Chapter 11 - Replication Is Connected to the Cell Cycle Flashcards
Frequency of Replication =
Rate of Cell Growth
Number of replication cycles is
connected with Cell Division.
Total time of Replication
60 minutes = 40 minutes for DNA Replication + 20 minutes lapse for chromosome segregation.
If cell division < 60 minutes
A new round of replication must be initiated before
the previous replication round is completed.
Daughter cell is produced with replication already occurring with the presence of multiforked
chromosomes.
Cell-septum components
Same components as cell envelop: inner membrane, outer
membrane (Gram-negative), and peptidoglycan disaccharides.
MreB
Forms cylindrical filament structures along the inner membrane and associates with the peptidoglycan synthesis
machinery, RodA & PBP.
FtsZ (C)
In cytosol; Initiates peptidoglycan synthesis with the recruitment of the pre-septal complex.
Recruits protein complex for the septum synthesis.
FtsZ forms a ring at midcell (defines septum position); this formation is the Z-ring or septal ring.
The GTPase activity of FtsZ is required for oligomerization of the monomeric FtsZ.
Env family
Hydrolyze the peptidoglycan to separate the daughter strands.
ftsZ mutations
Multinucleated, long filaments form when septum formation is inhibited but replication is unaffected.
Minicells
Septum formation is too rapid or not mid-cell.
Minicells different from anucleated mutants - have the same size as wild-type because septum formation is intact.
Observed with ftsZ over-expression.
Z-ring
Z-ring formation is the rate-limiting step in septum formation.
The Z-ring is a dynamic structure with continuous exchange of components. Critical components include cytoskeletal
proteins that link the Z-ring to the membrane.
ZipA
Integral membrane protein in the inner membrane.
FtsA
Membrane-recruited cytosolic protein.
FtsW
Integral membrane protein in the inner membrane.
Recruits FtsI to enhance peptidoglycan production, which in turn pushes inner membrane out and constricts the outer membrane.
FtsI/PBP3
Membrane-bound protein with transpeptidase
activity to promote peptidoglycan synthesis.
MinD
Activates MinC, which in turn prevents FtsZ polymerization.
Number and location of septa is
Determined by the ratio of MinE/MinC,D.
Dynamic movement of the Min proteins in the cell sets up a pattern in which inhibition of Z-ring assembly is highest at the poles and lowest at midcell.
MinC/D
is a division inhibitor whose action is confined to the poles
by MinE.
Noc/Slm
Ensure septum location with respect to the chromosome in the cell. Prevent septation (and consistently Z-ring
formation) from occurring in the space occupied by the bacterial chromosome (nucleoid occlusion).
SlmA
Binds to specific DNA sequences (SlmA binding sites).
Inhibits FtsZ polymerization.
Intermolecular recombination
Merges monomers into dimers.
Intramolecular recombination
Releases individual units from oligomers.
Xer site-specific recombination system
Targets a particular sequence to revert the dimeric
circle.
How does the cell know that a dimer has formed due to
recombination?
No recombination allows segregation of the chromosome
and eventually daughter cells.
Recombination confines the movement of the integrated
chromosomes.
Xer site-specific recombination system
Includes: 2 Recombinases: XerC and XerD; the dif gene sequence near the replication termination region.
Recombinases
Bind to the dif site and form Holliday junction.
FtsK
Resolves the junction, is located at the septum.
MukB
Essential component of the partitioning apparatus that recondenses the daughter nucleoids.
Proper Partitioning requires:
- Disentangling of the daughter chromosomes.
2. Segregation of the daughter chromosomes.
Cyclin-dependent kinases (Cdk)
Key mediator of cell cycle progression in eukaryotic cells.
Set of serine/threonine protein kinases that directly contribute control cell cycle progression. Inactive unless bound by Cyclins.
Cyclin proteins
Key mediator of cell cycle progression in eukaryotic cells. Required to activate Cdk proteins; Cdk are
Retinoblastoma Protein (Rb)
Key mediator of cell cycle progression in eukaryotic cells.
Inhibits gene transcriptional events required for progression from G1 to S-phase; activity is regulated through phosphorylation/de-phosphorylation PTM.
Cdk inhibitors (CKI)
Set of inhibitor proteins that negatively regulate the cyclin/CDKs.
Cdk-activating kinases (CAK)
A set of activator proteins that positively regulate the cyclin/Cdks.
Interplay between Cyclin/Cdk/Rb
- Cyclin D is expressed.
- Cyclin D > CKI in the cell.
- Cyclin D/Cdk4/6 complex forms.
- Cyclin D/Cdk4/6 à p-Rb.
- p-Rb released from complex with E2F.
- E2F initiates transcription of genes that
promote entry into S-phase and Cyclin E. - Cyclin E/Cdk2 complex form to maintain p-
Rb and continue progress to S-phase. - Cyclin E levels decline and Cyclin A levels
increase. - Cylin A/Cdk2 complex forms to carry cells
into G2 and inhibit E2F-mediated
transcription. - Cyclin A levels decline and Cyclin B levels
increase. - Cyclin B/Cdk1 complex is required for
mitosis.
Two factors dictate progression of the cells through the cell cycle:
- Cell growth/size
2. No damaged DNA.
p53
Activated by DNA damage with activation of the ATR/ATM
kinases.
Activated p53 prevents entry into the S phase of the cell cycle; stimulates DNA repair (GADD45 gene expression); can initiate cell apoptosis (inhibiting Rb).
Function of growth factor
Cause dimerization of its receptor and subsequent phosphorylation of the cytoplasmic domain of the receptor.
Function of growth factor receptor
Recruits the exchange factor SOS to the membrane to activate RAS.
Function of activated RAS
Recruits RAF to the membrane to become activated.
CNK
Serves as a molecular scaffold platform for KSR-mediated
RAF activation on the membrane after being recruited by active RAS-GTP.
The function of RAF
Initiates a phosphorylation cascade leading to the phosphorylation of a set of transcription factors that
can enter the nucleus and begin S phase.
Most of eukaryotic cells are
Are not growing or in the G0 stage. Exception: stem cells, embryonic cells, and cancer cells.
Replication and cell division is controlled via
Signal transductions pathways
Signal transduction pathway
The process by which a stimulus or cellular state is sensed by and transmitted to pathways within the cell. Example: The EGFR Signal Transduction Pathway.
Oncogene
A gene that when mutated may cause cancer.
