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Bacterial genetics and gene expression
Bacterial genetics and gene expression
44
Microbiology
Undergraduate 3
10/17/2013

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Term
Mutation
Definition
Heritable change in DNA sequence that can lead to a change in phenotype
Term
Mutant
Definition
A strain of any cell or virus differing from parental strain in genotype
Term
Wild-type strain
Definition
Typically refers to strain isolated from nature
Term
Spontaneous mutation
Definition
A mutation that occurs without external intervention
Term
Induced mutation
Definition
Made environmentally or deliberately

Can result from exposure to natural radiation or oxygen radicals
Term
Point mutations (base substitutions)
Definition
Mutations that change only one base pair

Can lead to a single amino acid change in a protein (missense), an incomplete protein (nonsense), or no change at all (silent)
Term
Missense mutation
Definition
Amino acid is changed; polypeptide is altered
Term
Nonsense mutation
Definition
Codon becomes stop codon; polypeptide is incomplete
Term
Silent mutation
Definition
Does not affect amino acid sequence
Term
Frameshift mutation
Definition
- Deletions or insertions that result in a shift in the reading frame

- Often result in complete loss of gene function
Term
Selectable mutations
Definition
- Give the mutant a growth advantage under certain conditions

- Useful in genetic research
Term
Non-selectable mutations
Definition
- Neither an advantage nor a disadvantage over the parent

- Detection of such mutations require examining a large number of colonies and looking for differences (screening)

- Screening is always more tedious than selection
Term
Methods to facilitate screening
Definition
Replica plating; useful for identification of cells with a nutritional requirement for growth (auxotroph)
Term
Direct reversal DNA repair
Definition
Mutated base is still recognizable and can be repaired without referring to other strand
Term
Single strand DNA repair
Definition
Damaged DNA is removed and repaired using opposite strand as a template
Term
Repair of double strand DNA damage
Definition
A break in the DNA requires more error-prone mechanisms, such as an SOS regulatory system
Term
Genetic recombination
Definition
Physical exchange of DNA between genetic elements
Term
Homologous recombination
Definition
Exchange of homologous DNA from different sources

- Crossing over occurs whe two chromosomes break and rejoin
Term
Vertical gene transfer
Definition
Passage of genes to offspring
Term
Horizontal gene transfer
Definition
Transfer of genes between cells of the same generation
Term
Different types of horizontal gene transfer
Definition
Transformation

Transduction

Conjugation
Term
Transformation
Definition
Genetic transfer process by which DNA is incorporated into a recipient cell and brings about genetic change

- Discovered by Fredrick Griffith in the late 1920s; worked with Streptococcus pneumonia
Term
Competent cells (transformation)
Definition
Cells capable of taking up DNA and being transformed

- Natural competence

- Induced competence: CaCl2 and heat shock; electroporation – electricity is used to force cells to take up DNA
Term
Transduction
Definition
Transfer of DNA from one cell to another by a bacteriophage

Two modes:
- Generalized transduction
- Specialized transduction
Term
Generalized transduction
Definition
DNA derived from virtually any portion of the host genome is packaged inside the mature virion

- Defective virus particle incorporates fragment of the cell’s chromosome randomly
- Virus can be temperate or virulent
- Low efficiency
Term
Specialized transduction
Definition
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
Term
Conjugation
Definition
Cell-to-cell contact

- plasmid-encoded mechanism
- Donor cell: contains conjugative plasmid
- Recipient cell
Term
F plasmid
Definition
F plasmid is an episome; can integrate into host chromosome

Cells possessing a non-integrated plasmid are called F+

Cells possessing an integrated F plasmid are called Hfr (high frequency of recombination)

- High rates of genetic recombination between genes on the donor chromosomes and those of the recipient
Term
Transposable elements
Definition
- 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

- Moves by a process called transposition; frequency of transposition is 1 in 1,000 to 1 in 10,000,000; first observed by Barbara McClintock
Term
Two main types of transposable elements in bacteria
Definition
Transposons
Insertion sequences

- Both carry genes encoding for transposase; both have inverted repeats at their ends
Term
Gene expression
Definition
Transcription of gene into mRNA followed by translation of mRNA into protein
Term
Expression regulation
Definition
Two major levels of regulation in the cell:

Post-translational

- Controls the activity of preexisting enzymes
- Very rapid process (seconds)
Term
What mechanisms control gene expression in bacteria?
Definition
The environment in which the organism is growing

Presence of absence of specific small molecules

Interactions between small molecules and DNA-binding proteins result in control of transcription or translation
Term
Negative control
Definition
A regulatory mechanism that stops transcription
Term
Types of negative control
Definition
Repression

Induction

Operon
Term
Repression
Definition
Preventing the synthesis of an enzyme in response to a signal

- Enzymes affected by repression make up a small fraction of total proteins

- Typically affects anabolic enzymes (arginine biosynthesis)
Term
Induction
Definition
Production of an enzyme in response to a signal

- Typically affects catabolic enzymes (e.g., lac operon)

- Enzymes are synthesized only when they are needed (no wasted energy)
Term
Operon
Definition
Cluster of genes arranged in a linear fashion and whose expression is under the control of a single operator

- the operator is located downstream of the promoter

- transcription is physically blocked when repressor binds to operator
Term
Positive control
Definition
regulator protein activates transcription by the binding of RNA polymerase to DNA

- Malstose catabolism in E. coli
-- Maltose activator protein cannot bind to DNA unless it first binds to maltose

- Activator proteins bind specifically to certain DNA sequence
-- called activator-binding site...not operator
Term
Global control systems
Definition
Systems that regulate expression of many different genes simultaneously
Term
Catabolite repression
Definition
An example of a global control system.

- synthesis of unrelated catabolic enzymes is repressed if glucose is present in growth medium

- lac operon is under control of catabolite repression

- ensures the "best" carbon and energy source is used first
Term
Diauxic growth
Definition
Two exponential growth phases
Term
Cyclic AMP receptor protein (CRP)
Definition
Activator protein that controls transcription in catabolite repression
Term
Cyclic AMP
Definition
Key molecule in many metabolic control systems

- It is derived from a nucleic acid precursor

- It is a regulatory nucleotide
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