Estructura y clasificación de virus | Khan Academy en Español

Key Concepts

Viruses, size, shape (capsid, capsomeres, icosahedron, helical, spherical), genetic information (double-stranded DNA, single-stranded RNA, single-stranded DNA, double-stranded RNA, nucleocapsid), obligate intracellular parasite, bacteriophage, host specificity, injection mechanism, receptor-mediated endocytosis, direct fusion.

Size

Viruses are significantly smaller than bacteria and eukaryotic cells. A typical bacterium is 100 times larger than a virus. A typical eukaryotic cell is 1000 times larger than a virus. There is also size variation among viruses themselves.

Shape

Viruses are differentiated by their shape, which is determined by their capsid, a protein layer composed of basic building blocks called capsomeres.

• Capsomeres: Small, identical protein subunits that assemble to form the capsid.
• Icosahedron: A three-dimensional, 20-sided structure formed by capsomeres.
• Helical: A cylindrical shape formed by monomers wrapped around each other in a helix-like fashion.
• Spherical: Some viruses have an outer cover or envelope surrounding the capsid, giving them a spherical shape. Not all viruses have this envelope.

Genetic Information

Viruses contain only one type of nucleic acid, which can be:

• Double-stranded DNA (dsDNA): Common in human cells.
• Single-stranded RNA (ssRNA): As in messenger RNA.
• Single-stranded DNA (ssDNA): Unique to viruses.
• Double-stranded RNA (dsRNA): Unique to viruses.

The nucleic acid is packaged inside the capsid, forming the nucleocapsid. Some viruses have an external envelope, while others do not.

Pirate Robots: Obligate Intracellular Parasites

Viruses are referred to as "pirate robots" because they lack organelles and cannot produce energy (ATP) or replicate on their own. They are officially termed obligate intracellular parasites, meaning they must invade a host cell to replicate.

• Obligate Intracellular Parasite: An organism that requires a host cell to reproduce and survive.

The fourth characteristic that distinguishes viruses is their host specificity, meaning they can only infect certain types of cells.

• Bacteriophages: Viruses that infect bacteria.
• Viruses infecting eukaryotic cells (e.g., smallpox, herpes, parvovirus) are distinct in size, shape, nucleic acid, and the diseases they cause.

Mechanisms of Cellular Entry

Viruses have specific adaptations to enter host cells and replicate.

• Bacteriophages: Use a complex structure with a sheath and tail to inject their nucleic acid into bacteria. The tail adheres to the host bacteria, and the sheath acts as a needle to inject the nucleic acid.
• Eukaryotic Viruses: Employ different strategies to enter cells:
  • Receptor-Mediated Endocytosis: Viruses exploit normal cell receptors by mimicking signals, causing the cell to engulf the virus in an endosome. The receptors cannot differentiate between normal signals and viral signals.
  • Direct Fusion: Enveloped viruses can directly merge with the cell membrane, releasing their contents into the cell. The viral envelope sends a signal to the membrane, causing it to fuse and allow entry.

Conclusion

Viruses are characterized by their small size, distinct shapes, unique genetic material, and their nature as obligate intracellular parasites. Their mechanisms for entering cells, such as injection, receptor-mediated endocytosis, and direct fusion, highlight their sophisticated adaptations for hijacking cellular machinery to replicate.