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12.2.2. Viral physiology

Viral physiology

We have already seen that viruses are not living beings because they do not fulfill the functions of living beings:

  • They do not have a nutritional function, since they do not need to develop any activity or matter to grow.
  • They do not have a relational function, since contact with cells is fortuitous.
  • They only perform the function of reproduction , although they need the metabolic mechanisms of the cells they parasitize in order to multiply.

Viruses are obligate intracellular parasites that can be in two phases, an inert extracellular phase and an active intracellular phase.

We will only study how viruses reproduce. The life cycle of viruses is called the lytic cycle, but a lysogenic or persistent infection cycle can also occur , when the virus remains inside the host cell without producing new viruses.

Viruses need to develop their life cycle, a host cell from which they obtain their matter and energy in order to synthesize their new nucleic acids and capsomeres. To do this, they penetrate inside the cell and use its machinery to produce new virions. This life cycle consists of these phases:

  1. Fixation or adsorption.
  2. Penetration.
  3. Eclipse phase.
    1. Nucleic acid replication.
    2. Capsomere synthesis.
  4. Assembly of new viruses.
  5. Lysis or release.

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

Aragon, June 2017, option A, question 4. The schemes in the accompanying figure represent a bacteriophage and an animal virus. (2 points)

c) What does the following sentence mean ?: "Viruses are obligate intracellular parasites". (0.75 points)

Aragon, September 2009, option B.3 .

Viruses (1 point):

a) Why are viruses obligate parasites?

b) How are their proteins and nucleic acid synthesized?

Aragon. September 2002, option B. Issue 2. -

Viruses (1 point):

a) Why are viruses obligate parasites?

b) How are their proteins and nucleic acid synthesized?

Basque Country, July 2020, extraordinary test, option 4B

In 2020 there has been a major international crisis as a result of the SARS-CoV-2 coronavirus outbreak declared by the WHO as a Public Health Emergency.

a) (1 point) What are viruses and how do they multiply?

b) (0.5 points) Are they considered living beings? Give reasons for your answer.

c) (0.5 points) Are antibiotics effective in treating viral infections? Give reasons for your answer.

d) (0.5 points) Do all viruses cause disease in humans? Are there useful viruses for technological development? Reason for your answers.

Asturias, July 2021, question 4B

In May 2019, British teenager Isabelle Holdaway saved her life thanks to a treatment with genetically modified phages that eliminated an infection resistant to antibiotics. Days later, in Brussels, a baby was similarly treated for an antibiotic-resistant liver and blood infection.

a. What relationship or link do you find between phages and antibiotic resistant infections? (Maximum score 0.5 points)

b. Justify to which phases of the life cycle of a phage the following cultures of the microorganisms they infect belong:

Culture A: The microorganisms continue to grow for several days without phage being observed in the culture medium.
Culture B: Phages are observed in the culture medium. (Maximum score 1 point)

c. Indicate two similarities between viruses and other microorganisms. (Maximum score 0.5 points)

Fundamental ideas about viral physiology. Viruses are obligate intracellular parasites

The viruses are obligate parasites because they can not meet alone the three vital functions of a living being by themselves such as nutrition, relationships and reproduction, but they need a host cell to reproduce.