Biomedical Sciences: Viral Replication (Lecture 7; Exam 2)

What are the steps of the virus replication cycle?

1. Attachment

2. Penetration

3. Uncoating

4. Transcription “mRNA production”

5. Translation

6. Genome Replication

7. Assembly

8. Release

What is tropism?

Refers to the specificity of a virus for particular host cells; target cells that a virus is able to infect.

The process of producing mRNA from a DNA template is called:

Transcription.

What is the process of creating protein from RNA?

Translation.

Do all viruses adhere to the central dogma?

NO. Not all viruses adhere to the central dogma.

What structure in the cell helps with translation?

Ribosomes.

What is a permissive cell?

A host cell that that allows a virus to enter, bypass its internal defenses, and fully replicate.

What needs to happen during every successful viral infection?

1. Viral mRNA must be made

2. Virus must use cellular components and energy to synthesize proteins from mRNA.

3. Must replicate its genomic nucleic acid to package in new virions.

What is Baltimore Classification?

Classification system on how viral genomes get to mRNA.

When do viruses cause disease?

Viruses cause disease AFTER:

• Breaking through the natural protective barriers of the body.

• Evading immune control

• Either kill cells of an important tissue or trigger destructive inflammatory response.

What kind of virus is the Human Parovirus B19?

ssDNA virus.

Why does replication always require a viral mRNA-dependent RNA polymerase (replicase)?

Because host cells lack enzymes that can use an RNA template to build a new RNA strand.

+sense (coding strand) RNA:

Genome functions as mRNA, proteins made immediately.

-sense (non-coding strand) RNA:

Must make complementary +sense RNA first.

What kind of virus is Human Rotavirus?

dsRNA virus.

What is Translation?

Ribosomes read mRNA (+sense) to make protein.

Types of Polymerases:

• DNA Polymerase

• DNA-dependent RNA Polymerase

• RNA-dependent RNA Polymerase

• RNA-dependent DNA Polymerase

What is DNA Polymerase?

Makes DNA from DNA template.

• Often provided by host cell (cellular DNA replication)

• Some viruses encode their own (Parvo, Pox, Herpes)

What is RNA-dependent RNA Polymerase?

Makes RNA from RNA template.

• NOT a cellular function (ever).

• Must be e encoded by virus.

What is DNA-dependent RNA Polymerase?

Makes RNA from DNA template.

• Host Cell: RNA Polymerase II (for mRNAs)

• Poxvirus RNA Polymerase

What is RNA-dependent DNA Polymerase?

Makes DNA from RNA template.

• NOT a cellular function (ever).

• Reverse transcriptase (HIV, HBV, cellular telomerase)

• Must be encoded by virus

What are DNA viruses?

Viruses that contain DNA in their virion.

What do DNA viruses contain?

May contain ssDNA, dsDNA or partially dsDNA.

What kind of infections do DNA viruses establish?

Persistent infections. (latent, chronic, etc)

What kind of genome do DNA viruses have?

Linear or circular.

Where does replication usually occur in DNA viruses?

Nucleus.

DNA viruses replicate in the nucleus. What virus is the one exception?

Poxviruses.

They usually set up “replication factories” in cytoplasm, complete w/DNA & RNA polymerase.

What viruses have a shorter replication time than DNA viruses?

RNA viruses.

Do RNA viruses mutate faster or slower than DNA viruses?

Faster.

Why do RNA viruses replicate faster than DNA viruses?

Their copying machine lacks proofreading tools/mechanisms.

Where do RNA viruses replicate?

In the cytoplasm (outside of nucleus).

Are RNA viruses double or single stranded?

Can be BOTH; may also be positive or negative sense.

ALL ssRNA (-) viruses are ____.

Enveloped.

Can RNA viruses replicate RNA?

NO. They must make or bring RNA-dependent RNA polymerase.

(RNA viruses may also have segmented genomes).

What is the mutation rate of RNA-dependent RNA Polymerases?

1 / every 10,000-100,000 nucleotides polymerized

(one million times higher than our DNA polymerase)

What role do RdRP’s play?

Play a role in immune evasion and antiviral resistance.

Do DNA viruses evolve faster or slower than RNA viruses? Why?

Slower; RNA viruses do not contain a proofreading mechanism.

Retrovirus Characteristics":

• RNA genome (ssRNA (+) genome)

• Virion contains reverse transcriptase (NEVER cellular function!!!)

• RNA goes in, makes DNA, makes new RNA

• Retroviridae (HIV, Human T cell leukemia virus)

Hepatitis B Virus Characteristics:

• Hepadnaviridae

• Circular, partially dsDNA genome

• DNA is “repaired” by host-cell machinery

• Long RNAs are transcribed (pre-genomic RNA, pgRNA)

• New partially dsDNA genomes are reverse-transcribed from long RNAs

  • -uses a reverse transcriptase

• DNA goes in, makes long RNA, makes new DNA (genomes)

What is the threshold for chronic infections?

6 months or longer.

SARS-CoV-2 Attachment protein:

Spike.

SARS-CoV-2 cellular receptor

Angiotensin Converting Enzyme II or (ACE2).

What is a notable benchmark for measuring immunogenicity of a vaccine?

The production of neutralizing antibodies.

What is the method of penetration for enveloped viruses?

Membrane fusion.

What method of penetration/uncoating can be done by both enveloped and non-enveloped viruses?

Receptor-mediated endocytosis.

What method of release is only done by enveloped viruses?

Budding.

What type of release is done by both enveloped and non-enveloped viruses?

Cell lysis and exocytosis.

What kind of virus is Human Parvovirus B19?

ssDNA virus.

What does RNA replication ALWAYS require?

A viral RNA-dependent RNA polymerase (replicase).

True/False: RNA polymerase lacks proofreading activity and makes more mistakes.

True.

What is DNA-dependent RNA polymerase?

The main enzyme responsible for copying a DNA template into an RNA strand during the process of transcription.

What is DNA-dependent DNA polymerase?

An enzyme that uses an existing DNA strand as a template to build a new, matching DNA strand.

Where do most RNA viruses replicate their genome?

In the cytoplasm!

ALL (-) ssRNA viruses are:

ENVELOPED.

A cell that a virus can successfully infect and replicate within is called a _____ cell.

Permissive.

What is the general name for a cellular component to which a viral attachment protein binds?

Receptor.

What are neutralizing antibodies?

Antibodies that block attachment of a virus to a host cell.

What kind of virus is HIV?

Retrovirus.

What kind of RNA genome does HIV have?

+ssRNA genome.

(Virion contains reverse transcriptase!!!)

HIV contains a +ssRNA genome but cannot be immediately translated. Why?

Because it is a retrovirus that must first convert its RNA into DNA.

What type of genome does Hepatitis B Virus have?

• Circular partially dsDNA genome

• Uses reverse transcriptase

Which type of virus must bring along a synthesized, pre-made RNA-dependent RNA polymerase in order to successfully infect a cell and eventually get new viral proteins made?

(-) ssRNA virus