Prokaryotic Genomes and Cell Division Flashcards
Describe the genome of Escherichia coli
- 4.6Mb of circular DNA
- 300 plasmids
- measured using 100minutes time mapping of Hfr mating
- approximately 40 minutes to replicate and divide
Describe chromosomal replication
- begins at the origin
- is bidirectional (there is a replication fork in both directions)
Describe the G1 phase of chromosomal replication
- DnaA-ATP binds to fully methylated oriC for initiation
- once oriC reaches critical concentration, it fires all the oriCs in the cell at one: it is an all-or-nothing response
- results in DNA replication and hemimethylation to form DMNA, allowing the S phase
DnaA exists at
low levels in the cell
Describe the S phase of chromosomal replication
- SeqA blocks the oriC regions, displacing DnaA and resulting in cell elongation by forming a ring at the centre of the cell
- DnaA must reach a critical concentration as the cell grows
- HdaA hydrolyses DnaA-ATP to Dna-ADP
Describe SeqA
homologous to tubulin
Describe the G2 phase of chromosomal replication
- results in chromosomal segregation and partitioning, and separation of the two nucleoids
- ParB is located at the old pole, and is bound by PopZ, whilst simultaneously binding to the DNA at the parS sequence
- drawn towards the new pole by the activity of ParA
- Z-ring forms, and the cell divides
Describe visualisation of the G2 phase of chromosomal replication
- usually modelled using the stalked cells of Caulobacter (since the Par genes are absent in E. Coli)
- most effectively visualised using fluorescent tags
Describe variation in replication fork usage
- in bacteria with a very fast generation time, up to 5 replication forks form on oriC
- others, such as E. Coli, use only one.
Where are very large plasmids found?
Rhizobia
Describe the shape of plasmids
- usually linear
- can be circular
Which genes do plasmids contain?
- not necessary for survival, but for accessory functions
- pathogenesis
- metabolism
- symbiogenesis
Describe R plasmids
- resistance plasmids
- confer antibiotic resistance
Describe bacteriocins
- found on plasmids
- kill related bacteria
Describe tra genes
make them mosibilisable and self-transmissible via the production of a conjugation bridge
Describe incompatibility groups
- a way of organising plasmids
- two plasmids from the same incompatibility group are very similar, and interfere with each other’s replication
Describe plasmid replication
plasmids are autonomous and replicate independently of the genome, sometimes poles away from the nucleoid
Give an example where a plasmid replicates poles away from the nuclei
large ColE1 plasmids
Describe bacterial cell division
- requires the divisome complex
- FtsZ assembles at the cell equator in a ring structure
- attached to the cell membrane by ZipA while converting GTP to GDP and Pi
- FtsA recruits FtsZ to transform ATP into ADP and Pi
- FtsZ polymerisation is prevented by MinCD, which oscillates in concentration mostly between poles as a ring
- oscillation induced by MinE, which disperses it at one pole, inducing reassembly at the other
- MinCD levels are lowest at the cell equator: this is where FdsZ polymerisation will occur.
Describe the divisome complex
comprised of FtsZ, FtsA, ZipA and other proteins
Describe FtsZ
tubulin-like
Describe FtsA
actin-like
Describe FtsI
- penicillin binding protein
- necessary for peptidoglycan synthesis
How is the rod shape in rod-shaped bacteria formed?
MreB forms in bands and spirals perpendicular to the cell wall, connecting to the cell membrane and directing peptidoglycan synthesis
How do concave bacteria such as Caulobacter crescentis create their shape?
contain proteins such as crescentin that localise at the curved surface to induce a concave structure, perpendicular to the cell wall
Describe dispersed cell wall synthesis
- lateral
- occurs at multiple points on the cell wall
- bands of peptidyglycan are dispersed along lines of the cell wall
- dependent upon the activity of MreB for connectivity.
- E. Coli and Bacillus
Which organisms use budding?
some alpha-proteobacteria such as
- Hirschia baltica
- Anacalomicrobium adentum
- Prosthecomicrobium hirschii
- Rhodoseudomonas plaustris
- Rhodomicrobrium vannielii
- Hyphonmicrobium denitrificans
- Sagittuis stellata
Describe cell wall synthesis in polar organisms
- polar bacteria do not hae MreB
- new cell wall is made at the pole(s).
- Agrobacterium tumefaciens
- Brucella suis
Describe cell wall synthesis in Gram positive Cocci
- lack MreB
- grow a band from the middle
- FtsZ ring grows in a growth zone between wall bands, creating a central deposition of new cell membrane and wall
- forms a septum and a septated cell.
- Streptococcus
Describe peptidoglycan dipeptide biosynthesis
- linking of the peptidoglycan precursor NAG-NAM-pentapeptide to the C55 alcohol bactoprenol in the cytoplasm
- hydrophobic bactoprenol carrier is then loaded, and can then carry the peptidoglycan unit across the cytoplasmic membranes lipid bilayer, to the peptidoglycan layer
- autolysins hydrolyse the glycosidic bonds in its backbone
- transglycosylases stitch in a new unit
- once D-Ala is released, transpeptidases crosslink DAP to D-Ala to form transpeptide cross-links, flipping the dipeptide into the chain
What is the peptidoglycan layer?
The growing point of the cell wall
Describe the action of penicillins
inhibit transpeptidation
Which antibiotics target the cell wall?
- Vancomycin (via DL/DR, causing cell breakdown)
- Bacitracin
Which antibiotics target the cytoplasmic membrane?
Daptomycin
Which antibiotics target the chromosome?
Quinolones
Which antibiotics target RNA polymerase?
- Rifampin
- Actinomycin
Which antibiotics target the 70s ribosomal subunit
- Puromycin (which terminates peptide synthesis)
- Streptomycin
List some resistance mechanisms
- beta-lactamase
- modified porins
- an alternative PBP
- an efflux pump
Describe vancomycin resistance
- vancyomycin binds in peptidoglycan synthesis to DL and DR, breaking down the membrane
- VanR has evolved to detect problems in the cell membrane, phosphorylating VanS and beginning rescue biochemistry
