.
How do viruses replicate?
Viruses use the machinery of the host cell. The basic steps are:
- Attachment: the virus binds to a specific receptor on the surface of the host cell.
- Entry: the virus or its genome enters the cell, by injection (phages), endocytosis or fusion with the cell membrane.
- Replication and synthesis: the host’s enzymes and ribosomes make copies of the viral genome and viral proteins.
- Assembly: new virus particles are assembled.
- Release: new viruses leave the cell, either by bursting it (lysis) or by budding from the membrane.
What is the lytic cycle?
In the lytic cycle, the virus takes over the cell at once and destroys it. Using the bacteriophage T4 as an example:
- The phage attaches to the surface of an E. coli cell and injects its DNA, leaving the empty capsid outside.
- Phage genes switch off the host’s own gene expression and degrade the host DNA.
- The host cell makes many copies of phage DNA and phage proteins.
- New phages are assembled.
- The phage makes an enzyme (lysozyme) that breaks down the bacterial cell wall. The cell bursts (lyses) and releases about 100 to 200 new phages. For T4, the whole cycle takes about 25 to 30 minutes.
Viruses that only use the lytic cycle are called virulent. Examples in humans include the influenza virus and rhinoviruses, which cause the common cold.
What is the lysogenic cycle?
In the lysogenic cycle, the virus does not destroy the host cell immediately. Instead:
- The phage DNA enters the cell and becomes integrated into the host chromosome. The integrated viral DNA is called a prophage.
- The prophage is copied each time the cell divides, and passed on to all daughter cells. The host cell stays alive, and the virus is dormant.
- Environmental stress, such as ultraviolet light or chemicals, can trigger the prophage to leave the chromosome. The virus then enters the lytic cycle and kills the host.
Phage lambda (λ) in E. coli is the classic example. Viruses that can use both cycles are called temperate. Lysogeny can change the host’s properties: the bacteria that cause diphtheria, cholera and botulism produce toxins only when they carry a prophage with a toxin gene. In animals, herpesviruses are similar: they remain dormant in nerve cells, and can reactivate, as in cold sores and shingles.
Lytic and lysogenic cycles compared
| Feature | Lytic cycle | Lysogenic cycle |
|---|---|---|
| Viral DNA | Replicates independently | Integrated into the host chromosome |
| Host cell | Destroyed (lysis) | Survives and divides |
| New viruses made? | Yes, immediately | No, until induced |
| Symptoms | Appear quickly | May be delayed or absent |
| Example | T4 phage, influenza | Lambda phage, herpesvirus (latency) |
How do retroviruses such as HIV replicate?
Retroviruses have an RNA genome. After entering the cell, the enzyme reverse transcriptase copies the viral RNA into DNA, which is the reverse of normal transcription. The DNA is integrated into the host chromosome by the enzyme integrase, and acts like a provirus. The host cell then makes new viral RNA and proteins. HIV targets T helper cells of the immune system and, without treatment, destroys them, which leads to AIDS. Antiretroviral drugs block the steps of replication, such as reverse transcriptase, integrase or attachment, and in combination they keep the virus under control.
Worked example: burst size of a bacteriophage
In a laboratory, 1 000 bacteria are infected with phages, and every infected cell releases 150 new phages after 30 minutes. Assume that each new phage infects a new bacterium and that bacteria are plentiful.
- After one cycle (30 min): 1 000 × 150 = 150 000 phages.
- After two cycles (60 min): 150 000 × 150 = 22 500 000 phages, if all 150 000 infect new bacteria and there are enough bacteria (at least 150 000).
- After three cycles (90 min): 22 500 000 × 150 = 3.375 × 10⁹ phages.
The numbers grow by a factor of 150 per cycle, so the phage population increases very rapidly until the host cells are used up. This is how a culture of bacteria can become clear within an hour or two. The method used in the lab to count phages is a plaque assay: phages are mixed with bacteria and spread on agar, and each infectious phage makes a clear spot (plaque) where the bacteria are destroyed. Counting plaques and correcting for the dilution gives the number of plaque-forming units (PFU) per milliliter.
Are viruses alive?
Viruses are not cells, have no metabolism and cannot reproduce on their own. Outside a host, a virus particle is inert. They do have genetic material, they evolve by natural selection and reproduce inside host cells. Most biologists regard viruses as being at the boundary of life, as discussed in the first article of this series. They are not included in the three domains of life.
How were viruses discovered?
Dmitri Ivanovsky (1892) and Martinus Beijerinck (1898) showed that the agent of tobacco mosaic disease passes through filters that hold back bacteria. Wendell Stanley crystallized tobacco mosaic virus in 1935, showing that it can be purified like a chemical. Felix d’Herelle discovered bacteriophages in 1917. The electron microscope made it possible to see viruses in the 1930s. HIV was identified in 1983 by teams led by Luc Montagnier and Françoise Barré-Sinoussi.
Which mistakes are common?
Mistake 1: Calling a virus a type of bacterium
Viruses are not cells and are much smaller. Antibiotics do not work on them.
Mistake 2: Saying that the lysogenic cycle never kills the host
The cell survives while the virus is dormant, but triggers can start the lytic cycle.
Mistake 3: Saying that viruses reproduce by dividing
They do not divide. They are assembled from components made by the host cell.
Mistake 4: Thinking all viruses contain DNA
Many viruses, including influenza, HIV and coronaviruses, have RNA genomes.