AP Bio: Chapter 19

The lytic cycle of phage T4, a virulent phage. (five steps)

1. ATTACHMENT - the T4 phage uses its tail fibers to bind to specific receptor sites on the outer surface of an E. Coli cell 2. ENTRY OF PHAGE DNA AND DEGRADATION OF HOST DNA - the sheath of the tail contracts, injecting the phage DNA into the cell and leaving an empty capsid outside. The cell's DNA is hydrolyzed 3. SYNTHESIS OF VIRAL GENOMES AND PROTEINS - The phage DNA directs production of phage proteins and copies of the phage genome by host and viral enzymes, using components within the cell 4. ASSEMBLY - three separate sets of proteins self-assemble to form phage heads, tails, and tail fibers. The phage genome is packaged inside the capsid as the head forms. 5. RELEASE - the phage directs production of an enzyme that damages the bacterial cell wall, allowing fluid to enter. the cell swells and finally bursts, releasing 100 to 200 phage particles

What are bacteriophages?

A bacteriophage is a virus that infects bacteria

Distinguish between virulent and temperate phages

A phage replicative cycle that culminates in death of the host cell is known as a lytic cycle. A phage that replicates only by the lytic cycle is a VIRULENT PHAGE. In contrast to the lytic cycle, which kills the host cell, the lysogenic cycle allows replication of the phage genome without destroying the host. Phages capable of using both modes of replicating within a bacterium are called TEMPERATE PHAGES.

What portion of the phage enters the host cell? How?

After the bacteriophage binds to a specific receptor on the outer surface of the host cell, the sheath of the tail contracts, injecting the phage DNA into the cell and leaving an empty capsid outside. The cell's DNA is hydrolyzed.

What might trigger the switch over from lysogenic to lytic mode?

An environmental signal, such as a certain chemical or high-energy radiation

Why were viruses difficult to study early on?

Because they are so small, it took years before they could be detected and studied

transposons

DNA segment that can move from one location to another within a cell's genome

What was Wendell Stanley's contribution to our knowledge of viruses?

He confirmed Beijerinck's concept of a virus by crystalizing the tobacco mosaic virus, which was then actually seen under an electron microscope.

What does examining the structure of a virus more closely reveal about its structure?

It is an infectious particle consisting of nucleic acid enclosed in a protein coat and, for some viruses, surrounded by a membranous envelope

What are three ways bacteria may win the battle against the phages?

Just as natural selection favors bacteria with mutant receptors or effective restriction enzymes, it also favors phage mutants that can bind the altered receptors or are resistant to particular restriction enzymes Thus, the parasite-host relationship is in constant evolutionary flux Moreover, instead of lysing their host cells, many phages coexist with them in a state called lysogeny

how does a DNA virus reproduce its genome?

Many DNA viruses use the DNA polymerases of the host cell to synthesize new genomes along the templates provided by the viral DNA

How do restriction enzymes help prevent viral infections of bacteria?

Restriction enzymes restrict the ability of the phage to infect a bacterium by cutting up the foreign DNA

Distinguish between a virus with a broad host range and one with an extremely limited host range, and give an example of each.

Some viruses have broad host ranges. For example, West Nile virus can infect mosquitoes, birds, horses, and humans. Other viruses have host ranges so narrow that they infect only a single species. Measles virus, for instance, can only infect humans. Viral infection of multicellular eukaryotes is usually limited to particular tissues.

T4 Bacteriophage: What does it's name mean? What is it's host? DNA or RNA?

T4 is a bacteriophage that's host is E. Coli and its genome consists of DNA

Why don't restriction enzymes destroy the DNA of the bacterial cells that produce them?

The bacterial cell's own DNA is methylated in a way that prevents attack by its own restriction enzymes

what components of the host cell does a virus use to reproduce itself?

The host provides the nucleotides for making viral nucleic acids, as well as enzymes, ribosomes, tRNAs, amino acids, ATP, and other components needed for making the viral proteins

describe the lytic and lysogenic modes of bacteriophage reproduction

The lytic mode of bacteriophage reproduction results in the release of new phages through the lysis of the host cell. The lysogenic mode allows the viral genome to be replicated along with the bacterial chromosome in the form of a prophage without killing the host.

Are viruses living or non living?

They are not living. They cannot reproduce or carry out metabolic activities outside of a host cell. They exist in a shady area between life-forms and chemicals - they lead a kind of "borrowed life"

how do most RNA viruses replicate their genome?

To replicate their genomes, RNA viruses use virally encoded RNA polymerases that can use RNA as a template

viruses are obligate intracellular parasites. What does that mean?

Viruses can only replicate within a host cell, subsisting on "borrowed life."

Compare host range for the rabies virus to that of the human cold virus

While human cold viruses infect only the cells lining the upper respiratory tract, rabies virus can infect most mammal species.

what are two elements that nearly all animal viruses have

a membranous envelope and an RNA genome

viral envelope

a membranous envelope that surrounds the capsid of some viruses they are derived from the membranes of the host cell, contain host cell phospholipids and membrane proteins they also contain proteins and glycoproteins of viral origin

lytic cycle

a phage replicative cycle that culminates in death of the host cell

virulent phage

a phage that replicates only by a lytic cycle

AIDS

acquired immunodeficiency syndrome caused by the retrovirus HIV

lysogenic cycle

allows replication of the phage genome without destroying the host

reverse transcriptase

an enzyme which transcribes an RNA template into DNA, providing an RNA to DNA information flow

restriction enzymes

cellular cutting enzymes whose activity restricts the ability of the phage to infect the bacterium

helical viruses

common name for rod-shaped viruses (the rods form a helix shape)

What are the four forms of viral genomes?

double stranded DNA single stranded DNA double stranded RNA single stranded RNA

host range

each particular virus can infect cells of only a limited number of host species, called the host range of the virus this host specificity results from the evolution of recognition systems by the virus

What is the role of an envelope in animal viruses?

it is an accessory structure that helps viruses infect their hosts

icosahedral viruses

name for viruses that have a polyhedral capsid with 20 triangular facets (icosahedron) EX: adenoviruses - infect upper respiratory tracts of animals, have 252 identical protein molecules arranged in a polyhedral capsid with 20 triangular facets - an icosahedron

temperate phages

phages capable of using both modes of replicating within a bacterium

plasmids

small, circular DNA molecule found in bacteria and unicellular eukaryotes (yeasts)

provirus

the integrated viral DNA that never leaves the host's genome, remaining a permanent resident of the cell

What is a capsid?

the protein shell enclosing the viral genome

What are capsomeres?

the protein subunits that build capsids

mobile genetic elements

transposons, plasmids, and viruses are all mobile genetic elements

compare and contrast prophage and provirus. Which are you likely to carry?

A prophage is a phage genome inserted into a specific site on a bacterial chromosome. A provirus is a viral genome permanently inserted into a host genome. Animal cells would most likely carry a provirus.

What different shapes may capsids have?

Depending on the type of virus, the capsid may be rodshaped, polyhedral, or more complex in shape, like T4.

What was some early evidence of viruses?

In 1883, Adolf Mayer discovered that he could transmit tobacco mosaic disease from plant to plant by rubbing sap extracted from diseased leaves into healthy plants After an unsuccessful search for an infectious microbe in the sap, Mayer suggested that the disease was caused by unusually small bacteria that were invisible under a microscope A decade later, Ivanowsky reasoned that such bacteria were small enough to pass through a porcelain filter or made a toxin that could do so The latter idea was ruled out when Martinus Beijerinck showed that the infectious agent in the filtered sap could replicate, but, unlike bacteria used in the lab at that time, could not be cultivated on nutrient media in test tubes or petri dishes Beijerinck imagined a replicating particle much smaller and simpler than a bacterium.

What property of a virus determines its attachment to a host cell membrane?

Viruses usually identify host cells by a "lock-and-key" fit between viral surface proteins and specific receptor molecules on the outside of cells

What are the components of a viral envelope? Which component is derived from the host cell, and which is of viral origin?

phospholipids and membrane proteins are derived from the HOST CELL proteins and glycoproteins are of VIRAL ORIGIN

the lytic and lysogenic cycles of lambda, a temperate phage

(lambda is a temperate phage used in research) After entering the bacterial cell and circularizing, the lambda DNa can immediately initiate the production of a large number or progeny phages (lytic) or integrate into the bacterial chromosome (lysogenic) In most cases, phage lambda follows the lytic pathway, however once a lysogenic cycle beings,, the prophage may be carried in the host cell's chromosome for many generations Phage lambda has one main tail fiber, which is short

the replicative cycle of HIV (10 steps)

1. the envelope glycoproteins enable the virus to bind to specific receptors on certain white blood cells 2. the virus fuses with the cell's plasma membrane. The capsid proteins are removed, releasing the viral proteins and RNA 3. reverse transcriptase catalyzes the synthesis of a DNA strand complementary to the viral RNA 4. reverse transcriptase catalyzes the synthesis of a second DNA strand complementary to the first 5. the double stranded DNA is incorporated as a provirus into the cell's DNA 6. proviral genes are transcribed into RNA molecules, which serve as genomes for the next viral generation and as mRNAs for translation into viral protein 7. the viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER) 8. vesicles transport the glycoproteins to the cell's plasma membrane 9. capsids are assembled around viral genomes and reverse transcriptase molecules 10. new viruses bud off from the host cell

The viral replicative cycle (4 steps)

1. virus enters cell and is uncoated, releasing viral DNA and capsid proteins 2. Host enzymes replicate the viral genome 3. Meanwhile, host enzymes transcribe the viral genome into viral mRNA, which host ribosomes use to make more capsid proteins 4. Viral genomes and capsid proteins self-assemble into new virus particles, which exit the cell

HIV

human immunodeficiency virus the retrovirus that causes AIDS

How do retroviruses, such as HIV, replicate their genome

The integrated viral DNA, called a provirus, never leaves the host's genome, remaining a permanent resident of the cell. The host's RNA polymerase transcribes the proviral DNA into RNA molecules, which can function both as mRNA for the synthesis of viral proteins and as genomes for the new viruses that will be assembled and released from the cell

retrovirus

an RNA virus equipped with an enzyme called a reverse transcriptase Retroviruses have the most complicated replicative cycles

virus

an infectious particle consisting of little more than genes packaged in a protein coat

what are the candidates for the original sources of viral genomes?

plasmids and transposons

prophage

viral DNA that is integrated into the bacterial chromosomes through the lysogenic cycle

bacteriophages (phages)

viruses that infect bacteria literally means "bacteria eater"