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## The Origin and Classification of Viruses - **Origin:** It remains unclear where viruses came from. - **Hypotheses:** - They might have originated from a trace of nucleic acid from a larger organism. - They might have once lived outside of a host but lost genes over time and became depen...

## The Origin and Classification of Viruses - **Origin:** It remains unclear where viruses came from. - **Hypotheses:** - They might have originated from a trace of nucleic acid from a larger organism. - They might have once lived outside of a host but lost genes over time and became dependent on a host. - **Classifying Viruses:** - Viruses are not considered organisms because they're not alive, so they are classified differently. - **Key Classification Categories:** - Size - Shape - Type of genetic material (DNA or RNA) ## Virus Shapes - **Helical:** Resembles a rod with rod-like proteins spiralling on the inside. - **Polyhedral:** Looks like a Rubik's cube with many sides. - **Complex:** Consists of a polyhedral capsid attached to a helical protein tail. ## RNA vs DNA Differences - **Viruses with RNA:** - Higher mutation rate - Examples: - Human Immunodeficiency Virus (HIV) - Influenza Viruses - Rabies - Measles, Mumps, Pneumonia, Polio, Common Cold - SARS - Coronaviruses - **Viruses with DNA:** - Generally more stable and constant - Often have high vaccine effectiveness - Examples: - Chickenpox - Cold sores - Genital Herpes - Mononucleosis - Hepatitis - Respiratory infections - Tumours ## Methods of Viral Infection - **Lytic Cycle (Simple):** - Virus attaches to a host cell and injects genetic copies. - The virus multiplies within the cell until it bursts, releasing new copies and infecting other cells. - This process takes about 30 minutes, leading to the creation of 20-500 new virus particles. - **Steps:** 1. Attachment 2. Injection/Entry 3. Replication 4. Assembly 5. Release (lysis = breaking open) - **Lysogenic Cycle (Simple):** - Virus integrates its genetic material into the host cell’s DNA. - The host cell then replicates, passing on the viral genes to its progeny. - The virus remains dormant in the host cell for an extended period, often providing a benefit to the host. - When triggered, the viral DNA is released, and the lytic cycle begins. - **Steps:** 1. Attachment 2. Injection/Entry 3. Integration into host cell’s DNA 4. Dormancy/Normal cell functions 5. Triggering of viral DNA release, leading to initiation of the lytic cycle