Cell Division Flashcards
Mother cell
the cell that is about to divide
events in cell division
- DNA replication and cell elongation
- chromosome partitioning
- determining the plane of division
- cytokinesis
Cell elongation genes
- MreB
Chromosome partitioning genes
- par
- xer
- muc
- tip
Mid-point determining genes
min
Min - midpoint
cytokinesis genes
- fts
- zip
Force and shape
- MreB
cell wall extension
A large multi enzyme complex
Murein hydrolyases
- lytic transglycolysase,
- endopeptidase
murein synthase
- transpeptidase
- transglycolysase
MreB
- skeleton that gives the bacterium its shape in bacilli (absent in cocci)
- MreB filaments polymerize in the presence of ATP, forming actin-like filaments
- as MreB polymerizes, a force is generated at the poles of the cell and it elongates.
Cell wall extension
- a large multi enzyme complex consisting of both murein hydrolyses and murein syntheses adds new polymerase to expand the cell wall without losing structural integrity.
- the complex synthesizes three new peptidoglycan strands underneath a “guide strand” via the transpeptidase and transglycolysase of the murein synthase
- and at the same time degrades the guide strand via the lytic transglycolyase and endopeptidase of the murein hydrolase.
Net result of cell wall extension
one strand is safely replaced by three new strands.
Chromosome separation
- After termination, the two DNA strands are connected or are concatamers
- The XerC and XerD recombinases bind at the dif locus and resolves the concatamers through a double stranded break in one DNA strand.
- The other DNA strand is passed through the break and the gap is resealed.
- ATP requiring process.
Partitioning
- the chromosomes must be translocated to the poles of the cell to make sure each daughter cell gets one.
- Done by Par and Muk proteins
Par
- bind to newly synthesized DNA molecules
- located near the origin of replication
Muk
- homologous to eukyarotic motors
MreB cytoskeleton
- used as a scaffold for the motor proteins to walk along
Par, Muk, and MreB process
- MreB polymers extend the length of the cell and are tracks along which the Muk motors walk, dragging the chromosomes via the Par proteins.
- origins are dragged to the poles as DNA is synthesized
Tip proteins
- bind to the end of the cell, anchor for Par
Determining the midpoint
- The Min family of proteins determines the division plane in rods.
- Min mutants originally isolate as strains that produced “minimills” that would divide at the poles instead of at the middle of the cells
MinC and Min D
- negative regulators of cell division
- cycle from pole to pole, but spend very little time in the middle of the cell.
- prevent formation of the Z ring that will physically divide the cell.
Z ring
- polymer of the the protein FtsZ, and can only form in an area where MinC and MinD are not present at high levels.
nucleon occlusion model
- the presence of a nucleoid also inhibits formation of the Z ring
- before the chromosomes have partitioned, they will occupy the center of the cell.
- The Z ring will not be able to form anywhere, being inhibited by MinCD at the poles and the nucleoid in the center
- Only after chromosomes have been partitioned does the Z ring start to assemble
FTS experiment
- filamentous temperature sensitive
- mutants are fine at normal growth temperatures but have mutations that express themselves at higher than normal temperatures.
- the cells grow as filaments at the non-permissive temperature because they can’t divide.
FTS proteins
- localize to the contractile ring that forms at the site of cytokinesis.
contractile ring
- make out of FtsZ, and thus the term Z ring.
FtsZ
- homologue of tubulin
- wil polymerize in the presence of GTP (self-assembles) to form a polymer akin to tubulin.
Cytokinesis
- starts with formation of Z ring
- ring is composed of FtsZ with multiple Fts protein anchors.
How does the ring contract?
- Z-ring forms
- Z-ring begins to shorten, and the shortening ring constricts the cell until the septum meets and daughter cells separate
- Straight filaments of FtsZ when GTP is bound
- hydrolysis to GDP introduces curvature
Alternative theories
- depolymerization model
- sliding filament model
depolymerization model
- individual monomers of FtsZ are removed from the ring one at a time, making the ring shorter.
- new data indicates that the FtsZ filaments are dynamic, but unknown how the force would be generated
sliding filament model
- FtsZ ring is broken and a motor slides the two ends past each other, thereby shortening the ring.
- does take into account force needed to shorten the ring, but no motor has been identified.
Spore
- a very stress resistant and metabolically inactive form of the bacterium that can lie dormant for many years until conditions improve, at which point it can germinate into a vegetative cell again.
Sporulation
- normal growth
- bacterial division
- asymmetric division
- engulfment
- cortex synthesis
- coat synthesis
- lysis of mother cell
- free spore and germination
Sporulation genes
- named with a letter and a roman numeral
- roman numer at what stage the mutant blocks sporulation
- letter is the order of gene discovered
mutation in spoIIIC
- block sporulation at Stage 3
- third gene found to block stage 3
Sporulation triggers
- starvation
- high cell density
starvation
- either carbon (energy), nitrogen, or phosphorus limitation
high cell density
- quorum sensing mechanism
- small protein or oligopeptide called extracellular differentiation factor PhrA (governed by spo0A, spo0B, and spo0H) secreted by the cell
- prior to sporulation when resources are limited.
Sporulation initiation
- The Spo0 proteins are involved in making the decision to sporulate
- Bacillus process takes several proteins actin in a phosphorelay
master regulation of sporulation
- Spo0A
- last protein in phosphorelay
KinA
- senses the nutrient status of the cell and starts the relay by phosphorylation of Spo0F
Spo0F
- passes the phosphate molecule to Spo0B
Spo0B
- donates the phosphate to Spo0A which initiates sporulation program.
RapA phosphatase
- block sporulation at Spo0F.
- will remove phosphate from Spo0F, and now Spo0B has no donor.
Spo0A
- sigma factor
- initiates polar division of SpoII genes
- blocks stationary phase.
Why have relay instead of just going on?
- gives places for more sensory input
1st step- asymmetric cell division stage II
- after sporulation is initiated, the first sign is the asymmetric cell division
- FtsZ and FtsA are transcribed from a sporulation specific promotor, p2, using Spo0H (sigmaH) - allows for an asymmetric plane of division
- chromosome partitioning (sigmaA) only 30 percent of the chromosome is in the foreshore at separation, the rest is actively pumped into the forespore.
regulation - sigma factors
- expressed prior to segregation
- differentially expressed in the mother and foreshore.
sigma E
- mother cell specific
- kept inactive to make sure it does not work in the wrong compartment
- expressed in a longer, inactive form pro-oE
- a protease only active in the mother cell will cleave pro-oE to the active form sigmaE
- controls the engulfment of the forespore to become an endospore.
sigma F
- forespore specific
- co-transcribed with SpoIIAB, an anti-sigma factor that will bind to sigmaF and keep it inactive until it is phosphorylated.
Engulfment
- sigmaE dependent genes in the mother spore control the engulfment of the forespore by the mother spore.
- The mother cell membrane will migrate around the forespore until they fuse on the other side, releasing the forespore into the cytoplasm - making it an endospore
two membranes of the endospore
- its own original and one derived from the mother cell.
Spore development
- once engulfed, the endospore specific sigma factor sigmaG directs spore development. Regulates transcription of
1. SASP (small acid soluble proteins)
2. PBP’s penicillin binding proteins
3. ger or germination specific genes
SASPs
will bind to and re-arragne the spore nucleoid, compacting it and making it less susceptible to desiccation
PBPs
responsible for making the spore cell wall, which has a lower degree of cross linking and thus is more flexible
sigma K
- control mother cell specific genes
- The most important of these are the cot genes that make up the spore coat
spore coat
- crystalline array of proteins that serves to keep out toxins
- not impermeable, so they spore must monitor its environment in order to decide when to germinate
- the coat contains metal oxidizes, which extract metals (Manganese and copper) from the environment and incorporate them into the coat.
- after spore coat is laid down, the mother cell degenerates and the spore is released.
Before septation
- sigmaA - DNA packing
- sigmaH - Asymmetric division
Mother
- sigmaE - engulfment, sigmaK
- sigmaK - coat proteins
Forespore
- sigmaF - engulfment, sigmaG
- sigmaG - SASP, germination
