Week 2: Bacterial Sex
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Evolutionary significance of gene exchange
The exchange of DNA between cells allows the exchange of genes and characteristics between cells, thus producing new strains of bacteria, beneficial for their own adaptive evolution.
Medical significance of gene exchange
Mechanisms of genetic transfer
which is most common?
- 1. transformation
- 2. transduction
- 3. conjugation
is the most common
and last transformation
list the exchangeable genetic materials in bacteria
1. Chromosomal DNA
- 2. Plasmid
- - extrachromosomal DNA
- - carry various genes including virulence genes, antibiotic resistance genes, or/and genes that mediate their own transfer between cells (i.e., F
- 3. Transposon (‘jumping gene’)
- - Mobile DNA elements that can jump from one place to another on the chromosomal DNA
- Some transponsons carry antibiotic resistance genes
Frederick Griffith experiment
what did he discover?
- DNA can exchange material from dead and live strains
- due to bacterial Transformation
- Viable bacteria absorb “naked” fragments
of DNA released from dead bacteria
and incorporate them into its own genome, thus changing its genetic makeup.
- Only a single strand DNA enters
the cell, exact mechanism is unknown.
- The process requires calcium ion (
- Cells capable of taking up exogenous DNA
are called competent cells.
- - occurs at extremely low frequency, but with
- large populations of bacteria, it offers a significant route for genetic transfer.
–In nutritional shortage or adverse condition, the bacteria develop a capacity to pick up and internalize DNA from the environment.
–Both G+ and G- bacteria can develop this ability
Artificial Transformation (in Lab)
–Use chemicals (Ca++) to make membrane permeable so DNA can be diffused into the cell (i.e. E. coli transformation)
–Use heat shock (42°C) to shoot the DNA into the cell
–Used mostly in genetic engineering
In what growth phase do bacteria develop competence?
Late log phase
- Genetic transfer during direct cell-to-cell contact through sex pilus
- Plays an important role in drug resistance gene transfer (e.g. R plasmid)
- Occurs with most bacteria, usually between the same or similar species; only between fertility factor positive (F+) and negative (F-) cell
Role of F plasmid in bacterial conjugation
- F plasmid encodes all the genes necessary for conjugation
F+ cell -> has sex pilus -> (donor; male)
F- cell -> (recipient; female)
Conjugation:Hfr (high frequency recombination) cell
- A bacterium with the F plasmid integrated into its chromosomal DNA.
- Partial or entire chromosome of Hfr cell can be transferred through pilus.
super F+ cell
- two types
- DNA exchange mediated by bacteriophages
- 1. Generalized transduction:
- - any host gene can be transferred
- 2. Specialized transduction:
- - only specific host genes (genes adjacent to the phage integration site) are transferred
- what is it?
- life cycles?
that infect bacteria
- Consist of protein and DNA
- Two life cycles:
- 1. lytic: virulent, resulting in bacterial lysis
- 2. lysogenic: non-virulent, viral DNA integrates into bacterial chromosome
- what cycle?
- Generalized transduction results from lytic cycle
- 1. Phage injects DNA
2. Phage enzymes degrade host DNA
synthesizes new phage that incorporates phage DNA and
mistakenly some host DNA
4. Transducing phage injects donor DNA into recipient cell
5. Donor DNA is incorporated
into recipient's chromosome by recombination
- what is it?
- - results from the combination of lysogenic and lytic cyclesFirst enters Lysogenic and then lytic cycle
Summary of bacteria gene exchange
via sex pilus
- 2. Transduction: requires virus
- (2 types: generalized and specialized)
: occurs between live and dead strains
- Genetic transfer within a bacterium
- Transposition is mediated by mobile genetic elements, transposons (jumping genes)
- Transposons can transfer DNA within a cell, from one position to another in the genome or btwn diff. molecules of DNA.
- If they are plasmid-located they can easily transfer from one bacterium to another.
what is it?
- Mobile genetic element
- Common structure:
- 1. palindromic sequences at its ends: 15 - 40 bp
2. insertion sequence (IS)
: 150 - 1500 bp
- Insertion sequence contains transposase
gene and/or antibiotic resistance
Consequence of gene transfer in bacteria
1. Colony morphology:
–Loss of pigmentation
–Smooth to rough (loss of capsule)
2. Biochemical activity:
–Change in enzyme production
–Change in ability to produce disease
4. Drug resistance
Eukaryotes vs prokaryote cells
Steps of Peptidoglycan Synthesis (bacteria)
: four a.a. are sequentially added to NAM
forming a tetrapeptide
(2) Cell Membrane
: NAM- tetrapeptide is attached to the bactoprenol (
- The NAG
is attached to the NAM-tetrapeptide on the bactoprenol to complete the peptidoglycan monomer.
(3) ) Peptidoglycan monomers are transported
into periplasmic space
(4) Periplasm: autolysins break
the glycosidic bonds
btwn the peptidoglycan monomers at the point of growth along the existing peptidoglycan.
- They also break the peptide cross-bridges (pentaglycine peptides) that link the rows of
- sugars together
In this way, new peptidoglycan monomers can be inserted.
(5) Transglycosylase (TG)
enzymes catalyze the formation of glycosidic bonds btwn NAM and NAG
of the peptidoglycan momomers and the NAG and NAM of the existing peptidoglycan.
(6) transpeptidase (TP)
enzymes reform the peptide cross-links (pentaglycine peptides) btwn the rows and layers of peptidoglycan to make the wall strong
euk vs bacteria
Histones -> condenses
- DNA gyrase -> condenses
- Topoisomerase IV -> de-condenses
Key enzyme in transcription
euk vs bacteria
euk: RNA polymerase II (12 subunits)
bacteria: RNA polymerase (4 subunits)
Ribosomes in Translation
euk vs bacteria
euk: 60s and 40s
bacteria: 50s and 30s
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