Q. Which of the following statements is/are correct

1. Viruses lack enzymes necessary for the generation of energy.
2. Viruses can be cultured in any synthetic medium.
3. Viruses are transmitted from one organism to another by biological vectors only.

Select the correct answer using the codes given below.

• 1 only
• 2 and 3 only
• 1 and 3 only
• 1, 2 and 3

Answer: (a) 1 only

Notes

• A virus is a small collection of genetic code (either DNA or RNA) surrounded by a protein coat.
• They lack metabolic machinery of their own to generate energy or synthesize proteins. They can reproduce only within a living cell – hence they are obligate intracellular parasites. They take over the functions of the host cell hence causing infection.
• Viruses are inactive outside the host (crystallized form) and active inside. This puts them in the unique criteria of living as well as the non-living entity or ‘not truly living’.
  • Unlike bacteria and other microorganisms which can be grown in artificial media, viruses cannot be grown on artificial medium but must be grown in living cells. Viruses cannot be grown on an ordinary culture medium because they are not actually living organisms. Viruses can be cultivated within suitable hosts, such as a living cell.
• A virus is an acellular organism which are 10 to 100 times smaller than bacteria, about 20-300 nm in size.
• Structure of a Virus
  • The core of the virus is made up of nucleic acids, which then make up the genetic information in the form of RNA or DNA. The protein layer that surrounds and protects the nucleic acids is called the capsid. Sometimes the capsid is surrounded by an additional spikey coat called the envelope.
  • Viruses are capable of latching onto host cells and getting inside them. When a single virus is in its complete form and has reached full infectivity outside of the cell, it is known as a virion.
• Spread:
  • The virus enters hosts through horizontal or vertical transmission, mostly horizontal. Examples of horizontal transmission include the following:
    • Direct contact transmission: This refers to transmission via physical contact between an infected and uninfected subject through kissing, biting, or sexual intercourse, for example.
    • Indirect transmission: Here, the virus is transmitted via contact with contaminated objects or materials such as medical equipment or shared eating utensils.
    • Common vehicle transmission:This transmission mode refers to when individuals pick up the virus from food and water supplies that are contaminated with feces. This often causes epidemic disease.
    • Airborne transmission refers to the respiratory infection that occurs when the virus is inhaled.
  • Once a virus has accessed its host, it recognizes and binds to a specific receptor on the surface of a target cell. One well-studied example is the interaction that occurs between the CCR5 receptor on human T lymphocytes and the gp41 protein present on the surface of the human immunodeficiency virus (HIV).
• Life cycle of a virus
  • Once a virus has infected a host cell, it can replicate within that cell thousands of times. Rather than dividing and reproducing in the way that cells do, viruses go through a process called the lytic cycle.
  • First, the virus replicates its DNA and protein coats, which are then assembled into new virus particles. This causes the host cell to burst or “lyse,” which is why the cycle is so-called. The new virus particles that are released once the cell has burst then infect surrounding host cells.
  • The process can take as little as twelve hours, as is the case with the norovirus, or as long as several days, as is the case with the Ebola virus.
  • Some complex viruses called phages bind their DNA to that of their host cell or deposit small pieces of their DNA in the cytoplasm. When the cell then divides, the viral DNA is copied into the daughter cells. This cycle, which is called the lysogenic cycle, is less common than the lytic cycle.