Transcript Chapter 1 Notes

Chapter 18 Notes
The Genetics of Viruses
and Bacteria
Concept 18.1
Researchers discovered viruses through
the understanding of tobacco mosaic
disease; which stunts growth and
causes discoloration of leaves
Concept 18.1
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Through many researchers, it was
found that the disease was caused by a
particle that was much simpler and
smaller than bacteria.
It was also found that this pathogen
could only reproduce within the host it
infected.
Comparing the sizes of a virus,
bacteria and animal cell.
Animation
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How Flu Viruses Attack
How A Virus Invades Your Body
Concept 18.1
A virus is a genome enclosed in a
protective coat
- a virus can be a DNA virus or an RNA
virus depending on the kind of nucleic
acid
Concept 18.1
The protein shell that encloses the viral
genome is called the capsid
 the capsid may be rod shaped,
polyhedral, or more complex
 Some viruses have viral envelopes,
membranes that covers their capsids
- Which are derived from the membrane
of a host cell
Concept 18.1
The most complex capsids are found
among bacteriophages (or phages),
viruses that infect bacteria
Bacteriophage Animation
Concept 18.1
Viruses are obligate intracellular
parasites: they can only reproduce
within a host cell
- viruses lack the enzymes for
metabolism and have no ribosomes for
making proteins
Viruses are merely packaged sets of
genes in transit from one host cell to
another.
Concept 18.1
Host range: the limited range of host
cells each type of virus can infect and
parasitize
- viruses identify their host cells by a
“lock-and-key” fit between proteins on
the outside of the virus and specific
receptor molecules on the surface of
the cell.
Concept 18.1
Viral reproductive cycle Animation
- the virus injects its DNA into the host
cell
- the viral DNA uses host nucleotides
and enzymes to replicate itself
- the viral DNA also uses host resources
to produce its capsid
- once assembled, the virus leaves
Concept 18.1
Phages replicate using either the lytic or
lysogenic cycle
Lytic cycle: reproductive cycle that
culminates in the death of the host.
- during the last stage of the cycle the
cell breaks open (or lyses) and releases
the phages that were produced
Concept 18.1
Lysogenic cycle: replication of the
phage genome without destroying the
host
Temperate phages: viruses that are
capable of using both modes of
reproducing within a bacterium
Concept 18.1
- phage l, the cause of e.coli, is an
example of a temperate phage.
- the phage DNA will incorporate
with the bacterial DNA to become a
prophage
- generally an environmental change
will cause a virus to go from the
lysogenic cycle to the lytic cycle
Concept 18.1
Reproductive Cycle of an Enveloped Virus
- Glycoproteins on the envelope bind to
receptor molecules on the host cell
- Viral envelope fuses with the cell’s
membrane and the capsid and genome
enter the cell
Concept 18.1
- enzymes remove the capsid and the
genome enters the cell
- genome uses the host organelles to
make and assemble new viruses
- viruses exit the cell
Concept 18.1
Retrovirus
- most complicated reproductive cycle
of viruses
- refers to the reverse directional flow
of the genetic information
- contain reverse transcriptase:
transcribes DNA from an RNA template
Concept 18.1
- RNA  DNA directional flow
- DNA integrates as a provirus in the
host cell
- ex. HIV (human immunodeficiency
virus)
Concept 18.2
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Many of the temporary symptoms
associated with viral infections, such as
fever and aches, are a result of the
body defending itself from infection.
Vaccines are harmless variants or
derivatives of pathogenic microbes that
stimulate the immune system to mount
defenses against infection. Animation
Viral Diseases in Animals
Emerging Viruses- recently seen in the last
25 yrs.
- ex. Ebola, Hantavirus, and SARS virus
Three processes contribute to new viruses
1. mutation of an existing virus
- RNA does not proofread = high rate of
mutation
Viral Diseases in Animals
2. spread of a virus from one host to
another
- ¾ of human diseases originated in
another animal
3. dissemination of a disease from a
small isolated population can lead to
widespread epidemics
- AIDS went unnoticed for decades
Viral Diseases in Plants
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More than 2000 types which results in
about $15 billion loss worldwide
Have the same structure and mode of
replication as animal viruses
Results are bleached or brown spots,
stunted growth, damaged flowers and
roots and the streaking of tulips
Viral Diseases in Plants
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Two modes of transmission:
Horizontal: Plant is infected from an
external source because the virus can
pass through the epidermis.
Vertical: The plant inherits a viral
infection from a parent (asexual
propagation or infected seeds).
The Simplest Infectious
Agents
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Viroids – circular RNA molecules, only
several hundred nucleotides long, that
infect plants
One viroid disease has killed over 10
million coconut palms in the Phillipines
They do not encode proteins but can
replicated in the host cell
The Simplest Infectious
Agents
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Prions – infectious proteins Animation
Appear to cause a number of
degenerative brain diseases in various
animal species, ex. Mad Cow Disease
They are slow-acting with long
incubation periods and they are virtually
indestructible
Concept 18.3
Bacteria have a short generation span
which helps them adapt to changing
environments
- the main part of a bacterial genome is
a double-stranded circular molecule
- bacteria also have plasmids: other
small circular DNA molecules separate
from the bacterial chromosome and can
be transferred between cells
Concept 18.3
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Bacteria reproduce asexually by binary
fission
Mutations can cause some of the
offspring to differ slightly in genetic
makeup
Replication of a bacterial
chromosome
Concept 18.3
Genetic recombination produces new
bacterial strains
- ex. two mutant strains of bacteria are
unable to grow on a nutrient medium
b/c they are unable to synthesize
necessary amino acids; if we mix the
two strains, and they grow, we will
know that the strains shared genes
Concept 18.3
Concept 18.3
Transformation: the alteration of a
bacterial cell’s genotype and phenotype
by the uptake of naked, foreign DNA
from the surrounding environment
Ex. Bacteria of a harmless strain of S.
pneumoniae can be transformed into
pneumonia causing cells
Concept 18.3
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Transduction: phages carry bacterial
genes form one host cell to another as
a result of aberrations in the phage
reproductive cycle
The transferred DNA may recombine
with the genome of the recipient cell
bringing about a recombinant cell
Concept 18.3
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Conjugation: sometimes referred to
as bacterial “sex”, is the direct transfer
of genetic material between two
bacterial cells that are temporarily
joined
One cell donates DNA and its “mate”
receives the DNA
Concept 18.4
The control of gene expression enables
bacteria to adjust their metabolism to
environmental change
Operon: The operator, the promoter
and the genes they control which is the
entire stretch of DNA required for
enzyme production
Concept 18.4
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The lac operon: regulates the synthesis
of the enzymes needed to metabolize
lactose.
Animation
Concept 18.4
Concept 18.4
-w/out lactose, no enzymes are made
for metabolism
- the regulatory gene, lacI is
producing a repressor (prevents the
transcription process) which blocks the
RNA polymerase from working
Concept 18.4
Concept 18.4
- when lactose is present, an inducer
binds to the repressor making it inactive
- RNA polymerase can then begin the
transcription process and the gene can
be expressed.