Chapter 18 Reading Quiz

1. Which viral reproductive cycle destroys the host cell?

2. A(n) ______ is a harmless variant or derivative of a pathogen that stimulates the immune system to mount a defense.

3. An infectious protein is called a(n) ______. 4. How many different species a virus can

infect is called its _______.5. Viruses that show up suddenly are

referred to as ______ viruses.

lytic

vaccine

prion

Host range

emerging

Discovery of viruses….

• Mayer demonstrated that the stunting disease of tobacco plants was contagious thought it was caused by an unusually small bacteria- later findings demonstrated that the disease could not be bacteria-caused, but must be a particle much smaller (and unlike) a bacterium- the infectious particle was finally crystallized and observed and is now known as the tobacco mosaic virus (TMV)

structural components of viruses and why viruses are obligate parasites.

• It is a genome enclosed in a protective coat

• It is organized as single nucleic acid molecules

• May have 4 to several hundred genes

• Simple composition 1. Capsid – protein coat that encloses the viral genome2. Envelope – membrane that cloaks some viral capsids (head, sheath, DNA, tail fibers)

• Viruses express their genes and reproduce only within a living cell

how viruses recognize host cells.

• They recognize their host cell by a complementary fit between external viral proteins and specific cell surface receptor sites

what happens when a virus infects a host cell.

• A viral infection begins when the genome of a virus makes its way into a host cell

the lytic and lysogenic reproductive cycles using phage T4 and phage as examples.

Lytic Cycle• Viral replication cycle that

results in the death (or lysis) of the host cell

• T4 phage attaches to cell surface, phage contracts sheath and injects DNA, hydrolytic enzymes destroy the host cell’s DNA, phage genome directs the host cell to make phage components and cell lyses and releases phage particles

Lysogenic Cycle• A viral replication cycle

that involves the incorporation of the viral genome into the host cell genome

• λ phage binds to the surface of ecoli and injects the DNA and inserts it by genetic recombination

defenses bacteria have against phage infection.

• Bacterial mutations can change receptor sites to avoid recognition- this in turn prevents infection

• Restriction nucleases in bacteria recognize and cut up foreign DNA- self-destruction is avoided because bacterial DNA is chemically altered (methylated)

variations in replication cycles of animal viruses.

Enveloped Viruses are characterized by:

1. Attachment2. Entry3. Uncoating4. Viral RNA & protein

synthesis5. Assembly and

release

RNA as viral genetic material:

- RNA viruses can be complicated (like retroviruses)

- mRNA or the strand that corresponds to mRNA is the + strand and it has the nucleotide sequence that codes for proteins

- The – strand is a template for synthesis of the + strand

the role of reverse transcriptase in retroviruses.

• It is the enzyme that transcribes DNA from an RNA template

• The newly integrated DNA is now a “provirus” and never leaves the host genome

Viral genomic RNA (reverse transcriptase)

Viral DNA

Evolution of viruses

• Probably evolved several times AFTER the first cells

• Probably came from little bits of nucleic acids that moved from cell to cell – maybe plasmids and/or transposons (mobile genetic elements)

evidence that viruses probably evolved from fragments of cellular nucleic acid.

• Genetic material of different viral families is more similar to host genomes than to that of other viral families

• Some viral genes are identical to cellular genes

• Viruses of eukaryotes are more similar in genomic structure to their cellular hosts than to bacterial viruses

• Viral genomes are similar to cellular genetic elements like plasmids and transposons

what vaccines are and how they work.

• Vaccine a “harmless” variant or derivative of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen

• Once you have a viral infection though, it’s pretty much up to your immune system

• There are some anti-viral drugs – AZT interferes with DNA synthesis by reverse

transcriptase– Acyclovir stops viral polymerase in herpesvirus

how viruses may cause disease symptoms

• Viruses damage or kill cells (viral infection lysosome releases hydrolytic enzymes)

• They can be toxic or cause infected cells to produce toxins

• Cause varying degrees of cell damage• Immune system reacts, causing fever,

aches, inflammation

Emerging viruses • HIV in San Francisco in the 1980s• Ebola in 1976 in Africa• West Nile virus in 1999 in North America• H1N1 (swine flu) in 2009; H5N1 (bird flu) in

1997 (mortality rate of 50%!)– Named for the surface proteins

• 16 types of hemaglutinin; 9 types of neuraminidase

• How they show up so suddenly…– Mutation of existing viruses– Dissemination from an isolated human population– Spread from other animals (about ¾ of new human

diseases)

horizontal and vertical routes of viral transmission in plants.

• Horizontal route of viral infection in which an organism receives the virus from an external source

• Vertical route in which an organism inherits a viral infection from its parent

Viroids & prions• Viroids – tinier circular RNA molecules

that cause errors in plant growth regulation

• Prions – infectious proteins - misfolded– Act very slowly (ten-year incubation period)– Pretty much indestructible– Cause mad-cow, Creutzfeldt-Jakob disease– Not much is known about these yet…