Bio Lecture Viruses chp. 24

Characteristics of viruses

• derived from cells of other organisms

• Have DNA and RNA

• Infect all cellular forms of life (Prokaryote and Eukaryote)

• Replicate, mutate, evolve

• Evolve independently of other organisms

Why is virus phylogeny difficult to resolve

• Viral genomes are tiny

• Rapid mutation rate. Mutations cloud evolutionary relationships

• No viral fossils found

• Very diverse

How are viruses classified

based on genome structure

Types of viruses (can be DNA or RNA)

• negative sense single stranded RNA virus (influenza)

• positive sense single stranded RNA virus (covid)

• RNA retrovirus (HIV): retrovirus needs reverse transcriptase

• Double stranded RNA virus

• Double strand DNA virus (Bacteriophage T2)

Viral reproductive cycles

Lytic cycles

Lysogenic cycles

Lytic cycles

soon after infection, the host cell lyses, releasing progeny virus

Lytic cycle steps

• virus infects host cell

• Uses host bacterium’s RNA polymerase to transcribe early genes

• one early protein shuts down host gene transcription

• another early protein stimulates viral genome replication

• Another early protein stimulates late gene transcription

• production of capsid proteins and a protein that lyses the host cell

capsid proteins

protein shell that encapsulates and protects viral genome

Lysogenic cycle

method used to delay lytic cycle. Viral DNA integrates with host DNA but is not expressed. Inserted bacteriophage genome is called a prophage

Prophage

bacteriophage genome that is integrated into bacterial chromosome but does not kill or get expressed.

When does a lysogenic cycle become a lytic cycle

If the cell is sick or environment has changed and host cell is not doing well than prophage can excise from chromosome and start reproducing starting the lytic cycle.