Viruses Have Its Unique Structural Features
Definition
Virus
A virus is a non-cellular particle composed of genetic material (DNA or RNA) enclosed in a protein coat called a capsid.
- A virus is a non-cellular particle composed of genetic material (DNA or RNA) enclosed in a protein coat (capsid).
- Viruses lack the structures and processes for independent life and can only reproduce by infecting a host cell.
- They are obligate intracellular parasites, depending entirely on host cells to replicate.
Core Features of Viruses
1. Small and Fixed Size
- Viruses range from 20 to 300 nanometers, much smaller than bacteria and cells.
- Their small size allows them to invade host cells and evade immune defenses.
- Viruses do not grow after assembly—their size is determined by capsid protein arrangement.
- Viruses are assembled inside host cells, much like a car is built in a factory.
- They do not grow or develop like living organisms.
2. Genetic Material: DNA or RNA
- All viruses contain one type of nucleic acid (DNA or RNA, never both).
- Genetic material varies:
- Single-stranded (ss) or double-stranded (ds)
- Linear or circular
- This genetic material directs the host cell's machinery to produce viral proteins and replicate the viral genome.
RNA viruses, such as coronaviruses, often mutate more quickly than DNA viruses because RNA replication lacks the proofreading mechanisms found in DNA replication.
3. Protein Capsid: A Protective Shell
- The capsid is a protein coat made of repeating subunits that encases the viral genome.
- Functions:
- Protection: Shields genome from environmental damage (e.g., enzymes).
- Host recognition: Helps virus attach to specific host cells.
- Efficiency: Simplifies assembly using minimal genetic information.
- Capsid shapes include icosahedral (spherical), helical (rod-like), and complex (e.g., bacteriophages).
- Influenza's capsid is made of protein subunits and is further enclosed in a lipid membrane (envelope) derived from the host cell.
- This envelope helps the virus evade the immune system, although not all viruses have this feature.
4. No Cytoplasm or Cellular Organelles
- Viruses lack cytoplasm, organelles, or membranes.
- They cannot perform metabolic processes like energy production or protein synthesis independently.
- Instead, they rely entirely on the host cell's machinery to replicate.
- Think of a virus as a USB drive containing a program.
- The USB drive cannot execute the program on its own as it needs to be plugged into a computer (the host cell) to run the code.
- This is why viruses are considered acellular.
- They do not have the internal structures of living cells.
5. Few or No Enzymes
- Viruses carry only essential enzymes for their replication.
- Most metabolic and synthetic processes are performed by the host cell.
- Reverse Transcriptase: In retroviruses, copies RNA into DNA.
- Neuraminidase: In influenza viruses, helps release new viral particles.
- Why must viruses be smaller than their host cells?
- Why are viruses described as acellular?
- What advantages do viruses gain by lacking cytoplasm and organelles?
- How does the capsid contribute to the success of a virus?
How do the minimal features of viruses challenge our definitions of life? Can something that is not "alive" still evolve and adapt? How might this connect to the Theory of Knowledge in biology?
