3

DNA viruses are either double stranded or single stranded.

True

Most DNA viruses are pluse sense but you can also have minus strand.

True

If it is a double stranded DNA virus they will have to be able to remake their genomes.

True

If there single stranded DNA viruses they have to go through a intermediate where they make the minus strand in order to make more plus strands just like an RNA virus.

True

Small DNA viruses tend to use cellular enzymes.

True

Smaller DNA viruses tend to use cellular enzymes. Smaller DNA viruses tend to steal enzymes from the cell to do there replication.

True

Larger DNA viruses tend to make their own enzymes.

True

• The enzymes used for replication generally have proofreading activity which will make synthesis more accurate.

  • This can potentially slow evolution in some ways. It gives the advantage of being more accurate and making more accurate progeny. At the same time, it takes a little bit away from diversity and diversification in the long run. This might be one of the reasons there are so many RNA viruses out there their able to diversify themselves.

True

DNA replications requires a primer it can either be DNA or RNA.

True

DNA replication is semiconservative.

True

DNA replication is 5’ → 3’ synthesis.

True

Small DNA viruses (eg. parvoviruses, polyomavirus, papilloma viruses) use host cell polymerase for replication and also may use many other host enzymes (ligases, topoisomerases, primases ssDNA binding proteins etc.) in the replication process.

True

Larger viruses (eg. adenovirus, herpesvirus, poxvirus) make their own polymerases and typically make other proteins involved in DNA replication as well although they often use host proteins too.

True

Almost all DNA viruses make their own origin binding protein that recognized the origin of DNA replication for the virus. This is the key protein in getting replication started and attracting all the other replication proteins.

True

What is the key protein in getting replication started and attracting all the other replication proteins?

Origin Binding Protein

Mechanisms of Viral DNA Synthesis

All viruses face the same problems (as does host):

 

1.Origin recognition and unwinding

2.Priming

3.Elongation

4.Termination

5.Resolution of intermediates.

Small DNA virus utilize host cell polymerases.

True

Larger DNA viruses encode their own polymerases but most use some host cell proteins.

True

Adenoviruses

Protein (primer)

Parvoviruses

DNA hairpin (primer)

Poxvirus

DNA hairpin (primer)

Mechanisms of viral DNA synthesis: primers

• Viral RNA primers-synthesized by viral or cellular primases (eg. polymavirus, papillomvirus, herpesvirus, poxvirus).

• Many DNA viruses have evolved to dispense with RNA priming. Replicate by strand displacement.

  • They can direct priming from either:

    • Their own DNA (self-priming) (eg. parvovirus)

    • Specific viral proteins (eg. adenovirus)

Mechanisms of exponential viral DNA replication

Many cells in vivo do not divide and permanently in a resting state termed G0

• Uncontrolled DNA replication is bad for cells → cancer

Cells in G0 do not have cellular machinery and components for high levels of replication of the virus.

How do the viruses circumvent this problem controls?

• Viruses encode the necessary components - herpesvirus, poxviruses

• Stimulate cell growth - adenoviruses

Tumor suppressor protein

Cellular proteins that negatively regulate cell proliferation

Retinoblastoma (Rb) protein blocks cell cycle progression

Blocks DNA replication

• Binds to transcription activators of the E2f family.

• Binds to promoter through E2f.

• Rb protein blocks transcription of these genes.

  • Loss of Rb function associated with retinoblastomas and other tumors in children/young adults

Rb protein blocks cell cycle progression and prevents cell division and growth. Since some DNA viruses need growing and dividing cells to get the components they need to replicate, they have to get around this problem.

True

Viral inactivation of Rb

Many viruses encode proteins that bind and inactivate Rb

• Allows uncontrolled DNA synthesis

DNA viruses make specific protein factors that prevent Rb from interacting with other cellular proteins and downregulates the synthesis of a number of host (and viral) proteins.

True

Parvoviruses are the smallest known DNA-containing viruses.

True

Parvoviruses are single stranded DNA viruses that replicate in the nucleus and carry no enzymes into the cell with the virion.

True

Parvovirus B19 causes 5th disease.

True

Some parvoviruses cannot cause an infection by themselves but can only replicate in the presence of another virus (adenoviruses, some can replicate along with hepeviruses).

True

The adeno-associated viruses (AAV's 1-5) have not been established to cause any disease, however, with co-infection with a helper virus (either adenovirus or a herpesvirus) results in host cell infection. Because of this lack of association with any known human disease and because of the simplicity of the genome and the ease with which it can integrate into host genome, AAV's recently have received much attention as possible vectors for gene therapy.

True

Inverted repeats at the ends of the genome allow the T - like structure to form and serve as a primer for DNA synthesis which is carried out by a host polymerase. This illustrates self-priming of DNA synthesis by a terminal inverted repeat.

True

Replication is complex (you do not have to know the process).  A key step in replicaton is carried out by the viral Rep78/68 dimer protein which nicks the dsDNA intermediate between the A’ and D regions of the DNA.  The “replication intermediate” serves as the template for production of viral mRNAs which is shown in C for the adenovirus-associated virus.  This virus has 3 different promoters that make mRNA which can be alternatively spliced to make 7 protein products.  Rep proteins are involved in replication as origin binding or nicking proteins (see above) and also serve to enhance transcription from the P40 promoter that makes the VP proteins which are virion proteins.  Other parvoviruses may have less promoters and make more or less proteins but the general theme is the same. 

True

Viral genomes have Inverted Terminal Repeats (ITR).

True

T - like inverted terminal repeat structure formed at the end of the genome that initiates DNA replication. The viral origin binding protein (Rep68/Rep78) recognizes this as the origin of replication and binds to initiate DNA replication

True

Adenoviruses cause upper respiratory tract infections. This virus does not bring enzymes in with the virion. Medium sized virus.

True

Splicing was discovered in Adenoviruses.

True

The genome of adenoviruses consist of linear, double-stranded DNA of 30-38kbp.

True