Viruses and Prions

Virion

A virus particle, not living

fully formed virus that can infect a cell.

nucleocapsid

capsid together with the nucleic acid it encloses

caspid

protein coat, made of subunits called capsomeres

protects the viral nucleic acid form enzymes and toxic chemicals in environment

capsomere

A protein subunit that joins with other protein subunits to form the viral capsid (protein coat).

temperate bacteriophage

A virus that infects bacteria and can either reproduce immediately by destroying the bacterial cell (lytic cycle) or insert its DNA into the bacterial DNA and remain dormant (lysogenic cycle).

prophage

Viral DNA from a bacteriophage that has inserted into a bacterium's DNA and remains dormant during the lysogenic cycle.

lysogen

A bacterial cell that contains a prophage (viral DNA integrated into its DNA) during the lysogenic cycle.

lysogeny

A process in which a bacteriophage inserts its DNA into a bacterial cell's DNA and remains dormant without immediately killing the cell.

provirus

Viral DNA that has integrated into a host cell's DNA and can remain dormant.

proto-oncogene

A normal gene that helps regulate cell growth and division but can become an oncogene if it mutates or becomes overactive, potentially causing uncontrolled cell growth and cancer.

oncogene

A gene that can cause uncontrolled cell growth and contribute to cancer when it becomes overactive or altered.

naked viris

composed of capsid whcoh surrounds the nucleic acid

What are the components of a naked virus?

1. Genetic material — DNA or RNA that contains the instructions for making more viruses.

2. Capsid — A protective protein coat surrounding the genetic material.

It does not have a lipid envelope surrounding its capsid.

What is the composition of a capsid?

made of capsomere

shapes can be: isometric, helical , complex

How are enveloped viruses different from naked viruses?

• Enveloped virus: Genetic material + capsid + lipid envelope.

• Naked virus: Genetic material + capsid only.

Be able to sketch the general structure of naked and enveloped viruses and label their parts

What is the composition of the chromosomes of viruses?

Viral chromosomes are made of either DNA or RNA (never both as their genome). They contain the genetic information needed for the virus to reproduce.

What is responsible for the host/tissue specificity of viruses?

Viral attachment proteins are responsible for host and tissue specificity because they bind to specific receptors on virus ligand/host cells.

Why can’t viruses replicate outside of a host cell?

Viruses cannot replicate outside a host cell because they lack the machinery needed to reproduce on their own, such as enzymes, ribosomes, nucleotides, proteins….

They must use a host cell's machinery to make new viruses.

Enveloped viruses

• Composed of a nucleocapsid surrounded by a lipid bilayer

• Many also have matrix proteins between envelope and capsid

Many naked viruses and envoloped viruses also have ___

Protein spikes projecting from their surface

• help it attach to and enter a host cell.

What are the possible outcomes of virus/host cell interactions?

1. Lytic infection: The virus replicates and destroys the host cell.

2. Persistent infection: The virus remains in the host for a long time, continuously producing viruses or becoming inactive for periods.

3. Latent infection: The virus remains dormant in the host cell and can reactivate later.

4. Transformation: The virus changes the host cell's growth or behavior, potentially causing cancer.

5. Abortive infection: The virus enters the host cell but cannot complete replication.

Virus structure

Nucleic Acid

• May be either DNA or RNA, never both

• May be single stranded or double stranded, never both

• DNA can be circular or linear, RNA only linear

Size virus

100- 1,000 times smaller than cells they infect

10nm - 500nm

List the steps that occur in bacteriophage lytic replication and describe the events that occur during each step.

• Attachment (adsorption): The bacteriophage attaches to specific receptors on the bacterial cell.

• Genome entry: The bacteriophage injects its genetic material (DNA or RNA) into the bacterial cell.

• Biosynthesis: The virus uses the bacterial cell's machinery to make viral genetic material and proteins.

• Assembly (maturation): The new viral parts are put together to form complete bacteriophages.

• Release (lysis): The bacteria

What events occur during lysogeny?

• Attachment and penetration: The bacteriophage attaches to a bacterial cell and injects its DNA.

• Integration: The viral DNA inserts into the bacterial chromosome, becoming a prophage.

• Replication: The bacterial cell copies its DNA, including the prophage, as it reproduces.

• Dormancy: The viral DNA remains inactive without immediately producing new viruses or killing the bacterial cell.

• Induction: Certain conditions can activate the prophage, causing it to leave the bacterial chromosome and enter the lytic cycle, where new viruses are produced and the cell is eventually destroyed.

Done only by temperate bacteriophage

What is the difference between lysogeny and lytic replication?

• Lytic = Makes new viruses and kills the cell.

• Lysogeny = Viral DNA hides in bacterial DNA and doesn't immediately kill the cell.

What causes lysogeny to end?

Induction: Environmental stress, such as UV radiation or DNA damage, activates the prophage, causing it to leave the bacterial DNA and enter the lytic cycle, where new viruses are produced.

What is meant by lysogenic conversion?

Lysogenic bacteriophage can carry genes that encode toxins

• bacteria carry the prophage produce the toxins, which cause disease

What are examples of diseases caused by bacteria that have undergone lysogenic conversion?

streptococcus pyogenes - scarlet fever

Clostridium botulinum- botulism

Corynebacterium diptheriae - diphtheria

Transduction

transfer of bacterial genetic information using bacteriophage

What is the difference between generalized transduction and specialized

transduction?

Generalized- a piece of host cell DNA is packaged instead of viral nucleic acid

Specialized - only occurs during lysogenic infections

Prophage takes some bacterial DNA when leaving the chromosome

Packaged

What types of genes can be transferred during each type of transduction?

• Generalized transduction: Any bacterial gene can be transferred because random pieces of bacterial DNA may be accidentally packaged into the virus.

• Specialized transduction: Only specific bacterial genes located near the prophage's insertion site can be transferred.

Be able to describe how generalized

Generalized Transduction

1. A bacteriophage infects a bacterium and enters the lytic cycle.

2. The bacterial DNA is broken into pieces as the virus replicates.

3. The virus accidentally packages a piece of bacterial DNA instead of viral DNA.

4. The bacteriophage infects another bacterium and injects the bacterial DNA.

5. The transferred DNA can recombine with the recipient bacterium's DNA, giving it new genes.

Be able to describe how specialized transduction occur.

Specialized Transduction

1. A temperate bacteriophage infects a bacterium and inserts its DNA into the bacterial chromosome, forming a prophage.

2. The prophage remains dormant during lysogeny.

3. When lysogeny ends, the prophage separates from the bacterial DNA incorrectly, taking nearby bacterial genes with it.

4. The virus replicates and packages the viral DNA along with the accidentally acquired bacterial genes.

5. The bacteriophage infects another bacterium and transfers those bacterial genes.

How are animal viruses classified?

Genome structure

• DNA or RNA, single stranded or Double, single molecule or segmented

Virus particle structure

• Isometric, helical, pleomorphic

Presence or absence of envelope

Be able to distinguish between family names and genus names

Family

• Family name ends in -Virida

• Based on genome, virus structure, envelope

Genus

• Genus name ends in -virus

• Enterovirus, Rhabdorvirus, Coronavirus

How are species names derived?

Name usually derived from disease caused: poliovirus, ebola virus, influenza virus

Be able to list and describe the steps in animal virus replication

Attachment

• Glycoproteins on cell membrane

• Function of glycoproteins to the host cell

Penetration and un coating

• Enveloped viruses: fusion or endocytosis

• Nake Viruses: endocytosis

• Uncoating: release of nucleic acid from nucleocapsid

Targeting to site of replication

• DNA viruses, RNA viruses

Assembly

• Release from host cell

• Budding, exocytosis, lysis

Shedding from the host

• Means of exit from host

• usually through same surface

Transmission

• Means of entry into new host

What are the ways in which enveloped animal viruses can enter host cells?

Fusion or endocytosis

What are the ways in which naked animal viruses enter host cells?

Endocytosis

Be able to list and describe the steps involved in fusion

Membrane Fusion

1. Attachment: The virus attaches to specific receptors on the host cell.

2. Fusion: The viral envelope fuses with the host cell membrane.

3. Penetration: The viral nucleocapsid enters the host cell.

4. Uncoating: The viral capsid is removed, releasing the viral genetic material into the cell.

Be able to list and describe the steps involved endocytosis

Endocytosis

1. Attachment: The virus attaches to specific receptors on the host cell.

2. Engulfment: The host cell membrane surrounds the virus and brings it inside in a vesicle.

3. Fusion: The viral envelope fuses with the vesicle membrane.

4. Release: The viral nucleocapsid enters the cell's cytoplasm.

5. Uncoating: The viral capsid is removed, releasing the viral genetic material.

What is meant by the shedding of viruses?

The release of newly produced viruses from an infected host cell so they can spread and infect other cells or hosts.

How is this accomplished?

• Lysis: The host cell bursts open, releasing newly made viruses. (Common with naked viruses.)

• Budding: The virus pushes through the host cell membrane and takes part of it to form its envelope. (Common with enveloped viruses.)

• Exocytosis: Viruses are transported inside vesicles that fuse with the host cell membrane, releasing the viruses outside the cell.

How is shedding different from transmission?

• Shedding: The release of viruses from an infected host into the environment (e.g., through saliva, mucus, or feces).

• Transmission: The spread of viruses from an infected host to another host (e.g., through coughing, touching, or contaminated surfaces).

What are the characteristics of acute animal virus infections?

Short Duration: days to months

Infected cells die: may or may not lysed

Infected host usually develops immunity

when do virus shed in acute infections

during infection

What are the

essential steps of an acute infection?

1. Incubation period: The virus enters the body and reproduces, but symptoms have not appeared yet.

2. Prodromal period: Early, mild symptoms begin, such as feeling tired or having a headache.

3. disease period: Symptoms become most severe as the infection progresses.

4. Decline period: Symptoms begin to improve as the immune system controls the infection.

5. Convalescence period: The body recovers and returns to normal.

Present infections

Virus continually present and released by budding

• may or may not cause disease

• People with persistent infection are carriers

• HIV, Hep B or C

Two categories of persistent infections

Latent infections - periodically

chronic infections - Life time

How are persistent viral infections different from acute infection?

Develops quickly and lasts a short time. The immune system usually clears the virus.

Persistent viral infection: The virus remains in the host's body for a long time, sometimes for life. It may continuously produce viruses or remain dormant and reactivate later.

Examples of HSV as a latent infection

• HSV-1: Causes oral herpes (cold sores). After the initial infection, the virus remains dormant in nerve cells and can reactivate later, causing cold sores to return.

• HSV-2: Commonly causes genital herpes. After infection, the virus remains dormant in nerve cells and can reactivate, causing genital sores to return.

Tumor (neoplasm)

Swelling caused by abnormal cell growth

Benign tumor

tumor remains confined to a defined region

Malignant tumor

Tumor spreads (mestasizes) to other parts of body

causes of tumors

Non-function or malfunction of cell growth controls

What are proto-oncogenes and what are the functions of the proteins produced from proto-oncogenes?

Genes that encode proteins that function in the regulation of cell growth or differentiation

They help make sure cells grow and divide when they are supposed to.

What is meant by viral transformation of cells?

when a virus changes a normal cell's behavior by inserting or affecting its genetic material (DNA).

Some viruses will cause cells to become tumor cells

How do viruses transform cells?

by changing the cell's normal growth controls using their genetic material (DNA or RNA).

Retrovirus

What are the characteristics of retroviruses?

RNA viruses that become proviruses when they infect cells

What enzyme allows them to convert their RNA genome into DNA?

Reverse transcriptase is the enzyme that allows certain viruses (called retroviruses) to convert their RNA genome into DNA.

Simple steps:

1. Virus enters the host cell.

2. Reverse transcriptase converts viral RNA → DNA.

3. The viral DNA can insert into the host cell's DNA.

What determines the host range for a given virus?

• Viral ligand: The attachment protein on the virus that binds to the host cell.

• Host cell receptor: The specific molecule on the outside of the host cell that the virus attaches to.

can viruses infect multiple hosts

yes

cells can be infected with more than one virus type at a time

Viruses can exchange genetic information

can result in a virus acquiring a new host specificity

• antigenic shift of influenza virus

Prions

Proteinaceous infectious particles

• composed of only proteins

What diseases are associated with prions?

cause transmissible spongiform encephalopathies (TSE)

• mad cow disease

• Kuru

• Scrapie

• Creutzfeldt-Jacob disease

Prions are highly resistent to….

to heat

chemicals

and more

How does one acquire a prion?

through consumption, handling of contaminated items

• some forms can be genetic

Hard thing about prions

Can not 100 % protect from prions

• no treatment

What are the characteristics of prions?

1. Abnormally folded: They have an incorrect shape.

2. Cause other proteins to misfold: They change normal proteins into abnormal ones.

3. Cause brain diseases: They damage brain tissue and cause neurological diseases.

4. Very difficult to destroy: They are resistant to many normal disinfection and sterilization methods.

Prolonged incubation time - months to years

How do prions “replicate”?

• A misfolded prion protein comes into contact with a normal protein.

• The prion causes the normal protein to fold incorrectly.

• The newly misfolded protein can then cause other normal proteins to misfold.

• This creates more abnormal prion proteins.

must have what for it

Normal protein to replication to occur