Download Viruses, Viroids, and Prions

Viruses, Viroids, and
Prions
Learning Objectives
Describe a protocell
 Define the main parts of a virus
 Describe major pathogenic viruses by:

 Enveloped
or nonenveloped
 RNA or DNA
 Major diseases caused
Early Earth

Earth’s early properties essential for life
 Gravity
high enough to retain atmosphere
 Water in liquid form (distance from Sun)
 Organic molecules from natural energy
sources

Oparin-Haldane hypothesis
 Early
earth had reducing atmosphere
 Allows complex organic molecules to form
and persist (prebiotic soup of organics)
24.2 The Origin of Cells

Protocells formed with some properties of life

Living cells may have developed from protocells

Prokaryotic cells were the first living cells

Subsequent events increased the oxidizing
nature of the atmosphere
Miller-Urey Apparatus
Early Macromolecules

Organic molecules are not alive by
themselves
 Macromolecules

and aggregration needed
Macromolecule formation by subunits
 Evaporation
of water concentrates subunits
 Dehydration synthesis connects subunits with
H and OH removal
Organic Molecule Aggregates


Clays facilitate organic molecule aggregates
 Layered structure absorbs molecules and
facilitates interactions, stores potential energy
 No lipid bilayer assembly
Phospholipids assemble into bilayers in water
 Formed spontaneously into vesicles
 Can incorporate proteins and make new
vesicles
Prokaryotic First Cells

Approximately one billion years for development
and evolution of first prokaryotic cells

Features required of first prokaryotes
 Membrane bound
 Nuclear region with DNA transcribed to RNA
 Cytoplasmic region with RNA translated to
amino acids/proteins
 Cytoplasmic region for energy transformation
 DNA replication and reproduction
Virus
The minimum necessary to transmit
nucleic acid molecules from one host cell
to another
 Viruses infect bacterial, animal, and plant
cells by similar pathways
 Viruses are NOT likely the first forms of
life, but rather a degenerative type of
evolution from early prokaryotes

Viruses

Nonliving infective agents
 No
metabolic system of their own
 Have minor to major effects on most organisms

Virus particle (virion) consists of a nucleic acid
genome enclosed in a protein coat (capsid)

Bacteriophages commonly infect bacteria
Viral Structure

Viral genome
 DNA or
RNA
 Double-stranded or single-stranded
 Few genes to 100 or more

Viral coat
 Made
of a single type of protein or up to 50
different proteins
 Includes recognition proteins that bind host
cell
Major Humantropic Viruses


Most viruses can be classified as follows:
Envelope (lipid bilayer coat) or naked
 Enveloped
viruses must stay moist, easily disinfected
 Naked viruses may last on surfaces for days


RNA or DNA, double or single, or retro
By the disease they cause (ex. Hepatitis viruses)
Infection of Animals:
Unenveloped Viruses

Virus binds to host using recognition
proteins
 Examples:
adenovirus and poliovirus
Whole virus taken into host by endocytosis
 Virus directs synthesis of new viruses like
in bacteria, kills host when cell ruptured

For RNA viruses




Single strand
Positive- Positive-sense (5' to 3') viral RNA signifies that
a particular viral RNA sequence may be directly
translated into the desired viral proteins
Negative-sense (3' to 5') viral RNA is complementary to
the viral mRNA and thus must be converted to positivesense RNA by an RNA polymerase prior to translation.
Ambisense- has both senses in one strand ex. Bunya
Infection of Animals:
Enveloped Viruses

Virus binds to host using recognition proteins
 Examples: herpes, pox, HIV, influenza

Whole virus taken into host
 Fusion of envelope with plasma membrane

Virus directs synthesis of new viruses

New viruses acquire envelope as they pass through
plasma membrane (does not injure host)
Viral Infection of Animals (1)

Pathogenic viruses cause diseases
 Some
cause cell death when cells rupture to
release viral progeny
 Some release cellular molecules that induce
fever or inflammation
 Some alter gene function of host cell, leading
to cancer or other abnormalities
Viral Infection of Animals (2)

Latent phase
 Viruses
remain in cell in an inactive form until
triggered to become active
 Similar to lysogenic cycle in bacteria

Most viral infections asymptomatic
Virus Families: Adenovirus
Enveloped
 DS DNA
 Respiratory infections (colds)

 Cause
tumors in non-human cells
 Part of the DNA integrates into the genome
Virus Family: Bunyavirus
Enveloped
 SS RNA
 Cause arthropod-bourne hemorrhagic
fevers

Virus Family: Calicivirus
Unenveloped
 SS RNA
 Most common human one is Norovirus, or
Norwalk virus
 Gastrointestinal illness

Virus Families: Flaviviruses
Enveloped
 SS RNA
 Yellow fever, Dengue (carried by
mosquitos)
 Hepatitis C- retrovirus, capable of causing
liver cancer, chronic disease
 Treated with peg interferon

Typical Retrovirus
Virus Family: Hepadnevirus
Enveloped
 DS DNA
 Hepatitis B
 Acute disease- very common
 Some patient develop chronic infections
which may eventually lead to liver failure
or cancer

Virus Families: Herpesviruses



Enveloped
DS DNA- large genome!
8 viruses
 HSV
I and II
 Epstein Barr
 Varicella-Zoster
 Cytomegalovirus (CMV)
 Roseola (HHV6 and 7)
 Kaposi’s sarcoma
Virus Families: Orthomyxovirus
Enveloped
 SS RNA
 Influenza
 Viral coat has two major proteins Hemeaglutinin (H)
 Neuraminidase (N)
 Differences in subtypes

Virus Family: Papovavirus
Unenveloped
 DS DNA
 Benign and malignant warts

Virus Family: Paramyxoviruses
Enveloped
 SS RNA
 Parainfluenza
 Measles
 Mumps

Virus Family: Picornaviruses
Enterovirus- polio, gastro illnesses
 Rhinoviruses- common cold
 Hepatitis A
 Foot and mouth disease

Virus Family: Poxvirus
Enveloped
 DS DNA
 Largest of viruses
 Smallpox, cowpox

Virus Family: Retroviruses
Enveloped
 SS RNA

 HTLV
 HIV
I and II
Virus Family: Rhabdovirus
Enveloped
 SS RNA

 Rabies,
VSV
Viroids

Plant pathogens
 Strands
or circles of RNA
 No protein coat

Viroid RNA may activate protein kinases
(adds phosphate groups to proteins)
 Leads
to reduction in protein synthesis
 Results in disease symptoms
Prions

Infectious proteins with no associated
nucleic acids

Misfolded versions of normal cellular
proteins that can induce other normal
proteins to misfold
Prion Diseases
Degenerate nervous system in mammals
 Scrapie:
Brain disease in sheep
 Mad cow disease (Bovine spongiform
encephalopathy): Spongy holes and protein
deposits in brain tissue
 Creutzfeldt-Jakob disease: Rapid mental
deterioration, loss of vision and speech,
paralysis
 Kuru-cannibalistic tribe in New Guinea,
Brain Tissue Damaged by BSE