Understanding Viruses: Composition, Types, and Cycles

Posted on Mar 11, 2025 in Biology

Concept of Virus

Viruses are very simple, microscopic particles that consist only of a nucleic acid (viral genome) and some envelope proteins (capsid). Therefore, they are not cells and are not surrounded by membranes, forming acellular organizational systems. They exist on the boundary between the living and the nonliving.

Composition of Viruses

The composition of a virus is very simple, consisting of a single molecule of nucleic acid, which can be:

  • Single-stranded RNA
  • Double-stranded RNA
  • Single-stranded DNA
  • Double-stranded DNA

Proteins

Surrounding the nucleic acid is a series of proteins; this wrapper is called a capsid. Some viruses carry enzyme proteins, which are often necessary in a host cell that cannot provide them. Some examples are:

  • The AIDS virus
  • Animal viruses that carry enzymes

Viral Envelope

Some viruses have a membrane roof. These viruses are called enveloped viruses, and those without are called naked or non-enveloped viruses.

Types of Viruses

A) By the Host They Infect

Viruses can be classified as:

  • Animal viruses
  • Plant viruses
  • Bacteriophage viruses

B) By Type of Nucleic Acid and Transcription Method

Another classification, given by Baltimore, groups viruses according to the form of their nucleic acid (DNA or RNA, and single- or double-stranded).

C) According to Structure

There are five types of virus structures:

  • Helical naked (without a casing): RNA (e.g., tobacco mosaic virus) or DNA.
  • Polyhedral naked: Nucleic acid can be RNA (like the poliovirus) or DNA (like those that cause warts).
  • Helical coated: Nucleic acid can be RNA (like that of influenza) or DNA (like that of smallpox).
  • Polyhedral coated: Nucleic acid can be RNA (like that of AIDS) or DNA (like that of herpes simplex).
  • Mixed or complex: Consisting of a helical and a polyhedral part, these are bacteriophages.

Lytic Cycle of Virus Infection

For didactic purposes, the cycle is divided into five phases, although in reality, it is a continuous process:

  1. Adsorption: The union between the virus and the host cell.
  2. Penetration: The entry of the virus’s genetic material. In phages, the capsid remains outside the bacteria.
  3. Eclipse: The phase during which the whole virus is undetected inside the cell, since nucleic acids and proteins are newly synthesized, and the capsid is outside.
  4. Assembly: The construction of new viruses from their constituent elements.
  5. Release: The exit of the new virus particles to the outside.

Lysogenic Infection Cycle (Lysogeny)

The most studied case is that of prophages, and it occurs as follows:

  • Absorption: As in the lytic cycle.
  • Entry: As in the lytic cycle.

The stages of eclipse, assembly, and release do not occur, but the virus’s DNA is inserted or fits into the DNA molecule of the bacterium. It remains there and is replicated when the bacterial DNA replicates. At a given moment, spontaneously or by induction caused by some agents (e.g., ultraviolet light), the virus can continue with a lytic cycle (i.e., eclipse, assembly, and release).

Cycle of Virus Infection of Eukaryotic Cells

  1. Absorption: Occurs in general by electrostatic attraction to the receptors of the cell coat.
  2. Penetration: May occur in several ways, e.g., by:
    • Endocytosis in animal cells.
    • Fusion of the viral envelope with the plasma membrane.
    • Through wall injuries (e.g., insect bites) in plant cells.
  3. Eclipse: The fundamental difference is that cellular DNA is not always considered.
  4. Assembly: Often spontaneous, and usually involves simply structured viruses.
  5. Release: May be by:
    • Cell lysis induced by viral enzymes.
    • Cell lysis induced by the immune system.
    • Enveloped viruses exit by budding.