Lecture 19: Viral Replication

Question 1

What is the eclipse phase?

Answer 1

The eclipse phase is a stage in the viral replication cycle during which the virus is actively replicating inside the host cell but no intact infectious virus particles (virions) can be detected.

Question 2

Give an overview of the viral replication of DNA viruses

Answer 2

• Viral nucleic acid synthesis and synthesis of viral mRNA takes place in the nucleus
• Translation of viral mRNA takes place in the cytoplasm
• The exception to these is Poxvirus (smallpox), where viral DNA replication takes place in the host cell cytoplasm

Question 3

Give an overview of the viral replication of RNA viruses

Answer 3

• Eukaryotic cells have no enzymes for RNA replciation
• So RNA viruses have their own enzyme called RNA replicase (ehich produces mRNA from RNA).
• Replication and translation of viral RNA will therefore take place in the cytoplasm (no transcription step for +ve RNA, can be used directly as mRNA)
• Negative RNA must be converted into positive RNA before it is used as mRNA.
• Exception is retroviruses, such as HIV, where +ve RNA is converted into double stranded DNA first, then converted into mRNA.

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Question 4

How are double stranded DNA viruses replicated?

Answer 4

• Double stranded DNA is transcribed into mRNA using RNA polymerase.
• This mRNA will then be translated into viral proteins, both structural and non-structural.

Question 5

For larger viruses, such as poxviruses, what copies the viral genome?

Answer 5

Viral DNA polymerase. Because they’re big enough ti have their own DNA polymerase.

Question 6

For small viruses, what copies the viral genome?

Answer 6

The host cell DNA polymerase

Question 7

How are single stranded DNA viruses replicated?

Answer 7

• They’re first replicated into double stranded DNA using host cell DNA polymerase.
• The host cell DNA polymerase is then used to replicate this DNA into mRNA.
• The mRNA can then be translated

Question 8

What is an example of a virus with a single stranded DNA?

Answer 8

Parvovirus B19, which causes slapped cheek syndrome

Question 9

How will the gapped double stranded DNA virus be replicated? (Hepatitis)

Answer 9

• It can’t be recognised so it can’t be copied into mRNA, so the host cell DNA polymerase will complete the incomplete strand.
• Dna polymerase then replciates this into positive mRNA
• Some of this mRNA will be transcribed straight into the viral proteins, which will consequently produce the viral particles, some will be used to produce the incomplete double stranded DNA, using enzyme reverse transcriptase.
• These will assemble to produce viral particles.

Question 10

Is it true that either RNA will have RNA replciase, or will have the genes encoding for RNA replicase, so that the host cell can transcribe the enzyme?

Answer 10

Yes

Question 11

How are ssRNA +ve sense replicated?

Answer 11

-+ ve sense, so will be used immediately by the ribosomes to produce the mRNA, the viral proteins, one of the structural proteins will be the RNA replicase.

• The RNA replciase will also be used to replicate the positive RNA

Question 12

Give examples of viruses that have ssRNA +ve sense genome?

Answer 12

Picornaviridae (which causes Poliovirus and Rhinovirus)

Caliciviridae (which causes gastroenteritis)

Coronaviridae (which causes SARS and COVID 19)

Flaviviridae (Yellow fever virus, West Nile Virus, Hepatitis C virus, Zika Virus)

Togaviridae (Rubelia Virus, Encephalitis virus)

Question 13

GIve examples of ssRNA, (-) sense

Answer 13

Paramyxoviridae (measles virus, mumps virus)

Rhabdoviridae (rabies virus)

Filoviridae (ebolavirus, marburg virus)

Orthomyxivirdae (influenza virus)

Question 14

How is single stranded RNA, negative sense, replicated?

Answer 14

• These viruses have the RNA replicase within them
• -ve RNA is converted into +ve RNA. This can then be translated into viral proteins

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Question 15

How are double stranded RNA viruses replicated?

Answer 15

• Has a capisd with 2 layers, one will be removed when the virus enters the host cell.
• The geneome is also segmented
• Replication takes place in the cytoplasm but within the capsid
• Transcription occurs from the -ve RNA to produce mRNA
• The mRNA then leaves the capsid through the pores at the vertices
• Translation into viral proteins (structural and non-structural)

Question 16

What is an example of a dsRNA virus?

Answer 16

Reovirus

Question 17

How do +ve sense single stranded RNA reteroviruses replicate?

Answer 17

• They are copied using reverse transcriptase into single starnded negative RNA
• The negative sense strand will then be used as a template to produce double stranded DNA.
• This DNA will then be integrated into the host genome(DNA) via the enzyme integrase.
• This will produce provirus, which will be transcribed to make viral proteins.