Topic 4-L1 - Cell division Flashcards
Most microbial cells divide by
binary fission
binary fission:
DNA replicated, cell grows, cytoplasm evenly divided, producing two (essentially) identical daughter cells
After replication, DNA must be
segregated to opposite sides of the cell so that each daughter gets one copy
In BF, Cells elongate and a septum begins to form to
separate the cell into two, division site contracts and cells pinched off
Cell division is a highly regulate process?
Yes
Things about cell division that need to be coordinated
- Division at appropriate times only
- Chromosome is faithfully replicated & each daughter gets one copy of genetic material
- septum forms at right location
- cell wall remains intact to prevent bursting
Nutritional status coordinated with
- DNA synthesis
- cell envelope synthesis
- division processes
DNA replication is first step in cell cycle. Starts at specific site on chromosome –
oriC. Protein called DnaA binds oriC to initiate DNA replication
Each strand of double-stranded chromosome acts as a
template to produce a new strand (DNA is fully replicated)
What does the z-ring form in cell cycle
Divisome, promotes formation of septum at center of cell. Septum’s forms then cells detach
DNA replication is controlled by binding of protein
DnaA to oriC.
One important mechanism (there are multiple) for controlling DnaA binding to oriC is via a protein called
SeqA – competes with DnaA for oriC binding site.
oriC DNA is
methylated by enzymes in
the cell. New DNA takes a while to get
methylated. Methylated DNA bound to SeqA, preventing DnaA form binding to oriC.
Bacteria can undergo multiple rounds of DNA replication at once?
Yes
For some bacteria, generation time (time to double your population) can be shorter than the amount of time it takes to
replicate the chromosome
In certain bacteria (but not all), such as the model bacteria E. coli and
Bacillus subtilis,
multiple DNA replication forks can be active at once
How can generation time be shorter then replication of DNA?
Multiple DNA replication forks
In multiple replication forks, OriC region starts getting replicated
again before the first fork is finished.
Daughter cells receive a complete chromosome, as well as a second one whose production is in progress
Bacteria build multi-protein complexes called divisomes to help identify the
center of the cell, building a septum, and separate the cell into two daughter cells
A central protein for divisome is FtsZ – polymerizes to from a
ring (FtsZ ring) around circumference of cell at the midcell where division will occur, ring constricts and help division.
Divisome helps build cell septum, including by
recruiting proteins to build cell envelope that separates the newly dividing cells
For finding the mid cell,
- nucleoid occlusion
- inhibitors of FtsZ ring formation
nucleoid occlusion
presence of the chromosome at midcell prevents divisome from forming there
During cell division, Cell wall must be
remodeled but remain intact to prevent bursting
How does the cell wall remain intact during division
- Peptidoglycan precursors brought across cytoplasmic membrane by
Bactoprenol (hydrophobic molecule – lipid) - Autolysin
- Transglycosylases enzymes
Autolysin:
breaks glycolytic bonds for insertion of new monomers
Transglycosylases enzymes form new
bonds in sugar backbone &
transpeptidase enzymes form new peptide crosslinks
How the new cell wall is built as cell grows largely dictates
cell shape
In rod-shaped cells, actin-like cytoskeleton protein
MreB ensures that new cell wall is added along its long axis – rod elongates
Spherical cells lack MreB – new cell wall added only at
midcell. Spherical shape is essentially the default
Alternative to binary fusion
Budding division
Budding division is an alternative to binary fission seen in a number
of microbes. How’s it work?
New cell grows from old by budding off from a particular site on mother cell.
multiple fission
For some species, new cells bud off from long extensions called hyphae. In some instances, long hyphal filaments will form multiple septa at once and break off into
many new cells
