Week 12: The Cell Cycle Flashcards
Section 2 Week 6
What is the basis of the cell cycle?
It is conserved in all eukaryotes. It is a sequence of events where contents of a cell are duplicated and divided in two
True or False: Animal cells all divide at the same time
False
True or False: When cells divide, they all follow the same stages in mitosis
True
What are the phases of the cell cycle?
G1 phase > S phase > G2 phase > M phase
What does the M phase consist of?
Both mitosis (nuclear division) and cytokinesis (cytoplasmic division)
What does interphase consist of?
It is the period of between cell divisions where metabolic activity, cell growth, and repair take place. It consists of G1 phase, S phase, and the G2 phase
Many mature cells __ ___ _____. What does this mean for the cell?
do not divide; This means they are termany differencitated cells (for example: nerve cells, muscle cells, red blood cells - they lost the ability to divide)
Some cells only divide when given an _____________ _________. What is an example of this?
appropriate stimulus; an example of this is when liver cells start to divide to replace damaged tissue
Some cells ________ divide on an ________ basis. What are examples?
normally; ongoing; e.g. hematopoeitic and epithelial stem cells
True or False: Cells in G0 are dead
False (tf) - they’re not dead they just don’t divide and are not preparing to
What do cells in G0 do?
They do not undergo cell division and instead are just metabolically active and carry out their cell function
What is the cell-cycle control system?
It is a system that delays later events until the earlier events are complete (e.g. finish DNA synthesis first before going though mitosis)
What are the major checkpoints of the cell-cycle control system and what do they check for at these checkpoints?
- Start transition (G1 > S): is the environment favourable? (e.g. sufficient nutrients, specific signal molecules)
- G2/M transition (G2 > M): Is all DNA replicated? Is all DNA damage repaired?
- Metaphase-to-anaphase transition: Are all chromosomes properly attached to the mitotic spindle?
What happens when the answer to the questions asked at each question is yes? What if the answer is no?
- Start transition (G1 > S): If yes, enter S phase
- G2/M transition (G2 > M): If yes, enter mitosis
- Metaphase-to-anaphase transition: If yes, pull duplicated chromosomes apart
- If no, for any of them, wait until the answer is yes
What would happen if you had problems at cell-cycle checkpoints or if they didn’t exist at all?
If you had problems at cell-cycle checkpoints (or if they didn’t exist) it can cause chromosome segregation defects
Cell cycle progression is controlled by _____________ _____________
molecular switches
How is entry to the next phase of the cell cycle triggered?
It is triggered by cyclin-dependent protein kinases (Cdks). The cyclin-Cdk complex is activated for entry then inactivated (molecular swtich)
What is an example of cyclin-Cdk activation?
The entry into M phase - M cyclin activates Cdk, which phosphorylates other regulatory proteins
Entry into next phase of cell cycle is paused by other _____________
regulators
During chromosome replication (during S phase), what is deposited to hold two sister chromatids together?
cohesins
What happens during the prophase of mitosis?
Chromosome condensation (chromatids compacted) and sister-chromatid resolution (separable units)
What happens to cohesins during prophase?
Cohesins are removed from chromosome arms, but not from centromeres
What is the function of condensins during prophase?
Condensins condense DNA in each sister chromatid
What is sister-chromatid resolution?
Sister-chromatid resolution is what happens after the removal of cohesins separate sister chromatids everywhere but the centromere
What happens to microtubules in dividing cells?
You must take the microtubule apart then put it back together as needed for mitosis
When does centrosome duplication occur?
The centrosome is duplicated once per cell cycle, it is initiated in G1 and completed by G2
How does centriole duplication work?
Each centriole in the pair of centrioles within a centrosome serves as a site for assembly of a new centriole
What happens once centrosomes are duplicated?
They form poles of mitotic spindle
Centriole duplication is ______-__________
semi-conservative
Mitotic splindle assembly starts in what mitosis phase?
prophase
What does mitotic spindle assembly and function require?
Microtubule disassembly and assembly and microtubule motor protein activity (kinesins, cytoplasmic dynein)
What happens during mitotic spindle assembly?
Duplicated centrosomes separate. Radial array of microtibules extend out from each to position centromeres, this will become the two spindle pores.
When does nuclear envelope breakdown occur?
Occurs at the boundary between prophase and prometaphase
What happens during nuclear envelope breakdown?
Phosphorylation of lamins and nuclear pore proteins. This triggers disassembly of the nuclear envelope into small membrane vesicles.
What is prometaphase?
- Nuclear envelope is now disassembled
- Mitotic spindle assembly can be completed
- Kinetochore microtubules in the mitotic spindle attach to duplicated chromsomes
- Chromosome movement begins
What are the three types of microtubules once mitotic spindle assembly is complete?
- Astral microtubules: help position the mitotic spindle (used by cytoplasmic dynein)
- Non-kinetochore microtubules: cross-linked microtubules throughout the mitotic spindle (used by kinesin-5, other microtubule-associated proteins)
- Kinetochore microtubules: attach duplicated chromosomes to the spindle poles
What do non-kinetochore microtubules do that the other microtubules do not?
They interdigitate with each other
Kinetochores are located at the ____________ of chromosomes
centromeres
How many kinetochores are at each sister chromatid?
There is one kinetochore for each sister chromatic in the duplicated chromosome
True or False: Microtubueles from both spindle poles must attach to kinetochores of sister chromatids
True - this generates equal tension on both sides (which is needed to line up chromsomes at equator of spindle)
True or False: Connecting protein complexes of kinetochores bind to sides of microtubule at the plus end
False - they bind near the plus end, not at the plus end
Why are kinetochore microtubule plus ends exposed?
This allows for growing or shrinking for chromosome movement
All chromosomes are aligned on the ____________ plate
metaphase
How is the metaphase spindle maintained?
- Continuous addition of tubulun subunits at plus end
- Continuous removal of tubulin subunits at minus end
- Length of kinetochore microtubules does not change (treadmilling! - see both actin filaments AND microtubules can undergo treadmilling)
How is tubulin flux observed?
- small amount of fluorescent tubulin added to observe microtubule flux
- time-lapse video microscopy used to follow fluourescent tubulin movement
What is the metaphase-anaphase transition also referred to as?
The spindle assembly checkpoint
What happens during Anaphase? (before anaphase A and B)
- Separase is activated
- Cohesin complex cleaved
What happens during anaphase A?
- Kinetochore microtubules are shortened (loss of tubulin at both ends)
- Sister chromatids are pulled apart towards opposite poles
What happens during anaphase B?
Spindle poles move outward (involves movement of kinesin and cytoplasmic dynein). Spindle poles are both pulled and pushed apart
How do kinesin-5 and cytoplasmic dynein contribute to sister chromatid separation?
- kinesin-5 push the plus ends apart
- cytoplasmic dynein pulls, dragging the two poles apart
What happens during telophase?
Nuclear envelope reasembly which happens through:
* Dephosphorylation of nuclear pore proteins, lamins
* Nuclear envelope reassembles
* Nucelar lamina forms
* Nuclear pores form
What do phosphatases do?
removes specific phosphate groups
What happens to the mitotic spindles and chromsomes during telophase?
the mitotic spindle disassembles and chromosomes decondense
Telophase marks the end of __________
mitosis
When does the contractile ring begin assembly?
It starts in anaphase and continues through telophase
How does cytokinesis work in animal cells?
The contractile ring divides the cytoplasm in two at the cleavage furrow - which is midway between the spindle poles. This happens from underneath the cell membrane
What is the contractile ring composed of?
It is assembled from actin and myosin filaments
What is the contractile force in cytokinesis?
The conctractile force is created by the contractile ring and brings the cell membrane in as the ring becomes smaller
What happens once the contractile ring disassembles?
The cell is divided into two daughter cells
What does each daughter cell have?
Its own nucleus and centrosome
What happens to the microtubules during the end of cytokinesis?
The interphase microtubules reform
Cytokinesis marks the end of _ _______
M phase
True or False: You cannot begin mitosis before finsihing cytokinesis
False
Cytoskeletal ________ are very important during M phase in animal cells
dynamics
What differentiates the M phase of animal cells vs plant cells?
Mitosis of plant cell is very similar to that of animal cells (just no centrosome - something else forms the mitotic spindle). Cytokinesis is very different due to a plant cell’s cell wall
What happens in the telophase of a plant cell?
- Chromosomes separated into two sets
- Phragmoplast starts to form
Describe a phragmoplast?
- It has specific structure to form cell plate
- It has microtubules, actin filaments, vesicles from Golgi
What happens during cytokinesis in plant cells?
- Nuclear envelope reassmbled, chromosomes decondensed
- Cell plate forms
What is a cell plate?
It is a transient membrane compartment (vesicles from Golgi fuse together) to divide cell in two
What can be seen in the G1 phase of a plant cell? (the interphase following cytokinesis)
The cell plate has matured into plasma mambranes and cell wall between two daughters
Differentiate between meiosis and mitosis
Meiosis:
* one round of DNA replication
* two rounds of cell division
* produces four haploid cells
Mitosis:
* one round of DNA replication
* one round of cell division
* produces two diploid cells
