Viruses
Viruses are not cells, yet they infect every form of life. They sit on the boundary between living and nonliving.
What a virus is
A virus is genetic material (DNA or RNA) inside a protein coat called a capsid; some also have an outer lipid envelope taken from the host cell. Viruses are far smaller than bacteria, often 20-300 nanometres. They cannot grow or reproduce on their own, which is why many biologists do not consider them alive.
How viruses replicate
A virus attaches to specific receptors on a host cell, enters or injects its genetic material, takes over the cell's machinery to copy its genome and make proteins, assembles new viruses, and releases them, often killing the cell. Receptor specificity explains why most viruses infect only certain species or tissues.
Lytic and lysogenic cycles
In the lytic cycle, a virus multiplies quickly and bursts the cell. In the lysogenic cycle, its DNA integrates into the host genome and is copied silently with each cell division, later switching to the lytic cycle. Herpes viruses hide in nerve cells and reactivate, causing cold sores.
Retroviruses
Retroviruses such as HIV carry RNA and use the enzyme reverse transcriptase to make DNA copies that insert into the host's genome. HIV destroys helper T cells, weakening the immune system. Antiretroviral drugs block steps like reverse transcription and now let people with HIV live long lives.
Viral evolution
Viruses, especially RNA viruses, mutate rapidly because their copying is error-prone. Influenza changes enough each year that new vaccines are needed. Occasionally a virus jumps from animals to humans (a zoonosis), as with HIV, Ebola, and SARS-CoV-2.
Treating and preventing viral disease
Antibiotics do not work on viruses, because viruses lack the bacterial structures antibiotics target. Antiviral drugs exist for some viruses, but prevention through vaccines has been most effective: smallpox was eradicated worldwide in 1980, and polio is close to eradication.
Bacteriophages
Bacteriophages are viruses that infect bacteria. They are the most abundant biological entities on Earth. Phage therapy, using phages to kill antibiotic-resistant bacteria, is being explored as a treatment when antibiotics fail.
Prions and viroids
Prions are misfolded proteins that cause normal proteins to misfold, leading to brain diseases such as mad cow disease. They contain no genetic material at all. Viroids are tiny circles of RNA without a coat that infect plants. Both stretch the definition of infectious agents.
Key takeaways
Viruses are DNA or RNA in a protein capsid and need host cells to reproduce. They attach to specific receptors, hijack the cell, and are released. Lytic cycles destroy cells quickly; lysogenic cycles hide genomes. Retroviruses like HIV use reverse transcriptase. Antibiotics do not work on viruses; vaccines have eradicated smallpox.
Flashcard
Genetic material in a protein coat
Check your understanding
Why are antibiotics ineffective against the common cold?