DNA virus replication

Question 1

Which one of the statements is false regarding DNA viruses?

1) They have DNA genomes that are replicated by either host or virally-encoded DNA polymerases.

2) Their DNA genomes are very diverse and the relatively higher stability of DNA allows for genomes much larger than possible for RNA viruses (complex lifecycle).

3) Genomes of DNA viruses that infect animals range from very small ssDNA and very large dsDNA.

4) DNA viruses are replicated with low fidelity because DNA polymerases like RNA polymerases do not have proofreading abilities.

Answer 1

4!

DNA viruses are replicated with HIGH fidelity because DNA polymerases have proofreading abilities.

RNA polymerases have low fidelity (more murations!).

Question 2

Fill in the blanks:

DNA viruses have a _______ of gene expression (early genes vs late genes).

DNA replication and transcription occurs in _______ __________ within the infected cell, in which viral proteins involved in these processes are concentrated.

Answer 2

Separation; Discrete compartments

Question 3

DNA viruses belong to group ___, ___, and ____ according to the Baltimore classification.

Answer 3

I (dsDNA), II (ssDNA), VII (gapped DNA)

Question 4

(T/F) DNA viruses come in different flavours; very diverse and complex!

Answer 4

True!

Question 5

What kinds of parasites are viruses?

Answer 5

Obligate intracellular parasites!

Viral DNA must be replicated efficiently in the host cell to provide genomes for assembly into progeny virions. The degree of dependency of a virus to a host cell depends on the type of virus!

Question 6

Eukaryotic cells replicate most of their DNA in the ______.

Answer 6

Nucleus (site of DNA replication and RNA synthesis).

However, there are some exceptions to this rule!

Question 7

Most DNA viruses replicate their genomes in the nucleus and some in the cytoplasm.

What are the two DNA virus families that replicate in the cytoplasm rather than the nucleus?

Answer 7

Poxviridae

Asfariviridae

Question 8

1) What poses a challenge for the DNA viruses that replicate in the nucleus?

2) How do they over come this?

Answer 8

1) Entry into the nucleus; NUCLEAR ENVELOPE (barrier)

2) They make use of the NUCLEAR PORE COMPLEX (embedded into the nuclear envelope) for the transport of the genome and accessory proteins into the nucleoplasm

Question 9

What is the difference between passive diffusion and facilitated translocation that occurs in the nuclear pore complex?

Answer 9

Passive diffusion; for small molecules not for viruses

Facilitated translocation; selective process used by viruses

Question 10

What are the four strategies used by enveloped viruses for nuclear entry of their genomes?

Answer 10

1) Intact entry (capsid enters and disassembles and is used by small viruses)

2) Ejection of DNA (capsid associates with importin B and docks on the nucleus and there is an injection + translocation of DNA; capsid not imported)

3) Disassembly of capsid in the cytoplasm and transport of genome/protein complexes

4) Avoid nuclear replication (large and complex viruses that are equipped with all the enzymes required for DNA replication + transcription replicate in the cytoplasm)

Question 11

(T/F) While small viruses undergo intact entry where the capsid disassembles in the nucleus, large viruses undergo ejection of DNA or disassembly in the cytoplasm and then transport of genome.

Answer 11

True!

*ejection of DNA: capsid is not imported

Question 12

(T/F) Replication of most DNA follows a set of universal rules. There are exceptions!

Answer 12

False!

Replication of ALL DNA (from the simplest virus to the most complex vertebrate cell) follows a set of universal rules .

Question 13

Fill in the blanks regarding the universal mechanism of DNA replication:

1) DNA synthesis is always _______ directed.

2) Synthesis occurs in the _________ direction. Incorporation of dNMPs from dNTPs into the _____ end of the growing DNA molecule.

3) Replication is _____________ (each parental strand of a DNA template is copied by base pairing to produce two daughter molecules identical to each other and to parental strand)

4) Replication initiates at specific sites on the template named ______________.

5) DNA replication is catalyzed by ________________________.

6) __________ proteins required for initiation and elongation.

Answer 13

1) Template

2) 5’-to-3’; 3’-OH

3) Semi-conservative

4) Origin of replication (ORI)

5) DNA-dependent DNA polymerases

6) Accessory

Question 14

Which one of the statement is true regarding the universal mechanism of DNA replication?

1) All DNA polymerases require a primer with a free 3’OH end to initiate replication

2) DNA replication ends at specific sites aka as termination sequences.

Answer 14

2!

Most DNA polymerases require a primer with a free 3’OH end to initiate replication but there is ONE exception (primer independent replication is seen in bacteriophage NrS-1)

Question 15

What are the five steps in the replication of viral DNA genomes?

Answer 15

1) Origin recognition

2) Assembly of replication complex

3) Priming DNA synthesis

4) Elongation

5) Termination

Question 16

Define:

1) Origin of replication

2) Replicon

Answer 16

1) Origin of replication is a region in the genome where replication is initiated. The structure of origins defer among species, but they do share some common characteristics.

2) A replicon is a unit of replication that is define by a distinct origin of replication (Ori) and terminus.

Question 17

(T/F) DNA replication can only be unidirectional.

Answer 17

False!

DNA replication can be bidirectional or unidirectional. Bidirectional replication forms replication ‘bubbles’.

Question 18

(T/F) As there is an increasing degree of DNA replication, there is a progressive movement of the replication forks from a single origin of replication.

Answer 18

True!

Question 19

What are the four common features shared by origins of DNA viruses?

Answer 19

1) AT-rich sequences (2 H bonds; easier to unwind the DNA helix during initiation)

2) Contains sequences recognized by VIRAL RECOGNITION PROTEINS which initiate DNA replication

3) Proximity to TRANSCRIPTIONAL REGULATORY SEQUENCES (these sequences increase replication efficient)

4) Many DNA viruses have two or three origins, the smallest DNA viruses have only one origin (no more than 3!)

Question 20

Match the following parts of the origin of SV40 DNA replication to their descriptions:

1) The core origin
2) LT binding site I
3) LT binding site II

A) found in the core origin. Contains 4 copies of a pentanucleotide-binding site for the SV40 origin recognition protein, LARGE T ANTIGEN (LT)

B) is 64 base pairs and is sufficient for initiation of DNA synthesis in infected cells. It is adjacent to transcription regulatory sequences. It included the AT-rich element, LT binding site II, and the early imperfect palindrome which are essential for replication!

C) modestly stimulates replication IN VIVO.

Answer 20

The core origin: the origin is 64 base pairs and is sufficient for initiation of DNA synthesis in infected cells. It is adjacent to transcription regulatory sequences. It included the AT-rich element, LT binding site II, and the early imperfect palindrome which are essential for replication!

LT binding site I: modestly stimulates replication IN VIVO.

LT binding site II: found in the core origin. Contains 4 copies of a pentanucleotide-binding site for the SV40 origin recognition protein, LARGE T ANTIGEN (LT)

Question 21

(T/F) The SV40 DNA contains one origin of replication and is one of the most simplest ORI structures.

Answer 21

True!

Question 22

Origin recognition proteins are __________!

Answer 22

Multifunctional

Origin recognition proteins of other DNA viruses share similar properties as SV40 LT (i.e., many have ATPase & helicase activity, origin-binding sites), although some possess additional activities or features!

Question 23

What is the source of DNA and RNA polymerase in:

1) Viruses with small genomes
2) Viruses with large genomes
3) Viruses with intermediate genomes

Answer 23

Viruses with small genomes: both encoded by the host (replicate in the nucleus)!

Viruses with large genomes: both encoded by themselves (replicate in the cytoplasm)!

Viruses with intermediate genomes: DNA polymerase encoded by themselves, RNA polymerase encoded by the host (replicate in the nucleus)!

Question 24

(T/F) The genomes of DNA viruses span a considerable size range and may be single- or double- stranded DNA molecules that are linear or circular. Whatever their physical nature, viral DNA molecules must be replicated within an infected cell to provide genomes for assembly into progeny virus particles.

Answer 24

True!

Question 25

Why is viral DNA replication delayed until viral replication proteins are produced?

Answer 25

Viral DNA replication requires synthesis of at least one, but usually multiple, viral proteins.

These proteins are involved in viral replication or host cell modulation.

Question 26

Match the steps of DNA virus life cycle

1) Step 1
2) Step 2
3) Step 3
4) Step 4
5) Step 5

A) Early mRNAs synthesized prior to DNA replication

B) Late proteins expressed from late mRNAs (structural proteins)

C) Late mRNAs synthesized after genome replication

D) Early proteins expressed from early mRNAs (involved in 1) viral replication, 2) host cell modulation)

E) Early proteins direct genome synthesis

Answer 26

Step 1: Early mRNAs synthesized prior to DNA replication

Step 2: Early proteins expressed from early mRNAs (involved in 1) viral replication, 2) host cell modulation)

Step 3: Early proteins direct genome synthesis

Step 4: Late mRNAs synthesized after genome replication

Step 5: Late proteins expressed from late mRNAs (structural proteins)

*A common theme among DNA viruses is separation of gene expression into early and late phases

Question 27

What is different about Hepadnaviruses?

Answer 27

Usually small DNA viruses use host enzymes for replication and transcription, while large DNA viruses use their own. Intermediate use their own for replication but host enzymes for transcription.

Hepadnaviruses break all these general trends! They have very small genome but they encode their own DNA polymerase /reverse-transcriptase (RT) that executes their unique mechanism of DNA replication via an ssRNA intermediate.

Question 28

What are the three primers that DNA viruses can direct priming from?

Answer 28

1) RNA primers (classical cellular way!)

2) Their own DNA

3) Via virally-expressed proteins (e.g., adenoviruses that encode a “pre-terminal” protein (pTP), which attaches to the 3’ end of the genome)

Question 29

Differentiate the two mechanisms of viral DNA synthesis.

Answer 29

Replication fork: DNA replication proceeds by copying BOTH strands at a replication fork. There are continuous & lagging strands and is initiated form an RNA PRIMER.

Strand displacement: DNA replication proceeds by copying of ONE strand while its complement is displaced (which can be replicated later). It is initiated from a primer but IS NEVER RNA. It can be a DNA structure or PROTEIN.

All universal elements of DNA replication apply to both mechanisms! Even though the mechanisms are different, both are semi-conservative, require a primer, undergo 5’-3’ synthesis and are template directed.

Question 30

Which viruses undergo replication fork mechanism of viral DNA synthesis? Which undergo strand displacement?

Answer 30

Replication fork: Papillomaviruses, Polyomaviruses, Herpesviruses

Strand displacement: Adenoviruses (protein), Parvoviruses (DNA hairpin), Poxviruses (DNA hairpin)

Question 31

Match the steps of Simian Virus 40 leading & lagging strand synthesis (an example of replication with RNA primer).

1) Step 1
2) Step 2
3) Step 3

A) The replicative DNA polymerase ε then binds to the Rfc/PCNA complex which enables continuous synthesis of the leading strand. Lagging strand synthesis begins with the first Okazaki fragment by DNA polymerase α-primase.

B) DNA polymerase α-primase binds to the ssDNA binding protein Rp-A and to SV40 LT antigen assembled at the origin in the pre-synthesis complex. Once bound, the enzymes generate the leading strand RNA primers that are then extended as DNA.

C) The 3’OH group of the nascent RNA-DNA fragment is then bound by the REPLICATION FACTOR C (Rf-C). Rf-C then allows ATP-dependent opening of the proliferating-cell nuclear antigen (PCNA) ring and its loading onto the template. This process induces dissociation of DNA polymerase α-primase.

Answer 31

Step 1: DNA polymerase α-primase binds to the ssDNA binding protein Rp-A and to SV40 LT antigen assembled at the origin in the pre-synthesis complex. Once bound, the enzymes generate the leading strand RNA primers that are then extended as DNA.

Step 2: The 3’OH group of the nascent RNA-DNA fragment is then bound by the REPLICATION FACTOR C (Rf-C). Rf-C then allows ATP-dependent opening of the proliferating-cell nuclear antigen (PCNA) ring and its loading onto the template. This process induces dissociation of DNA polymerase α-primase.

Step 3: The replicative DNA polymerase ε then binds to the Rfc/PCNA complex which enables continuous synthesis of the leading strand. Lagging strand synthesis begins with the first Okazaki fragment by DNA polymerase α-primase.

Question 32

For lagging strand synthesis, DNA polymerase ________ synthesizes RNA primers and then the Rf-C/PCNA and the DNA polymerase ____ begins the elongation on the 3’ OH free group of the RNA primer.

DNA polymerase ____ synthesizes the leading strand while DNA polymerase _____ synthesizes the lagging strand.

Answer 32

DNA polymerase α-primase; γ

ε(epsilon); δ(delta)

Question 33

What does spooling of a loop of the template DNA strand for discontinuous synthesis allow for?

Answer 33

Spooling of a loop of the template DNA strand for discontinuous synthesis would allow the complex to copy the two strands in opposite directions simultaneously.

Question 34

1) What is the 5’ end problem?

2) How do eukaryotes and prokaryotes over come this problem?

Answer 34

1) RNA primer near the 5’ end on the lagging strand can’t be replaced with DNA since DNA polymerase only adds to the primer sequence and hence the DNA gets shorter and shorter.

2) Eukaryotes use telomerases to fix the problem, while prokaryotes have circular genomes with no ends (no ends = no problems)

Question 35

How do DNA viruses with no telomerases combat the 5’ end problem?

Answer 35

DNA viruses have circular genomes with no ends (Rolling circle model) or use special primers (protein/DNA primers).

Question 36

How do parvovirus overcome the 5’ end problem?

Answer 36

Parvovirus (parvoviridae) replicate using a DNA primer.

Its genome is a ssDNA molecule that has “inverted terminal repetitions” (ITRs).

The ITRs contain PALINDROMIC sequences that form base pairs which ultimately create T-shaped structures (hairpins); No pol α-primase needed, uses ITRs to self-prime.

Generation of these ITRs provides a free 3’OH which is ideal for serving as a primer for the initiation of DNA synthesis. Referred to as “self priming”

Replication is continuous; no replication fork; no end problem!

Question 37

How do adenovirus and hepadnavirus overcome the 5’ end problem?

Answer 37

They have a protein primer with a 3’ OH!

There are origins at both ends!

There is no lagging strand synthesis –> the viral E2 ssDNA binding protein coats the displaced strand.

They undergo strand displacement synthesis and do not have a 5’ end problem!

Question 38

(T/F) For dsDNA viruses (group I), proteins are formed right after entering a cell as there are no key intermediate steps.

Answer 38

True!

The dsDNA can be used as a template for mRNA to make proteins!

Question 39

Which viruses are the largest group of viruses?

Answer 39

dsDNA viruses

Question 40

Polyomavirus (aka SV40) is a ______ virus with three capsid proteins that is _______.

It is usually mild but can cause long term diseases in ___________ patients.

Answer 40

dsDNA; non-enveloped

Immunocompromised

Question 41

Briefly answer the questions regarding the Polyomavirus genome.

1) Describe the genome (the type, what it is associated with, what it encodes for).

2) Why can it lead to malignant tumors?

3) Describe its transcription (its phases, proteins expressed, mechanism used)

4) Which enzyme is used to transcribe the genes for this virus?

Answer 41

1) It has a very small CIRCULAR dsDNA that is associated with cellular HISTONES in a chromatin-like complex. It encodes 5-9 proteins.

2) The viral genome can be integrated in host cell chromosomes. This event inactivates the integrated virus but can give the host cell a REPLICATIVE ADVANTAGE, leading to malignant tumors.

3) Transcription IS NUCLEAR, in two phases: EARLY (replication) and LATE (assembly/exit). 5-9 proteins are expressed from the TWO pre-mRNA genes by ALTERNATIVE SPLICING.

4) All genes are transcribed by HOST RNA POLYMERASE II (small virus).

Question 42

(T/F) The 5-9 proteins of Polyomavirus (SV40) are multifunctional proteins.

Answer 42

True!

Question 43

Match the steps of SV40 infection cycle:

1) Step 1
2) Step 2
3) Step 3
4) Step 4
5) Step 5
6) Step 6
7) Step 7
8) Step 8
9) Step 9
10) Step 10

A) Virion transits through the ER where host proteins rearrange its capsid
structure

B) Transcription of EARLY GENES

C) Virions are released by lysis of the cell

D) Transcription of LATE GENES encoding for structural proteins

E) Attachment of the viral proteins to host receptors triggers LIPID-MEDIATED ENDOCYTOSIS of the virus into host cell

F) Assembly of new virions in NUCLEAR VIRAL FACTORIES

G) Replication of the DNA into the nucleus

H) Export of misfolded virion to the cytoplasm

I) Lost of VP1 in the cytosol

J) Import of genomic DNA into the nucleus

Answer 43

Step 1: Attachment of the viral proteins to host receptors triggers LIPID-MEDIATED ENDOCYTOSIS of the virus into host cell

Step 2: Virion transits through the ER where host proteins rearrange its capsid structure

Step 3: Export of misfolded virion to the cytoplasm

Step 4: Lost of VP1 in the cytosol

Step 5: Import of genomic DNA into the nucleus

Step 6: Transcription of EARLY GENES

Step 7: Replication of the DNA into the nucleus

Step 8: Transcription of LATE GENES encoding for structural proteins

Step 9: Assembly of new virions in NUCLEAR VIRAL FACTORIES

Step 10: Virions are released by lysis of the cell

Question 44

What kind of promoter is found in Polyomavirus?

Answer 44

Bidirectional promoter!

Early and late transcription units on opposite strands: two promoters: EARLY (PE) and PROMOTER LATE (PL).

Question 45

How many early transcripts and late transcripts found in polyomavirus (SV40)?

What mechanisms used for each?

Answer 45

3 early transcripts by ALTERNATIVE SPLICING

single late transcript; alternative splicing, alternate translation initiation, and regulatory miRNAs

Question 46

Through alternative splicing, three early proteins are made in Polyomavirus. Briefly describe each.

Answer 46

Large T (LT/ T ag): major regulator of viral transcription and replication; it represses transcription from early promoter and stimulates transcription from late promoter. it is NUCLEAR + phosphoprotein

Small T (ST/ t ag): shares some sequences with LT, but unqiue C-terminus. Found in the NUCLEUS to help with replication and in the CYTOPLASM to control cellular antiviral pathways. It binds cellular protein phosphatase, reducing cap-dependent translation.

17kT and tiny t: interacts with key cell cycle regulator proteins and thus PROMOTES CELL CYCLE PROGRESSION (allows more product of viral particles)

Question 47

Fill in the blanks regarding the late mRNA transcription of SV40:

1) It occurs after DNA replication and uses complementary DNA strand (_______).

2) The large T antigen binds near ____ and blocks ____ mRNA transcription.

3) The large T antigen also binds to _______ region and turns on ______ promoter (PL).

4) There is ____ late mRNA and _______ ______ leads to two mRNAs coding for the capsid proteins (VP1,2,3).

Answer 47

1) Clockwise

2) Ori; early

3) Enhancer; late

4) one; alternative splicing

Question 48

(T/F) Agno protein is a very early protein of SV40 with no known function.

Answer 48

False!

It is a very LATE protein that may be involved in the release of the virion.

Question 49

Poxviruses are large viruses that are ________ and have a _____-_____ virion.

The surface membrane display surface ________/________.

There are two distinct infectious particles: _________ and ______.

Answer 49

enveloped; brick-shaped

tubules/filaments

intracellular mature virion (MV); extracellular enveloped virion (EV)

Question 50

Vaccinia virus (monkeypox) and variola virus (smallpox) are examples of:

Answer 50

poxviruses

Question 51

(T/F) The vaccinia virus has a circular, dsDNA genome flanked by inverted terminal repeats (ITR) sequences which are covalently-closed at their extremities and serve to prime the transcription.

In the conserved central region, there are genes that code for all the enzymes needed for RNA/DNA synthesis.

Answer 51

False!

The vaccinia virus has a LINEAR, dsDNA genome flanked by inverted terminal repeats (ITR) sequences which are covalently-closed at their extremities and serve to prime the DNA REPLICATION.

In the conserved central region, there are genes that code for all the enzymes needed for RNA/DNA synthesis.

Question 52

The vaccinia virus has three phases of gene expression: early, intermediate, and late. Describe what happens in each.

Answer 52

Early: promote cell growth, counteract host cell responses, proteins for DNA replication, and promote expression of intermediate genes

Intermediate: proteins necessary for late gene expression

Late: structural proteins, enzymes required for assembly, and transcription factors

Question 53

Briefly answer the following questions regarding replication of the Vaccinia virus:

1) How many oris?

2) How is it primed?

3) What kind of mechanism is used?

4) What types of enzymes used (viral or cellular)?

Answer 53

1) One origin for DNA replication

2) DNA self priming from 3’ terminal hairpin

3) Continuous/strand displacement mechanism

4) NO cellular replication enzymes used!

Question 54

What does the replication complex of the vaccinia virus consist of?

Answer 54

It consists of virally encoded proteins:

DNA polymerase
uracil DNA glycosylase (UDG)
nucleoside triphosphatase
A20 protein (forms heterodimer with UDG to enhance processivity)

Question 55

Match the steps of Poxviruses replication cycle:

1) Step 1
2) Step 2
3) Step 3
4) Step 4
5) Step 5
6) Step 6
7) Step 7
8) Step 8

A) EARLY PHASE: early genes are transcribed in the cytoplasm by viral RNA polymerases

B) LATE PHASE: late genes are expressed to produce the structural virions

C) Attachment of the viral proteins to host GLYCOSAMINOGLYCANS MEDIATED ENDOCYTOSIS of the virus into host cell. Can also be uptaken by APOPTOTIC MIMICRY

D) IMV virion can be released upon cell lysis (MV) or can acquire a second double membrane from the trans-golgi and bus as an external enveloped virion (EV)

E) Assembly of virions start in cytoplasmic viral factories, producing immature spherical particles. They mature into brick-shaped mature virions (MV)

F) Core is completely uncoated as early expression ends, viral genome is now free in the cytoplasm

G) Fusion with the plasma membrane to release the core into the host cytoplasm

H) INTERMEDIATE PHASE: intermediate genes expressed, triggering genomic DNA replication

Answer 55

Step 1: Attachment of the viral proteins to host GLYCOSAMINOGLYCANS MEDIATED ENDOCYTOSIS of the virus into host cell. Can also be uptaken by APOPTOTIC MIMICRY

Step 2: Fusion with the plasma membrane to release the core into the host cytoplasm

Step 3: EARLY PHASE: early genes are transcribed in the cytoplasm by viral RNA polymerases

Step 4: Core is completely uncoated as early expression ends, viral genome is now free in the cytoplasm

Step 5: INTERMEDIATE PHASE: intermediate genes expressed, triggering genomic DNA replication

Step 6: LATE PHASE: late genes are expressed to produce the structural virions

Step 7: Assembly of virions start in cytoplasmic viral factories, producing immature spherical particles. They mature into brick-shaped mature virions (MV)

Step 8: IMV virion can be released upon cell lysis (MV) or can acquire a second double membrane from the trans-golgi and bud as an external enveloped virion (EV)

Question 56

How does Vaccinia virus spread from cell to cell?

Answer 56

Vaccinia virus hijacks cellular movements to spread - there is formation of ACTIN TAILS that allows virus to spread from cell to cell.

Also help the virus to avoid the immune system!

It is a unique trait of poxviruses!

Question 57

Which one of the statements is false?

1) For ssDNA viruses, there always needs to be an intermediate step to produce dsDNA for genome replication and for production of mRNA.

2) The ssDNA viral genome is produced by viral DNA polymerases.

3) The best known ssDNA viruses are adneo-associated virus and parvovirus.

Answer 57

2!

The ssDNA viral genome is produced by CELLULAR DNA polymerases.

Question 58

Human parvovirus is a ___________ virus with a small ______ ssDNA genome that has two identical _____ _____ ____ that serve as origins of DNA replication (DNA primers).

Answer 58

non-enveloped; linear; inverted terminal repeats (ITRs)

Question 59

(T/F) Human parvovirus do not cause humans any harm.

Answer 59

False!

They do not normally cause diseases in adults but they do in infants.

Question 60

The ITRs of parvovirus contain _______ sequences that form base pairs, creating __-shaped structures (hairpins).

These structures provide a free 3’____ which is ideal for serving as a primer for the initiation of DNA synthesis (self-priming).

Answer 60

palindromic; T; OH

Question 61

Briefly answer the following questions regarding gene expression of human parvovirus:

1) Which proteins transcribe the genes into mRNAs?

2) What mechanism allows for expression of both structural and non-structural proteins?

3) What does the genome encode?

Answer 61

1) HOST proteins! The virus depends on host enzymes.

2) ALTERNATIVE SPLICING allows expression of both structural and non-structural proteins.

3) The genome encodes a SINGLE NONSTRUCTURAL PROTEIN (NS1) with a large open reading frame (ORF) in its left half and two capsid proteins (VP1, VP2) from another ORF in the right half. It also encodes at least two other smaller proteins with functions unknown.

Question 62

Match the following steps of human parvovirus replication cycle.

Step 1
Step 2
Step 3
Step 4
Step 5
Step 6
Step 7

A) entire virion enters through NUCLEAR PORE (very small)

B) mRNA + new virions produced

C) virus enters the cell via ENDOCYTOSIS (only infects some bone marrow cells)

D) cell lysis (does not acquire an envelope)

E) virions travel to the NUCLEUS (for replication and transcription)

F) makes dsDNA by host cell enzymes

G) replication occurs through rolling-hairpin mechanism

Answer 62

Step 1: virus enters the cell via ENDOCYTOSIS (only infects some bone marrow cells)

Step 2: virions travel to the NUCLEUS (for replication and transcription)

Step 3: entire virion enters through NUCLEAR PORE; intact entry (very small)

Step 4: makes dsDNA by host cell enzymes

Step 5: mRNA + new virions produced

Step 6: replication occurs through rolling-hairpin mechanism

Step 7: cell lysis (does not acquire an envelope)

Question 63

(T/F) For gapped DNA viruses, the DNA gaps are first filled to make a perfect dsDNA, which is transcribed into an mRNA. The mRNA is used to make proteins or for genome replication using reverse transcriptase.

Answer 63

True!

Question 64

1) Hepatitis B virus is an ______, spherical virus that can cause ______ cancer. It is part of the __________ family.

2) It has a gapped dsDNA that is ______.

3) The longer, (-) strand is covalently linked to a _______ _______.

4) The shorter, (+) strand is primed by an _____ _________.

5) The genome has extensive __________ ____.

6) The genome encodes for ____ proteins.

Answer 64

1) enveloped; liver; hepadnavirus

2) circular

3) protein primer

4) RNA primer

5) overlapping ORFs

6) 7

Question 65

Match the following hepadnaviruses proteins to their functions.

1) Polymerase
2) Small S, Medium S, Late S
3) Core
4) E
5) X

A) envelope proteins produced late

B) regulatory protein, helps develop liver cancer

C) terminal protein domain primes DNA synthesis, RT synthesizes DNA; RNAse H cleaves RNA during reverse transcription

D) secreted from infected cells and not found in virions

E) forms the capsid

Answer 65

Polymerase: terminal protein domain primes DNA synthesis, RT synthesizes DNA; RNAse H cleaves RNA during reverse transcription

Small S, Medium S, Late S: envelope proteins produced late

Core: forms the capsid

E: secreted from infected cells and not found in virions

X: regulatory protein, helps develop liver cancer

Question 66

Match the following steps of hepatitis B replication cycle.

Step 1
Step 2
Step 3
Step 4
Step 5
Step 6

A) Virions exit by exocytosis

B) Virus attaches to a particular receptor in the hepatocyte

C) Translation occurs in the cytoplasm

D) Transcription occurs by polymerase II

E) Relaxed circular DNA gets transported in the capsid through the cytoskeleton and into the nucleus through the nuclear pore

F) DNA is released and the gap gets repaired by host enzymes

Answer 66

Step 1: Virus attaches to a particular receptor in the hepatocyte

Step 2: Relaxed circular DNA gets transported in the capsid through the cytoskeleton and into the nucleus through the nuclear pore

Step 3: DNA is released and the gap gets repaired by host enzymes

Step 4: Transcription occurs by polymerase II

Step 5: Translation occurs in the cytoplasm

Step 6: Virions exit by exocytosis