Cell Cycle Flashcards
What is mitosis ?
It is the process when the mitotic spindles segregate the duplicated condensed chromosomes into 2 daughter nuclei
What is cytokinesis ?
When the cytoplasm divides producing 2 genetically identical daughter cells
What are the 2 stages of the cell cycle ?
Interphase and Mitotic phase
What are the different processes that occur in interphase ?
G1
synthesis of interphase
G2
What is the longest part of the cell cycle ?
Interphase
What happens in Gap 1 ?
All organelles and the cytoplasm are replicated
The cell doubles in size
What happens in synthesis of interphase ?
The DNA is replicated
What happens in Gap 2 ?
Enzymes are created for cell division
What is the length of Gap 1 ?
~ 10 hours
What is the length of synthesis of interphase ?
~ 5-6 hours
What is the length of Gap 2 ?
~ 3-4 hours
What is the G0 phase ?
It is a temporary cell cycle state where the population of cells rest and do not replicate
What is another name for the G0 phase ?
Quiescence
What is terminal differentiation ?
It is a way of describing that cells change their phenotypic characteristics as they migrate to the surface and fully differentiate
How long do cells spend in mitosis ?
Less than one hour
What is generating time ?
The length of the cell cycle is called the generation time
How do cells divide ?
Cells have exponential growth
What is the mitotic index ?
It is the percentage of time that cells spend in the mitotic phase
How is mitotic index calculated ?
Cells in mitosis / total number of cells
How is mitosis divided into stages ?
Mitosis is divided into stages based on the changing appearance and behaviour of chromosomes
What are the different stages of mitosis ?
- prophase : individual chromosomes become visible
- prometaphase : fragmentation of the nuclear envelope
- metaphase : chromosomes align at the metaphase plate
- anaphase : sister chromatids move to opposite poles
- telophase : nuclear envelopes and cleavage furrow forms
Describe what happens in prophase
- chromosomes condense to become compact
- chromosomes are composed of two tightly attached chromatids
- microtubule organising centres begin to migrate away from each other
Describe what happens in prometaphase
Microtubules attach to chromosomes in the centromere region
Describe what happens in metaphase
- fully condensed chromosomes align at the metaphase plate
- sister chromatids are actively tugged towards opposite poles
Describe what happens in anaphase
- anaphase is the shortest phase of mitosis
- sister chromatids abruptly separate and move towards opposite poles
- in anaphase A the chromosomes are pulled towards spindle poles
- in anaphase B the spindle poles move away from each other as microtubules lengthen
Describe what happens in telophase
- daughter chromosomes arrive at the poles of the spindle
- chromosomes uncoil into chromatin
- nuclei reappear and nuclear envelopes re form
- cytokinesis also takes place simultaneously
Describe what happens in cytokinesis
- a ring of actin and myosin forms which divides the cytoplasm into 2 cells
- this ring is known as a contractile ring
How does the cell cycle vary ?
- the overall length of the cell cycle is variable
- the relative length of time spent in various phases of the cell cycle is variable
- how closely mitosis and cytokinesis are coupled is variable
Why is the cell cycle regulated ?
The cell cycle is regulated to meet the needs of growth and repair per cell type and organism
What are checkpoints ?
Parts of the cell cycle where stop or go signals can help or inhibit the cell from passing into the next stage of its life
What are the different checkpoints ?
1) restriction point found between G1 and S phase
2) G2-M transition point found between G2 and mitosis
3) metaphase-anaphase transition point found between metaphase and anaphase
How is the progression of cells in the cell cycle controlled ?
- controlled by cyclin-dependent kinases (Cdks)
- phosphorylation of target proteins by protein kinases activates these proteins and dephosphorylation by protein phosphatases inactivates the proteins
- progression is only driven when the protein kinases are bound to cyclin
What are the different types of cyclins ?
- mitotic cyclins : required for G2 - M transition point
- G1 cyclins : required for passage through the G1 restriction point
- S cyclins : required for DNA replication
What are the two levels of control on cyclin dependent kinase - cyclin activity?
- availability of cyclin molecules
- phosphorylation of Cdks
Describe the process of Cdk phosphorylation
1) M-Cdk and M cyclin bind to form an inactive complex
2) two inhibitory phosphate groups attach to Cdk by inhibiting kinases
3) an activating phosphate group is added by activating kinase but Cdk remains inactive when an inhibitory phosphate group is present
4) phosphatase removes the inhibitory phosphate to activate the mitotic Cdk - cyclin complex
What happens when the nuclear lamina is phosphorylated ?
The nuclear envelope is destabilised and breaks down
What happens when condensin is phosphorylated ?
Causes chromosome condensation from euchromatin to heterochromatin
What happens when microtubule proteins are phosphorylated ?
Mitotic spindles assemble
What happens when the anaphase promoting complex is phosphorylated ?
Cyclin is degraded and mitosis is complete
What is aneuploidy ?
When the daughter cell receives the incorrect number of chromosomes
Describe the role of Mad & Bub proteins in relation to aneuploidy
- Mad & Bub proteins accumulate at unattached kinetochores
- they inhibit anaphase-promoting complex
- once all chromosomes are attached Mad & Bub leave kinetochores and inhibition is lifted
- errors cause aneuploidy
