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56 Cards in this Set
- Front
- Back
• Mutations can be divided into three main classes
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1) Point mutations
2) Frame-shift mutations 3) Inversions |
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1) Point mutation is
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is a change in single nucleotide
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i) Transition = A mutation in which a
DNA level |
purine is replaced by another purine or a pyrimidine is replaced by another pyrimidine
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ii) Transversion = A mutation in which a
DNA level |
purine is replaced by a pyrimidine or vice versa
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i) Silent mutation =
At the protein level |
i) Silent mutation = The resulting triplet codes for the same amino acid
- AAA (Lysine) → AAG (Lysine) |
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ii) Missense mutation
At the protein level |
The resulting triplet codes for a different amino acid
- AAA (Lysine) → GAA (Glutamic acid) - If the resulting triplet codes for a functionally equivalent amino acid, the mutation is said to be a neutral mutation - If the missense change decreases or eliminates protein activity, the mutation is said to be a loss of function mutation - If the missense change allows the protein to gain a new activity, the mutation is said to be a gain of function mutation |
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iii) Nonsense mutation =
At the protein level |
The resulting triplet codes for a stop codon- AAA (Lysine) → TAA (Stop Codon)
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2) Frame-shift mutation is a
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change in the ORF that changes all the downstream amino acids
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3) Inversion occurs when _____
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a fragment of DNA is rotated 180o
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• The effect of mutations on proteins depends on
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type of mutation and position of mutation
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A mutation away from the wild-type
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forward mutation
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reverse mutation
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A second mutation that makes the mutant appear to be a wild-type organism again
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• Non-ionizing radiation
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- Includes UV light
- Characterized by long wavelength and thus low energy - Forms pyrimidine dimers (Fig. 9.21) Covalent cross-linking between adjacent pyrimidines |
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• Ionizing radiation
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- Includes X-ray, gamma rays
- Characterized by very short wavelength and thus high energy - Forms highly-reactive free radicals which break the DNA |
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• Base Analogs =
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Structurally similar to nitrogenous bases and can be incorporated in the growing polynucleotide chains during replication
- Cause point mutations - Example = 5-bromouracil |
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• Base Modifiers =
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Change a base’s structure and thus alters its base-pairing properties
- Cause point mutations - Example = Nitrous acid |
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• Intercalating Agents =
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Planar molecules that insert themselves between the stacked bases
- Cause frame-shift mutations/ adding or removing of a base - Example = ethidium bromide |
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• Transposons =
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DNA elements that have the ability to move from one site of the genome to
another - Are found in all 3 domains of life - Cause HUGE frame-shift mutations - Please refer to Figures 9.31 and 9.32 |
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DNA TRANSFER IN PROKARYOTES
1. Conjugation = |
Transfer of DNA from bacterium to another following cell-to-cell contact
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DNA TRANSFER IN PROKARYOTES
2. Transduction = |
Transfer of DNA from one bacterium to another via a bacteriophage
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DNA TRANSFER IN PROKARYOTES
3. Transformation = |
Transfer of free DNA from one bacterium to another via the “environment”
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• Competence =
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The particular physiological state in which cells can take up DNA and be genetically changed by it
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TRANSFORMATION
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• Transformation is the uptake of extracellular naked DNA by a cell
• It is mediated by chromosomal genes |
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Natural Transformation
2) Haemophilus influenzae = A Gram-- |
- The mechanism differs in several respects from that in Streptococcus
- To begin with, no competence factor is involved - Changes in the cell envelope accompany the development of the competent state - Numerous vesicles called transformasomes bud from the surface - These contain proteins that react specifically with DNA from closely-related species - The specificity is due to an 11-bp sequence (5′ AAGTGCGGTCA 3′) present at 4 Kb intervals in the Haemophilus genome - DNA is taken into cells as intact duplex molecules - However, only one strand participates in subsequent recombination |
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Artificial competence can be induced by
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heatshock, electroporation
- Both perturb the membrane allowing the DNA to enter the cell |
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TRANSDUCTION
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• Transduction is the unidirectional transfer of DNA from one bacterium to another via a bacteriophage
• It is mediated by phage genes |
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PLASMIDS
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• Plasmids are typically double-stranded, circular, extra-chromosomal DNA molecules (Fig. 7.22)
- Linear plasmids have been found in certain bacteria • Plasmids are characteristic of prokaryotes, although some eukaryotes possess them • The plasmids are typically 1-5 %of the size of the host chromosome • They are capable of autonomous replication - However, they “borrow” the proteins involved from the host - Only a small region surrounding the plasmid ori is required for replication |
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Plasmids replicate in one of two basic ways (Fig. 7.23; p. 244)
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roling circle/ bi-directional
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1. Host range =
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The types of bacteria in which the plasmid can replicate
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2. Copy number =
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Average number of a particular plasmid per cell
- Please refer to Figure 1 (p. 245) |
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1. Resistance plasmids
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- Confer resistance to antibiotics, and various other inhibitors of growth
- R100 plasmid of enteric bacteria - Carries resistance-genes for a number of antibiotics and heavy metals |
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2. Bacteriocinogenic plasmids
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- Carry genes that code for bacteriocins
- Proteins that are produced by bacteria and that adversely affect closely-related bacteria - COL plasmids of E. coli encode colicins |
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3. Metabolic plasmids
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- Code for proteins involved in the catabolism of unusual substances
-Pseudomonas has plasmids encoding genes for the degradation of octane, camphor and naphthalene |
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4. Virulence plasmids
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- Code for proteins involved in pathogenesis
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4. Virulence plasmids
- Bacillus anthracis |
pX01 encodes the anthrax toxin
- pX02 encodes the capsule |
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4. Virulence plasmids
- E. coli |
- ENT plasmids traveler’s diarhea
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5. Fertility plasmids
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- Found in Escherichia coli in 1-3 copies per cell
- Are about 95 Kb in size - Contains genes responsible for DNA replication, DNA transfer, and episome function |
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- The F plasmid of E. coli has two replication origins
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- orN is used to replicate and maintain the plasmid in non-conjugating cells
- orT is used to replicate the plasmid during conjugation |
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CONJUGATION
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• Conjugation is the unidirectional transfer of DNA from one bacterium to another following cell-to-cell contact
• Conjugation is mediated by plasmid genes |
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• F+ cell
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- A cell that has an unintegrated F-plasmid
- Also termed male and donor |
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1. Resistance plasmids
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- Confer resistance to antibiotics, and various other inhibitors of growth
- R100 plasmid of enteric bacteria - Carries resistance-genes for a number of antibiotics and heavy metals |
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2. Bacteriocinogenic plasmids
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- Carry genes that code for bacteriocins
- Proteins that are produced by bacteria and that adversely affect closely-related bacteria - COL plasmids of E. coli encode colicins |
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3. Metabolic plasmids
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- Code for proteins involved in the catabolism of unusual substances
-Pseudomonas has plasmids encoding genes for the degradation of octane, camphor and naphthalene |
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4. Virulence plasmids
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- Code for proteins involved in pathogenesis
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4. Virulence plasmids
- Bacillus anthracis |
pX01 encodes the anthrax toxin
- pX02 encodes the capsule |
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4. Virulence plasmids
- E. coli |
- ENT plasmids traveler’s diarhea
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5. Fertility plasmids
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- Found in Escherichia coli in 1-3 copies per cell
- Are about 95 Kb in size - Contains genes responsible for DNA replication, DNA transfer, and episome function |
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- The F plasmid of E. coli has two replication origins
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- orN is used to replicate and maintain the plasmid in non-conjugating cells
- orT is used to replicate the plasmid during conjugation |
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CONJUGATION
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• Conjugation is the unidirectional transfer of DNA from one bacterium to another following cell-to-cell contact
• Conjugation is mediated by plasmid genes |
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• F+ cell
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- A cell that has an unintegrated F-plasmid
- Also termed male and donor |
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• F– cell
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- A cell that does not contain an F-plasmid
- Also termed female and recipient |
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• F+ X F–
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- The tip of the F pilus attaches to a receptor on the F– cell
- The pilus contracts to bring cells closer - The two cell envelopes fuse and generate a conjugation bridge - This triggers the synthesis of an F plasmid helicase/endonuclease that nicks the phosphodiester backbone at OriT - DNA POL III is recruited to the nicked site, where replication begins - It occurs unidirectionally by rolling circle replication - The 5′ end of the nicked F plasmid strand moves through a pore in the conjugation bridge - In the donor cell, DNA synthesis restores the double-stranded plasmid - In the recipient cell, DNA synthesis converts the transferred single-stranded into double-stranded DNA - The F factor then circularizes to reform the plasmid => F– cell becomes F + - Finally, the conjugation complex spontaneously comes apart, and the membranes seal |
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• Hfr (High Frequency of Recombination) cell
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- A bacterium that has a chromosome-integrated F plasmid
- Hfr cells can transfer chromosomal genes at a much higher frequency than F+ cells do - Nick in the Hfr chromosome at OriT => The F plasmid is now divided into two portions separated by the entire bacterial chromosome - First, a part of F is transferred, then chromosomal genes in order - Conjugation does not last long enough for the second part of the F to be transferred => F– cell stays F- - In the donor cell, DNA synthesis restores the double-stranded Hfr chromosome - In the recipient cell, DNA synthesis converts the transferred single-stranded into double stranded DNA - Recombination occurs between the endogenote and exogenote |
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• F′ cell
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- Once an F factor has been integrated into a host chromosome, it can also be excised
- Usually the F factor is excised completely and restored to its original form - In rare cases, an aberrant excision takes place - The result is a plasmid which carries adjacent bacterial chromosomal DNA - This plasmid is called the F′ plasmid - Please refer to Fig. 9.5 |
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• F′ X F–
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- The process is very similar to that of F+ X F–
- One difference is that it leads to the formation of a stable merozygote - A cell that contains two copies of some genes One set on the chromosome and the other on the F′ factor |
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• Enterococcus faecalis
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Sex pili are not involved
- Recipient cells excrete pheromones - These are small peptides which enter the donor cell through special oligopeptide permeases - Once inside, the pheromone stimulates transcription of the plasmid transfer genes - The proteins produced include an AS- Aggregation substances - AS of the Donor Cell combines with Binding sub. S of the Recipient Cell - This results in the formation of donor-recipient mating aggregates - Plasmids are transferred by a mechanism that is not very well understood - However, plasmid transfer results in repression of pheromone specific for that plasmid |