Chapter 12: The Structure and Infection Cycle of Viruses

Things have to reproduce

What is a biological imperative?

80% of infectious diseases (we can't do a lot about the very common viruses)

Why the Structure and Infection Cycle of Viruses is Important: How many infectious diseases are caused by viruses?

Viruses

Why the Structure and Infection Cycle of Viruses is Important: As a health professional, what will most of the infectious diseases you see be caused by?

Viral structure and the infection cycle caused by viruses

Why the Structure and Infection Cycle of Viruses is Important: It is important you have an understanding of what?

Obligate intracellular Parasites

Why the Structure and Infection Cycle of Viruses is Important: Viruses are defined as what, meaning they can't live or reproduce outside a cellular host

A productive infection

Why the Structure and Infection Cycle of Viruses is Important: What are the one goal of viruses?

Alive, as they are acellular

Viruses are not actually what and why?

After all, just one virus on a computer is too many- Glenn Turner

What was the quote in this chapter of the book?

"A piece of bad news wrapped in a protein coat"

According to Peter Medawar, what is a virus for cellular hosts?

Nucleic acids, so DNA or RNA (never both)

What is wrapped in the protein coat of the virus?

Capsomeres are the individual proteins, protein coat is capsid

What makes up the protein coat and what is it called?

DNA viruses and RNA viruses (based on type of nucleic acid they contain)

What are the 2 types of viruses?

Capsid

protein coat that encloses genetic material

An envelope structure made up of lipids obtained from internal membranes or the plasma membrane of the infected host cell

What do some viruses wrap themselves in?

Enveloped viruses

nucleocapsids surrounded by a lipid bilayer with proteins

A lipid bilayer (NOT a plasma membrane, but an envelope) surrounding a protein coat and nucleic acid genome

What does an enveloped virus have?

Naked (polio, rhinovirus, norovirus); phospholipids could be melted away in envelope of enveloped viruses (HIV, influenza) so "when it dries it dies"

Which is more stable: naked or enveloped viruses?

Virion

a single, mature, infective viral particle

1. Strength to withstand the environment in which it must survive to infect the host cell

2. Must be able to shed the protective protein coat upon entry to the host cell

The design of the virion is based on what 2 challenges that contradict each other?

Protein coat

What protects the virion from elemental changes?

So they can disassemble after entering the host cell (and release their genome)

Why is the assembly of virions reversible?

Capsomere

protein molecule forming capsid

Envelope

"viral membrane"; phospholipid bilayer with embedded glycoproteins surrounding the capsid in the enveloped virus

Nucleocapsid virus

nucleic acid, capsid

Virion

"viral particle"; complete infectious viral structure; nucleic acid plus capsid for non-enveloped virus, nucleic acid plus capsid plus envelope for enveloped virus

1. Double-stranded DNA (dsDNA virus)

2. Single-stranded DNA (ssDNA virus)

3. dsRNA or ssRNA

What are 3 common ways to classify viruses by describing the nucleic acids they contain?

capsomeres

Capsid is built of ________________, which are identical protein subunits used to build capsids that are arranged to provide maximal contact and bonding among the subunits

Helical, icosahedral, complex

What are 3 categories of virus shape?

Families or virdae (virus family)

What are animal viruses arranged in?

Because they aren't living/aren't cells

Why do viruses not have genus or species names?

1. DNA or RNA

2. Single stranded or double-stranded

3. Icosahedral, helical, or complex

4. Enveloped or naked

5. ssRNA: + or -

6. Size in nm

7. Site of assembly in the nucleus or cytoplasm

What do animal viruses either have to help classify them?

+ means it can serve directly as mRNA; - means it is complementary to mRNA

What does + and - mean for the viruses with ssRNA?

in reo and bima virus families

Where is double-stranded RNA seen that shows how strange viral genomes can be?

Parvovirus family

Where is single-stranded DNA seen that shows how strange viral genomes can be?

1. Bacteria (called bacteriophages)

2. Plant cells and fungal cells

3. Animal cells (including human cells)

Criteria for Classification of Viruses: What can viruses infect (3)?

A certain cell type

Criteria for Classification of Viruses: What are viruses specific for?

Obligately intracellular parasites

Criteria for Classification of Viruses: What kind of parasites are viruses?

Not composed of cells, living things are/have cells, so probably not living

Are viruses living or not living (tough question)?

So far every living organism that has been studied has had atleast one virus associated with it, and viruses outnumber all other life forms by at least an order of magnitude

Criteria for Classification of Viruses: What is true about the sheer amount of viruses?

Rod (ridged/straight) or filamentous (curved/coiled)

Shape of helical viruses is either...

Segmented, each segment encloses its own helical capsid that is held in place by nucleoproteins

Helical Viruses: What is the RNA of Influenza like?

Matrix Protein

Helical Viruses: second layer or protein just inside the envelope of influenza viruses, gives virus spherical shape

Presence of hemagglutinin and neuraminidase

Helical Viruses: What are they categorized by?

Hemagglutinin

Helical Viruses: bind to and entering the host cell

Neuraminidase

Helical Viruses: release from the host cell

20 triangular faces that make up the capsid

What are Icosahedral viruses derived from?

Simple or Complex

Icosahedral viruses can be what or what?

Simple

Icosahedral viruses: capsid is made up of repetitive polypeptide capsomeres; polio is an example, survives in digestive tract because of stable protein-protein interactions in capsid

Complex

Icosahedral viruses: use of additional proteins and lipids surrounding the capsid

Herpesvirus; 3 groups of proteins

Icosahedral viruses: What is an example of a complex one and what does it contain?

1. Proteins of the capsid

2. Proteins forming the tegument

3. Glycoproteins that form spikes on the surface of the envelope

Icosahedral viruses: What are the 3 groups of proteins that herpesvirus contains?

Tegument

Icosahedral viruses: protein layer located b/w the capsid and the envelope

Complex viruses

viruses that do not have either helical or icosahedral symmetry; ex: smallpox (variola virus)- smooth rectangular shape in the envelope

Envelopes

formed when viral glycoproteins associate with membranes of the host cell; foundation is the phospholipid bilayer

Envelope glycoproteins

comprise the envelope; integral membrane proteins that are firmly embedded in the lipid bilayer of the envelope

Spikes or other surface structures

What can envelope glycoproteins form that the virion can use to attach to the host cell?

A variety of sizes and shapes

Viruses come in what?

Produce more virions, which are then released via exocytosis when the host cell is filled (AKA lysis)

In infection of animal viruses, the host cell is used to do what?

The lytic infection cycle

What is lysis a hallmark of?

The production of new virions, virus can also become latent

Infection of animal viruses does not always result in what?

The host cell DNA and viral genome combine to make a provirus

What happens during the latent phase?

Reproduces as it usually would, but also including the viral genome into new host cells

What does the host cell do reproductively during the latent phase?

Latency

dormant state

1. Lytic infection cycle

2. Integrating of the phage genome into that of the host cell, producing a dormant state known as lysogeny

What are the 2 infection cycles in infection of bacteria with viruses?

Lytic infection cycle

destroys bacteria; treated with phage therapy

Attachment, penetration, uncoating, biosynthesis, maturation, release

Infection cycle of animal cells consists of the following steps:

Attachment

occurs as a virion binds to specific receptors on the host cell; not a passive process, relies on the virus taking over normal cell functions (vesicular transport)

capsomeres

Remember that the capsid is made up of ___________, which are three-dimensional proteins

Through random collisions and are essentially governed by chance

When the virus meets the host: How do viral interactions with the host cell occur?

More virus = more collisions = increased chance of infection

When the virus meets the host: What does more virus do for attachment?

Promiscuous (nonspecific) or specific

When the virus meets the host: Viruses can either be what or what?

Specific viruses

When the virus meets the host: host cell must have the appropriate receptor and the cellular components needed to produce new virions

Certain areas of the host plasma membrane, called lipid rafts

When the virus meets the host: Where do many viruses only attach to?

Areas that are rick in cholesterol, fatty acids, and other lipids; more densely packed and less watery

When the virus meets the host: What are areas that are more reliable for stable attachment like?

Areas for the release site

When the virus meets the host: What are lipid rafts also in some viruses?

electrostatic; high-affinity

When the virus meets the host: Virus-receptor interactions begin as ______________ and become _______________________ binding

Amplify the attachment step; inflammation increasing the presence of ICAM-1, which facilitates more binding to the host cell virus

When the virus meets the host: Sometimes the host's response to the attachment of a virus can what, and give an example of this

localized; released to underlying tissues and usually spread to other sites

Viral infection that occur at apical surface of epithelial cells are usually _____________; viruses at the bottom of the host cell are ______________________________________________

Co-receptors

Attachment: host components that interact with the virus and the primary receptor to allow continuation of the infection cycle

complete infections without these molecules (HIV, herpesvirus)

Attachment: Viruses that require co-receptors cannot..

Attachment is between the host cell's receptor and structures on the viral capsid

What is receptor binding like for non-enveloped viruses?

Viral proteins previously inserted into the envelopes bind to specific host cell receptors (Ex: influenza and HIV-1)

What is receptor binding like for enveloped viruses?

Penetration and uncoating

getting into the interior of the cell and uncoating the viral genetic material

1. Receptor-mediated endocytosis

2. Formation of a pore in the host plasma membrane to gain entrance into the cytoplasm

3. Uncoating can also be very complex and require use of endocytosis or docking at the nucleus

How is penetration and uncoating done in non-enveloped viruses (3 possible ways)?

1. Virion binds to a receptor on the host cell's plasma membrane, causing a pit to form in the membrane

2. Membrane then encloses the virion to form a vesicle called an endosome

3. Travel through the host cytoplasm and contain an acidic acid interior environment that begins uncoating the virion

4. Endosome fuses with the lysosomes in the cytoplasm and the uncoating is complete

What are the 4 steps of receptor-mediated endocytosis?

1. Envelope is essentially a plasma membrane and can fuse with the host plasma membrane

2. Virion receptor binding cause conformational changes in the host cell that expose fusion proteins

3. Fusion requires the presence of co-receptor

How is penetration and uncoating done in enveloped viruses (3 possible ways)?

Special fusion proteins and results in the formation of a large opening called a fusion pore in the host cell

What is the envelope fusing with the host viral membrane mediated by?

The virion to move across the plasma membrane and into the host cell

What does opening the fusion pore in the host cell permit?

compartmentalization

Cytoplasmic transport of viral components: Viral infections involve _____________________

In specific locations in the cell

Cytoplasmic transport of viral components: Where does DNA or RNA synthesis, as well as the synthesis of new capsids and other viral proteins, occur?

To other sites for assembly of the virion (this means that there needs to be mechanisms of movement for viral components through the host's cytoplasm)

Cytoplasmic transport of viral components: After synthesis, where are the viral components moved?

1. Uses membrane-enclosed vesicles in conjunction with host cell cytoskeletal structures (ex: microtubules)

2. Direct association of viral components with cellular transport mechanisms

Cytoplasmic transport of viral components: What are the 2 ways that viral components are moved to other sites for assembly (after the virus has uncoated)?

The use of specialized lipids and proteins called chaperones

Cytoplasmic transport of viral components: What can other cases include that move the virus through the host cytoplasm?

For replication of the viral genome

Transport of the viral genome into the nucleus: Why must some viruses enter the host cell's nucleus?

Need to bypass the double phospholipid bilayer membrane

Transport of the viral genome into the nucleus: What must happen for some viruses to enter the host cells nucleus?

Pathways or pores

Transport of the viral genome into the nucleus: What is transport through the double phospholipid bilayer membrane accomplished using?

They dock there and deliver their genetic material

Transport of the viral genome into the nucleus: What happens since DNA viruses are too large to get into the nucleus?

Retroviruses

Transport of the viral genome into the nucleus: What are RNA viruses called that also need to replicate their genomes in the nucleus?

Reverse transcriptase

Transport of the viral genome into the nucleus: Retroviruses carry this enzyme that converts RNA to DNA, after which DNA moves into the nucleus and integrates with the host chromosome

A small fraction of cell membrane proteins

Host cell receptors used by viruses represent what?