viruses 20.1 ahhh chhooo – you are soooo good looking !
TRANSCRIPT
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