Lecture 19 - Viruses

What is a virion?

Complete virus particle composed of a nucleic acid surrounded by a protein coat

What is a capsid and what is it made of

Protein coat that surrounds a virus

Made of individual proteins (capsomeres)

4 common capsid shapes

Helical, polyhedral, spherical, and complex

Naked vs enveloped virus

Naked: no envelope surrounding its capsid

Enveloped: has a lipid membrane derived from the host cell.

Whats a viral infection

when a virus commandeers a host cell’s machinery and replicates itself

What does it mean for a virus to have a lifecycle

set of steps in which a particular virus enters a cell, reprograms its activities, and uses its resources to make more virions

Viral lifecycle: Entry and uncoating

a virus binds to specific receptors on a cell surface, then the virus enters the cell

capsid proteins released (entry via endocytosis or fusion w/ membrane)

Viral lifecycle: Replication

viral genome is copied using the cell’s enzymes

the genomes copies put inside new viral particles

Viral lifecycle: Gene expression

viral genome is expressed

host’s ribosomes are used to make proteins such as capsid proteins, membrane proteins, or other proteins that interfere w/ normal cell activities

Viral lifecycle: Self-assembly and release

capsid proteins assemble around a copy of a genome

virus particles then exit the cell

What is the Baltimore classification for viruses?

Organizes viruses based on their viral nucleic acid and how they make mRNA

ex: double stranded RNA: Rotavirus

What are bacteriophages? What kinds of cells do they infect?

Viruses that infect bacterial cells

inject DNA into cells rather than entering bacterial cells

Explain the steps of a lytic phage lifecycle

1. attachment: phage binds to specific surface protein

2. entry of phage DNA and degradation of host DNA: injects DNA, cell’s DNA is hydrolyzed

3. Synthesis of viral genomes and proteins: DNA directs production of page proteins and copies

4. Self-assembly: 3 separate sets of proteins self-assemble to form head, tails, and tail fibers

5. release: phage directs production of an enzyme that damages cell wall, allowing fluid to enter. The cell swells and bursts, releasing 100-200 phage particles

Explain the steps of a lysogenic phage lifecycle

1. Phage DNA integrates into chromosome, becoming prophage

2. Bacterium produces normally, copying prophage and transmitting it to daughter cells.

3. Occasionally, a prophage exits the chromosome, initiating lytic cycle

What is a virus’ host range?

the different species that a virus can infect; narrow (smallpox) or broad (flu)

What determines which species or cells can be infected by a virus?

Viruses can only infect a subset of a complex organism’s cells (e.g. influenza infects respiratory epithelial cells)

Factors: compatibility between a virus and cell surface receptors, compatibility between the virus and the cell’s biochemistry

What is zoonosis?

when a pathogen that came from another animal causes disease in humans

Phylogenetic distance is a major determinant

What happens during a spillover event

when a virus jumps to another species

Virus’ virulence vs infectivity

virulence: degree to which the virus weakens, sickens, or kills the host

infectivity: ability to establish an infection in a host

What is the HIV virus? Which cells does it infect?

Human Immunodeficiency virus

Infects human immune cells (helper T cells)

Explain what makes HIV a retrovirus

it contains RNA and its nucleic acid used to make DNA via reverse transcriptase

HIV’s DNA is incorporated into host’s DNA, becoming a provirus

Explain the steps of a HIV’s lifecycle

1. HIV fuses to host-cell surface

2. HIV RNA, reverse transcriptase, integrase and other vital proteins enter the host cell

3. Viral DNA is formed by reverse transcriptase

4. Viral DNA is transported across nucleus and integrates into host DNA

5. New viral DNA is used as a genomic RNA and to make viral proteins

6. New viral RNA and proteins move to the cell surface and a new immature HIV forms

7. The virus matures when protease releases the proteins that form the mature HIV

What is the influenza virus? What types are there?

Enveloped virus w/ 8 RNA strands

Causes upper respiratory tract infections

Influenza A-D

IAV and IBV are responsible for most annual infections, epidemics and pandemics

What are Hemagglutinin and Neuraminidase in the influenza virus?

Glycoproteins found on the surface of the influenza A virus

H: helps virus attach to host cells, 18 types

N: facilitates the release of new virus particles from infected cells allowing a virus to spread, 11 types

The combination of different types determines the specific strain of the virus

Viral antigenic drift vs antigenic shift

Antigenic drift: small, gradual mutations in influenza’s surface proteins

Antigenic shift: new hybrid virus strains forms when 2 different strains infect the same host

Reservoir host vs amplifier host (ex: Hendra virus)

Reservoir host: harbors a pathogen without being adversely affected by it (e.g. bats)

Amplifier host: virus replicates in them to very high loads. Virus can more easily spillover to other species (e.g. horses eating the bat poop)

Virus first hypothesis

selfish replicating genetic elements evolves before cells

Regression hypothesis

some early cells degenerated into a parasitic lifestyle

Escaped genes hypothesis

some cellular genes ability to selfishly replicate and escaped the cell

What role do bacteriophages and other viruses play in marine environments

70% of ocean biomass are planktonic microbes (algae and bacteria)

20% of this biomass is killed everyday by viruses

Contribute to the regulation of microbial populations and nutrient cycling in marine ecosystems.

What is a virus shunt?

when viruses lyse bacteria, nutrients are released from fresh microbial and algae growth

What is a biological pump?

ocean sequestration of carbon

Describe some possible application or uses of viruses in medicine

Gene therapy: use virus to deliver correct DNA segment into target cells of a patient with a disease (e.g. cystic fibrosis)

Drug delivery: use a virus to deliver specific drugs into certain cells

Oncolytic viruses: target and kill cancer cells