Microbiology Ch 17.1: Viruses 17.2: Monera

Virus: A non- cellular particle made up of genetic material and protein that can invade cells

Characteristics of Living Things To be Alive you need to: 1. Have cells (or be a cell!) 2. Ingest nutrients and energy 3. Be homeostatic 4. Respond to stimuli 5. Reproduce 6. Grow and develop 7. Have adaptations to the environment

Viruses – are they alive? Technically, no! They cannot reproduce on their own. Are obligate intracellular parasites Viruses rely on host cells for: transportation, cellular machinery, and reproduction.

Viruses Are Small!

Viruses can infect virtually every kind of organism, including humans, birds, insects, and plants.

Viral Structure 1. Nucleic Acid Core DNA or RNA, but never both Contains several 100’s of genes 2. Capsid A protein coat or shell (some viruses have a protective envelope outside the capsid)

Different Viruses

E.g. Bacteriophage T4 This virus attacks bacteria DNA inside a protein coat Head and tail regions

Bacteriophages – Infect Bacteria

To reproduce, viruses must invade, or infect a living host cell, which is often killed in the process. Viruses are specific. Particular viruses will infect particular organisms.

Viruses Are “Host Specific” To infect a cell, the virus must first attach to it. This attachment is specific – the virus must find the type of cell that it can “fit” onto

Lytic Cycle 1. Infection 2. Growth 3. Replication

1.. Lytic Lifecycle The inserted genetic material instructs the host cell to produce and assemble viral structures The cell will do as it is told, until it literally bursts (= lysis)

1. Infection Virus is activated by chance contact with a specific host cell Virus injects its DNA into the cell

2. Growth Viral DNA (or RNA) take over the host cell A) viral DNA is indistinguishable by the host cell B) produce enzymes that can destroy the host cell’s DNA

3. Replication Viruses then direct the production of it’s own components (genetic material and protein coats) New viruses are produced so much that the host cell lyses, releasing hundreds of new viruses to carry on the same process

b. Lysogenic Cycle

Lysogenic Lifecycle E.g. Lysogenic bacteriophage 1. Viral DNA (or RNA) is injected into a host cell 2. Viral DNA (or RNA) is incorporated into the host DNA There are two pathways the bacteria may take:

Prophage Viral DNA is incorporated into the host DNA Both types of DNA are replicated for many generations Lytic Cycle Viral DNA and proteins are produced New viruses are assembled Host cell lyses

Lysogenic At an unknown time, stimulated by certain environmental factors, the prophage will enter the lytic cycle

Animal Viruses Similar to bacteriophage replication Have envelopes and spikes Spikes attach to the host cell membrane Once inside, uncoating releases the genetic material from the envelope and the capsid

Retroviruses Have RNA instead of DNA Carry reverse transcriptase – an enzyme that will convert single- stranded RNA back to double-stranded DNA This cDNA is integrated into the host cell DNA and instructs the cell to produce and assemble new viruses

Retrovirus Replication 9 STEPS: 1. Attachment 2. Penetration (and uncoating) 3. Reverse transcription 4. Replication (makes cDNA 5. Integration 6. Transcription of integrated cDNA 7. Biosynthesis 8. Maturation 9. Release (gets new envelope from host)

HIV is a Retrovirus that attacks T-cells