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  • A virus can be describe as "genes in a box". It was a bit of nucleic acid wrapped in a protein coat.
  • It can only survive by infecting a living cell, making it create other viruses


  • An adenovirus is a virus that infects the human respiratory system. 
  • Shaped like a polyhedron


Viruses that attack bacteria are called bacteriaphages or phages. Once they infect a bacterium, they enter a reproductive cycle called the lytic cycle. In the lytic cycle, they insert their DNA into a cell, and that cell begins reproducing phages until overfill the cell and cell breaks open, releasing viruses. 

Some viruses use the lysogenic cycle. In the lysogenic cycle, the phage's DNA gets inserted into the cell, and it combines with the cell's DNA. Now, the phage's DNA are called prophages, and they are mainly harmless. So, the host cell reproduces itself, and the prophage DNA is multiplied. This may cause a large population of bacteria to have prophage DNA. This is dangerous, because if a prophage leaves the chromosome, it may enter the lytic cycle, leading to the destruction of the cell. This may be due to an environmental stimulus, so it could potentially destroy an organism.

Plant Viruses

Viruses that infect plant cells can diminish crop yields and stunt plant growth. Most of them have RNA instead of DNA as their genetic material. They usually have a rod shape, like the Tobacco Mosaic Virus.

The virus enters the plant by penetrating the epidermis. So, if a plant is damaged by wind, injury, or insects can be infected easier than a healthy cell. There is no cure for most viral plant diseases, so scientists try to breed plants that are resistant to various viruses. 

Animal Viruses

Viruses that infect animals are a common cause of disease. Many animal viruses have RNA instead of DNA, such as HIV and polio. Diseases caused by DNA include hepatitis, chicken pox, and herpes. 

Reproduction of an RNA virus

When the virus contacts a cell, the protein spike attaches to the receptor proteins on the cell's plasma membrane.  The protein-coated RNA enters the cytoplasm. Then, enzymes remove the protein coat. The RNA is replicated within the cell, and a new coat assembles around the new viral RNA. Finally, the viruses leave by cloaking themselves in plasma membrane. So, instead of destroying the cell, they sneak out.

Reproduction of a DNA virus

DNA viruses, like herpes, reproduce in the cell nucleus, and copies of their DNA usually remain there, usually latent, until physical stress triggers the DNA to begin producing the virus. 

HIV, the AIDS Virus

HIV is a type of an RNA virus with certain unique characteristics. Its RNA enables it to enter and leave a cell like the aforementioned method of an RNA virus reproducing. HIV is a retrovirus, meaning it reproduces like a DNA virus. 

They reverse the usual DNA-> RNA flow of genetic information. They carry molecules of an enzyme called reverse transcriptase, which catalyzes reverse transcription. The reverse transcriptase uses the RNA as a template to make a DNA strand, then it adds the second, complementary DNA strand. The resulting DNA then enters the cell nucleus and inserts itself in the the chromosomal DNA, becoming a provirus. Occasionally, the provirus is transcribed into RNA and translated into viral proteins. New viruses made from that RNA eventually leave the cell, undisturbed, and then infect other cells.

AIDS stands for acquired immunodeficiency syndrome, meaning when many symptoms are present in a patient with HIV, they are said to have AIDS. The first medicine invented for HIV is called AZT, and is still widely used today. 

The biggest pandemic in history was in 1918. With people going between continents, the flu was easily able to spread, infecting around 500 million people worldwide. 


David Witten

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