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Chapter 18 Notes
The Genetics of Viruses
and Bacteria
Concept 18.1
Researchers discovered viruses through
the understanding of tobacco mosaic
disease
- stunts growth and causes
discoloration of leaves
It was found that the pathogen could only
reproduce within the host it infected
Concept 18.1
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
- the protein shell that encloses the
viral genome is called the capsid
Concept 18.1
- the capsid may be rod shaped,
polyhedral, or more complex
Some viruses have viral envelopes,
membranes that cloak their capsids
- 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
Concept 18.1
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 packages sets of
genes in transit between hosts
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
- 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
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
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 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
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
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 18.1
Concept 18.2
Emerging Viruses
- ex. Ebola, Hantavirus, and Nipah virus
- recently seen in the last 25 yrs.
3 process contribute to new viruses
- mutation of an existing virus
- RNA does not proofread = high
rate of mutation
Concept 18.2
- spread of a virus from one host to
another
- ¾ of human diseases originated in
another animal
- dissemination of a disease from a
small isolated population
- AIDS went unnoticed for decades
Concept 18.2
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 moecules separate
from the bacterial chromosome
Concept 18.3
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 by the uptake
of naked (no cell w/ it), foreign DNA
from the surrounding environment
Concept 18.4
The control of gene expression enables
bacteria to adjust their metabolism to
environmental change
The lac operon: regulates the synthesis of
the enzymes needed to metabolize
lactose.
Concept 18.4
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.