Ch, 19 Viruses

Virus

an infectious, not alive particle consisting of genes packaged in a protein coat

• simple (nucleic acid w/ protein coat/membranous envelope) + cannot reproduce or carry out metabolism w/out host

Tobacco Mosaic Disease (TMV)

1935 - Wendell Stanley

Confirmed first cite of virus

• latter hypothesis by crystallizing the infectious particle

Virus Structure

double or single strand DNA/RNA

• single linear or circular molecules

• 3-2,000 genes in genome

Capsid

“helical or icosahedral viruses”

protein shell surrounding the viral genome

• built from protein subunits (Capsomeres)

Viral Envelope

(derived from membranes of host cells) surround the capsids of influenza viruses and many other viruses found in animals

• contain viral + host cell molecules

Bacteriophages/phages

viruses that infect bacteria (lytic + lysogenic repo cycle)

• elongated capsid head that encloses their DNA

• protein tail attaches phage to host + inject phage DNA

Host Range

• spillover or CST = rare

limited number of host cells that it can infect

• Once entered manufactures viral proteins = new viruses

Lytic Cycle

a phage replicative cycle that culminates in the death of the host cell

• new phages produced, lyses host cell wall to release progeny viruses

• Virulent phage: only repo by lytic cycle

Lysogenic Cycle

replicates the phage genome without destroying the host cells

• Viral DNA incorporated into host cell’s chromosome

• Temperate phage: both lytic + lyso

Prophage

integrated viral DNA

• Every division of host leads to copies in daughter cell

Restriction Enzyme (RE)

cellular enzyme that identifies and cuts up foreign DNA/double stranded DNA

• cleaves DNA molecules at specific base sequences

• Methylated = protection from cuts

Palindromic Sequence

Same forward and backwards (5’-3’)

CRISPR -Cas System

“clustered regularly interspaced short palindromic repeats”

Protects bacteria + archea from viral infection

How to classify viruses that infect animals (2)

1. RNA or DNA genome, single- or double-stranded

2. The presence or absence of membranous envelope

• Viral glycoproteins bind to host

Retroviruses

use reverse transcriptase to copy their RNA genome into DNA

• HIV causes AIDS

Provirus

viral DNA that is integrated into the host genome

• remains a permanent resident of host

• RNA polymerase transcribes DNA to RNA

Evolution of Viruses (3)

1. evolved from cellular nucleic acid

2. sourced from plasmids + transposons (jumping gene)

3. Largest = Megavirus chilensis (440nm

Prions

Smaller, less complex entities that cause disease in plants and animals

• release hydrolytic enzymes from lysosomes to damage/kill

• Some release toxins + envelope proteins

Vaccine

harmless derivative of pathogenic microbes that stimulate the immune system to mount defenses against the harmful pathogen

• Viruses cannot be treated by antibiotics but antiviral drugs can

Emerging Viruses

those that suddenly become apparent

• Ebola virus, chikungunya virus, zika virus

4 Types of Transmission

• MEMORIZE PIC

1. Epidemic: disease in community at particular time

2. Pandemic: global epidemic (black death, 1918 spanish flu)

3. Endemic: exists permanently in particular region (malaria)

4. Sporadic: occasional disease (tetanus, rabies)

What contributes to the emergence of new viral diseases (3)

1. RNA viruses have high rate of mutation

2. disease come from small, iso pop and spread worldwide

3. ¾ of new human diseases come from animals

R-naught (R₀)

a value that can be calculated for communicable diseases. It represents, on average, the number of people that a single infected person can be expected to transmit that disease to.

Flu Epidemics

caused by type A influenza viruses; infect birds, pigs, horses, and humans, but be prevented by insecticides

• names based on viral surface proteins (HA or NA)

• H1N1 caused 2009 flu pandemic

Viral Diseases in Plants

Spread by vertical transmission (virus from parent) or horizontal transmission (damaged cell wall)

• RNA Genome, Helical capsid, Icosahedral capsid

• tomato (A) + TMV (B)

Prions

infectious mis-folded proteins that appear to cause degenerative brain diseases in animals from food

• Scrapie in sheep, mad cow disease, and Creutzfeldt-Jakob

• Alzheimer’s + Parkinson’s