Chapter 18: Genetics of Viruses

With regard to promoting the lytic or lysogenic cycle, what would happen if the following genes were missing from the λ genome? A. Cro B. cI C. cII D. int E. cII and cro

A. The lysogenic cycle would occur because Cro is req. to start the lytic cycle. B. The lytic cycle would occur because cI encodes the λ repressor C. The lytic cycle would occur because cII protein is req. to start the lysogenic cycle. D. Both cycles could try to initiate, but the lysogenic cycle would fail because it would be unable to integrate into the host chromosome. E. Neither cycles could occur.

What do the terms host cell and host range mean?

A host cell is a cell that can be infected by a virus and allows the virus to replicate. Host range refers the diff organisms the virus is able to infect.

What is the difference between a temperate phage vs a virulent phage?

A temperate phage can follow either the lytic or lysogenic cycle, whereas a virulent phage can follow only the lytic cycle.

What is a viral envelope? Describe how it is made.

A viral envelop is composed of a membrane with embedded proteins. It is made when the virus buds from the host cell, taking with it a portion of the host cell's plasma membrane.

What are the similarities and differences among viral genomes?

All viral genomes are composed of nucleic acid and carry genes. They differ in that some genomes are DNA whereas others are RNA. Some are single stranded and others are double stranded. Also, Viral genomes differ greatly in their relative sizes.

What structural features are common to all viruses? Which features are found only in certain types of viruses?

All viruses have a nucleic acid genome and a capsid composed of protein. Some eukaryotic viruses are surrounded by an envelop that is composed of a membrane with embedded proteins.

What technique must be used to visualize a virus?

Because they're so small we should use Electron microscopy techniques Two types: 1. Transmission electron microscopy 2. Scanning electron microscopy - more magnified and can see 3D

What's the entry step of the viral reproductive cycle of phage Lambda?

For phage λ, the virus attaches to the surface of the host cell and then injects its genetic material into the cytoplasm. The entire virus does not enter.

What key features distinguish the lytic from the lysogenic cycle?

In the lytic cycle, the viral genome does not incorporate into the host genome. In the lysogenic cycle, the viral genome incorporates into the host genome and stays there throughout replication until the lytic cycle is triggered.

A researcher identified a mutation in PR of phage λ that causes its transcription rate to be increased 10-fold. Do you think this mutation would favor the lytic or lysogenic cycle? Explain your answer.

It would first increase the amount of Cro protein so that the lytic cycle would be favored.

Describe how lytic bacteriophages are released from their host cells.

Lytic bacteriophages encode an enzyme called lysozyme that digests the bacterial cell wall. This causes the bacteria to burst, thereby releasing the newly made phages.

What is a prophage, a provirus, and an episome? What is their common role in a viral reproductive cycle?

Prophage- phage DNA intergrated into the host genome Provirus- another term for prophage, where viral DNA is integrated into host cell's genomic DNA Episome-

What is the role of reverse transcriptase in the reproductive cycle of retroviruses?

Reverse transcriptase is used to copy the viral RNA into DNA so it can be integrated into a chromosome of the host cell.

Viruses may be latent for a long period of time. For example, HIV may be latent for many years, during which new viruses are not made. What are three different mechanisms of viral latency?

Some bacteriophages enter the lysogenic cycle, in which phage DNA becomes incorporated into the bacterial chromosome as a prophage. In this case, the phage DNA is directly incorporated into the host-cell chromosome. Eukaryotic viruses can become latent in other ways. Retroviruses, which have an RNA genome, are reverse transcribed and the viral DNA is incorporated into a host-cell chromosome as a provirus. Alternatively, herpesviruses can exist as episomes that remain latent for long periods of time.

Describe why the attachment step in a viral reproductive cycle is usually specific for one or just a few cell types.

The attachment step usually involves the binding of the virus to a specific protein on the surface of the host cell. Only certain cell types will make that specific protein.

Discuss why viruses are considered nonliving.

Viruses do not possess many of the characteristics that are associated with living organisms. They do not carry out metabolism, reproduce, or utilize energy by themselves