MCAT Viruses and Viral Particles

Question 1

What are viroids?

Answer 1

Short, circular, single- stranded RNA molecules act as viral agents by binding to and silencing host genes.

They replication by using the RNA polymerases of the host.

Question 2

What are prions?

Answer 2

Prions ( PrP^sc) are misfolded surface proteings ( PrP^c). They can cause misfolding and aggregation of other Prp^c proteins.

Prp^c proteins are highly expressed in the CNS and so PrP^sc cause neurodegenerative diseases.

Change from PrPc to PrP^c happens with a change in the secondary structure from alpha helices to beta sheets.

Question 3

Critical threshold

Answer 3

PrP^c to PrP^sc of additional PrP^c proteins is how they replicate, and they will aggregate because they’re less soluble forming amyloid fibrils.

Critical threshold is the point of aggregation where cellular functions are disrupted causing disease.

Question 4

What are viruses considered?

Answer 4

Obligate intracellular parasites because they need a host to carry out viral activities.

Question 5

Do viruses undergo natural selection and evolution?

Answer 5

Yes, but they vary in the type of genetic material they have.

Question 6

Capsid

Answer 6

The protein coat of viruses that surrounds the genetic material of the virus. Contains protein spikes.

Capsomeres are the units that makes up the capsid and creates a shape (i.e. polyhedral capsomeres makes up the icosahedral virus shape).

Question 7

Complex viruses

Answer 7

More complex viral structures.

Question 8

Naked viruses v. Enveloped viruses

Answer 8

Naked viruses are viruses that only has the capsid while enveloped viruses have an envelope surrounding the capsid made of the phospholipid bilayer of the host- can contain protein spikes from the host.

Question 9

Possible types of viral genetic material

Answer 9

Can be RNA or DNA, circular or linearly, double stranded or single stranded, or hybridization of DNA and RNA.

Question 10

Determine how dsDNA viruses replicate?

Answer 10

They incorporate their DNA into the host genome, they can use their own RNA polymerase or host’s to replication, and then use host RNA polymerase to synthesize proteins.

Question 11

Determine how ssDNA viruses replicate?

Answer 11

They use the ssDNA as a template to make the complementary strand creating a dsDNA, they can use their own DNA polymerase or the hosts.

Once this happens, they incorporate their DNA into host genome and perform the same steps as dsDNA.

Question 12

How does naked viruses and enveloped viruses leave the cell?

Answer 12

Naked viruses either exit through exocytosis or lysis (results in cell death).

Enveloped viruses leave through budding in which part of the plasma membrane of the host leaves with the virus.
- Protein spikes are shuttled to plasma membrane so that when it buds off its incorporated into the envelope.

Exocytosis and budding don’t result in cell death while lysis does.

Question 13

Retroviruses

Answer 13

Viruses that have the ability to go from +SSRNA to dsDNA so that it can be incorporated into the host genome.

-The enzyme reverse transcriptase makes +SSRNA to dsDNA in the cytoplasm of the host.

Question 14

Provirus

Answer 14

When a virus integrates their genome into the host genome the virus is turned into the provirus.

Question 15

Retrotransposons

Answer 15

When transposable elements move around the genome through the use of mRNA intermediates mediated by reverse transcriptase.

Process:
1. Retrotransposon DNA is transcribed to DNA to mRNA in nucleus.

2. The mRNA is transcribed into integrase and reverse transcriptase by ribosomes in cytoplasm.
3. The retrotransposon mRNA is converted back into dsDNA by reverse transcriptase enzyme.
4. The dsDNA is then integrated somewhere else in the genome by the enzyme integrase.

Question 16

When does the viral life cycle begin v. when does it end?

Answer 16

It begins with virus entry into the host and ends with viral progeny release (virions).

Question 17

Describe bacteriophages and what are their replication cycle?

Answer 17

Viruses that only effect bacteriophages. They are naked viruses and usually contain DNA as their genetic material.

1. Attachment- bacteriophage attaches its tail fibers on the bacteria.
2. Entry- The tail sheath injects its DNA into the cytoplasm of the bacterium.
3. Synthesis - bacterial genome is degraded and viral proteins are made.
4. Assembly - viral components are made and packaged inside.
5. Release - Host cell lysis, releasing virions and causing the death of the bacteria.

Can switch from the lytic replication cycle to the lysogenic cycle.

Question 18

Describe the lysogenic and lytic cycles?

Answer 18

The lysogenic cycle begins when viral DNA is incorporated into bacterial genome where it becomes a prophage. With each replication cycle of the bacterium the viral genome also replicates. This is the latent phase where the viral genome is not expressed.

At some time the prophage DNA is excised from the bacterial genome where it’s expressed and created virions.

Question 19

What are the similarities between animal viruses and bacteriophages?

Answer 19

Both animal viruses attach to specific receptors on the host that’s complementary to virus spike proteins (glycoproteins).

Difference is that sense bacteriophages are naked viruses that can enter through direct penetration (injection of viral DNA) or endocytosis. Animal viruses can enter through endocytosis or membrane fusion.

Question 20

Describe RNA viruses

Answer 20

They contain negative sense or positive sense RNA.

• Positive sense (+) SSRNA has the same directionality as host mRNA so they can be translated directly to proteins.
• Negative sense (-) SSRNA does not have the same directionality and sequence as host mRNA and so the virus needs RNA dependent RNA polymerase to transcribe -SSRNA to +SSRNA.

To make more of a specific type of mRNA for a virus it uses the other mRNA strand type as a template.

Question 21

How does dsDNA replicate?

Answer 21

They’re denatured upon host entry into +SSRNA and -SSRNA. The +SSRNA is used as a template to make proteins. Both strands are replicated to those replicated strands hybridize to make viral progeny.