S2W6 - The Cell Cycle Flashcards
cell cycle
- conserved in all eukaryotes
- sequence of events where contents of a cell are duplicated and divided into two
state the whole order of the cell cycle
- interphase (G1, S, G2 phase)
- prophase
- prometaphase
- metaphase
- anaphase
- telophase
- cytokinesis (NB: partly occurs alongside half of anaphase and telophase)
observing animal cell division in culture
- cells do not divide at the same time
- when cells do divide, all cells follow the same stages in mitosis
3 broad cell cycle stages conserved in all eukaryotes
- cell growth and chromosome duplication
- chromosome segregation
- cell division
M phase
- nucleus and cytoplasm divide:
1. mitosis (nuclear division)
2. cytokinesis (cytoplasmic division)
interphase
period between cell divisions (metabolic activity, cell growth, repair)
- G1 phase
- S phase (synthesis)
- G2 phase
do all mature cells divide in multicellular organisms?
no; many mature cells do not divide
- eg terminally differentiated cells - nerve cells, muscle cells, red blood cells
- as they become differentiated, they lose the ability to divide
some cells only divide when given
an appropriate stimulus:
- eg when damaged, liver cells start to divide to replace damage tissue
examples of cells that normally divide on an ongoing basis
hematopoietic and epithelial stem cells
G0
cells that do not divide are in G0
- no cell division
- metabolically active, carry out cell function
3 checkpoints/transitions in the cell cycle
- start transition (G1 -> S)
- G2/M transition (G2 -> M)
- metaphase to anaphase transition (aka spindle assembly checkpoint)
what is the purpose of the cell-cycle control system?
to delay later events until the earlier events are complete
problems in checkpoints can cause
chromosome segregation defects
start transition
- decision to enter S phase
- is the environment favourable? eg sufficient nutrients, specific signal molecules
G2/M transition
- decision to enter mitosis
- is all DNA replicated? is all DNA damage repaired?
metaphase to anaphase transition?
- decision to pull duplicated chromosomes apart
- are all chromosomes properly attached to the mitotic spindle?
cell cycle progression is controlled by
molecular switches
how is entry into the next phase of the cell cycle ensured?
- triggered by cyclin-dependent protein kinases (Cdks)
- cyclin-Cdk complex is activated for entry, then inactivated (molecular switch)
entry into the M phase
M-Cdk (Idk activated by M cyclin) phosphorylates other regulatory proteins
how is entry into the next phase of the cell cycle paused?
by other regulators, if any answer to the questions is no:
- Cdk inhibitors block entry to the S phase
- inhibition of activating phosphatase (Cdc25) blocks entry to mitosis
- inhibition of APC/C activation delays exit from mitosis
interphase - G1 phase
centrosome duplication initiated and completed by G2
interphase - S phase
chromosomes replicated (decondensed)
- cohesions deposited to hold two sister chromatids together
interphase - G2 phase
- by the end of G2, the replicated chromosomes are dispersed and tangled
- need to reorganise and condense for mitosis
3 main steps of prophase
- replicated chromosomes condense
- centrosome duplication
- mitotic spindle assembly
prophase - replicated chromosomes condense
chromosome condensation (chromatids compacted) and sister-chromatid resolution (separable units)
- cohesins removed from chromosome arms, but not from centromeres
- condensins condense DNA in each sister chromatid
- sister chromatids are resolved but remain associated at the centromere by cohesins
how are microtubules arranged in a non-dividing cell?
- in a radial pattern
- plus ends radiating out
- minus ends stabilised at the MTOC (centrosome)
what are the two conditions required for mitotic spindle assembly to start during prophase?
- disassembly and reassembly of microtubules (to go from radiating from a single centrosome to radiating from two)
- duplicated centrosomes
pair of centrioles in the centrosome
- organised at right angles to each other
- composed of nine fibrils of three microtubules each
centrosome structure
- centrosome matrix surrounds the pair of centrioles
- contains y-tubulin ring complexes (y-TuRCs), which are nucleating sites to assemble new microtubules
prophase - centrosome duplication
- centrosome duplicated once per cell cycle during interphase; initiated in G1 and completed by G2
- each centriole in the pair of centrioles in centrosome serves as a site for assembly of a new centriole
- duplicated centrosomes form poles of mitotic spindle
prophase - mitotic spindle assembly
- mitotic spindle assembly starts in prophase (M phase)
- requires microtubule dynamics (disassembly and assembly)
- duplicated centrosomes separate
- radial array of microtubules extend out from each to position centrosome, and will become the two spindle poles
nuclear envelope breakdown
occurs at the boundary between prophase and prometaphase
- phosphorylation of lamins and nuclear pore proteins triggers disassembly of nuclear envelope into small membrane vesicles
nuclear lamina
- meshwork of interconnected nuclear lamina proteins
- form a two dimensional lattice on the inner nuclear membrane
pro metaphase
- nuclear envelope is now disassembled
- mitotic spindle assembly can now be completed
- kinetochore microtubules in the mitotic spindle attach to duplicated chromosomes
- chromosome movement begins
mitotic spindle assembly and function requires
- microtubule dynamics (disassembly and assembly)
- microtubule motor protein activity (kinesics, cytoplasmic dynein)
3 microtubules involved in the mitotic spindle
- astra microtubules: anchored at the centrosome and help position the mitotic spindle. cytoplasmic dynein (motor protein) attached to PM and moves to minus ends, thus pulling centrosome towards PM
- non-kinetochore microtubules: cross-linked microtubules throughout the mitotic spindle. microtubule-associated proteins (kinesin-5 and others) hold the microtubules together
- kinetochore microtubules: attach duplicated chromosomes to the spindle poles
kinesin-5
walks towards the plus end of the non-kinetochore microtubules that it associates with, which have opposite orientations. this pushes the microtubules apart, pushing the centrosomes apart.
describe how kinetochore microtubules attach to chromosomes
- kinetochores are located at the centromeres of chromosomes
- there is one kinetochore for each sister chromatid in the duplicated chromosome
- microtubules from both spindle poles must attach to kinetochores of sister chromatids
- generates equal tension on both sides to line up chromosomes at equator of spindle
connecting protein complexes
- bind to sides of microtubule near plus end
- the exposed plus end of the microtubule allows for growing or shrinking for chromosome movement so it can be placed at the centre of the cell before separation
metaphase
- all chromosomes are aligned on the metaphase plate (equator of the spindle)
- microtubule dynamos continue to maintain the metaphase spindle (tubulin flux)
tubulin flux through microtubules
to maintain the metaphase spindle, there is a continuous:
- addition of tubulin subunits at plus end
- removal of tubulin subunits at minus end
- length of kinetochore microtubules does not change
this is an example of treadmilling
how was tubulin flux confirmed?
- a small amount of fluorescent tubulin (‘speckles’) was added to observe microtubule flux
- a time lapse video microscopy was used to follow the fluorescent tubulin movement
- added at plus end
- depolymerises are removing the tubulin heterodimers from the minus end
metaphase-anaphase transition (spindle assembly checkpoint)
anaphase does not start until all the chromosomes are aligned on the metaphase plate
anaphase
separation of sister chromatids:
- separate activated, which then cleaves the cohesin complex
anaphase A
- kinetochore microtubules are shortened (loss of tubulin at both ends) due to depolymerisation
- sister chromatids are pulled apart towards opposite poles
anaphase B
- kinesin causes a sliding force between non-kinetochore microtubules from opposite poles, pushing the poles apart
- cytoplasmic dynein generates a pulling force at the cell cortex, dragging the two poles apart
- microtubule growth at the plus ends of non-kinetochore microtubules also helps push the poles apart
telophase
- chromosomes are now separated into two groups, one at each spindle pole
- nuclear envelope reassembly takes place: there is dephospho rylation of nuclear pore proteins and lamins by phosphatase, causing the envelope lamina, and pores to reform
- mitotic spindle disassembles
- chromosomes decondense
- end of mitosis!
- contractile ring for cytokinesis is being assembled (starts in anaphase)
cytokinesis in animal cells
- contractile ring divides the cytoplasm in two
- contractile force by contractile ring brings cell membrane in as contractile ring becomes smaller
- contractile ring disassembles once there are two daughter cells
structure of contractile ring
assembled from actin and myosin filaments at the cleavage furrow (midway between spindle poles and underneath cell membrane)
describe how actin and myosin II motor proteins are involved in causing contractile force
- Actin filaments form a band at the division site.
- Myosin II uses ATP to slide actin filaments towards each other; myosin moves toward plus end, pushing actin towards respective minus ends.
- This sliding action constricts the ring and pulls in the membrane until cell division is complete
role of non-kinetochore microtubules in animal cell cytokinesis
send a signal to the plasma membrane at the plane of cleavage to say this is where we need the contractile ring to assemble
how does mitosis in a plant cell differ from that in an animal cell?
- very similar but no centrosome (it has another mechanism to form the mitotic spindle)
- cytokinesis in the plant cell is different due to cell wall
telophase in plant cells
- chromosomes separated into two sets
- phragmoplast starts to form
what is the phragmoplast?
- specific structure to form cell plate
- has microtubules, actin filaments, vesicles from Golgi
cytokinesis in plant cells
- nuclear envelope reassembled, chromosomes decondensed
- cell plate forms
- this is a transient membrane compartment (vesicles from Golgi fuse together) to divide into two
G1 in plants
cell plate has matured into plasma membranes and cell wall between two daughter cells
compare cell division in meiosis and mitosis
meiosis:
- one round of DNA replication (chromosome duplication)
- two rounds of cell division
- produces 4 haploid cells
- homologous chromosomes are paired at the metaphase plate
mitosis:
- one round of DNA replication (chromosome duplication)
- one round of cell division
- produces 2 diploid cells
- homologous chromosomes are not paired at the metaphase plate
meiosis cell division rounds 1 and 2
Round 1: homologous chromosome are segregated into two daughter cells (sister chromatids remain attached)
Round 2: sister chromatids are segregated, producing 4 haploid cells
give an example of how studying cell division in a multicellular organisms helps us understand the complexities of the cell cycle
biochemical studies:
- injecting cytoplasm from fertilised Xenopus eggs into Xenopus oocytes led to the discovery of cyclin-dependent protein kinases
