Micro Exam 2

Viruses

Minute parasites that seize control of the synthetic and genetic machinery of cells

Ivanovski and Beijerinck showed that a

disease in tobacco was caused by a virus; Tobacco Mosaic virus

Friedrich Loeffler and Paul Frosch discovered an

animal virus that causes foot-and-mouth disease in cattle

Filterable virus

Found that when fluids from host organisms passed through porcelain filters (trap bacteria), filtrate remained infectious

Filterable virus result proved that a

cell-free fluid could contain agents that could cause infection

Viruses infect

every type of cell, including bacteria, algae, fungi, protozoa, plants, and animals

Seawater can contain

100 million viruses/mL

Louis Pasteur postulated that a

“living thing” smaller than bacteria caused these diseases

Louis Pasteur also proposed the term

virus, which is Latin for poison

Since viruses are unable to replicate independently from the host cell

they are not living things and should be called infectious molecules

Even though viruses do not exhibit most of the life process of cells

they can direct them and thus are certainly more than inert and lifeless molecules

Viruses are better described as

active or inactive rather than alive or dead

Viruses infect

cells and influence genetic makeup

10% of the human genome consists of

sequences that come from viruses

10-20% of bacterial DNA contains

viral sequences

Viruses have either

DNA or RNA, not both

Viruses are either

single stranded or double stranded, not both

For many years, animal viruses were classified on the basis of

their hosts and the diseases they caused

The newer virus classification system is based on

• Host and disease they cause

• Structure

• Chemical composition

• Similarities in genetic makeup

International committee of the taxonomy of viruses

3 orders, 73 families, and 283 genera

Viruses range from

20-450nm

Virus particle contains both a

covering and central core

A virus covering is a

capsid and in some viruses also an envelope

A virus central core is

nucleic acid molecule (DNA or RNA), matrix proteins, and in some viruses enzymes

Viruses have no

resemblance to cells; no protein synthesizing machinery

Viruses have regular

structure, repeating subunits; crystalline appearance

Viruses contain only those parts needed to

invade and take over host cells

Capsid

protein shell that surrounds the nucleic acid

Nucleocapsid

capsid + nucleic acid

Envelope

external covering of a nucleocapsid, usually a modified piece of the host’s cell membrane (from nuclear envelope or ER)

Spikes are on

naked or enveloped viruses

Spikes project from the

nucleocapsid of the envelope

Spikes allow viruses to

bind host cells

Virion

a fully formed virus able to establish an infection in a host cell

Generalized Structure of Viruses

Naked or enveloped viruses

Capsid structure is made of

identical protein subunits called capsomeres that spontaneously self-assemble

Capsid structure

Helical, icosahedral, or complex capsid

There are 4 genes in

hepatitis B virus

There are 100s of genes in some

herpesvirus

Viruses posses only the genes needed to

invade host cells and redirect their activity

Viruses exhibit a wide variety of configurations of

DNA or RNA

DNA viruses

single-stranded, double-stranded, linear, circular

RNA viruses

double-stranded, usually SS, Positive-sense RNA: ready for immediate translation, Negative-sense RNA: must be converted before translation

Retrovirus

carry their own enzymes to create DNA out of RNA

Viruses lack

genes for synthesis of metabolic enzymes

polymerase synthesize

DNA and RNA

Replicase copy

RNA

Reverse transcriptase synthesize

DNA from RNA

Some viruses can carry away substances from their

host cell

retroviruses borrow the host’s

tRNA molecules

Animal virus replication cycle varies from

8 hours in polio viruses to 36 hours in herpes virus

Life cycle of animal viruses

1. Absorption

2. Penetration and Uncoating

3. Synthesis phase in dsDNA virus

4. Assembly

5. Maturation and release of enveloped viruses

Absorption of animal viruses

a virus can invade its host cell only through making an exact fit with a specific host molecule

Host range

the limited range of cells that a virus can infect

Hepatitis B infects

live cells of humans

Poliovirus infects

intestinal and nerve cells of primates

Tropisms

specificites of viruses for certain tissues

Cells that lack compatible virus receptors are resistant to

absorption and invasion by that virus

Penetration of animal virus

whole virus (endocytosis) or its nucleic acid enter host cell and can fuse with membrane and release nucleocapsid or enzyme dissolved in vacuole

Uncoating of animal viruses

Vacuole enzymes dissolve the capsid and/or envelope. Virus fuses with the wall of the vesicle and viral nucleic acid is released into the cytoplasm

Synthesis phase in dsDNA virus, early phase

Viral DNA goes into nucleus where genes are transcribed then the RNA transcripts go into the cytoplasm for translation, translated into viral proteins needed to replicate viral DNA (host cell’s DNA polymerase is involved in this phase)

Synthesis phase in dsDNA virus, late phase

proteins required to form the capsid and other structures made, new viral genomes and capsids are assembled, and mature viruses are released by budding

Assembly of animal viruses

put together the new viruses using the parts manufactured in the synthesis process, new capsids and nucleic acids

The number of released enveloped viruses is based on

size and host cells health

Poxvirus-infected cell contains

3,000-4,000 virions

Poliovirus-infected cell contains

100,000 virons

Even if a small number of virions meet another cell

the potential for rapid viral proliferation is immense

Cytopathic effectors

virus-induced damage to the cell that alters its microscopic appearance

Cytopathic effectors can lead to

gross changes in shape and size, intracellular changes, inclusion bodies, and syncytia

Inclusion bodies

compacted masses of viruses or damaged cell organelles

Syncytia

Fusion of multiple host cells into single large cells containing multiple nuclei

Persistent infections are due to

cell harboring the virus and not being immediately lysed

Persistent infections can last from

a few weeks to the remainder of the host’s life

Persistent infections can remain latent in

cytoplasm

Provirus

a persistent infection in which the viral DNA is incorporated into the DNA of the host

Proviruses may remain hidden in

brain cells for many years, causing progressive damage and loss of function

Chronic latent stage

Viruses go into a period of inactivation in cells and then later periodically emerge under the influence of various stimuli

Oncogenic viruses

experts estimate that up to 20% of human cancers are caused by viruses

Transformation has effects on the cell caused by

oncogenic viruses

Transformation factors

increased rate growth, alterations in chromosomes, changes in cell’s surface molecules, capacity to divide for an indefinite period

Viruses carry genes that directly cause

cancer

Viruses produce proteins that induce a loss of

growth

Cancer-causing viruses

papillomaviruses and Epstein-Barr virus

Some retroviruses are viral oncogenes that incorporate into

host DNA, produce proteins that lead to uncontrolled cell growth

Other retroviruses are viral genes that affect

expression of host oncogene leading to uncontrolled cell growth

DNA tumor viruses are viral genes that directly produce

proteins that lead to uncontrolled cell growth

Bacteriophage

every bacterial species is parasitized by various bacteriophages

Bacteriophages often make the bacteria they infect more

pathogenic

T-even bacteriophage infect

Escherichia coli

Lytic cycle of T-Even Bacteriophages

1. Absorption

2. Penetration

3. Duplication of phage components

4. Assembly of new virions

5. Maturation

6. Lysis of weakened cell and release viruses

Lysogenic state

The viral DNA molecule is inserted at specific sites on the bacterial chromosome. The viral DNA is duplicated along with the regular genome and can provide adaptive genes for the host bacterium.

Lysogeny

a condition in which the host chromosome carries viral DNA

Phage genes can insert in the

bacterial chromosomes and cause the production of toxins or enzymes that cause pathology in the human

Lysogenic conversion

when a bacterium acquires a new trait from its temperate phage

Viruses require

living cells as their medium

Viruses in vivo

lab-bred animals and embryonic bird tissues

Viruses in vitro

cell or tissue culture methods

Primary purposes of viral cultivation

isolate and identify viruses in clinical specimens, prepare viruses for vaccines, and do detailed research on viral culture, multiplication cycles, genetics, and effects on host cells

Spongiform encephalopathies is smaller and simpler than

viruses

Spongiform encephalopathies is implicated in

chronic, persistent diseases in humans and animals