paleontology10: What are viruses? (v1.0)
In my paleontology essays, I talked about life and started with a cell as the smallest unit. All life has DNA. But I made no mention of viruses. What are viruses?
Viruses are not really life. They have a small strand of genetic material, DNA or RNA, and many have a protein coat around them called an envelope. The protein may be spiky and help it to bind onto a cell and penetrate it. They don't have a cell or cellular structure, cannot reproduce by themselves, cannot synthesize proteins by themselves and do not have any of the organelles we associate with life. But they are often very good at making their way into cells and hijack the cells machinery to create copies of themselves and create viral proteins and create other genetic material. They are a parasite that only operates meaningfully in a host cell. Viruses can infect plants or animals, and some may cause disease, while others may be helpful. A virus may also affect one organism in one way but a different one in another way. This explains why a virus that causes illness in a cat may not affect a human.
Although viruses are most often studied as pathogens, many are beneficial to their hosts, providing essential functions in some cases and conditionally beneficial functions in others. Beneficial viruses have been discovered in many different hosts, including bacteria, insects, plants, fungi and animals. How these beneficial interactions evolve is still a mystery.
Three competing theories
- The regressive, or reduction, hypothesis suggests that viruses started as independent biological entities that became parasites. Over time, they shed genes that did not help them parasitize, and became entirely dependent on the cells they inhabit.
- The progressive, or escape, hypothesis postulates that viruses evolved from sections of DNA or RNA that “escaped” from the genes of larger entities. In this way, they gained the ability to become independent and move between cells.
- The virus-first hypothesis suggests that viruses evolved from complex molecules of nucleic acid and proteins either before or at the same time as the first cells on Earth appeared, billions of years ago.
Viruses often change over time. One key method is as they reproduce, “copying errors” and genetic changes naturally occur. Some of these changes are very small and do not cause concern, but others can be more significant.
Viruses cause many human diseases. For example, the Epstein-Barr virus can lead to mononucleosis. Other viral diseases include:
- COVID-19
- smallpox
- the common cold
- different types of flu
- measles
- mumps
- rubella
- chickenpox
- hepatitis
- herpes simplex virus (HSV)
- polio
- rabies
- Ebola
- hantaviruses (spread primarily by rodents)
- HIV
- SARS
- dengue fever
- Zika
When the body’s immune system detects a virus, it starts taking measures to protect the body. As viruses enter the body’s cells, the immune system cannot “see” the virus. However, special T cells, known as cytotoxic T cells, can recognize cells that contain viruses, and release substances that kill those cells. Some viruses can escape detection by cytotoxic T cells, but other immune cells — natural killer cells — can cause the cell containing the virus to die. In addition, body cells that contain a virus emit proteins called interferons, which warn other cells that a virus is present. This gives healthy cells a chance to defend themselves by changing the molecular makeup of their surface. Antibodies can also help fight a virus before it enters a cell. They do this by neutralizing or damaging the virus or by changing its features so that it can no longer enter healthy cells. People may have antibodies if they have already had the virus or if they have received a vaccine.
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