Viruses

Introduction to Viruses and Cells

• To be considered a living organism, one must consist of cells.

• Viruses do not fulfill this criterion:

  • They are not considered cells.

  • They lack the capability for independent metabolism and cannot perform cellular functions without a host.

  • Example: Viruses cannot hydrolyze lactose.

Characteristics of Viruses

• Viruses rely entirely on their host for replication.

• They cannot carry out transcription or translation independently.

Phages

• Definition: A type of virus that specifically infects bacteria, commonly referred to as bacteriophage.

  • Termed as obligate intracellular parasites since they only replicate within a host cell.

  • They induce a negative impact on the host’s health.

• Outside a host cell, they are referred to as infectious particles and sometimes called virions.

Viruses and Host Cells

• Infection Process:

  • Viruses must hijack the host's biochemical machinery to reproduce themselves.

  • Two types of cycles that phages follow upon infecting a host (bacteria):

Lytic Cycle

• Process:

  1. Phage attaches to the host cell and injects its DNA.

  2. The phage's nucleic acid is replicated, and phage genes are expressed.

  3. This results in the production of phage proteins.

  4. Mature phage particles are assembled.

  5. The host cell lyses, releasing the phage particles.

Lysogenic Cycle

• Process:

  1. The phage DNA integrates into the host cell genome.

  2. The host cell replicates, carrying the integrated phage genome.

  3. The integrated phage DNA is called prophage DNA.

• When exposed to ultraviolet light or certain chemicals, phage induction occurs:

  • The prophage DNA is excised and can enter the lytic cycle.

Examples of Viruses

• Coronavirus 2:

  • An RNA virus that caused the pandemic.

  • Characterized by spike proteins on its surface.

  • The spikes can mutate, leading to new variants.

  • Implication: Vaccination may be necessary when spike proteins change, as the immune system may not recognize new variants.

Mechanisms of Viral Entry into Host Cells

• Two mechanisms for enveloped viruses:

1. Fusion

• The virus attaches to host cell receptors via specific spike proteins.

• The envelope fuses with the plasma membrane of the host cell, releasing the nucleocapsid into the cytoplasm.

• The nucleic acid separates from the protein coat.

2. Endocytosis

• The virus binds to the host cell and is engulfed in a vesicle.

• The envelope fuses with the vesicle’s membrane, releasing the nucleocapsid into the cytoplasm.

• For naked virions, the process is similar but without fusion; the nucleic acid is released after being engulfed.

Conclusion

• The understanding of viral replication and infection is crucial in combating viral diseases.

• Knowledge of cycles and entry mechanisms serves as the foundation for developing treatments and vaccines against various viral infections.