Title: VIRUSES
1VIRUSES
2Size Comparisons
3The Genetics of Viruses
- A virus is a genome enclosed in a protective
coat. - Genome is an entire set of genes (DNA or RNA)
- Capsid is the protein coat that encloses the
viral genome. - Capsids are built from a large number of protein
subunits called capsomeres. - Viral Envelopes accessory structures that help
viruses infect their host these are membranes
cloaking the capsid. - Envelopes are derived from the membrane of the
host cell.
4Viral Structures Figure 19.3
5Viral Genomes
- The genome of a virus may be single-or- double
stranded DNA or single-or-double stranded RNA. - Called a DNA-like or RNA-like virus depending on
the nucleic acid found in the genome. - The genome is usually organized as a single
linear or circular molecule of nucleic acid.
6Viral Reproduction
- Viruses can reproduce ONLY within a HOST cell
- Lack enzymes for metabolism or ribosomes for
protein synthesis - Sothey use enzymes, ribosomes, and small
molecules of host cells to synthesize progeny
viruses. - Each type of virus can infect and parasitize only
a limited range of host cells (called a host
range). - Some viruses (like rabies) have a broad enough
host range to infect several species, while
others infect only a single species. - Most viruses of eukaryotes attack specific
tissues - Human cold viruses infect only the cells lining
the upper respiratory tract. - The AIDS virus binds only to certain white blood
cells.
7Overview of Viral Reproduction - Figure
19.4http//www.sumanasinc.com/webcontent/animatio
ns/content/herpessimplex.html
- After entering the cell, the viral DNA uses host
nucleotides and enzymes to replicate itself. - The viral DNA uses other host resources to
produce its capsid proteins by transcription and
translation. - The new viral DNA and capsid proteins assemble
into new virus particles, which leave the cell.
85 Steps of Virus Replication
- 1. Attachment
- 2. Penetration
- 3. Replication and Synthesis
- 4. Assembly
- 5. Release
- Virus uses hosts nucleotides and enzymes to
replicate itself. - At the same time, other host resources are used
to make new capsid proteins by transcription and
translation. - The new viral genomes and capsids are assembled
into new virus particles when the number exceeds
the cells surface area limitations, the cell
bursts open and new viruses are released to
infect other cells exponential increase .
9Bacteriophages (Phage Virus)
- Phages are viruses that infect bacterial cells.
- Phages are the most complex viruses
- They are the best understood viruses
- Phages reproduce using lytic or lysogenic cycles
10Speed of Viral Takeover
- Lytic Cycle A phage reproductive cycle that
culminates in the death of the host cell. - In the lytic cycle, the virus takes over the host
cell immediately and reproduces quickly the
host cell can lyse within a few minutes - Ex. Cold virus
- Viruses that reproduce by lytic cycles are called
virulent viruses - Lysogenic Cycle A phage reproductive cycle that
replicates the phage genome without destroying
the host. - In the lysogenic cycle, the virus hides in the
original host cells DNA until optimal conditions
for viral survival are present (provirus or
prophage) then, because the host cell has
reproduced, the virus will reproduce and emerge
from MULTIPLE cells at once, causing much more
severe cellular damage. Once free from the cell,
the phage will initiate a lytic cycle. - Ex. E. coli infection
- Viruses that reproduce by both lytic and
lysogenic cycles are called temperate viruses
11Lytic Lysogenic Cycles Figure
19.6http//highered.mcgraw-hill.com/sites/0072556
781/student_view0/chapter17/animation_quiz_2.html
12Defense System of Bacteria
- While phages have the potential to wipe out a
bacterial colony in just hours, bacteria have
defenses against phages. - Natural selection favors bacterial mutants with
receptor sites that are no longer recognized by a
particular type of phage. - Bacteria produce restriction nucleases that
recognize and cut up foreign DNA, including
certain phage DNA. - BUTnatural selection also favors resistant phage
mutants!
13Animal Viruses Table 18.1
- Many animal viruses have a membranous envelope
present that is used to enter and exit the host
cell. - This envelope is a lipid bilayer with
glycoproteins that bind to specific receptors
molecules on the surface of the host cell (for
attachment). - Viral envelope is derived from the host cells
plasma membrane, so host cell may not be killed. - Lets look at figure 18.6 on page 334 in the
textbook.
14Retroviruses
- Retroviruses are viruses that contain RNA instead
of DNA and replicate in an unusual way. - Have most complicated reproductive cycles
- Following infection of the host cell, their RNA
serves as a template for the synthesis of
complementary DNA (called cDNA because it is
complementary to the RNA from which it was
copied). - THUS, THESE RETROVIRUSES REVERSE THE USUAL FLOW
OF INFORMATION FROM DNA TO RNA. - This reverse transcription occurs under the
direction of an enzyme called reverse
transcriptase. - Retroviruses usually insert themselves into the
host genome, become permanent residents, and are
capable of making multiple copies of the viral
genome for years. - Examples of retroviruses are the polio virus and
the HIV virus, which causes AIDS.
15Reproductive Cycle of HIV - Figure
19.8http//www.sumanasinc.com/webcontent/animatio
ns/content/lifecyclehiv.swf
16Plant Viruses
- Plant viruses are serious agricultural pests
because they can stunt plant growth and diminish
crop yields. - Most plant viruses are single stranded RNA
viruses. - They enter plant cells through damaged cell walls
or are inherited from a parent.
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18Controlling Viruses
- Diseases causes by viruses are difficult to
treat - Drugs are only used to treat SYMPTOMS, not cure
the disease (just make patient feel better for
short duration) - The only methods to control viruses are to
PREVENT illness - Vaccines and antibody production, use of
interferon in body.
19Antibodies Vaccines Interferons
- Antibodies are made by hosts immune system after
infection occurs (if host survives the infection) - Help inactivate viruses and destroy harmful
bacteria - Are specific for viruses or bacteria
- Once an antibody is produced that recognizes a
specific virus or bacteria, then that strain will
be ineffective on that individual organism - Vaccines are harmless variants or derivatives of
pathogenic microbes - Stimulate the immune system to mount defenses
against a specific pathogen - Developed by Edward Jenner cowpox used to
develop smallpox vaccination - Vaccinated or immunized again disease
- Ex. MMR, DPT, polio, smallpox, influenze,rabies,
hepatitis C - Interferons are chemicals in the body that are
activated when cells are attacked - Cell under seige produces interferon which binds
to neighboring cells cell membranes to warn them
of the dangerous pathogen
20Antibiotics and Viruses
- Antibiotics are powerless against viruses!
- Antibiotics kill bacteria by inhibiting enzymes
or processes specific to the pathogens since
viruses have no metabolism of their own, the
antibiotics do not work. - Only drugs that have any effect on viruses are
ones that interfere with nucleic acid synthesis
AZT (with HIV), acyclovir (with herpesvirus)or
with protein production (protease inhibitors with
AIDS)
21Emerging Viruses
- Emerging viruses that cause new outbreaks of
disease are usually existing viruses that manage
to expand their host territory. - AIDS
- Hantavirus
- Ebola (hemorrhagic fever)
- Nipah virus
- Influenza
- What contributes to spread of emerging viruses?
- 1. Mutation
- 2. Spread from one species to another
- 3. Dissemination from small, isolated population
-
22Viroids
- Viroids tiny molecules of naked circular DNA
that infect plants - do not encode proteins
- can replicate in host plant cells using
cellular enzymes - disrupt the metabolism of cell and stunt growth
of whole plant - Point is that MOLECULES can be an infectious
agent.
23Prions
- Prions are infectious proteins misfolded form of
a protein normally found in brain cells - Cause degenerative brain diseases
- Ex. Scrapie, mad cow disease, Creutzfeldt-Jakob
disease - When prion gets into a cell containing the normal
form of the brain cell protein, prion converts
the normal protein to the prion version
24Evolution of Viruses
- Viruses exist somewhere between life and
non-life - They display many (but not all) characteristics
of lifeincluding the ability to EVOLVE! - Viruses probably evolved AFTER the first cells
appeared - They are naked bits of nucleic acid that perhaps
evolved from plasmids or transposons MOBILE
GENETIC ELEMENTS!
25Bacteria
26The Genetics of Bacteria
- The major component of the bacterial genome is
one DOUBLE STRANDED, CIRCULAR DNA molecule which
is smaller and less complex than that of
eukaryotes. - Different from eukaryotic chromosomes which have
linear DNA molecules associated with large
amounts of protein. - Within bacterium, the chromosome is so tightly
packed that it fills only part of the cell
dense region called nucleoid NOT bound by
membrane like the nucleus of eukaryotic cell. - Replication of DNA occurs from single origin of
replication on circular DNA and
transcription/translation can be coupled in
prokaryotes. - Generally have few or no introns so majority of
genome is expressed. - Gene regulation is controlled using operons.
- In addition, many bacteria have PLASMIDS, much
smaller circles of DNA. - Each plasmid has only a small number of genes,
from just a few to several dozen.
27Replication of the Bacterial Chromosome Figure
18.11
- Bacterial cells divide via binary fission.
- This is preceded by replication of the bacterial
chromosome from a single origin of replication. - From a single origin of replication DNA
synthesis progresses in both directions around
the circular chromosome. - Because binary fission is asexual, most bacterial
colonies are genetically identical to the parent
cell.
28Producing New Bacterial Strains
- Bacteria do not undergo meiosis and fertilization
as do eukaryotic organisms they reproduce via
means of genetic recombination - The genetic recombination in bacteria includes
- Transformation
- http//highered.mcgraw-hill.com/sites/0072556781/s
tudent_view0/chapter13/animation_quiz_1.html - Transduction
- http//highered.mcgraw-hill.com/sites/0072556781/s
tudent_view0/chapter13/animation_quiz_2.html - Conjugation
- http//highered.mcgraw-hill.com/sites/0072556781/s
tudent_view0/chapter13/animation_quiz_3.html
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30Figure 18.12 Detecting genetic recombination in
bacteria
31Important Definitionshttp//highered.mcgraw-hill.
com/sites/0072556781/student_view0/chapter13/anima
tion_quiz_1.html
- Transformation alteration of bacterial cells
genotype by the uptake of naked, foreign DNA from
the surrounding environment.
32Important Definitionshttp//highered.mcgraw-hill.
com/sites/0072556781/student_view0/chapter13/anima
tion_quiz_2.html
- Transduction process where phages carry
bacterial genes from one host cell to another (2
types generalized and specialized) - Generalized phage transfers bacterial genes
randomly - http//highered.mcgraw-hill.com/classware/ala.do?i
sbn0072464631alaidala_661332 - Specialized - Only certain genes are transferred
the ones near the prophage site on the
bacterial chromosome - http//highered.mcgraw-hill.com/sites/0072556781/s
tudent_view0/chapter17/animation_quiz_3.html?isbn
0072556781firstNameMIlastNamemyEmailmySty
leprofEmailprofStyletaEmailtaStyleotherE
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33Figure 18.13 Transduction (Layer 1)
34Figure 18.13 Transduction (Layer 2)
35Figure 18.13 Transduction (Layer 3)
36Figure 18.13 Transduction (Layer 4)
37Important Definitionshttp//highered.mcgraw-hill.
com/sites/0072556781/student_view0/chapter13/anima
tion_quiz_3.html
- Conjugation direct transfer of genetic material
between 2 bacterial cells that are temporarily
joined - DNA transfer is one-way (one cell donating DNA
and its mate receiving the genes) - The donor (male) uses pili to attach to the
female - maleness the ability to form sex pili and
donate DNA during conjugation results from the
presence of an F factor (F for fertility) - F factors can either be a segment of DNA within
the bacterial chromosome or as a plasmid
38Conjugation Figure 18.14
- The E. coli male (right) extends sex pili, one
of which is attached to a female cell. - The two cells will be drawn close together,
allowing a cytoplasmic bridge to form between
them. - Through this tube, the male will transfer DNA
to the female. - This mechanism of DNA transfer is called
conjugation.
39Plasmids
- Plasmids are small, circular, self-replicating
DNA molecules separate from the bacterial
chromosome - If a genetic element can exist as either a
plasmid or as a part of the bacterial chromosome,
that genetic element is called an EPISOME - Plasmids have only a small number of genes, and
these are not necessary for the survival and
reproduction of the bacterium BUT, they can
confer advantages F plasmids and R plasmids - F plasmid fertility bacteria that contain F
plasmids are F and carry genes for production of
pili. - R plasmid resistance to antibodies such as
ampicillin or tetracycline.
40Figure 18.15 Conjugation and recombination in E.
coli (Layer 1)
- Cells that carry an F plasmid are called F
cells. They are male because they can transfer
an F plasmid to a female F- cell. - In this way, an F- cell can become F.
- The F plasmid replicates as it transfers, so that
the donor cell remains F.
41Figure 18.15 Conjugation and recombination in E.
coli (Layer 2)
This process is similar to phage DNA joining the
host chromosome as a prophage. Crossing over
occurs between the two DNA circles at a specific
site on each.
42Figure 18.15 Conjugation and recombination in E.
coli (Layer 3)
Replication and transfer of the Hfr chromosome
begins at a fixed point within the F factor. The
conjugation bridge usually breaks well before the
entire chromosome and most of the F factor are
transferred.
43Figure 18.15 Conjugation and recombination in E.
coli (Layer 4)
Crossing over can occur between genes on the
fragment of bacterial chromosome transferred from
the Hfr cell and the same genes on the recipient
F- cells chromosome. A recombinant F- cell will
result. Pieces of DNA ending up outside the
bacterial chromosome will eventually be degraded
by the cells enzymes or lost in cell
division. THE DNA REPLICATION THAT ACCOMPANIES
TRANSFER OF AN F PLASMID OR PART OF AN Hfr
BACTERIAL CHROMOSOME IS CALLED ROLLING-CIRCLE
REPLICATION.
44Transposonshttp//highered.mcgraw-hill.com/sites/
0072556781/student_view0/chapter13/animation_quiz_
5.html
- Transposons are pieces of DNA that can move from
one location to another jumping genes - These NEVER exist independently
- Movement of transposons occurs as a type of
recombination between the transposon and another
DNA site (target site) that comes in contact with
the transposon - Ability to scatter certain genes throughout the
genome makes transposition fundamentally
different from other mechanisms of genetic
shuffling DOES NOT depend on complementary base
sequences
45Types of Transposonshttp//highered.mcgraw-hill.c
om/olc/dl/120082/bio36.swf
- Insertion Sequences consist of only one gene,
which codes for transposase the enzyme
responsible for moving the sequence from one
place to another. - can cause mutations when they happen to land
within the coding sequence of a gene or within a
DNA region that regulates gene expression. - Composite Transposon are longer and include
extra genes, such as a gene for antibiotic
resistance or for seed color. - benefit bacteria by helping them to adapt to new
environments
46Barbara McClintock
- American geneticist
- Worked with Indian corn (maize) in 1940s and
50s - Identified changes in the color of corn kernels
that made sense only if there were mobile genetic
elements capable of moving from one location to
another in the genome - Changes in color of corn kernels
- Awarded Nobel Prize in 1983
47Insertion Sequence Transposons
- Insertion Sequences simplest of the bacterial
transposons - consist ONLY of the DNA necessary for
transposition - Sometimes called jumping genes
- codes for transposase
- bracketed by inverted repeats (non coding
sequences of DNA about 20 to 40 nucleotides long) - See page 345
48Figure 18.16 Insertion sequences, the simplest
transposons
- The one gene of an insertion sequence codes for
transposase, which catalyzes the transposons
movement. - The inverted repeats are backward, upside-down
versions of each other. - In transposition, transposases bind to the
inverted repeats and catalyze the cutting and
resealing of DNA required for insertion of the
transposon at a target site.
49Figure 18.17 Insertion of a transposon and
creation of direct repeats
- First, transposase enzyme makes staggered cuts in
the 2 DNA strands at the target site. - The transposon is then joined to the
single-stranded ends at the target site. - Finally, the gaps in the DNA strands are filled
in by DNA polymerase and sealed by ligase. This
results in direct repeats, identical segments of
DNA on either side of the transposon.
50Composite Transposons
- Composite Transposon include extra genes in
addition to the transposition DNA - benefit bacteria by helping them to adapt to new
environments
51Figure 18.18 Anatomy of a composite transposon
A composite transposon consists of one or more
genes located between twin insertion
sequences. The transposon here has a gene for
resistance to an antibiotic, which is carried
along as part of the transposon when the
transposon is inserted at a new site in the
genome.