Week 2: Bacterial Sex

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Week 2: Bacterial Sex
2014-01-19 00:41:30
Bacterial Sex
micro lecture: Week 2
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  1. 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.
  2. Medical significance of gene exchange
    Antibiotic resistance
  3. Mechanisms of genetic transfer

    which is most common?
    • 1. transformation
    • 2. transduction
    • 3. conjugation

    Conjugation is the most common then transduction and last transformation
  4. 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
    • plasmid)

    • 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
  5. Frederick Griffith experiment

    what did he discover?

    - DNA can exchange material from dead and live strains

    - due to bacterial Transformation
  6. 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 (Ca2+). 

    - 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.
  7. Natural Transformation
    –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
  8. 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
  9. In what growth phase do bacteria develop competence?
    Late log phase
  10. Conjugation
    - 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
  11. 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)
  12. 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
  13. Transduction

    - definition
    - two types
    - DNA exchange mediated by bacteriophages (bacterial viruses)

    • Types:
    • 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
  14. Bacteriophages

    - what is it?
    - contents?
    - life cycles?
    - Viruses 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
  15. Generalized Transduction

    - what cycle?
    - steps?
    - Generalized transduction results from lytic cycle

    • Steps:
    • 1. Phage injects DNA

    2. Phage enzymes degrade host DNA

    3. Cell 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
  16. Specialized transduction

    - what is it?
    • - results from the combination of lysogenic and lytic cycles
    • First enters Lysogenic and  then lytic cycle
  17. Summary of bacteria gene exchange
    1. Conjugation via sex pilus

    • 2. Transduction: requires virus
    • (2 types: generalized and specialized)

    3. Transformation: occurs between live and dead strains
  18. Transposition
    - 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.
  19. Transposon

    what is it?
    common structure?
    - 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 genes.
  20. 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

    3. Virulence:

    –Change in ability to produce disease

    4. Drug resistance
  21. Eukaryotes vs prokaryote cells

    cell wall?
    nuclear membrane?
    DNA replication?
    Cell membrane?
  22. Steps of Peptidoglycan Synthesis (bacteria)
    1)  cytoplasm: four a.a. are sequentially added to NAM forming a tetrapeptide.


    (2) Cell Membrane: NAM- tetrapeptide is attached to the bactoprenol (carrier molecule)

    - The NAG is attached to the NAM-tetrapeptide on the bactoprenol to complete the peptidoglycan monomer.

    (3) ) Peptidoglycan monomers are transported into periplasmic space by bactoprenol

    (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
  23. DNA condensation

    euk vs bacteria
    Eukaryotes: Histones -> condenses

    • Bacteria:
    • DNA gyrase -> condenses
    • Topoisomerase IV -> de-condenses
  24. Key enzyme in transcription

    euk vs bacteria
    euk: RNA polymerase II (12 subunits)

    bacteria: RNA polymerase (4 subunits)
  25. Ribosomes in Translation

    euk vs bacteria
    euk: 60s and 40s

    bacteria: 50s and 30s