VIRUSES 20.1

AHHH CHHOOO – You are SOOOO Good Looking!

Viruses

Objectives:

• Describe the structure of viruses.• Discuss the two methods by which

viruses infect living cells.

Introduction to Viruses

• Tobacco mosaic virus – Occurred 100 years ago– Seriously threatened

the tobacco crop– No visible agent could

be determined

• Martinus Beijerinck named viruses– Discovered tiny

particles in the juice

• Virus – comes from the Latin word for “poison”

What is a Virus?

• Wendell Stanley (1935) – American biochemist that discovered the nature of a virus and chemically isolated it (D.Ivanovski – demonstrated the cause of the plant disease was in the liquid extracted from the plants – but could not pin down the culprit)

• Virus – a nonliving particle made up of nucleic acids and protein that can invade cells (sometimes lipids)– Can only reproduce INSIDE a LIVING CELL

Structure of a Virus

• Typical virus is composed of:– Core – nucleic acid

(either DNA or RNA)– Capsid – protein coat

that protects the core (some viruses have an envelope – addition membrane that surrounds the capsid)

– Several to several hundred genes

• Bacteriophage (T4) – type of virus that infects bacteria– Head region –

nucleic acid + capsid

– Tail region – used to attach to host

Characteristics

• Small (10-400 nanometers) electron micro.• No nucleus, cytoplasm, or membrane• No cellular functions• Only reproduce inside a living host cell

– Specificity – viruses can only infect specific hosts (animal viruses cannot infect plants, etc.)

• Come in a variety of shapes– Rod, cube, helical

• Some have ENVELOPES, which help virus enter host

Viral Infections

• Lytic Infection – Active/Virulent• Lysogenic Infection – Latent/Temperate

Life Cycle of a Lytic Virus

• Lytic Cycle– process by which host cell is invaded, burst, and destroyed by a virulent virus

• Virulent Virus – immediately disease-causing

Life Cycle of a Lytic Virus (cont’d)

1. ATTACHMENT – Virus attaches to cell wall (specific spot, “lock & key”

2. ENTRY – Viral nucleic acid enters cell through weak spot in cell wall

3. REPLICATION – viral DNA take over replication, make new viral DNA, “hijack”

4. ASSEMBLY – new virus particles are put together

5. LYSIS/RELEASE – cell wall digested, new virus is liberated

Lytic Cycle

Life Cycle of a Lysogenic Virus

• Lysogenic Cycle – process by which viral DNA is incorporated into a host cell’s DNA where it lies dormant for many generations before becoming active

• Temperate Virus – not immediately disease causing

• Prophage – viral DNA that has been incorporated into host

Life Cycle of a Lysogenic Virus (cont’d)

• A phage enters a cell and remains inactive until an external stimulus (i.e. change in temperature, availability of nutrients, etc.) causes the phage to become virulent

Lysogenic Cycle

Lytic and Lysogenic Cycle

Retrovirus

• RETROVIRUS– RNA virus that copies its information backwards (RNA into DNA), i.e. HIV

• Uses enzyme reverse transcriptase to

make DNA from an RNA template

HIV Virus