1. STARTER… 15 minutes READ page 105 – 107 Read Viral Replication
We will take notes on this today!
Take jot notes on Viruses and Human Health
Three important points or more!
Define Antibiotic and Vaccines
Answer 5 questions on Influenza

2. Viruses and Human Health
Most viral infections are difficult to treat, they are not destroyed by antibiotics.
ANTIBIOTICS - inhibits the growth of or destroys microorganisms.
Some viruses can remain dormant for years before symptoms appear
Certain viruses cause cancer by adding specific genes to an infected cell.
VACCINES - liquid preparations of dead or weakened viral or bacterial cells that stimulate the body’s immune system to fight back.

3. Influenza (FLU) Usually considered to be more annoying than dangerous
Spread by direct contact and can live for hours in dry mucus
First host: cells in the upper respiratory tract
Soar throat, congested lungs, chills, fever, pain, sweating…
Contagious 1 day before symptoms appear, up to seven days after.
Incubation 1 – 4 days
Last up to 2 weeks
Influenza vaccine can be 90% effective

4. Viral Reproduction
February 5, 2008 4

5. Viral Reproduction
Viruses can reproduce only within a host cell because:
a) lack specific enzymes and ribosomes
(necessary to make new cores & capsids)
b) have no energy of their own
(needed to synthesize their parts)
There are 2 modes of viral reproduction
Lytic Cycle: all DNA viruses reproduce this way
Lysogenic Cycle: mostly RNA viruses use this cycle 5 5

6. Difference between RNA and DNA viruses
RNA viruses
DNA viruses
common cold
Unstable they change their protein coat and “fool” the immune system
Chickenpox
Stable and are the same throughout
Why do we need a Flu shot once a year?

7. The Lytic Cycle
February 5, 2008
SBI3U - A.Y. Jackson S.S. 7 7

8. 1. Attachment 1a. Attaches to host cell by its receptor sites
1b. Injects its core (DNA) into the host cell 8 8

9. 2. Synthesis Viral DNA stops host protein synthesis
Viral protein coats are made
Viral DNA is also replicated 100’s of times using cell machinery 9 9

10. 3. Assembly Capsids surround the viral DNA cores
100's of virus particles are assembled in the cytoplasm of the host cell 10 10

11. 4. Release
Host cell is destroyed (bursts) as 100’s of viruses are released
Each virus now infects a neighbouring cell 11 11

12. The Lytic Cycle
All DNA viruses reproduce with the lytic cycle and are called
virulent (capable of causing disease & extremely infectious) 12 12

13. The Lysogenic Cycle
RNA viruses cannot take over the host cell machinery directly
They must first change their RNA to DNA.
To do this they have a special enzyme called reverse transcriptase.
Because this is a reverse process RNA viruses are called retroviruses.
Retroviruses are especially lethal to their hosts because the viral DNA produced by this enzyme can splice into the host chromosome and stay dormant or latent for months or years. 13 13

14. The Lysogenic Cycle
When each host cell divides, it also copies the viral DNA 'hiding' within its own chromosome.
During this dormant period, the host's immune system does not 'see' the viral DNA and produces no antibodies against it.
The host feels no symptoms, but carries the disease and may infect others.
Examples of diseases caused by retroviruses are AIDS, genital Herpes and cold sores.
Some unknown stimulus will 'trigger' the viral genes to become active, take over the cell's protein synthesis and continue as in the lytic cycle. 14 14

15. reverse transcriptase
Steps Of The Lysogenic Cycle
reverse transcriptase
dormancy period of variable length
viral RNA
lysogenic cycle
viral DNA
viral DNA splices into host chromosome & becomes dormant
viral DNA activated and takes over host protein synthesis
lytic cycle
viral DNA is copied 100's of times
host ribosomes are used to make viral protein coats
newly made viruses are assembled and released by destroying their host cell 15 15

16. (DO NOT WRITE) The Lysogenic Cycle
Still unclear what "triggers“ the dormant viral DNA to enter lytic cycle.
Hormones and high stress levels (adrenaline) often associated with activation 16 16

17. The Lysogenic Cycle 17 17

18. Homework Read “Sex Slaves for Science” article
Answer questions # 1 – 11
April 24, 2017 18 18 18 18