Bacterial genetics by E. Börje Lindström This learning object has been funded by the European...
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Transcript of Bacterial genetics by E. Börje Lindström This learning object has been funded by the European...
Bacterial genetics
by
E. Börje Lindström
This learning object has been funded by the European Commissions FP6 BioMinE project
Definitions• Gene:
- the aa-sequence in a peptide/protein
- rRNA
- tRNA
- operator (O)
- promoter (P)
a DNA sequence that determines
• Allele:
- a different form of a gene (wt, mutant)
• Genotype: - the total amount of genes in an organism
• Phenotype:
- those properties that are expressed
Definitions, cont.• Mutation: - a permanent change in the genotype
- spontaneous (frequency 10-10—10-5)
- induced (frequency >10-5)
• Mutant: - an organism that has a changed genotype
• Two types of mutants: -Gain of a property
(possible to select for)
-Loss of a property
(not selectable)
How to find the mutant? • Those who are selectable: -e.g. Antibiotic sensitive resistant
- streak out the culture on a plate containing the antibiotic.
• Those who are not selectable:
1) Enrichment with the e.g. penicillin-method
2) Screening with ’replica plating’
- e.g. Prototroph (wt) auxotroph (mutant)
Penicillin-method
MM-medium + pen
Inoculum
- Only prototrophs can grow are killed
Complex medium without penicillin
-All bacteria will grow
- both prototrophs and auxotrophs
- the auxotrophs are enriched
- Change the medium
Replica plating
Replica plating, cont.
Nomenclature
• Genotype: - trp- (trp+); (A, B, etc.)
• Phenotype: - Trp+; Trp-
Model system: tryptophan
• Amino acid:
try
Transfer of DNA between bacteria
Three processes:
1) Transformation:
2) Transduction:
3) Conjugation:
- f ree donor-DNA recipient
Donor Recipient
- donor-DNA within a virus (Phage) recipient
- cell-to-cell contact between donor and recipient
Transformation
Transformation, cont.The Donor
- lyse and cut the DNA into pieces
Free DNA (10 genes/ fragment)
The Recipient
- Cultivate, make competent cells
ds DNA - Uptake of DNA (ds or ss)
- homologue integration - transformed cell
TransductionTwo types:
• General transduction
• Specific transduction
-Model system: P1- E. coli
- P1 a virulent phage
- Model system: phage – E. coli
General Transduction
Specifik transduction
Specifik transduction, cont.
ConjugationThree different bacteria:
F-plasmid
FreeIntegrated
F- F+ Hfr (high frequency of recombination)
Recipient (female) Donors (males)
pili
Integration of the F-plasmid
oriTIS-element
Cross no. 1• F+ x F- F- F+ (all)
The F-plasmid is transferred:
- a ss-DNA is transferred
- new DNA synthesis occur in both cells
- the complete F-plasmid is transferred
- rapid spread of the F-plasmid
Cross no. 1, cont.
Cross no. 1, cont.
Cross no. 2• Hfr x F- F- F- (all)
Course of events:
1) Cell-to-cell contact - pili promote a conjugation bridge (Week)
2) ss-brake at oriT in Hfr
3) One of the Hfr-strands is transferred – ori T first
-new synthesis in both cells
- constant transfer speed (takes 100 min for E. coli)
4) Pairing of homologue DNA-segments
5) Integration and exchange of genetic material
6) Brake down of linear extra-chromosomal DNA
7) A conjugant/ recombinant has been constructed
Cross no. 2, cont.
Cross no. 2, cont.
A-B-
C-
A+ Ori T B+ C+
- Partial diploid40)
50)A+
B+
C-
A- Ori T B- C+
- Integration and exchange
Cross no. 2, cont.
Q: Why is streptomycin added to the plate?
Summary
• Obtaining a recombinant/conjugant require:
- genes must have been transferred from a donor to a recipient
- exchange of genes must occur