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.

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

Viral Reproduction February 5, 2008 3

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 4 4

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?

The Lytic Cycle February 5, 2008 SBI3U - A.Y. Jackson S.S. 6 6

1. Attachment 1a. Attaches to host cell by its receptor sites 1b. Injects its core (DNA) into the host cell 7 7

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 8 8

3. Assembly Capsids surround the viral DNA cores 100's of virus particles are assembled in the cytoplasm of the host cell 9 9

4. Release Host cell is destroyed (bursts) as 100’s of viruses are released Each virus now infects a neighbouring cell 10 10

The Lytic Cycle All DNA viruses reproduce with the lytic cycle and are called virulent (capable of causing disease & extremely infectious) 11 11

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. 12 12

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. 13 13

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 14 14

(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 15 15

The Lysogenic Cycle 16 16