9/16 Chapter 5 Part 1: Viruses and Prions

What is an obligate intracellular parasite? Is this what a virus is? where must they be in order to use resources and reproduce?

Cannot multiply unless they invade the inside of a specific host cell and instruct its genetic and metabolic machinery to make and release new viruses

• A cell is inactive, if there is no host and they live at the expense of the host

What things can a virus infect? Are viruses ubiquitous in nature? Are they ultramicroscopic in size and what does this mean? Lastly, do they independently fill the characteristics of life?

• Viruses infect not only humans but also bacteria, protozoa, fungi, algae, plants, and animals

• Are ubiquitous (found everywhere) in nature and have had major impact on development of biological life

• Are ultramicroscopic in size

• Viruses do not independently fulfill the characteristics of life because they need a host

What are two common things that all viruses have that is their basic structure? What three things are unique about their nucleic acid?

1. Basic structure consists of protein shell (capsid) surrounding nucleic acid core

• They don’t follow the same rules as human cells

• Nucleic acid can be either DNA or RNA, but not both

• Nucleic acids can be double-stranded DNA, single-stranded DNA, single-stranded RNA, or double-stranded RNA, segmented RNA (several short RNA segments and is more likely to evolve into diseases)

What two things do viruses lack?

1. Viruses lack enzymes for most metabolic processes (Uses host enzymes exception HIV)

2. Lack cellular machinery for synthesizing proteins, ATP (ex: mitochondria)

How do viruses multiply? Why are the molecules on viruses Surface important?

• Multiply by taking control of host cell’s genetic material and regulating the synthesis and assembly of new viruses

• Molecules on virus surfaces give them high specificity for attachment to host cell (viruses have specificity For where they work in human cells ex: GI tract)

Are viruses the smallest infectious agents? And what is needed to see them?

They are the smallest infectious agents (prions are smaller), and visibility is limited without an electron microscope

Do viruses bear any resemblance to cells and what machinery do they lack? What are the Structures that are needed to invade and control a Host cell?

1. Viruses bear no resemblance to cells and lack any of the protein synthesizing machinery found in cells

2. The structure contains only those parts needed to invade and control a host cell:

• External coating

• Core containing one or more nucleic acid strands of DNA OR RNA

• Sometimes one or two enzymes

What are the three viral components and describe their composition/what they do? What is a nucleocapsid? And what do naked viruses consist of?

1. Capsid: protein shell that surrounds the nucleic acid:

• Nucleocapsid: the capsid together with the nucleic acid

• Naked viruses consist only of a nucleocapsid.

2. Envelope: external lipid coat covering a capsid, (usually a modified piece of the host’s cell membrane)

3. Spikes can be found on naked or enveloped viruses:

• Project from the nucleocapsid or the envelope

• Allow viruses to dock with host cells (Creates specificity for certain cells)

What is a viral capaid made of? And what does it determine of a virus? What are the two types?

• Constructed from identical protein subunits called

capsomeres

• Capsomeres spontaneously self-assemble into the

finished capsid. Determine shape of a virus

Two different types:

• Helical

• Icosahedral (20-sided figure with 12 corners)

Describe how an envelope virus forms it its layer. When an envelope virus buds from its host, what are sources of the envelope it can take and is it more flexible than a capsid? Are they pleomorphic?

1. Take a bit of the host cell membrane when they are released from a host cell

2. Enveloped viruses can bud from host:

• Cell membrane, Nuclear membrane, Endoplasmic reticulum

• More flexible than the capsid so enveloped viruses

are pleomorphic (can take multiple shapes but usually globular)

What is a complex capsid structure? what are these also called?

Complex capsids used tail fibers instead of spikes to stab into a cell

• these are also called bacteriophages or phages

What is a genome and do viruses contain DNA or RNA or both? What is the amount of genes that viruses possess?

• Genome: the sum total of the genetic information carried by an organism

• Viruses contain DNA or RNA, but not both

• Possess only the genes needed to invade host cells and redirect their activity

For DNA viruses are they single shredded or double stranded? If they are double stranded or are they linear or circular or both? how is this different from humans and bacteria?

DNA viruses: Single-stranded (ss) or double-stranded (ds; linear or

circular)

• humans have linear DNA whereas bacteria have circular DNA

Which is more common for RNA viruses double stranded or single stranded DNA? What are the four types of viruses that use RNA? Describe how they use RNA.

• RNA viruses: can be double-stranded, but more often single-stranded:

1. Positive-sense RNA: ready for immediate translation (mRNA)

2. Negative-sense RNA: must be converted before translation can occur (-RNA into mRNA)

3. Segmented: individual genes exist on separate pieces of RNA

4. Retroviruses: carry their own enzymes to create DNA out of their RNA (EX: HIV turns RNA to DNA which is why it’s so hard to stop)

What type of RNA virus was COVID-19?

Covid19: SARS-CoV-2, like all coronaviruses, is a single-stranded (+) sense RNA virus.

What are 3 enzymes for specific operations within their whole cell that a virus might use? most viruses lack the gene for what?

• Polymerases that synthesize DNA and RNA

• Replicases that copy RNA

• Reverse transcriptase synthesizes DNA from RNA

• The vast majority of viruses completely lack the genes for synthesis

of metabolic enzymes