Download Chapter 24 ppt

• Nonliving particle that acts as an
“obligate intracellular parasite”
• A piece of nucleic acid (either
DNA or RNA)
• Surrounded by a protein coat
called a “capsid”
• Cause many diseases to living
organisms
• Can change how cells function
• Originally thought viruses were tiny
primitive cells (bacteria ancestors)
• Ideas of something smaller than
bacteria causing disease in late 1800’s
• Virology: study of viruses
• 1935 Wendell Stanley: nature of
viruses
– Crystallized tobacco mosaic virus
– Found it was made of chemicals, rather
than tiny cells
Viruses
Cells
1. Contain either DNA or RNA
2. Structure: capsid, no cell
membrane or wall
3. Does not grow (in size)
4. Requires a host cell for
reproduction
5. Does NOT carry out metabolic
activity
1. Always have both DNA and
RNA
2. Structure: cell wall/membrane,
cytoplasm, organelles...etc...
3. Grows
4. Able to reproduce by
mitosis/meiosis
5. Carries out metabolic activity
• Viruses are generally classified according
to their:
–
–
–
–
SIZE (nm)
capsid SHAPE
TYPE of nucleic acid (DNA, RNA) they carry.
Presence of an envelope
• Diameter: 20 nm (poliovirus) to 250 nm
(smallpox)
• Viral nucleic acid may be DNA or RNA
– Shape of nucleic acid may be helical, closed
loop, or long strand; ds or ss
• Capsid: protein coat surrounding nucleic acid
• Envelope: membrane-like structure outside
capsid
- Taken from host cell membrane during
replication
- Allows new viruses to attach/infect host
cells during 1st stage of viral replication
• Shape can be determined by capsid or
nucleic acid
• Icosahedron: geometric shape with 20
triangular faces
– Herpes simplex, polio, chickenpox
• Helix: coiled spring shape
• Viral nucleic acid responsible for
shape
• Rabies, measles, tobacco mosaic
virus, helical virus
• DNA & RNA viruses differ in the way
they use host cell’s machinery to
produce new viruses
• DNA virus
– Directly produce RNA that makes viral
proteins
– Join with host’s DNA to direct synthesis of
new viruses
• RNA virus
– Releases RNA into host’s cytoplasm
– Uses host’s ribosomes to produce new viral
proteins
• Retrovirus: contains RNA & reverse
transcriptase
• Reverse transcriptase is an enzyme
that uses RNA as template to make
DNA (reverse)
Examples: HIV,
leukemia
•
•
•
•
Smallest known particles able to replicate
Disease-causing agents
Short, single strand of RNA, no capsid
Disrupt plant cell metabolism & damage
entire crops
– Potatoes, cucumbers, avocados, oranges
• Abnormal forms of proteins that clump
together inside a cell
• Clumps block cell traffic; kills cell
• Found on surface of mammalian cells &
brains of host
– Scrapie, Mad Cow Disease (BSE),
Creutzfeld-Jacob syndrome
• Can replicate only by invading host cell &
using enzymes & organelles of host to
make more viruses
• “Obligate intracellular parasites”;
depend on host cells for replication
• Outside host, virus is lifeless particle, no
control of movements
• Spread by direct contact (body fluids),
wind, water, food
• Viruses that infect bacteria
• Commonly studied: T phages
– infect bacterium in human
digestive tract, E. coli
• Icosahedral capsid contains nucleic
acid
• Contractile tail; collar & sheath
– Inject nucleic acid into host
• Base plate; tail fibers attach to host
• Virus invades host cell, produces
new viruses, destroys host cell,
releases new virus
• “Virulent” virus - undergoing lytic
cycle, causes disease
1) Attachment- phage
attaches to cell surface
2) Entry- phage injects DNA
into host cell
3) Replication- phage DNA
"tells" host to make more
phage DNA and protein
coats
4) Assembly- new viruses are
assembled (host cell becomes
a "virus factory"
5) Release- cell lyses (breaks
open), releasing viruses
• Viruses can infect cell without
causing immediate destruction
• Stay in host cell for days, months,
or years
• Called “Temperate” Viruses
• Examples:
•
•
•
•
Herpes simplex -> Herpes, Cold Sores
Varicella Zoster Virus -> Shingles
Human Papilloma Virus -> Warts
Human Immunideficiency Virus -> HIV / AIDS
• After attachment and entry, Integration:
virus integrates its instructions (genes)
within the host cell’s DNA
• Prophage: (provirus) viral DNA
segment that integrates itself into
specific site of host cell’s genome
• Viral Multiplication: each time the host
cell DNA replicates so does viral DNA,
each bacterial offspring infected with
prophage
• Prophage does not initially harm host cell
• When triggered, a prophage becomes
virulent, enters lytic cycle, replicating and
destroying cell
• Viruses are usually contracted through
direct contact.
•
• Antibiotics work against bacteria but
NOT viruses.
• Some mammalian cells are capable of
making small amounts of a special
protein called INTERFERON, which can
block (“interfere” with) viral activity
when cells are attacked.
Other defenses:
• Phagocytes: large white blood
cells that can engulf and
destroy "invaders"
• Lymphocytes: white blood
cells capable of making
ANTIBODIES which surround
and disable invaders
(recognize foreign "antigens")
• Viral diseases among most
widespread illnesses in humans
• Mild fevers, forms of cancer, fatal
diseases
• Transmitted
– Human Contact
– Water
– Insect Bite (parasite vectors)
• Common human viral diseases
• Chickenpox, Measles, Mumps,
Polio, Rabies, Hepatitis, Common
Cold
• Viral infections can affect brain,
liver, heart, lungs, & skin
• Rabies
– Animal Bite (saliva)
– Virus travels, wound to central
nervous system
– Fever, headache, throat spasms,
paralysis, coma
– Lethal if untreated
• Chickenpox
– Highly contagious (direct contact & air)
– Multiplies in lungs & networks through
blood to skin
– Fever, skin rash
– Mild, recovery followed by lifelong
resistance
• Shingles
– Chickenpox not destroyed, persists in nerve
cells as provirus & cause shingles later
during adulthood
– Higher fever, Immune system weakens,
pneumonia can occur
– Rash in particular area (one side of chest)
– Shed chickenpox viruses
• Control of viral diseases accomplished
by vaccination and antiviral drugs
• Antiviral drugs: interfere with viral nucleic
acid synthesis (few compared to
bacteria)
• Vaccination is most successful
• Vaccine: preparation of pathogens or
other materials that stimulates body’s
immune system to provide protection
against that pathogen
• Inactivated
– Do not replicate in a host system
• Attenuated
– Genetically altered so incapable of
causing disease under normal
conditions
– Vaccines from attenuated preferred
– Protection is greater & lasts longer
• Booster: additional dose of vaccine,
extend person’s protection
•
•
•
•
1960’s Measles, Mumps, Rubella
1980’s Hepatitis B
1990’s Hepatitis A & Chicken pox
Continuous work done for AIDS
vaccine
– Genetic diversity & mutability
– Best tool for now is educating people
about HIV virus
• Once killed 40% of people infected
• Other 60% scarred & blind
• DNA virus transmitted by nasal droplets
emitted during sneezing & coughing
• Fever, Headache, backache, skin rash
• Eradication program 1967-1980
• Last case 1977 in Somalia
• Control of animals that spread viral
disease (“vector control”)
– Ex: yellow fever eradicated by mosquito
control
• Rabies vaccinations kept pets free of
infection; wildlife (meat with oral
rabies vaccine)
• Antiviral agents (like acyclovir)
interfere with synthesis of viral
nucleic acids & synthesis of viral
capsids during replication
• Some viruses contain oncogenes,
which disrupt the cell cycle of
infected host cells
• Human Papilloma Virus (HPV) can
cause cervical cancer
• Hepatitis B Virus can cause liver
cancer
• Epstein-Barr Virus linked to
Burkitt’s lymphoma (malignant
tumors)
• Ebola
• Causes hemorrhagic fever,
death
• 1st identified in Zaire, 1977
• Avian influenza (bird flu) H1N1
• Outbreak spread to humans,
2008