Viruses, Disease And Vaccines

Lecture 11

Viruses, disease and Vaccines Biology, Campbell & Reece. 7th Edn. Ch 18

Dr Mohamed Abumaree Molecular Reproductive Biologist & Immunologist College of Medicine King Saud bin Abdulaziz University for Health Science 2009

How do viruses kill cells? 1.Hydrolytic enzymes released lysosomes

from

2.Toxins produced by infected cells 3.Toxic compounds (such as envelope proteins) produced by viruses

The extent of damage  Depends on the ability of infected tissues to regenerate or not!!!!……for !!!!…… example 1.People recover from colds because the infected epithelium of the respiratory tract can repair itself 2.But, nerve cells damaged by poliovirus is permanent

Many of the temporary symptoms associated with viral infections, such as fever, fever result from the body’s natural defenses (immune system) which is the basis for the major medical tool (vaccines) for preventing viral infections

Vaccines

 Harmless microbes

derivatives

of

pathogenic

 Stimulate the immune system to mount defenses against the actual pathogen  Effective vaccines: available hepatitis B & other viral diseases  Cannot prevent all viral illnesses

against

Antibiotics  Kill bacteria: inhibit enzyme involved in the pathogens  Cannot kill viruses: viruses have few/no enzymes!!  Drugs fight viruses: block viral nucleic acid synthesis, synthesis such as azidothymidine (AZT) AZT control HIV reproduction

Emerging Viruses Viruses that appear suddenly, such as HIV

7

How viral diseases appeared?

(1) Mutation of existing viruses: a major source of new diseases

 RNA viruses have a high rate of mutation…………  Because they cannot repair errors during replication process  Mutations enable existing viruses to evolve into new genetic strains………..  That can cause disease in individuals who had developed immunity to the ancestral virus

For example, flu epidemics: Caused by new strains of influenza virus genetically different from earlier strains that people have little immunity to them 11

(2) The spread of existing viruses from one host species to another

 About three–quarters of new human diseases originate in other animals  For example, people infected with a flu virus previously seen only in birds

(3) The distribution of a viral disease from a small isolated population can lead to widespread epidemics (AIDS)

 Thus, emerging viruses are not new viruses but they are existing viruses that have: 1. Mutate 2. Spread to new host species 3. Distribute more widely in the current host species 15

(4) Changes in host behavior or environmental changes can increase the viral traffic responsible for emerging diseases

Viral Diseases in Plants  Plant viruses have the same basic structure and mode of replication as animal viruses  Most plant viruses discovered including tobacco mosaic virus (TMV), have an RNA genome

Viroids  Small and simple molecules that infect plants  Circular RNA molecules  Do not encode proteins but can replicate in plant cells by using cellular enzymes  Affect plant growth, such as abnormal development & small growth

Prions 1. Infectious proteins 2. Very slow acting agents: incubation time ~10 years 3. Not destroyed or deactivated by heating to normal cooking temperatures 4. Cause degenerative brain diseases in animals, such as mad cow disease 5. Can be transmitted in food 6. There is no known cure for prion diseases

How can a protein, which cannot replicate itself, be a transmissible pathogen?

Hypothesis 1) A prion is a misfolded form of a protein normally present in brain cells 2) When the prion gets into a cell containing the normal form of the protein, the prion converts the normal protein to the prion version

So prions may repeatedly trigger chain reactions that increase their numbers

Rapid reproduction, mutation and genetic recombination contribute to the genetic diversity of bacteria

Bacterial Genome Replication  Bacteria have one double–stranded circular DNA molecule that is associated with a small amount of protein, protein the bacterial chromosome, which is tightly coiled and densely packed  The dense region of DNA (nucleoid) is not bounded by membrane  Many bacteria also have plasmids (much smaller circles of DNA)

 Plasmid has a small number of genes, genes up to several dozen  Bacterial cells divide by binary fission, fission which is preceded by replication of the bacterial chromosome  From a single origin of replication, DNA synthesis progresses in both directions around the circular chromosome until the entire chromosome is reproduced  Bacteria can proliferate very rapidly in a favorable environment (E. coli)

Sources of Genetic Variation  Binary fission: an asexual process (production of offspring from a single parent)  So most of the bacteria are genetically identical to the parent cell  However, rare mutation can occurs to cause slight genetic variation  Bacteria are fast reproducing organisms  So the new mutations cause significant genetic diversity

 The genetic diversity affects the development of bacterial populations (reproduction) unlike the slowly reproducing organisms (humans)  Most of the heritable variation in humans is not due to the mutations, but because of recombination of existing ones during sexual reproduction  Genetic diversity in bacteria also arises from genetic recombination, the combining of DNA from two sources