MICROBIO- CHAPTER 8 & 10

Bacteriophage T4 lacks its own RNA polymerase. How do T4 genes get expressed or converted to mRNA? What host barriers must be broken before release of virions from the host cell?

Bacteriophage T4 contains a double-stranded DNA genome that is both circularly permuted and terminally redundant. T4 encodes its own DNA polymerase and several other replication proteins. After a T4 virion penetrates a host cell, viral genes are expressed and regulated so as to redirect the host synthetic machinery to make viral nucleic acid and protein. Early viral genes encode viral genome replication events; middle and late viral genes encode structural proteins and capsid assembly. Once T4 components have been synthe- sized, new virions are made, primarily by self-assembly, and the virions are released after lysis of the host cell.

Where does the envelope surrounding animal viruses originate?

In the virion of a naked virus, only nucleic acid and protein are present; the entire unit is called the nucleocapsid. Enveloped viruses have one or more lipoprotein layers surrounding the nucleocapsid. The nucleocapsid is arranged in a symmetric fashion, with the icosahedron being a common morphology. Although virus particles are metabolically inert, one or more key enzymes are present within the virion in some viruses.

How do viral numbers compare to those of bacteria in seawater?

The number of viruses on Earth is greater than the number of cells by 10-fold. Most of the genetic diversity on Earth resides in virus genomes, most of which are still to be investigated. Viruses affect their host cells by either culling the host population or by carrying out horizontal gene transfer from one bacterial cell to another. In the oceans, both Bacteria and Archaea are likely to be infected with viruses.

Briefly describe the two approaches a bacteriophage can take to trigger an infection once inside its host cell.

A virus is an obligate intracellular parasite that requires a suitable host cell for replication. A virion is the extracellular form of a virus and contains either an RNA or a DNA genome inside a protein shell. Once the virus is inside the cell, the viral nucleic acid, and sometimes viral enzymes, redirects host metabolism to support virus replication. Viruses are classified by the characteristics of their genome and hosts. Bacteriophages infect bacterial cells.

Explain CRISPR and its mechanism.

Prokaryotic cells employ various mechanisms to defend against viral infection. However, the fast mutation rate and genome plasticity of viruses allows them to adaptto these strategies. The CRISPR system is a type of immune system for Bacteria and Archaea that resists viral infection by recognizing and degrading incoming foreign DNA.

What enzyme is required to form a prophage, and what are the two main transcriptional repressors that control whether lambda proceeds to lysogeny or lysis?

Some bacteriophages are temperate, meaning that they can initiate lytic events or integrate into the host genome as a prophage. Integration initiates a state called lysogeny in which the virus does not destroy the cell. A well-studied lysogenic virus of Escherichia coli is phage lambda; this phage uses an intricate regulatory system to govern whether the lytic or lysogenic state is initiated following infection.

What is required for a bacteriophage T4 virion to attach to an Escherichia coli cell?

The attachment of a virion to a host cell is a highly specific process. Recognition proteins on the virus recognize specific receptors on the host cell. Sometimes the entire virion enters the host cell, whereas in other cases, as with most bacteriophages, only the viral genome enters. Host cells employ restriction enzymes in attempts to destroy viral and other foreign DNA, but T4 has chemically modified its DNA to make it resistant to such attack. Cells also modify their own DNA to protect it from their own restriction enzymes.

Why does a one-step growth curve differ in shape from that of a bacterial growth curve?

The virus replication cycle can be divided into five major stages: attachment (adsorption), penetration (uptake of the entire virion or injection of the nucleic acid only), protein and nucleic acid synthesis, assembly and packaging, and virion release.

Why can it be said that the retrovirus genome is unique in all of biology?

There are animal viruses with all known modes of viral genome replication. Many animal viruses are enveloped, picking up portions of host membrane as they leave the cell. Viral infection of animal host cells can result in cell lysis, but latent or persistent infections are also common, and a few animal viruses can cause cancer. Retroviruses like the AIDS virus are RNA viruses that employ the enzyme reverse transcriptase to replicate their RNA genome through a DNA intermediate. The DNA can integrate into the host chromosome where it can later be transcribed to yield viral mRNA and genomic RNA.

Draw a diagram of the one-step growth curve of virus replication, and briefly explain why the growth curve differs from a typical bacterial growth curve.

Viruses can replicate only in their correct host cells. Bacte- rial viruses have proved useful as model systems because their host cells are easy to grow and manipulate in culture. Many animal viruses can be grown in cultured animal cells. Viruses can be quantified (titered) by a plaque assay. Plaques are clearings that develop on lawns of host cells, and in analogy to bacterial colonies, arise from the viral infection of a single cell.