Viruses are among the simplest living organisms, and their structures are also very simple.
The main component of a virus is the core, which contains the genetic material. This genetic material can be either DNA or RNA, and is essential for the virus's ability to reproduce and infect host cells.
Surrounding the core is the capsid, a protective protein coat. The capsid shields the genetic material from the external environment and helps the virus attach to host cells.
The capsid is made up of multiple capsomeres, which are the smallest protein subunits that come together to form the complete capsid. Just as many monomers form a polymer, many capsomeres join to create the capsid.
The combined structure of the capsid and the core is called the nucleocapsid. This term refers to the entire unit of genetic material enclosed within its protective protein shell.
Other Layers of Virus
Some viruses, especially animal viruses like HIV and Influenza, have an additional lipoprotein layer called the envelope. This envelope surrounds the capsid and is derived from the host cell's surface membrane as the virus exits the cell.
The envelope is made of lipids and proteins, and its main function is to protect the virus and help it evade the host's immune system. Because the envelope comes from the host cell, the immune system may not recognize the virus as foreign, allowing it to survive longer.
Many enveloped viruses also have glycoproteins embedded in the envelope. These glycoproteins form specific patterns on the virus surface and play a crucial role in recognizing and binding to specific host cells.
Glycoproteins help the virus identify and attach to the correct host cell, ensuring infection occurs only in suitable cells. For example, a virus targeting respiratory cells will have glycoproteins that match receptors in the respiratory tract, similar to how a key fits a specific lock.
Structure of Bacteriophage
A bacteriophage is a virus that infects bacteria. The name comes from "bacteria eater." Bacteriophages vary in size, typically ranging from 24 to 200 nanometers in diameter.
Bacteriophages are also known as "complex viruses" or "tadpole viruses" due to their unique structure.
The structure of a typical bacteriophage consists of two main parts: the head and the tail.
The head contains the nucleocapsid, which includes the capsid (protein coat) and the core (genetic material, usually DNA). The capsid protects the genetic material inside.
The tail is helical (spiral-shaped) and rod-like. It includes several components:
Collar: Connects the head and tail.
Hollow tube: Runs through the center of the tail, serving as a channel for genetic material to pass into the host.
Sheath: A flexible, contractile covering around the hollow tube. The sheath can contract and relax, helping inject the genetic material into the bacterial cell.
Base plate: Located at the end of the tail, it contains the enzyme lysozyme (also called lysin), which breaks down the bacterial cell wall to allow entry of the viral DNA.
Tail pins: Needle-like structures that help anchor the bacteriophage to the bacterial surface and assist in injecting the genetic material.
Tail fibers: Long, thin fibers that help the bacteriophage attach firmly to the host bacterium, ensuring a stable connection during infection.
The most well-studied bacteriophage is the T4 type, which has both a head and a tail and serves as a model for understanding bacteriophage structure.
Key Terms & Definitions
Core: The genetic material (DNA or RNA) of the virus.
Capsid: The protective protein coat surrounding the core.
Capsomeres: The smallest protein units that make up the capsid.
Nucleocapsid: The combined structure of the capsid and core.
Envelope: The outer lipoprotein layer of some viruses, derived from the host cell membrane.
Glycoprotein: Surface proteins embedded in the envelope that help the virus recognize and attach to specific host cells.
Bacteriophage: A virus that infects bacteria, also known as a complex or tadpole virus.
Action Items / Next Steps
Draw a detailed diagram of the bacteriophage structure, labeling all key parts (head, capsid, core, collar, hollow tube, sheath, base plate, tail pins, tail fibers).
Review and study the virus life cycle in the next lecture, focusing on how viruses infect host cells and replicate.