Information flow - DNA transfer

• what is the smallest bacterial genome

  Nasuia deltocephalinicola (112Kbp)
• what is the largest bacterial genome

  Minicystis rosea (16Mbp)
• what is the largest eukaryotic genome

  Paris japonica (149 Gbp)
• gene structure and organisation of bacteria

  • no introns and exons - no splicing required
  • 70% present as single genes and 6% have four or more genes in an operon
  • no nucleus allowing transcription and translation to be coupled within the cell
• what do plasmids contain

  double stranded circular DNA, contain genes to direct initiation of plasmid replication and partition of plasmids
• how are plasmids replicated

  bi-directionally creating a theta structure or rolling circle
• what does direct transfer of genetic material allow

  allows bacteria to acquire new genetic material rapidly without the need for vertical inheritance from parent to offspring 

  • can acquire and evolve new traits enabling bacteria to adapt to new environments and stresses
  • accelerated rate of evolution compared to organisms reliant on vertical gene transfer through sexual reproduction
• why is horizontal gene transfer unlike sexual reproduction

  1. Unidirectional (Donor > Recipient)
  2. Asymmetrical (small amount of DNA)
  3. Not constrained by species boundaries (can be distantly related)
• what is horizontal gene transfer mediated by

  RecA protein
• outcomes of horizontal gene transfer

  patches and splices
• what is transformation mediated by

  a pilus or pseudopilus

  Tightly regulated often by quorum sensing mechanisms (cell density)
• what is transduction mediated by

  bacteriophages
• generalised trandcution

  1. normal lytic events produces a transducing particle
  2. transducing particle attaches to host cell and injects DNA
  3. homologous recombination
• examples of generalised transduction

  P1 Phage E. coli and P22 Phage Salmonella enteritica
• specialised transduction

  1. induction
  2. phage DNA circularises and detaches from host DNA
  3. detached DNA replicates
  4. phage replication is completed and cell lyses
• what is conjugation

  transfer of genetic material through direct contact
• conjugation

  1. pilus retracts
  2. donor plasmid is nicked in one strand
  3. transfer of one strand from the donor to recipient cell. plasmid simultaneously replicated in donor cell
  4. synthesis of the complimentary strand begins in the recipient cell
  5. completion of DNA transfer, cells separate
• barriers to HGT

  • Genomes between different species differ in GC content and codon usage
  • Expression of restriction enzymes
  • CRISPR-cas9
• transposable elements

  • Movement of discrete segments of DNA from one location to another
  • Mobilised by transposase enzyme via inverted repeat sequences
  • Replicated by host DNA replication machinary
  • Two types:
  • Insertion Sequences (IS)
  • Transposons (Tn)
• transposition

  • Transposase recognise specific target sequence
  • Transposase cuts and ligates DNA during transposition leading to duplication of the target sequence
  • Excision of the transposon leaves a scar at the site of insertion
• insertion sequences

  • Maybe present in multiple copies on host genome/plasmids
  • Can lead to wide-spread DNA rearrangements
  • Responsible for mobilisation of vancomycin resistance genes between Enterococcus faecium plasmids (IS216)
• transposons

  • Similar to IS elements but are larger
  • Often comprise IS elements flanking antibiotic resistance genes
  • Some transposons encode Tra genes enabling mobilisation via conjugation