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The genotype of bacteria can change through either
mutation or horizontal gene transfer.
Bacteria are _______, so any changes in DNA can easily alter the ________.
haploid
phenotype
A mutant that requires a growth factor ;
auxotroph;
in contrast, a _______ does not require growth factors.
prototroph
Spontaneous mutations
occur as a result of normal cell processes.
are stable but occasionally revert back to the non-mutant form.
Spontaneous mutations
The chance that two given spontaneous mutations will occur within the same cell is the
product of
the individual mutation rates
Base substitutions occur during ____________
DNA synthesis
_____result in synonymous, missense, and nonsense mutations
Base substitutions
frameshift mutation
Deleting or adding one or two nucleotides
,changing the reading frame of the encoded protein.
________results in a shortened non-functional protein.
frameshift mutation
Transposons
can move from one location to another in a cell’s genome.
The gene into which the transposon jumps is
insertionally inactivated by the event
Induced mutations
are caused by mutagen
Some chemicals modify nucleobases, altering their
hydrogen-bonding propertie
Base analogs can be mistakenly incorporated in place of the usual _______, and they have different hydrogen-bonding properties.
nucleobases
Intercalating agents insert between
adjacent bases in the double helix and push nucleotides apart,
resulting in frameshift mutations
Transposons can be introduced intentionally into a cell in order to
inactivate genes.
thymine dimer formation cause by
Exposure to ultraviolet light
The repair mechanism can cause mutations.
thymine dimer formation
X rays cause
single- and double-strand breaks
Repair of Errors in Nucleotide Incorporation
DNA polymerases have a.
proofreading function.
Mismatch repair
removes a portion of the strand that has a
misincorporated nucleotide.
As a result of Mismatch repair
A new DNA strand is then synthesized
Base excision repair
uses specific DNA glycosylases to remove damaged nucleobases in DNA
photoreactivation
an enzyme uses the energy of light to break the bonds of the thymine dimer.
In nucleotide excision repair
the damaged single-stranded segment is removed and replaced.
SOS repair
is a last-effort repair mechanism that uses a special DNA polymerase that has no proofreading
ability but can bypass the damaged DNA.
SOS repair results in DNA that
has many mutations
Genes can be transferred by
bacterial transformation, transduction, and conjugatio
For newly acquired DNA to replicate in a cell, it must either be a
replicon or integrate into the cell’s genome
Bacterial transformation
transfers “naked” DNA
A cell must be competent to
take up DNA, and only certain species naturally become competent.
The Process of Natural Transformation:
Short strands of double-stranded DNA
bind to cells, but only one strand enters
Transduction.
is the transfer of bacterial DNA by a bacteriophage
Two types of Transduction
generalized transduction and specialized transduction
Conjugation requires
cell-to-cell contact
___ cells synthesize an____,
F+
F pilus
F pilus encoded on an
F plasmid
the F plasmid is transferred from an ______ to an _____
F+ cel
F− cell.
Hfr cells have the ______ integrated into the chromosome.
F plasmid
When the F plasmid is transferred, __________ moves into a recipient cell along with it.
chromosomal DNA
An F′ donor carries
a modified F plasmid
An F′ donor carries a modified F plasmid that contains
piece of chromosomal DNA
pan-genome.
The sum total of genes encoded by the various strains of a given species is the
What make up the core genome
Genes common to all strains of the species
What are significant contributors to genome variability
Mobile genetic elements (MGEs), including plasmids, transposons, genomic islands, and phage DNA,
Restriction-modification systems
allow bacterial cells to quickly distinguish and destroy invading DNA.
Restriction enzymes ;
recognize and cut specific DNA sequence
modification enzymes
protect the hosts’ DNA from the action of the restriction enzymes by adding methyl groups to certain nucleobases.
CRISPR systems .
allow bacterial cells to destroy invading DNA that has been encountered previously
CRISPR array contains
; processing of the transcript generates RNA fragments that bind to a Cas nuclease, allowing it to recognize and
destroy that DNA
The cell transcribes a chromosomal region
CRISPR array contains
contains fragments captured from invading DNA;
The cell transcribes a chromosomal region called a CRISPR array that contains fragments captured from invading DNA;processing of the transcript generates
DNA fragments that bind to a Cas nuclease, allowing it to recognize and
destroy that DNA.
Most viruses are approximately
100-fold to 1,000-fold smaller than the cells they infect
Virion (viral particle) .
consists of nucleic acid surrounded by a protein capsid
Capsids are composed of .
capsomeres
Some viruses have an envelope surrounding the ________; other viruses are non-
enveloped or naked. Viruses contain either RNA or DNA, but never both. The shape of a virus is generally
icosahedral, helical, or complex
nucleocapsid
Viruses contain either
RNA or DNA, but never both.
The shape of a virus is generally
icosahedral, helical, or complex
Viruses are classified primarily
on the basis of their genome structure and hosts they infect.
The names of virus families end in _____.
-viridae
Viruses are also given informal names and are sometimes grouped based on their
routes of transmission.
Lytic or virulent phages exit
the host at the end of the cycle by lysing the host, resulting in a productive
infection.
Lytic or virulent phages exit the host at the end of the cycle by lysing the host resulting in
productive infection
The infection proceeds through five steps
attachment, genome entry, synthesis of phage proteins and
genome, assembly (maturation), and release.
Temperate phages have the option of either
directing a productive infection or initiating a lysogenic infection(lysogen)
A repressor
.
maintains the prophage in an integrated state, but the prophage can
be excised to initiate a lytic infection.
Lysogens are immune to
superinfection
Lysogenic conversion occurs if
a prophage carries genes that change the phenotype of the host cell
Filamentous phages cause ,
productive infections
Generalized transduction results from .
a packaging error during phage assembly
Generalized transducting particles can
transfer any gene of a donor cell to a recipient cell
Specialized transduction results from
.
an excision mistake made by a temperate phage during its transition from a lysogenic cycle to a lytic cycle
Only genes located near the site at which the temperate phage integrates are
transduced
The generalized infection cycle of animal viruses can be viewed as a five-step process
Attachment
Entry and Uncoating
Synthesis of Viral Proteins and Replication of the Genome
Assembly (Maturation)
Release
Attachment
Attachment proteins or spikes on the viral particle attach to specific receptors on the cell surface.
Entry and Uncoating: Enveloped viruses
fuse with the host membrane or are taken in by receptor-mediated endocytosis.
Entry and Uncoating - animal viruses
the entire virion enters the cell.
Entry and Uncoating: Non-enveloped
Non-enveloped virions enter by receptor-mediated endocytosis.
Entry and Uncoating
releases the nucleic acid from the protein coat
Synthesis of Viral Proteins and Replication of the Genome
DNA viruses generally replicate in _______ and use the _____________ as well as
_________, although they often encode their own DNA polymerase.
the nucleus
host cell machinery for DNA synthesis
gene expression
Synthesis of Viral Proteins and Replication of the Genome
Replication requires a _______________ to ____________________. This enzyme lacks ___________ and makes more mistakes in replication than DNA polymerases do.
virally encoded replicase
synthesize the complementary RNA strand.
proofreading ability
Synthesis of Viral Proteins and Replication of the Genome
Reverse-transcribing viruses encode _________, which
synthesizes _____________. As with replicases, these enzymes are ________.
reverse transcriptase
DNA from an RNA template
error prone