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Viruses
• In 2009, a general outbreak (epidemic) of a flulike illness first appeared in Mexico and the
United States
– Caused by an influenza virus H1N1
• Flu epidemics= caused by new
strains of influenza virus
– People have little immunity to
new strain and more susceptible
to infection
1 m
(a) 2009 pandemic H1N1
influenza A virus
Viruses
• New viruses arise from mutations of original
virus into different form
– Viral diseases in a small isolated population can
emerge and become global
– New viral diseases can emerge when viruses
spread from animals to humans
• Often requires a mutation that changes host range
– Viral strains that jump species can exchange
genetic information with other viruses
• Humans may have no immunity, increasing severity of
disease
Viruses
• H1N1 continued to spread and infected
people around the world
– Large-scale spread of these strains can cause
pandemics (global epidemics)
• 2009 flu pandemic= likely passed to humans
from pigs
– “Swine flu”
What are viruses?
• Viruses are not cells
– Small infectious particle consisting of nucleic acid
enclosed in a protein coat
– Obligate intracellular parasites
Bacteriophages
attacking E. coli
cell
0.5 mm
What are viruses?
• Viruses lead a “borrowed life” between lifeforms and chemicals
– Viruses can not reproduce without using a host
cell
– Viruses do not have any metabolic activity
• Viruses do contain genetic material
– DNA virus= Double- or single-stranded DNA
– RNA virus= Double- or single-stranded RNA
What are viruses?
• A capsid is the protein shell that encloses the
viral genome
– Built from protein subunits called capsomeres
• Viral envelopes= membranous envelopes found
in some viruses
– Help infect hosts
– Found in influenza viruses and many other animal
viruses
– Derived from the host cell’s membrane
• Contain a combination of viral and host cell molecules
Viruses and Hosts
• Each virus has a host range
– Limited number of host cells that it can infect
• Recognition systems for host cells
– Surface proteins of virus recognize specific
receptor molecules on outside of cells
Some viruses have broad host range
Zoonosis= infectious
disease transmitted
between humans
and other animals
Rabies Virus
Some viruses have limited host range
Smallpox
Some viruses have limited host range
HIV only infects certain types
of human white blood cells
Viruses
• Viruses “take over” the cell machinery to replicate
– Once a viral genome has entered a cell, the cell begins to
manufacture viral proteins, DNA, RNA
– Uses host enzymes, ribosomes, tRNAs, amino acids, ATP,
and other molecules
– Viral nucleic acid molecules and capsomeres
spontaneously self-assemble into new viruses
Viruses
• Bacteriophages, also called phages, are
viruses that infect bacteria
• Most complex capsids
– Elongated capsid head that
encloses their DNA
– Protein “tail” attaches the
phage to the host and injects
the phage DNA inside
50 nm
(d) Bacteriophage T4
Bacteriophages
• Two reproductive mechanisms: the lytic cycle and
the lysogenic cycle
1. Lytic cycle
– Causes death of host cell
• New phages produced in cell
• Cell lyses (breaks open) to release new viruses
– Releases large amount of viruses at one time
• Virulent phage= reproduces only by the lytic
cycle is called a
• Bacteria have defenses against phages
– Restriction enzymes that recognize and cut up certain
phage DNA
Bacteriophages
• Two reproductive mechanisms: the lytic cycle and the
lysogenic cycle
2. Lysogenic cycle
– Does not result in death of host cell
• Viral DNA incorporated into the host cell’s
chromosome
– Known as a prophage
• Every time the host divides, it copies the phage DNA
and passes the copies to daughter cells
– Environmental signal can trigger the virus genome to exit
the bacterial chromosome and switch to the lytic mode
• Temperate Phages= use both the lytic and lysogenic
cycles
Phage
DNA
The phage attaches to a
host cell and injects its DNA.
Bacterial
chromosome
Lytic cycle
• Virulent or temperate phage
• Destruction of host DNA
• Production of new phages
• Lysis of host cell causes release
of progeny phages
Prophage
Lysogenic cycle
• Temperate phage only
• Genome integrates into bacterial
chromosome as prophage, which
(1) is replicated and passed on to
daughter cells and
(2) can be induced to leave the chromosome and initiate a lytic cycle
Animal Viruses
• Viruses can cause disease in hosts
– May damage or kill infected cells
– Cause infected cells to produce toxins that lead to
disease symptoms
– Molecular components, like envelope proteins,
that are toxic
Animal Viruses
• Antiviral drugs help to treat, but not cure, viral
infections
– Viral infections cannot be treated by antibiotics
• Vaccines can prevent certain viral illnesses
– Harmless derivatives of pathogenic microbes
– Stimulates immune system to mount defenses
against pathogen
Animal Viruses
• Classified by
• DNA or RNA
• Single-stranded or double-stranded
Herpes
Double-strand DNA
HIV
Single-strand RNA
Herpes Virus
• Double-stranded DNA genome
• Reproduce within host cell nucleus
– Copies of viral DNA remain in nuclei of nerve cells
– Remains dormant in host system until stress
triggers new round of virus production
– Infection of other cells by virus results in blisters
characteristic of herpes virus
Retroviruses
• HIV (human immunodeficiency virus) is type
of retrovirus
– Single-strand RNA virus
– RNA acts as template for DNA synthesis
• Uses reverse transcriptase
• Changes flow of genetic information RNA to DNA
– Viral DNA incorporated into DNA of host
chromosome
• Provirus- integrated viral DNA
• Never leaves host’s genome
Plant Viruses
• Over 2,000 known types of viral diseases of
plants cause
– Discoloration of leaves and fruits
– Stunted growth
– Damaged flowers, roots
• Most plant viruses have
an RNA genome
Plant Viruses
• Plants are sessile, limiting modes of transmission of virus
• Spread disease in two major modes:
1. Horizontal transmission= infection from external
source
–
–
Entering through damaged
cell walls
Vectors transmit virus
• insects, worms, bacteria
–
Once inside of cell,
viruses can spread to adjacent
cells through plasmodesmata
Plant Viruses
• Plants are sessile, limiting modes of transmission of
virus
• Spread disease in two major modes:
2. Vertical transmission= inheriting the virus from a
parent
– Asexual reproduction- infected cells present in
clone or fragment
– Sexual reproduction- infected seeds
Plant Viroids
• Viroids= smaller than viruses
– Circular single-stranded RNA molecules
• No capsid
– Infect plants
• Cause errors in regulatory system of plant
growth
– Abnormal development
– Stunted growth