Ch. 10 Flashcards
Mutation -
Heritable change in DNA sequence that can lead to a change in phenotype
Phenotype -
Observable properties of an organism
Mutant -
A strain of any cell or virus differing from parental strain in genotype
Genotype -
Nucleotide sequence of genome
Wild-type strain -
Typically refers to strain isolated from nature
Selectable mutations -
- those that give the mutant a growth advantage under certain conditions
- useful in genetic research
Nonselectable mutations -
- those that usually have neither an advantage not a disadvantage over the parent
Detecting such mutations requires examining a large number of colonies and looking for differences -
Screening
Antibiotic resistance is an easy ___ marker.
Selectable
UV-radiation-induced nonpigmented mutants of Serratia marcescens is a ___ mutation.
Nonselectable
Halobacterium, a member of Archaea.
White colonies are the ___ and orangish-brown colonies are mutants that lack gas vesicles.
Wild-type
___ is always more tedious than selection.
Screening
Methods are available to facilitate screening:
Ex. Replica plating
___ ___ is useful for identifying cells with a nutritional requirement for growth.
Replica plating
Auxotroph -
An organism that has developed a nutritional requirement through mutation
A ___ is a parent from which an auxotroph mutant has been derived.
Prototroph
Induced mutations -
- those made environmentally or deliberately
- can result from exposure to natural radiation or oxygen radicals
Spontaneous mutations -
Those that occur without external intervention (occasional errors due to DNA polymerase during replication)
Point mutations -
- mutations that change only one base pair
- can lead to a single amino acid change in a protein, an incomplete protein, or no change at all
Silent mutation -
Does not affect amino acid sequence
Missense mutation -
Amino acid changed; polypeptide altered
Nonsense mutation -
Codon becomes stop codon; polypeptide is incomplete
___ and ___ cause more dramatic changes in DNA.
Deletions and insertions
Frameshift mutations -
- deletions or insertions that result in a shift in the reading frame
- often result in complete loss of gene function
What often result in complete loss of gene function?
Frameshift mutations
Point mutations are typically ___.
Reversible
Reversion -
Alteration in DNA that reverses the effects of a prior mutation
Revertant -
Strain in which original phenotype is restored
Two types of revertant:
- same-site revertant
- second-site revertant
Same-site revertant -
Mutation is at the same site as original mutation
Second-site mutation -
Mutation is at a different site in the DNA
Suppressor -
Mutation that compensates for the effect of the original mutation
For most microorganisms, errors in DNA replication occur at a frequency of ..
10^-6 to 10^-7 per kilobase
DNA viruses have error rates ____ greater.
100-1000X
The mutation rate in RNA genomes is ___ higher than in DNA genomes.
1000-fold
Some RNA polymerases have ___ capabilities.
Proofreading
RNA ___ ___ similar to DNA ___ ___ do not exist.
Repair mechanisms
The ___ ___ makes practical use of bacterial mutations to detect for potentially hazardous chemicals.
Ames test
The Ames test
- looks for an increase in mutation of bacteria in the presence of suspected mutagen
- a wide variety of chemicals have been screened for toxicity and carcinogenicity
Mutagens -
Chemical, physical, or biological agents that increase mutation rates
Nucleotide base analogs -
Resemble nucleotides
Chemical mutagens that induce chemical modifications
Ex. Alkylating agents such as nitrosoguanidine
Chemical mutagens that cause frameshift mutations
Ex. Intercalating agents such as acridines and Ethidium Bromide
Two main categories of mutagenic electromagnetic radiation:
- nonionizing
- ionizing
Nonionizing:
- purines and pyrimidines strongly absorb UV
- pyrimidine dimer is one effect of UV radiation
Example of nonionizing
UV radiation
Ionizing:
- more powerful than UV radiation
- ionize water and produce free radicals
- free radicals damage macromolecules in the cell
Example of ionizing
- x-rays
- cosmic rays
- gamma rays
Three types of DNA repair systems:
- direct reversal
- repair of single-strand damage
- repair of double-strand damage
Direct reversal -
Mutated base is still recognizable and can be repaired without referring to other strand
Repair of single-strand damage -
Damaged DNA is removed and repaired using opposite strand as template
Repair of double-strand damage -
A break in the DNA
- requires more error-prone repair mechanisms
Perfect ___ in organisms is counterproductive because it prevents evolution.
Fidelity
The ___ ___ of an organism is subject to change.
Mutation rate
Mutator strains -
Bacteria that benefit from increased mutation rates
Mutants can be isolated that are ___ or have __ mutation rates.
Hyperaccurate; increased
Recombination -
Physical exchange of DNA between genetic elements
Homologous recombination -
Process that results in genetic exchange between homologous DNA from two different sources
___ ___ can be used to detect rare genetic recombinants.
Selective medium
Transformation -
Genetic transfer process by which DNA is incorporated into a recipient cell and brings about genetic change
Competent -
Cells are capable of taking up DNA and being transformed
In naturally transformable bacteria, ___ is regulated.
Competence
Electroporation -
Electricity can be used to force cells to take up DNA.
During ___ ___, integration of transforming DNA is a highly regulated, multi step process.
Natural transformation
Transfection -
Transformation of bacteria with DNA extracted from a bacterial virus
___ is a physical technique that is used to get DNA into organisms that are difficult to transform.
Electroporation
Electroporation:
Cells are mixed with DNA and then exposed to brief high-voltage electrical pulses, which makes the cell envelope permeable and allows entry of the DNA
Transduction-
Transfer of DNA from one cell to another by a bacteriophage
Two modes of transduction:
Generalized transduction
Specialized transduction
Generalized transduction -
DNA from any portion of the host genome is packaged inside the virion
- defective virus particle incorporates fragment of the cells chromosome randomly
- virus can be temperate or virulent
- low efficiency
Specialized transduction -
DNA from a specific region of the host chromosome is integrated directly into the virus genome
- DNA of temperate virus excises incorrectly and takes adjacent host genes along with it
- transducing efficiency can be high
Phage conversion-
Alteration of the phenotype of a host cell by lysogenization
Nondefective temperate phage lysogenizes a cell and becomes a ___.
Prophage
Bacterial conjugation (mating) -
Mechanism of genetic transfer that involves cell-to-cell contact
- plasmid-encoded mechanism
Donor cell -
Contains conjugative plasmid
Recipient cell -
Does not contain plasmid
F (fertility) plasmid -
- circular DNA molecule; about 100 kpb
- contains genes that regulate DNA replication
- contains several transposable elements that allows the plasmid to integrate into the host chromosome
- contains tra genes that encode transfer functions
___ ___ is essential for conjugation.
Sex pilus
DNA ___ is necessary for DNA transfer by conjugation.
Synthesis
DNA synthesized by ___ ___ ___.
Rolling circle replication
F plasmid is a ___; can integrate into host chromosome
Episome
Cells possessing a nonintegrated F plasmid are called ___.
F+
Cells possessing an integrated F plasmid are called ___.
Hfr (high frequency of recombination)
Presence of the F plasmid results in alterations of cell properties:
- ability to synthesize F pilus
- mobilization of DNA for transfer to another cell
- alteration of surface receptors so that cell can no longer act as a recipient in conjugation
___ ___ (mobile elements) are present in both the F plasmid and E. coil chromosome.
Insertion sequences
Plasmid is now part of ___.
Chromosome
Chromosomal genes transferred with ___.
Plasmid
Recipient cell does not become Hfr because …
Only a portion of the integrated F plasmid is transferred by the donor
Hfr strains that differ in the __ __ of the F plasmid in the chromosome transfer genes in different orders.
Integration position
Genetic crosses with __ __ can be used to map the order of genes on the chromosome.
Hfr strains
F’ plasmids -
Previously integrated F plasmids that have excised and captured some chromosomal genes
Discrete segments of DNA that move as a unit from one location to another within other DNA molecules are ___ ___.
Transposable elements
Transposable elements can be found in all three..
Domains of life
Transposable elements move by a process called ___.
Transposition
Transposition was first observed by who?
Barbara McClintock
Two main types of transposable elements in bacteria are ___ and ___ ___.
Transposons and insertion sequences
Transposons and insertion sequences
- both carry genes encoding transposase
- both have inverted repeats at their ends
___ ___ are the simplest transposable element.
Insertion sequences
Insertion sequences:
- about 1000 nucleotides long
- inverted repeats are 10-50 base pairs
- only gene is for the transposase
- found in plasmids and chromosomes of bacteria and archaea
- found in some bacteriophages
___ are larger than insertion sequences.
Transposons
Transposons:
- transposase moves any DNA between inverted repeats
- insertion of a transposable element generates a duplicate target sequence
- may include antibiotic resistance
Two types of mechanisms of transposition:
- conservative
- replicative
Conservative -
Transposon is excised from one location and reinserted at a second location
- number of transposons stays constant
Replicative -
A new copy of transposon is produced and inserted at a second location
- number of transposons present doubles
Cells capable of growing on selective medium likely acquired ___.
Transposon
