Viruses sit at the boundary between living and non-living. They can't reproduce on their own, yet they hijack host cells with remarkable precision. Understanding their structure reveals how they infect cells, and understanding their replication cycles reveals why they're so effective at spreading.
Viral Structure
Components of a Virus
Every virus has at least two core components: a genome and a capsid. Some viruses add a third layer, the envelope.
- Capsid: A protein shell built from repeating subunits called capsomeres. The capsid protects the viral genome and, in non-enveloped viruses, helps the virus attach to host cells.
- Viral genome: The genetic material inside the capsid. This can be either DNA or RNA, and either single-stranded or double-stranded, depending on the virus. It carries all the instructions needed for replication.
- Envelope: Some viruses are surrounded by a lipid bilayer stolen from the host cell's membrane. Embedded in this envelope are glycoproteins that help the virus recognize and bind to specific host cells. Influenza is a classic enveloped virus.
Variations in Viral Structure
Not all viruses look the same. Their capsid shapes fall into a few major categories:
- Icosahedral: A roughly spherical shape with 20 triangular faces (poliovirus)
- Helical: A rod-shaped capsid with capsomeres spiraling around the genome (tobacco mosaic virus)
- Complex: Irregular structures that don't fit neatly into the other categories. Bacteriophage T4 is the go-to example, with its icosahedral head, tail sheath, tail fibers, and base plate.
Some viruses also have extra structural features. Influenza, for instance, has a matrix protein layer between the capsid and envelope that helps maintain the virus's shape and plays a role in assembly.
The enveloped vs. non-enveloped distinction matters practically: enveloped viruses are easier to destroy with soap and disinfectants because disrupting the lipid bilayer inactivates the virus. Non-enveloped viruses tend to be hardier outside a host.
Viral Replication Cycles
Viruses can't replicate on their own because they lack ribosomes and the metabolic machinery needed to make proteins or generate energy. Instead, they force host cells to do the work. The two main replication strategies are the lytic cycle and the lysogenic cycle.
Lytic Cycle
The lytic cycle is fast and destructive. It ends with the host cell bursting open (lysing) and releasing new virus particles. Here's how it works:
- Attachment: The virus binds to specific receptors on the host cell's surface.
- Entry/Penetration: The virus injects its genetic material into the host cell (or, in some cases, the whole virus is taken in).
- Replication and synthesis: The host cell's machinery is redirected to copy the viral genome and produce viral proteins. The host's own DNA may be degraded.
- Assembly: New viral genomes are packaged into newly made capsids, forming complete virus particles (called virions).
- Lysis and release: The host cell bursts, releasing hundreds of new virions that can infect neighboring cells.
Bacteriophage T4 infecting E. coli is the textbook example. A single T4 phage can produce roughly 100-200 new phages before the bacterial cell lyses.
Lysogenic Cycle
The lysogenic cycle is quieter. Instead of immediately destroying the host, the virus integrates its genome into the host's DNA and waits.
- Attachment and entry: Same initial steps as the lytic cycle.
- Integration: The viral DNA inserts itself into the host cell's chromosome. At this point, the integrated viral DNA is called a provirus (or prophage in bacteriophages).
- Dormancy and replication: Every time the host cell divides, it copies the provirus right along with its own DNA. The viral genes are essentially silent, and the host cell functions normally.
- Induction: Certain triggers, such as UV radiation, chemical stress, or damage to the host cell's DNA, can cause the provirus to excise from the chromosome and enter the lytic cycle.
Lambda phage in E. coli is the classic example. It can persist as a prophage through many generations of bacterial division before environmental stress flips it into lytic mode.
The key difference: the lytic cycle destroys the host cell quickly, while the lysogenic cycle allows the virus to hide inside the host genome and spread passively through cell division until conditions favor active replication.
Viral Assembly and Release
How new viruses are put together and exit the cell depends on the type of virus:
- Assembly location varies. Herpesviruses assemble in the nucleus, while picornaviruses (like the common cold rhinovirus) assemble in the cytoplasm.
- Non-enveloped viruses typically exit through lysis, rupturing and killing the host cell.
- Enveloped viruses exit through budding: the assembled capsid pushes through the host cell's plasma membrane, wrapping itself in a piece of that membrane (complete with viral glycoproteins already inserted into it). This is how influenza and HIV leave the cell. Budding doesn't necessarily kill the host cell immediately, which means the cell can continue producing viruses for a longer period.
Types of Viruses
Retroviruses
Retroviruses are a special category of RNA viruses that replicate through a DNA intermediate. The name "retro" refers to the reverse direction of information flow: RNA → DNA, the opposite of the usual DNA → RNA.
- The retrovirus enters the host cell carrying its RNA genome and a key enzyme called reverse transcriptase.
- Reverse transcriptase converts the viral RNA into double-stranded DNA.
- This viral DNA integrates into the host cell's chromosome, becoming a provirus.
- The provirus can be transcribed by the host's normal machinery to produce new viral RNA and viral proteins.
HIV (Human Immunodeficiency Virus) is the most well-known retrovirus. It targets helper T cells ( cells), gradually weakening the immune system and leading to AIDS if untreated. Human T-cell leukemia virus (HTLV) is another retrovirus that can cause certain cancers.
Retroviruses are particularly difficult to eliminate because once the provirus integrates into the host genome, it becomes a permanent part of that cell's DNA.
Bacteriophages
Bacteriophages (or just "phages") are viruses that specifically infect bacteria. They've been enormously important in the history of molecular biology.
- Lytic phages like T4 replicate aggressively and destroy the bacterial cell.
- Temperate phages like lambda can choose between the lytic and lysogenic pathways depending on host cell conditions (nutrient availability, cell stress, etc.).
- Phages are highly specific. Each type typically infects only certain bacterial species or even certain strains, because attachment depends on matching surface receptors.
Beyond the classroom, phages matter in two practical areas. They've been essential research tools for understanding DNA replication, gene regulation, and molecular genetics (the Hershey-Chase experiment used T2 phage to confirm DNA as the genetic material). Phage therapy, using phages to treat antibiotic-resistant bacterial infections, is also an active area of medical research.