DD-Viral Structure and Replication

What are the consequences of the universal properties of viruses?

-Not living -Must use host cells processes to synthesize viral components -Self-assemble (NOT replicate by division) -Cannot be cultured on artificial media

All viruses share what characteristics?

-Obligate intracellular parasites (can only grow in living cells -Filterable infectious agents (so small they retain infectivity after passing thru filters that trap and retain the smallest of bacteria) -At least two components: a nucleic acid genome and a protein coat called the capsid (some also surrounded by lipid envelope) -Viral genome composed of DNA or RNA but never both

The capsid consists of individual protein subunits called...?

Capsomeres

Viral genomes are composed of what type of nucleic acid: DNA, or RNA?

Either one (DNA OR RNA) but not both

The envelope is derived from cellular membranes and is composed of...?

Lipid and protein (glycoproteins)

Name all the possible outcomes of viral infection.

Lytic infection Persistent infection Latent infection Abortive infection Viral transformation

DNA genomes may be circular or linear. Most DNA viruses have a _______ genome. Papillomaviruses, which cause warts, are an example of viruses with a __________ DNA genome.

Most DNA viruses have a linear genome. Papillomaviruses (cause warts) are an exception: they have a circular DNA genome.

RNA genomes may also be single or double stranded. Most RNA viruses are ______-stranded.

Most RNA viruses are single stranded (as opposed to DNA viruses, which are mostly double-stranded).

Which is more environmentally stable, viruses with an envelope, or "naked" viruses without an envelope?

Naked viruses (without an envelope) are more stable and resistant to harsh conditions. Therefore transmission may not require direct contact.

The capsid plus the viral nucleic acid is called...?

Nucleocapsid

DNA genomes may be single or double-stranded. For example, __________ have a single-stranded DNA genome while most other DNA viruses have a double-stranded genome.

Parvoviruses are the exception to the rule that DNA viruses have double stranded genomes, in that parvoviruses have single-stranded DNA genomes.

Provide two examples of ss-RNA viruses that have positive (+) polarity.

Picornaviruses and togaviruses can be directly translated on cellular ribosomes (+ polarity)

What is an exception to the rule that DNA viruses usually replicate in the cell nucleus?

Poxviruses

What is polarity, regarding single-stranded RNA genomes?

Single-stranded RNA genomes may (or may not) be directly translated on cellular ribosomes to produce viral proteins. ss-RNA viruses that have this ability are, by convention, positive (+) polarity. In contrast, RNA genomes of other viruses cannot be directly translated and have negative (-) polarity.

What are peplomers?

Some of the viral envelope proteins (peplomers), protrude from the envelope surface. They play a role in attachment to host cells. They are also expressed on infected cells and are major antigenic targets for the immune response.

Describe helical symmetry

The capsomeres and the nucleic acid are coiled around each other.

Most types of RNA viruses have a nonsegmented genome. What is a "segmented" RNA genome?

The genome may consist of 2 or more distinct fragments or segments. The segments may be single or double-stranded. Ex. Rotavirus consists of 10 ds-RNA segments; influenza virus has 8 ss-RNA segments.

Which virus type has complex symmetry?

The poxviruses are the largest viruses and possess a complex, brick-shaped morphology

The envelope surrounds...?

The viral capsid

Describe persistent infection.

The virus completes the replication cycle and progeny virions are produced (same as lytic infection). However, the host is not killed in the process and viral infection proceeds for an extended period of time.

Describe lytic infection

The virus completes the replication cycle and progeny virions are produced. The cell is lysed or killed in the process. Tissue destruction in viral infections is associated with lytic infection.

Describe viral transformation.

The virus does not kill the cell, but rather causes the cell to lose normal growth regulation leading to tumors.

The proteins within the envelope are encoded by...?

The virus genome

True/False: Enveloped viruses are labile and transmission generally requires close person-to-person contact.

True.

Describe latent infection.

Upon infection, the virus enters a dormant state in the cell. Viral gene expression is limited and infectious virus cannot be detected in the cell. The virus can be reactivated to cause lytic infection at a later time.

Describe abortive infection.

Virus replication is blocked at some step and infectious virus progeny are not produced. Some viruses will lytically infect only specific species or specific tissues.

Cells cannot replicate RNA. How then is an RNA virus genome replicated?

RNA viruses must encode an RNA polymerase (transcriptase).

Provide two examples of viruses with helical symmetry.

Rabies Measles virus

Describe what makes retroviruses unique.

Retroviruses are a special group of RNA viruses- as part of their life-cycle, a DNA intermediate is inserted into the host cell genome.

What is the exception to the rule that most RNA viruses are single stranded?

Rotavirus, which causes gastroenteritis in children, has a double-stranded RNA genome.

The viral protein capsid has what 4 basic functions?

-Confer structural symmetry and packaging -Protect nucleic acid from damage -Permit transport from cell to cell -Permit attachment to susceptible cells

What are the general steps of viral replication?

1. Adsorption/attachment 2. Penetration 3. Uncoating 4. Biosynthesis 5. Assembly 6. Budding and Release

Describe each of the general steps of viral replication.

1. Adsorption/attachment: Receptors on the virus capsid or envelope irreversibly bind to cellular receptors on the cell membrane 2. Penetration: Virions pass through the cell membrane into the cell cytoplasm. In some cases endocytosis facilitates viral entry. 3. Uncoating: The viral capsid is broken down by viral or cellular enzymes and the viral nucleic acid is released. 4. Biosythesis: The viral genome is transcribed to mRNA which is translated on cell ribosomes to viral proteins (capsid and envelope components and enzymes). The parental viral genome is replicated into an extensive number of progeny genomes. 5. Assembly. The viral genome is incorporated into the viral capsid. Some nucleocapsids acquire an envelope as the bud from the cellular nuclear or plasma membrane. 6. Budding (enveloped viruses only) and Release: Progeny virions are released from the cell, in some cases by cell lysis.

Viral multiplication can be divided into what phases?

1. Eclipse period- Intra- or extra-cellular infectious virus cannot be detected within infected cells. Ends upon generation of infectious virus within cells and indicates the time is takes to make mature, infectious virus within the infected host cell 2. Latent period- Extracellular infectious viruses cannot be detected. Ends upon release of infectious virus into the media or extracellular environment and indicates the times it takes to make and release mature, infectious virus from the host infected cell. 3. Rise period. Time during which the extracellular virus concentration (virus titer) increases until it reaches a constant titer.

Gene expression in replication of DNA viruses is regulated in what 3 phases?

1. Immediate early (these genes encode proteins that regulate gene expression) 2. Early (these genes encode proteins involved in DNA replication- DNA polymerase, thymidine kinase) 3. Late (these genes encode structural protens - capsids, envelope glycoproteins)

What are 2 examples of viruses with icosahedral capsid symmetry?

Adenovirus Poliovirus

For positive (+) polarity viruses, the viral RNA genome can serve as a mRNA, directly binding ribosomes and being translated into protein. How do negative (-) stranded viruses manage?

An endogenous viral RNA transcriptase (within the entering viral capsid) must be present to copy the RNA genome into a complementary positive-strand RNA which can then be translated into protein.

Virus growth curves for particular viruses may differ significantly. _________ may complete their replication cycle in 20 minutes, which _________ replication may take 18-24 hours.

Bacteriophages: 20 min Human cytomegalovirus: 18-24 hours

RNA viruses, which must encode an RNA polymerase to be replicated, may evolve viral mutants rapidly. Why?

Because the viral RNA polymerase (transcriptase) does not have proof-reading ability.

Describe icosahedral symmetry.

Geometric shape with 20 sides or faces and 12 vertices. The viral nucleic acid is packaged within the capsid.

Provide two examples of viruses that cause a latent infection.

Herpes simplex virus and varicella zoster virus establish a latent infection in the host neural tissue and can be reactivated to cause disease.

Some viruses have an envelope. Provide 3 examples of viruses with an envelope.

Herpesvirus HIV Hepatitis B

Provide 3 examples of viruses which can oncogenically transform cells.

Human papilloma virus (HPV) Retroviruses like HTLV Herpesvirus such as Epstein-Barr virus (EBV)

What are the 3 basic capsid shapes/symmetry?

Icosahedral symmetry Helical symmetry Complex symmetry

DNA viruses usually replicated where?

In cell nucleus

Where do RNA viruses replicate?

In the cell cytoplasm

What is the exception to the rule that RNA viruses replicate in the cell cytoplasm?

Influenza, which replicates in the nucleus