Unit 7 Microbial Genetics Flashcards
Bacterial genome
Total collection of genes carried by a bacterium:
- chromosome
- extra chromosomal material like plasmids or bacteriophages
Gene
Sequence of nucleotides that has a biological function
Phenotype
Morphological or physiological manifestation of the genotype
By what is the phenotype determined
By the interaction of the expression of the genome and the external influence of the environment
NOT every character is encoded genetically
Genotype
Specific set of gens of an organism
Inherited
Not all genes are expressed phenotypically
Phenotypic variations properties
- High frequency, affecting the entire population exposed to the environmental pressure
- Reversible, returning to the original state when the generating cause disappears
- Not inherited, no change in the DNA
Types of phenotypic variations
Morphological : apparition of mucus layer in media with sucrose
Chromogenic : color changes depending on temperature
Enzymatic : capacity to produce enzymes only in the presence of the substrate. E g production of penicillinase in the presence of penicillin
Mutation definition
Causes
Persistant change of DNA bases without addition of exogenous DNA
Arise
-spontaneously,
-because of mutagens or
-transposable elements
Random facts bacterial mutations
-High speed duplication
-DNA repair system of bacteria
produces higher frequency of errors than any species
-Lots of mutations are incompatible with life
-Some spécificité mutation can provide thé mutant BIOLOGICAL ADVANTAGE
Characteristics of mutation
-Low frequency : but can be increased by mutagen agent
- Irreversible
- Inherited
- Spécificité: Affects specific character
- spontaneous mutation
Selector agent
Selects a spontaneous mutation by killing the specimens that aren’t resistant to it
Exemple of mutations in bacteria
- Alteration of surface components such as capsule or flagella
- Loss of ability to synthesize growth factor( auxotroph)
-Resistance to antibiotics
Types of mutations
Substitution
Deletion
Insertion
How is a mutation called when it affects a single base
Point mutation
After the translation it can result in three types of mutations
- Non sens ( no protein/ terminal codon )
-Missense ( altered protein) - Silent
Non sense mutation
Premature terminal codon
No protein
Missense
Altered protein
Silent mutation
Same protein
Frameshift mutations
Insertion or deletion of number of nucleotides that is not a multiple of 3
Spontaneous mutation
Random errors in replication
Induced mutations
Why
Types
From exposure to known mutagens
- Physical agents : ionizing, UV
- Chemical agents ( substitution or insertion )
Chemical agents that cause mutations
Substitutions:
- Nitrous acid ( transforms adenine in hypoxanthine that binds to cytosine)
- 5-bromouracil (replaces thymine and binds to guanine)
Insertion : Acridine orange that is an intercalant agent
Transposable elements/transposons
Segment of DNA that has the capacity to “jump” to different locations in the genome. Produce insertions and deletions
Transposable elements/Transposon are made of
IR inverted repeat
Transposases, enzymes that cut the DNA
Inverted repeat
Small nucleotide sequence located at the ends of the transposon. Inverted
Where do transposases cut
( DR )direct repeats
Few nucleotides on both ends, not inverted
Types of Transposable elements
Insertion sequences
Complex Transposon
Other
DNA repair system made of
Specific enzymes
Excision repair system
Recombination system
SOS repair system
What’s a reversion
Reverse mutation returns to the wild type genotype
Exemple specific enzyme repair system
Photolyase corrects the damage done by UV radiation
Excision repair system
Damage chain gets cut by endonuclease and re-synthesized by polymerase and ligase
Recombination system
Damaged chain gets cut, replace by the homologous fragment that is then resynthesized
SOS repair system
System of enzymes that repairs severely damaged DNA
Genetic transfer
Entry of foreign DNA within the bacteria from the surroundings
3 types
Environment : Transformation
Another bacteria : Conjugation
Bacteriophage : Transduction
Exogenous DNA can react in three ways
Degradation
Circulation
Recombination
Degradation
The new DNA is destroyed by endonucleases that recognize it as foreign
Circulation
Methylated to avoid the action of endonucleases
Then it can replicate independently of bacterial DNA (transmitted to daughter cells)
Or don’t replicate and will be lost in successive divisions (abortive process)
Plasmid can be both
Endonucleases
Enzyme that destroys foreign DNA
Recombination
Genetic exchange between bacterial DNA and exogenous DNA
➡️ Mixed genetic information
Transformation
The uptake by a cell of a naked DNA molecule of the medium and it’s incorporation into the recipient chromosome in a heritable form
Happens after cell lysis, that releases its compounds in the environment
Transformation usually occurs between cells that share the same
Ecological niche
E g 2 bacteria in colon
Bacteria able to take up the DNA from environment
Competent bacteria
How to artificially induce transformation
Electrical shock or calcium chloride
Conjugation
Transfer of genetic material between two bacteria by direct contact between them
Different for gram + or -
Conjugation gram-
Transfer of plasmid via sex pilus
After the conjugation, the receptor f- can become f+
Transduction
Induced by bacteriophages, that inject the viral DNA
Virulent phages
Attenuated phages
Virulent phages
Use the enzymatic machinery to synthesize bacterial phage structures.
They create viral progeny breaking the bacteria to get out
Lytic cycle
Attenuated phages
DNA of the phage is integrated into the cell genome and doesn’t produce lysis
Lysogenic cycle
Lytic cycle
Results in the destruction of the infected cell
Insertion or deletion of a number of nucleotides that’s not a multiple of 3
Frameshift mutation
Name cycle attenuated phages
Lysogenic cycle
When the bacteria is killed because of bacteriophage
Lytic cycle
