Transcript Chapter 17

Chapter 17
Microbial Models:
The Genetics of Viruses and Bacteria
Viruses
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Def’n:
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infectious particles consisting usually of only the viral
genes enclosed in a protein shell
VIRUSES ARE NOT ALIVE!!!
Viruses can be classified according to the type of
nucleic acid they contain:
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Double-stranded DNA
Single-stranded DNA
Double-stranded RNA
Single-stranded RNA
Viruses
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The protein shell
protects the genome
and is called the
capsid
The capsid can come
in many different
shapes… see photo at
right!
Viruses
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Viral envelopes
surround some viral
capsids
Viruses that infect
bacteria are known as
bacteriophages
(phages)
Viral Reproduction
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Viruses can only reproduce within a
host cell
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an isolated virus is unable to replicate itself or
do anything else
Unless it’s in a host cell, it can’t do anything…
Viral Reproduction
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Each type of virus can infect and parasitize
only a limited range of host cells  known
as its host range
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Identified by “lock-and-key” mechanism…
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Why does that sound familiar?? 
Example…
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Rabies virus can only affect a number of
mammalian species (rodents, dogs, humans)
Lytic Cycle
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Viral reproductive cycle that ends in the death
of the host cell
Basic Breakdown:
1.
2.
3.
4.
5.
Phage sticks to outside of bacterial cell
Phage injects its DNA into the cell
Empty capsid is left outside
Cell’s metabolic machinery produces phage proteins
Phage directs the production of an enzyme that
breaks down the bacterial cell wall, causing the cell
to burst – phage particles are released
Lysogenic Cycle
Viral reproductive cycle that does NOT end in the
death of the host cell
Basic Breakdown:
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1.
2.
3.
Phage binds to the surface of the cell
Phage injects its DNA into the cell
Phage DNA is incorporated into the host cell’s chromosome 
now known as a prophage
1.
4.
5.
Phage DNA is “silent”
Every time the bacterial cell prepares to divide, it replicates
the viral genome in addition to its own DNA, passing on the
viral genome to its daughter cells
One day, the prophage commands the host cell to
manufacture phages and then self-destruct
Retroviruses
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Important retrovirus: HIV
RNA viruses in which genetic information
flows backwards
Reverse transcriptase:
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An enzyme that can transcribe DNA from an
RNA template
This changes the cell’s genome!
Viroids and Prions
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Viroids:
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Simple infectious molecules of naked RNA
Prions:
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Simple infectious proteins
Bacterial Genetics
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Bacterial genome is a single double-stranded
DNA molecule arranged in a circle
(chromosome)
The chromosome is found within the nucleoid
region of a bacterial cell
Bacterial cells divide by binary fission
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This is an asexual process
Rapid reproduction
Therefore, most bacteria in a colony are genetically
identical
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Sources of Genetic Diversity
in Bacteria
Mutation
Transformation
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Transduction
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Alteration of a bacterial cell’s genotype by the uptake
of naked, foreign DNA from the surrounding
environment
This is what we did in lab!
Phages transfer genes from one host cell to another
Conjugation and Plasmids
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Direct transfer of genetic material between two
bacterial cells that are temporarily joined
Transposons
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Transposon:
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Pieces of DNA that can move from location to
another in a cell’s genome
Often called “jumping genes”
Operons
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Structural genes:
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Operator:
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REVIEW!!
OPERON:
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Segment of DNA that controls the access of RNA polymerase to
the structural genes
Promoter:
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Genes that code for polypeptides
The entire stretch of DNA required for enzyme production of a
certain polypeptide – including the structural genes, operator,
and promoter
Repressor:
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If the repressor is bound to the operator, the operon is blocked
Kind of “off/on” switch for operon
Chapter 18
Genome Organization and Expression
in Eukaryotes
Cellular Differentiation
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Each cell of a multicellular eukaryote
expresses only a small fraction of its
genome
50,000 – 100,000 genes in every human
cell
Cellular differentiation:
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The divergence in structure and function of
different types of cells as they become
specialized during an organism’s development
and remain that way
The Structure of Chromatin
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Eukaryotic chromatin consists of DNA and
LOTS of protein
If extended, a eukaryotic DNA molecule
would be about 6 cm long… WAY bigger
than a cell’s nucleus
Lots of elaborate packing is required to fit
the DNA into the nucleus
The Structure of Chromatin
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Proteins (called
histones) are
responsible for the first
level of DNA packing in
chromatin
Nucleosome:
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Looks like beads on a
string
Consists of DNA wound
around histones
The Structure of Chromatin
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Nucleosomes are then
coiled into a fiber,
which is then folded
into looped domains
All this to save space!
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Repetitive Sequences
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10 – 25 % of total DNA of multicellular
eukaryotes is made up of short sequences
repeated 1000s or 1,000,000s of times
Known as satellite DNA
Telomeres
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Satellite sequences located at the ends of
chromosomes
During replication of lagging strand, RNA
primers must be “laid down”  impossible at
the very end of a linear chromosome
If this was not “dealt with” the chromosome
would get shorter every time it was replicated
That’s the role of telomeres! 
Analogy: ends of shoelaces
Gene Amplification
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Def’n:
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The selective replication of certain genes
The number of copies of a gene or gene family
may temporarily increase in some tissues
during a particular stage of development
Example:
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Ovum producing lots of extra rRNA so tons of
ribosomes can be made
Cancer: It’s in the Genes?!
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Cancer is caused by uncontrolled cell growth and division
(mitosis)
Certain genes control cell growth and division
These genes can be altered by environmental
influences/mutations (sun, smoke, etc.)
Oncogenes:
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Proto-oncogenes:
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Cancer-causing genes
Normal cellular genes that normally regulate cell growth and
division
Tumor-suppressor genes:
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Genes that encode proteins that normally help prevent
uncontrolled cell growth
The Genetics of Cancer
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A genetic predisposition to cancer might be
inherited as a defect in a single tumor-suppressor
gene
An individual inheriting such a recessive mutant
allele will be one step closer to accumulating the
necessary mutations that will result in abnormal
cell growth and division
Pre-screening/identification of these mutant
alleles??
The Progression of
Colorectal Cancer