Unit 2: Bacterial Replication Flashcards
What is the main difference from prokaryote and eukaryotic replication?
how the replication is controlled and linked to the cell cycle. Bacteria divide by binary fission.
What are the steps of bacterial replication?
1) A unit cell has a circular chromosome
2) Replication initiates at origin when cell grows to a critical size
3) Replication begins at the origin and produces catenated daughter chromosomes
4) septum divides cell and new chromosomes are partitioned to opposite ends of cell
5) daughter cells separate
How does the cell know when to initiate replication?
Possible theories:
1) Initiator protein levels: initiative protein is synthesized constitutively throughout the cell cycle but when it accumulates past a threshold amount it can trigger initiation
2) Inhibitor protein levels: an inhibitor protein could be synthesized or activated at a point when its concentration falls below a critical point by the increasing cell volume
Current model suggest a combo of both
How often does initiation of replication occur in bacterial cell?
occurs only once every cycle and it occurs at the same time in every cycle
What is doubling time in bacterial replication?
it represents the rate of bacterial growth in case of E coli it can vary anywhere between 80 minutes and 180 minutes.
it takes 40 minutes to replicate the bacterial chromosome which means that the replication fork is moving at a rate of approximately 15 base pairs per minute.
it then 20 minutes from the completion of a round of replication to complete cell division. During these 20 minutes the cell is possibly assembling the components needed for the division into the daughter cells
Fast rates of bacterial cell growth produce what?
multiforked chromosomes.
If it takes about 60 minutes for each replication cycle then what happens if the doubling time for the bacteria is less than 60 minutes ?
what happens is that the next round of initiation begins even before cell division from the current cycle is completed. For bacteria that has a doubling time of 35 minutes, it shows the status of the bacterial DNA at 5 minute intervals of the cell cycle and the start of the replication cycle which is 0 minutes the cell already has a partially replicated chromosome as a DNA replication would have been initiated 25 minutes before the previous cycle was completed. At about 10 minutes into the cycle even though the replication fork has not reached the terminus the daughter chromosomes have already started the next round of replication producing these multiforked chromosomes. By 15 minutes the replication fork has reached the terminus because DNA replication takes about 40 minutes there. it takes 20 minutes from the completion of DNA replication to cell division so by the end of 35 minutes you have two daughter cells each having an already replicating chromosome and can start the next cycle thus recreating the point at which we started
Poles:
the ends of a bacterial cell, usually containing specializes structures for positioning structures
Septum:
formation is initiated mid-cell, 50% of the distance from the septum to each end of the bacterium.
this is the site at which the cell divides into the daughter cells
Anucleate cell:
bacterial that lack a nucleoid, but are similar shape to wild-type bacteria. Comprise 0.03% of bacterial population.
TRUE OR FALSE: In the case of bacteria, the DNA itself is involved in its segregation unlike eukaryotes where they have an apparatus involved
TRUE:
What is a nucleoid?
the bacterial chromosome is compacted into an organized structure of protein and DNA called a nucleoid which occupies most of the cell
how do the origin and terminus move around a newly formed cell?
in the newly formed cell the origin and terminus of the bacterial chromosome is located in the middle of the cell
as replication is initiated the new origins move apart towards the poles with the terminus remaining in the center
the two origins occupied the 1/4 and 3/4 positions at the time of cell division
following cell division the origin and terminus of each daughter cell comes back to the middle of the cell
Which proteins help maintain the cylindrical shape of E.coli?
MreB, PBP2, and RodA
What is the role of MreB
MreB is similar to actin protein that is part of the eukaryotic cytoskeleton like actin, MREB forms polymers that moves around the circumference of the cell interacting with PBP2 and other parts of the peptidoglycan synthesis machinery. if MREB is absent cells no longer have a rod shape but become round
What is the family of proteins called SEDS?
SEDS (Shape, elongation, division, and sporulation)
found in all bacteria that have a peptidoglycan cell wall, of which RodA is a member
Each member of the SED family is linked to a specific transpeptidase for its function. a transpeptidase catalyzes the formation of the crosslinks in the peptidoglycan layer
What is the role of RodA?
Rod a functions along with its specific transpeptidase PBP2
how do we know that these proteins (MreB, PBP2, RodA) are necessary for the elongated structure of the bacterial cell wall?
mutations and any one of the genes that encode for these proteins and or depletion of these proteins results in the cells no longer retaining their cylindrical shape and instead look more rounded
What is FtsZ and what is it required for?
FtsZ is a GTPase (hydrolyzes GTP and uses energy to form oligomeric ring) that forms a ring (the Z-ring or septal ring) on the inside of the bacterial envelope.
The product of ftsZ is required for septum formation at pre-existing sites. FtsZ recruits other proteins needed for the synthesis of the septum
Where is ftsZ found before the cell divides?
FtsZ is found throughout the cytoplasm but before the cell divides it localizes along the circumference at the midpoint of the cell forming a ring shaped structure called the Z ring.
What determines position of the septum?
Position of the Z ring.
FtsZ is analogous to what in eukaryotes?
FTSZ resembles tubular and the formation of the ring could be analogous to the formation of the microtubules in eukaryotes
How do ZipA and FtsA interact with FtsZ?
ZipA is links FtsZ ring to the membrane.
Once the FtsA has been incorporated other Fts proteins joined the Z ring to form the septal ring.
This multi protein structure is thought to have the ability to constrict the membrane
TRUE OR FALSE: The Z ring will form if both ZipA and FtsA are absent
FALSE:
The z ring will form if either ZipA or FtsA are absent but does not form if both are absent suggesting that they may play a role in stabilizing the Z ring and possibly linking it to the membrane
How do we know that ftsZ is necessary for septum formation?
because in Fts mutants the septum does not form but instead long filaments are formed.
Surprisingly chromosome replication and segregation are not affected.
What happens if FtsZ is over-expressed in a cell?
if FTSZ is overexpressed then too many septum are produced resulting in the formation of minicells.
What is a minicell and how does it differ from anucleate cells?
the mini cell is small and lacks DNA
Minicell: small in size and lacks DNA
Anucleate: retains normal size and lacks DNA
Minicells: too many septum affect it
Anucleate: generated b/c of the failure of chromosomes to segregate properly
Genes that regulate the location of the septum were determined by studying what?
minicell mutants;
The minC, D, and E are part of the minB locus. so in cells that were lacking the minB locus the septum formed at the poles instead of the middle. this means that MinB was needed to prevent formation of the septum at the poles.
What is the role of MinC and MinD?
proteins minC and MinD together form what is called a division inhibitor.
MinD activates minC with
MinC preventing FtsZ from forming the Z ring.
