Microbiology: The Genetics of Viruses
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Transcript of Microbiology: The Genetics of Viruses
Microbiology: The Genetics of Viruses
Viral Genetics Microbiology: The Genetics of Viruses
https://www.youtube.com/watch?v=UEKS4w9bfJg Viruses &Bacteria:
simplest biological systems
smaller/ simpler than bacteria Contain nucleic acid and protein
ONLY Not considered to be living b/c they need a host and NOT make
out of a cell Bacteria: larger than viruses Contain ALL
marcomolecules considered living Virus Bacterium Animal cell Animal
cell nucleus 0.25 m Bacteria with Bacteriophages infecting the
cell: What is a virus? Has a genome but
can reproduce only within a host cell Scientists detected viruses
indirectly long before they could see them The story of how viruses
were discovered begins in the late 1800s: Tobacco mosaic disease
stunts growth of tobacco plants gives their leaves a mosaic
coloration In late 1800s, researchers hypothesized that a particle
smaller than bacteria caused the disease In 1935, Wendell Stanley
confirmed this hypothesis Tobacco Mosaic Disease Viruses are not
(living) cells!
Viral structure Viruses are not (living) cells! Virus in Latin
means poison Made up of infectious particles consisting of a
nucleic acid (RNA or DNA) in a protein coat (and sometimes another
membrane) Viruses have a Capsid protein shell encloses viral genome
(nucleic acid) A capsid can have various structures Viral genomes
may consist of Double- or single-stranded DNA (dsDNA, ssDNA)
Double- or single-stranded RNA (dsRNA, ssRNA) Viral Structure
Images Depending on viral type of nucleic acid
A virus is called a DNA virus or an RNA virus The smallest viruses
have only 4 genes, while others have 100s HOST CELL
https://www.youtube.com/watch?v=cE0qdqoBFa8 Some viruses have
membranous envelopes help them infect hosts
Surround capsids of influenza viruses & other viruses found in
animals Viral envelopes (made from host cells membrane) contains a
combo of viral and host cell molecules Examples of Viruses and
their Hosts
Some viruses have a broad range to infect several species, others
infect only a single species West Nile virus can infect mosquitoes,
birds, horses and humans Measles virus can infect only humans Most
viruses of eukaryotes attack specific tissues Human cold viruses
infect only cells lining upper respiratory tract HIV (the virus
that causes AIDS) binds only to certain white blood cells (immune
cells) General Features of Viral Reproductive Cycles
Viruses are intracellular parasites they can reproduce only within
a host cell Each virus has a host range, a limited number of host
cells that it can infect Viruses identify host by a lock and key
fit between proteins on the outside of the virus and key receptors
on the hosts surface Viruses use enzymes, ribosomes, and other HOST
molecules to synthesize progeny (offspring) viruses Entry into cell
and uncoating of DNA VIRUS LE 18-5 DNA Capsid Replication
Transcription HOST CELL Viral DNA mRNA Viral DNA Capsid proteins
Self-assembly of new virus particles and their exit from cell Viral
infections begin when
Viral genome enters the host cell Once inside, genome takes over
its host, reprograms cell to copy viral nucleic acid/manufacture
viral proteins instead of host cell needs Host provides
nucleotides, ribosomes, tRNAs, amino acids, ATP and other
components for making viral components dictated by viral genes Most
DNA viruses use DNA polymerases of host cell to synthesize new
viral DNA along the templates provided by the viral DNA RNA viruses
use special virus-encoded polymerases that can use RNA as a
template Once all the parts of the virus are manufactured the virus
parts are assembled into new viruses Reproductive Cycles of
Phages
Phages = best understood of all viruses Research on phages shows
they can reproduce by 2 alternative mechanisms: lytic cycle vs.
lysogenic cycle Phages have potential to wipe out bacterial
colonies but bacteria also have defenses against them BACTERIRAL
DEFENCES INCLUDE: 1. Some bacteria have mutant receptor sites that
phage can no longer bind to 2. Bacteria also produce restriction
endonucleases, or restriction enzymes, that recognize and cut up
foreign DNA, including phage DNA Viral reproduction: Lytic
Cycle
A phage reproductive cycle ends in DEATH of host cell Produces new
phages and digests the hosts cell wall, releasing the progeny
(offspring) viruses A phage that reproduces only by the lytic cycle
is called a virulent (strong) phage There are 6 classes A bigger
perspective of the Lytic Cycle:
Attachment Entry of phage DNA and degradation of host DNA Phage
assembly Release Head Tails Tail fibers Assembly Synthesis of viral
genomes and proteins Viral reproduction: Lysogenic Cycle
Replicates the phage genome (DNA)without destroying the host cell
Genetic material of virus becomes incorporated into the host cells
DNA (called prophage DNA) Every time host divides, phage DNA is
copied and passes the copies to the daughter cell The viruses
spread without killing the host cells Temperate virus Are phages
capable of using the lytic and lysogenic cycles. The term lysogenic
implies that prophages are capable of giving rise to active phages
that lyse their host cells A bigger perspective of the Lysogenic
Cycle:
Phage DNA Daughter cell with prophage The phage attaches to a host
cell and injects its DNA. Many cell divisions produce a large
population of bacteria infected with the prophage. Phage Phage DNA
circularizes Occasionally, a prophage exits the bacterial
chromosome, initiating a lytic cycle. Bacterial chromosome Lytic
cycle Lysogenic cycle Certain factors determine whether Prophage
The bacterium reproduces normally, copying the prophage and
transmitting it to daughter cells. The cell lyses, releasing
phages. Lysogenic cycle is entered Lytic cycle is induced or Phage
DNA integrates into the bacterial chromosomes, becoming a prophage.
New phage DNA and proteins are synthesized and assembled into
phages. Viral Envelopes Many viruses that infect animals have a
membranous envelope Viral glycoproteins on the membrane envelope
bind to specific receptor molecules on the surface of a host cell
Capsid Capsid and viral genome enter cell RNA HOST CELL Envelope
(with glycoproteins) Viral genome (RNA) Template mRNA ER Capsid
proteins Glyco- proteins Copy of genome (RNA) New virus RNA viruses
= Retroviruses
Retroviruses: transcribe DNA from an RNA template (RNA DNA) These
viruses use Reverse transcriptase (catalyzing enzyme) Ex: HIV AIDS
Retrovirus (HIV) Viral envelope Glycoprotein Capsid RNA (two
identical
strands) Reverse transcriptase RNA viruses mechanisms
The viral DNA that is integrated into a eukaryotic host genome is
called a provirus remains a permanent resident of host cell The
hosts RNA polymerase transcribes the proviral DNA into RNA
molecules The RNA molecules function both as mRNA for synthesis of
viral proteins and as genomes for new virus particles released from
the cell LE 18-10 HOST CELL Reverse transcription Viral RNA RNA-DNA
hybrid
Membrane of white blood cell HIV HOST CELL Reverse transcription
Viral RNA RNA-DNA hybrid 0.25 m HIV entering a cell DNA NUCLEUS
Provirus Chromosomal DNA RNA genome for the next viral generation
mRNA New HIV leaving a cell Retrovirus Animations
Viral Evolution Viruses are intracellular parasites CANT reproduce
independently reasonable to assume that viruses evolved after 1st
cells appeared Hypothesis for virus origin originate from fragments
of cellular nucleic acids that could move from one cell to another
original sources of viral genomes? Plasmids (circular pieces of
DNA) Transposable elements Combating Viral Infection
Occasionally viral damage can be repaired (ex: respiratory
epithelium after a cold) Some infections cause permanent damage
(ex: nerve cells after polio) Modern medicine has developed
vaccines Vaccines https://www.youtube.com/watch?v=rb7TVW77ZCs
Vaccines harmless variants or derivatives (ex: viral proteins) of
pathogenic microbes stimulate immune system to mount defenses
against actual pathogen (ex: create antibodies) Vaccination has
eradicated smallpox Effective vaccines are available against polio,
measles, rubella, mumps, hepatitis B, and more Medical Technology
cannot
Treat a viral infection that has already occurred (vaccines do not
kill viruses) Antibiotics do not treat viral infections Antiviral
drugs resemble nucleosides and interfere with the viral nucleic
acid synthesis Valtrex stops herpes virus reproduction by
inhibiting viral DNA polymerase that synthesizes viral DNA
Azidothymidine (AZT) curbs HIV reproduction by interfering with DNA
synthesis by reverse transcriptase Currently the most effective
treatment is a cocktail multidrug treatment method Emerging Viruses
Emerging viruses are those that appear suddenly or suddenly come to
the attention of scientists Ex: Severe acute respiratory syndrome
(SARS) recently appeared in China Ebola Outbreaks of new viral
diseases in humans are usually caused by existing viruses that
expand their host territory/range or mutant Recent virus trends HIV
appear suddenly in the early 1980s
Each year new strains of influenza are emerging seemingly stronger
than previous strains Deadly ebola virus has caused hemorrhagic
fevers in central Africa beginning in 1976 West Nile virus appeared
for the first time in North America in 1999 Severe acute
respiratory syndrome (SARS) appears first in 2003 Avian Influenza
(Bird Flu) appears in birds around 1997 and human to human in 2004
(Asia) Young ballet students in Hong Kong wear face masks to
protect themselves from the virus causing SARS. The SARS-causing
agent is a coronavirus like this one (colorized TEM), so named for
the corona of glyco-protein spikes protruding from the envelope.
Where do viruses come from?
About of new human diseases originated in other animals Hantavirus
1993 (mice; SW USA) SARS (exotic animal markets in China) Bird Flu
(domesticated chickens, geese, ducks) SIV(Chimpanzees) Changes in
host behavior and environmental changes can increase viral
infections Destruction of forests and land may bring humans into
contact with other animals that may host viruses that can infect us
too. Plant Viruses 2,000+ types of known viral diseases of
plants
These diseases account for annual loss of $15 billion worldwide
Some symptoms are spots on leaves and fruits, stunted growth, and
damaged flowers or roots Most are RNA viruses with rod-shaped or
polyhedral capsids Plant Viruses spread by 2 major routes
Horizontal transmission Plant is infected with virus by an external
source Plants are more susceptible if their protective epidermis
(cuticle) is damaged Insects are often carriers Vertical
transmission Occurs by asexual propagation or in sexual
reproduction via infected seeds Other Pathogens Smaller, less
complex things called viroids and prions also cause disease in
plants and animals These are DIFFERENT than viruses Viroids tiny,
naked circular RNA that infect plants
plant RNA virus w/o capsid do NOT code for proteins, but use
cellular enzymes to reproduce stunt plant growth Prions
slow-acting, virtually indestructible infectious proteins that
cause brain diseases in mammals Prions spread by converting normal
proteins into the prion version a transmissible protein Ex: mad cow
disease Prions in depth Prions are proteins that spread
disease
Appear to cause several degenerative brain diseases scrapie in
sheep, mad cow disease, and Creutzfeldt-Jakob disease in humans
Likely transmitted in food Have 2 alarming characteristics: VERY
slow-acting agents (incubation up to 10 yrs) Virtually
indestructible (cannot be destroyed or deactivated by heating to
normal cooking temperatures) Prions How can a nonreplicating
protein be a transmissable pathogen?
It is a misfolded form of a normal protein When the prion gets into
a cell with the normal form of the protein, the prion can convert
the normal protein into the prion version, creating a chain
reaction that increases their numbers. Small Pox Polio Polio Herpes
Simplex Hepatitis Varicella Zoster (chicken pox) Mumps Measles -
Rubeola