L11: The Cell Cycle Flashcards
How do prokaryotes divide?
Binary Fission
4 purposes of the cell cycle
1) Copy the genome & partition the copies
2) Enable a multicellular organism to grow to adult size
3) Maintain the total cell no. of an adult organism
4) Replace lost/damaged cells
2 pathways of binary fission
1) Replication of DNA
2) Cytokinesis
Stages of binary fission
1) DNA attached to cytoplasmic membrane
2) Cell enlarges & DNA duplicates
3) Septum forms
4) Cell divides into 2, DNA partitioned into each cell
5) Cells separate
How many origin of replication do prokaryotes have?
Circular chromosome of prokaryotes has 1 origin (ori) of replication
3 stages of replication of DNA
1) Circular chromosome of prokaryotes has 1 origin (ori) of replication
2) 2 replication forks (RF) form at the origin
3) 2 identical copies of circular chromosome
What does it mean when replication is bidirectional?
Opposite directions
What forms during the early steps in bacterial cytokinesis?
Ring of a protein, FtsZ
Where is the FtsZ found?
On the inner surface of the cytoplasmic membrane
What does the paradox resolve?
All cells have DNA
What does multi-fork replication ensure?
At least 1 round of replication is finished before cytokinesis
Features of the eukaryotic cell cycle that proposes complications
1) Genome is composed of multiple linear chromosomes
2) Multicellularity
3) Numerous organelles (rER, sER, golgi etc.)
Characteristics of the eukaryotic cell cycle
1) DNA must be accurate
2) Replicated chromosomes must be accurately segregated
What happens during G1 ?
Growth phase
What happens during S phase?
DNA replication
What happens during G2 phase?
Cell prepares for mitosis
What is produced at the end of S phase?
Each replicated chromosome has a pair of identical sister chromatids
What can’t the sister chromatids do?
Separate from each other, or else bipolar attachment to the mitotic spindle would be difficult to happen
What protein complex is used to make sure sister chromatids do not separate?
Cohesin
2 events in the beginning of mitosis?
1st event: Chromosome condensation
2nd event: Formation of the mitotic spindle
1st stage of chromosome condensation
Interphase: Chromosomes are not visible
2nd stage of chromosome condensation
M phase begins; where chromosomes are condensed & become visible
Function of condensin
Protein complex that encircles loops of DNA & compresses the sister chromatids to give a compact structure
What happens at the end of G2 phase?
Making the network of protein fibres (SPINDLE FIBRES)
1st stage of the formation of mitotic spindle
Mitotic spindle fibres grab onto a protein complex called kinetochore which is attached to the centromere
What is the mitotic spindle made by?
Spindle pole body
What else happens before mitosis to the nuclear membrane?
It breaks down so the spindle has access to the chromosomes
What happens to the kleisin when the cell segregates?
Degrades part of the kleisin subunit by cohesin
2 things that happens when the sister chromatids have reached opposite poles of the cell
1) Nuclear membrane reforms
2) Cytoplasm divided in 2 by contractile ring of filaments composed of actin & myosin II
What happens to the contractile ring in animal cells?
Divides the cytoplasm from the OUTSIDE IN
What happens to the contractile ring in plant cells?
Contractile ring DOES NOT form
New cell wall is constructed between daughter nucleii, cytoplasm partitioned from INSIDE OUT
What is new wall synthesis guided by?
Phragmoplast: structure that contains microtubules from mitotic spindle
What does it mean when unicellular organisms operate a closed mitosis?
Don’t need to degrade nuclear membrane
What does it mean when multicellular organisms operate a open mitosis?
Nuclear membrane breaks down & reforms
What are stem cells attached to ?
Niche cell, which blocks their differentiation, but cell division is allowed
What happens during polarity stem cell division?
1 daughter is released & free to differentiate, other daughter stays attached to the niche cell & remains a stem cell
2 cell cycle control
1) Anchorage Dependence
2) Density-Dependent Inhibition
Define anchorage dependence
Cells must be attached to a layer or substance in order to divide
Define density-dependent inhibition
Once cells are in contact with each other, they stop dividing
3 aspects of the cell cycle control system
1) Cell cycle engine
2) Co-ordination
3) Checkpoints
Define cell cycle engine
Protein complex that drives the cell division in every eukaryote cell
Define coordination in the cell cycle control system
Cell has to know where it is
Define checkpoints in the cell cycle control system
Cycle will stop if the DNA is damaged
2 proteins that drive the cell cycle engine
1) Cyclin
2) Cyclin dependent protein kinase (CDK)
Why is cyclin key to regulating the cell cycle?
1) Undergo cycles of synthesis & degradation
2) Different CDK’s & cyclins, each pair activating different phase of the cell cycle
What degrades cyclin?
Proteolysis
Role of coordination referring to S phase, G1 and G2 phase cells
If S phase fuses with G1 cell, the G1 DNA will enter S phase
If S phase fuses with G2 cell, the G2 DNA will NOT be forced to enter S phase
Why does surveillance mechanisms (checkpoints) operate continually?
Ensure next phase is not initiated before the previous one is finished
What is the restriction point checkpoint?
A positive signal from outside that instructs the cell to divide
What is the checkpoint during mitosis?
Cell ask itself is DNA synthesis complete?
If NOT, cell cycle is frozen
What is the checkpoint in M?
Spindle checkpoint
What is the spindle checkpoint?
Cell ask itself, is each chromosome attached to the spindle?
If NO, cell cycle is suspended
2 consequences of checkpoint failure
1) Failure of DNA damage checkpoint
2) Spindle checkpoint failure
What is DNA damage checkpoint?
Operates throughout the cell cycle to make sure there is no DNA damage
What cell cycle does cancer feature?
De-regulated cell cycle
