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12.2. Viruses

Viruses are said to be obligate intracellular parasites because they lack their own metabolism, since they do not have the necessary enzymes to do so. That is why they need the matter, energy and enzymes of the cell they parasitize.

Although the viral organization is simple, viruses contain the genetic information necessary to control their replication and synthesize the proteins they need, some of them structural, and others necessary for their reproduction.

Viruses are small structures that are responsible for transporting nucleic acid from one host cell to another. Viruses, outside the cell, are called virions. In the extracellular phase, viruses are inert, since they do not have the necessary enzymes to have their own metabolism.

Viruses adhere to the surface of the cells to which they are going to parasitize, and introduce their genetic material (DNA or RNA), where the new envelopes for the new viruses are reproduced and synthesized (intracellular phase), taking advantage of the matter, enzymes and energy of the parasitized cell. As we have said, they are obligate intracellular parasites.

Various types of viruses are distinguished, depending on the cell they parasitize. Thus, phages parasitic bacteria, plant virusesplant cells, and animal viruses, which infect animal cells.

Viruses cause many diseases such as influenza, the hepatitis or AIDS, but it may also be beneficial. The biotechnology and genetic engineering can be used as virus vectors in the cloning of genes for therapeutic or industrial purposes. They have also had an important role in the evolution of living beings, by inserting themselves into the DNA of some organisms and carrying information to others.

Living beings perform the functions of nutritioninteraction and reproduction. Reproduction is the only function that viruses perform, even if it is using the matter, energy and mechanisms of the host cell, which is why we have said that they were considered obligate parasites. As they do not have their own metabolism or cellular organization, they are not considered living beings.

Questions that have come out in University entrance exams (Selectividad, EBAU, EvAU)

Cantabria, June 2021, question 6

Question 6 [1.25 POINTS]

Viruses: definition, composition and structure (drawing), lifestyle and role of viruses in nature. Why are viruses not considered living beings in biology?

Extremadura, June 2021, question 7

7.- In relation to viruses:

A. Briefly explain why a virus remains inert if it is not in contact with a host cell (0.5 points).
B. List two differences between viruses and prokaryotes. (0.5 points)
C. Give two arguments for and two against viruses being considered living organisms. (1 point).

Basque Country, June 2021, question 3A

In 2020, he awarded the Nobel Prize in Physiology or Medicine to three scientists, Harvey J. Alter, Michael Houghton, and Charles M. Rice, for their important contribution to the fight against blood-borne hepatitis that causes cirrhosis and liver cancer. This chronic infection, which affects 71 million people worldwide, is caused by the hepatitis C virus.

a) (1 point) Draw a diagram of the structure of a virus and indicate each of its components.

b) (0.5 points) Indicate the main differences between a virus and a bacterium.

c) (0.5 points) How do viruses multiply if their components do not show any metabolic activity? Give reasons for your answer.

d) (0.5 points) Are antibiotics effective in treating diseases caused by a virus? Give reasons for your answer.

Canary Islands, July 2021, question 18

18. A finger prick, a smartphone, and the new $ 34 device created by biomedical engineers at Columbia University (USA) are all it takes to detect the presence of the HIV AIDS virus and Syphilis bacteria Treponema pallidum in the blood. (Source: Science Translational Medicine).

a. Indicates the different types of viral nucleic acids, specify if they are single or double stranded.
b. Why do viruses need to invade a cell to multiply?
c. Viruses have two strategies of action: lytic cycle and lysogenic cycle. What difference is there in the result of each cycle?


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