Viruses

A **non-cellular infectious particle**: a **nucleic acid genome (DNA or RNA)** inside a **protein capsid**, sometimes with a lipid envelope. It replicates only inside a host cell.

It has **no cytoplasm, organelles, ribosomes or metabolism** — it is not built from cells.

A **genome** (DNA or RNA) and a **protein capsid** (built from capsomere sub-units).

The **protein coat** surrounding a virus's genome, made of repeating sub-units called **capsomeres**; it protects the genome and helps attach to a host.

The **lipid envelope** (from the host membrane) surrounds the capsid; the **spikes** bind **specific host-cell receptors** so the virus can attach and enter.

**Size**; **capsid shape** (helical / icosahedral / complex); and **genome type** (DNA vs RNA, single- vs double-stranded).

The virus **has no choice** but to be inside a **host cell** to replicate, using the host's **ribosomes and machinery**.

A virus that immediately runs the **lytic cycle**, **killing the host cell** to reproduce.

**Attachment → Entry → Replication/synthesis → Assembly → Lysis/release.**

The virus **binds a specific receptor** on the host surface. The match decides which cells it can infect — it sets the **host range**.

Only the **viral genome** (DNA or RNA) is injected; the empty protein coat is left outside.

It **hijacks the host's machinery** — enzymes, ribosomes, nucleotides and ATP — to copy its genome and make capsid proteins.

**Lysis** = the host cell **bursting and dying**, releasing many new viruses. Net effect = **rapid amplification of the virus + host-cell death**.

A bacteriophage that can take **either** the **lytic** cycle (kill the host now) **or** the **lysogenic** cycle (integrate and lie dormant).

The pathway where a phage's DNA **integrates into the host chromosome** and is copied with it, **without** making new virus or killing the cell.

Phage DNA that has **integrated into the host chromosome** and lies **dormant** there.

**Passively** — the host's own machinery copies the whole chromosome (prophage included) at every cell division, so it passes into **all daughter cells**.

A trigger (**stress, UV light or DNA damage**) that makes the prophage **excise** from the chromosome and switch to the **lytic** cycle.

**Lytic** = make virus **now** and **kill** the host. **Lysogenic** = **integrate** as a prophage, no immediate harm, can **later** turn lytic.

It is **copied passively** with the host chromosome at each division, so every **daughter cell inherits it** — vertical transmission, no virions released.

An **enveloped RNA virus** that carries the enzyme **reverse transcriptase**. HIV is the classic example.

It makes a **DNA copy from an RNA template** (RNA → DNA) — the **reverse** of normal transcription (which goes DNA → RNA).

Normal transcription goes **DNA → RNA**; reverse transcription goes **RNA → DNA** — the opposite direction.

The **viral DNA after it has integrated** into the host cell's own DNA. The host then transcribes it to make new virus.

HIV infects **helper T-lymphocytes (CD4 cells)**. Destroying them weakens the immune system, causing **AIDS**.

**Reverse transcriptase has no proofreading**, so its copying errors are not corrected — giving a **high mutation rate** and rapid evolution.

Its fast mutation lets it **evolve drug resistance** (so combination antiretrovirals are used) and **escape the immune system** (so no effective vaccine yet).

It is an enzyme **your own cells don't have**, so blocking it stops the virus copying its RNA into DNA while largely sparing your cells.

They have **multiple independent origins** — viruses probably arose **several separate times**, not from one common ancestor.

**Escape** (host genes/mobile elements gained a capsid), **reduction** (free-living parasitic cells lost genes), and **virus-first** (self-replicating molecules co-evolved with early cells).

Viruses are non-cellular and leave no fossils, and each hypothesis explains only **some** viruses — so there is **no single agreed origin**.

**Huge populations + very short generation times + high mutation rates** mean **natural selection** acts on them extremely fast.

Their **polymerases lack proofreading**, so many copying errors (mutations) accumulate each generation.

**Drift** = small, gradual mutations in the surface proteins. **Shift** = a large, sudden change when two strains **swap whole genome segments**.

Variants with **changed surface proteins** escape existing immunity and are **selected for**, so the circulating strain no longer matches the old vaccine.