CH. 19 Viruses Flashcards
cannot reproduce independently
infect every type of cell, including algae, bacteria, fungi, protozoa, plants and animals
seawater can contain 10 million viruses per milliliter
estimated 10^31 virus particles on earth
approximately 10 times the number of bacteria and archaea on earth
Miniscule, acellular infectious agents having either DNA or RNA
Cause most of the diseases that plague the industrialized world
cannot carry out any metabolic pathway
Neither grow nor respond to the environment
Recruit the host cell’s metabolic pathways to increase their numbers
No cytoplasmic membrane, cytosol, or organelles
Have extracellular and intracellular state
Not considered living due to the fact that they cannot reproduce but rather replicate. However they are not inert molecules by infectious molecules
Evolution of viruses most likely arose as bits of cellular nucleic acid
Plasmids and transposons or “jumping genes” serve as potential candidates for the source of viral genomes
plasmids, transposons and viruses are all mobile genetic elements
Viruses infect cells and influence their genetic makeup
8% of the human genome contains sequences that come from viruses
10 to 20% of bacterial DNA contains viral sequences
obligate intracellular parasites:
CANNOT multiply unless they invade a specific host cell and instruct its genetic and metabolic machinery to make and release new viruses
smallest infectious agents
smallest viruses: parvoviruses around 20 nm in diameter
Largest viruses: Herpes simplex virus around 150 nm in length
Pandoravirus~ 1 Micron
Megavirus ~680 nm
Viruses bear no resemblance to cells and lack machinery found in cells
The structure contains only those needed to invade and control a host cell
Viruses exist as extracellular and intracellular state
Extracellular state called virion: a fully formed virus that is able to establish an infection in the host cell
protein coat (capsid) surrounding nucleic acid
Nucleic acid and capsid also called nucleocapsid
Some have phospholipid envelope (enveloped vs. naked virus)
outermost layer provides protection and recognition site for host cells
intracellular state: capsid removed and virus exists as nucleic acid +
viral genomes may consist of either:
double or single stranded DNA
double or single stranded RNA
viruses are classified as either DNA or RNA
The genome of viruses are either single linear or circular molecule of the nucleic acid
viruses have between 3 and 2,000 genes in their genome
For many years animal viruses were classified on the basis of their hosts and the diseases they caused
Newer classification systems emphasize the following when classifying viruses:
- Hosts and diseases they cause
- Structure
- Chemical composition
- Similarities in genetic makeup
International committee on the taxonomy of viruses:
8 orders 38 families (another 84 families not yet assigned to any order)
Most viral capsids come in two simple shapes\ Helical- rodlike or threadlike Icosahedral -generally spherical viral morphology is highly diverse: Naked viruses (poliovirus) some virsues are complex poxiviruses- multilayered capsid T-even bacteriophages - binal (two fold) symmetry enveloped virsues (influenza)
Viruses are obligate intracellular parasites in every kind of organism investigated
Host range:
Each type of virus has a limited host range
Tissue tropism- inside a host the virus may only infect certain tissues
Viruses can remain dormant or latent for years ex. chicken pox can reemerge as shingles
More kinds of viruses exists than kinds of organisms
Viral replication #1
Viral tricks host genome into making new viruses
cell with a virus is often damaged with infection
viruses can only reproduce inside cells
outside, they metabolically inert virions
Viral replication #2
Viruses lack their own ribosomes and enzymes for protein and nucleic acid synthesis
virus hijacks the cell’s transcription and translation machineries to express
End result is assembly and release of viruses
viral infections: persistent vs. acute
categorized based -
How rapid or frequent it is produced
appearance of associated symptoms
persistent infections-latent or chronic
acute infections-rapid replication of virus and can lead to sudden symptom onset
can cause outbreaks, epidemics and pandemics
Influenza
One of the most lethal viruses in human history
Influenza pandemic of 1918 and 1919 thought to have infected 1/3 of the world’s population
flu viruses are enveloped animal viruses
Influenza 3 "types" based on capsid protein -Type A -serious epidemics in humans and other animals - Types B and C - mild human infections subtypes differ in protein spikes -Hemagglutinin (H)-aids in viral entry -Neuraminidase (N)-aids in viral exit
Antigenic drift and shift
H and N molecules accumulate random mutations
-thus we have annual flu shots and not a single vaccine
-antigenic shift produces strains of virus with novel combinations of HA and NA
Spanish flu of 1918, A (H1N1) -killed 50- 100 million worldwide -Asian flu of 1957, A (H2N2) -killed over 100,000 Americans Hong Kong flu of 1968, A (H3N2)
Human immunodeficiency virus
HIV causes acquired immune deficiency syndrome (AIDS)
HIV infection is Chronic
HIV infects and ultimately destroys certain types of immune cells
Without CD4+ cells and helper T cells the body cannot mount an effective immune response
Host may ultimately die from a variety of opportunistic
infections
-do not normally cause disease
Evolution of HIV
During an infection, HIV is constantly replicating and mutating
Eventually virus mutates to infect a broader range of cells, leading to a loss of T-Helper cells
Destruction of T cells block the body’s immunity response to opportunistic infections and cancer
