CH. 9: The Cell Cycle

G1 checkpoint

- a go ahead signal usually indicates that the cell will complete the cycle and divide - in the abscence of this signal, the cell may exit the cell cycle, switching to the nondividing state called G0 phase - many cells of the human body are in teh G0 phase- muscle and nerve cells will remain n G0 untill they die, liver cell smay be recruited back to teh cell cycle under certain cues such as growth factors

binary fision

- a process during which a bacteria replicates its chromosome and equally distributes copies between the 2 daughter cells. - the chromosome is replicated. each copy remains attached to PM at adjacent sides by special proteins - between the attachment sites the membrane grows and separates the 2 copies of the chromosome - the bacterium grows to about twice its initial size,the PM pinches inward (tubulin) - a cell wall forms across across the bacterium between the 2 chromosomes, dividing the original cell into 2 daughter cells

cyclin dependent kinases

- active only when attached to particular cyclin - CDK conc stays the same throughout the cell cycle - its activity changes in response to the changes in cyclin concentration

cyclins

- also control cyclical changes - their concentrations change cyclically during the cell cycle

prokayotes

- bacteria - smaller than eukaryotes - came before eukaryotes - evidence of evolution of mitosis from bacterial cell division includes the relatedness of several proteins involved in both types of division and the possible intermediate stages seen in some modern unicellular algae and fungi - most genes in prokaryotes are found on a single circular chromosome

cytokinesis

- begins during telophase of mitosis - animal cells: cleavage - a cleavage furrow forms as a shallow groove in the cell surface near the old metaphase plate - a contractile ring of actin microfilaments forms on the cytoplasmic side of the furrow... this ring contracts until it pinches the parent cell in two - finally, the remaining mitotic spindle breaks and the two cells become completely separate

plant cell cytokinesis

- cell plate formation across the plant cell midline - Golgi-derived vesicles move along the microtubules to the cell's center where they fuse in a disc-like plate - additional vesicles, fuse around the edge of the plate, expanding it laterally until its membranes touch and fuse with the existing plant cell PM - a new cell wall forms as cellulose is deposited between the 2 members of the cell plate

metaphase

- centrosomes are positioned at opposite poles of the cell - chromosomes move to the metaphase plate, the plane equidistant between the spindle poles - centromeres of all chromosomes are aligned on the metaphase plate - the long axis of each chromosome is roughly at a right angle to the spindle axis - kinetochores of sister chromatids face opposite poles, so identical chromatids are attached to kinetochore fibers radiating from opposite ends of the parent cell - entire structure formed by nonkinetochore microtubules plus kinetochore microtubules is called the spindle

anaphase

- characterized by movement - begins when paired centromeres of each chromosome move apart - sister chromatids split apart into separate chromosomes and move towards opposite poles of the cell - because kinetochore fibers are attached to the centromeres, the chromosomes move centromere first in a "V" shape - kinetochore microtubules shorten at the kinetochore end as chromosomes approach the poles - simultaneously, the poles of the cell move farther apart, elongating the cell - at the anaphase, the two poles have identical collections of chromosomes

chromosome

- consists of various proteins which serve to maintain teh structure of the chormosome or are involved with the expression of genes, DNA replication, and DNA repair

density dependent inhibition

- cultured cells stop dividing when they form a single layer on the container's inner surface - if some cells are removed, those bordering the open space divide again untill the vacancy is filled - consequence of the fact that quantities of nutrients and growth regulators may be insufficient to support cell division

MPF

- example of cyclin dependent kinases - control the cell's progress through G2 checkpoint - cyclin's rhythmic changes in concentration regualte MPF activity, and thus act as a mitotic clock that regulate the sequential changes in a dividing cell

external cues- chemical factors

- if essential nutrients are left out of the culture, cells will not divide - specific regulatory substances called growth factors are necessary for most cultured mammalian cells to divide, even if all other conditions are favorable. - differnet cell types respond to different growth factors

MPF activating

- in later half of mitosis, MPF activates proteolytic enzymes which destroy cyclin which leads to the reduction of MPF activity (the cdk portion of MPF is not degraded). - the proteolytic enzymes also are involved in driving the cell cycle past the M-phase checkpoint which controls the onset of anaphase - continuing cyclin synthesis raises the conc. again during interphase and the newly synthesized cyclin binds to CDK to form MPF and mitosis begins again

internal cues

- kinetochores provide internal cues that signal the M-phase checkpoint about he status of chromosome-spindle interactions - all chromosomes must be attached to spindle microtubules before the M-phase checkpoint allows the cycles to proceed to anaphase... this ensures that daughter cells do not end up with missing or extra chromosomes

molecular control system

- molecular signaling system which cyclically switches on the appropriate parts of the cell-cycle machinery and then switches them off - consists of cell-cycle molecular clock and a set of checkpoints, or switches, that ensure that appropriate conditions have been met before the cycle advances - when control system malfunctions, cancer may result

interphase

- nondividing phase, includes most of cell's growth and metabolic activities - 90% of cell cycle - intense biochemical activity during which the cell grows and copies its chromosomes in preparation for cell division - has G1, S, G2

telophase (during process)

- nonkinetochore microtubules further elongate the cell - daughter nuclei begin to form at the 2 poles - nuclear envelope form around the chromosomes from fragments of the parent cell's nuclear envelope and portions of the endomembrane system - nucleoli reappear - chromatin fiber of each chromosome uncoils and the chromosomes become less distinct (by the end of telophase) - mitosis, the equal division of one nucleus into 2 genetically identical nuclei, is complete. - cytokinesis has begun and the appearance of 2 separate daugher cells occurs shortly after mitosis is completed

prometaphase

- nuclear envelope fragments, which allows microtubules to interact with the highly condensed chromosomes - spindle fibers extend from each pole toward the cell's equator - each chromatid now has a speicalized structure, the kinetochore, located at the centromere region - kinetochore microtubules radiate from each centrosome toward the metaphase plate without attaching to chromosomes; nonkinetochore microtubules radiating from one pole overlap with those from the opposite pole

Prophase

- nucleoli disappear - chromatin fibers condense into discrete, observable chromosomes, composed of 2 identical sister chromatids joined at the centromere - mitotic spindle forms; it is composed of microtubules between the 2 centrosomes or microtubule-organizing centers - centrosomes move apart, apparently propelled along the nuclear surface by lengthening of the microtubule bundles between

cyclin production

- produced at uniform rate throughout cell cycle - accumulates during interphase - cyclin combines with CDK to form active MPF, so as cyclin concentration rises and falls, the amount of active MPF changes in a similar way - MPF phosphorylates proteins that participate in mitosis and initiatie chromosome condensation during prophase and nuclear envelope dispersion during prometaphase

M-phase

- shortest part of cell cucle and phase during which cell divides... includes mitosis (division of nucleus) and cytokineis (division of cytoplasm)

checkpont

- signals registered at the checkpoint report the status of the various cellular conditions - checkpoints integrate a variety of internal (intracellular) and external (extracellular) information - for many cells, the G1 checkpoint, is the most important

metastasis

- spread of cancer cells beyond original sites

protein kinases

- the ordered sequence of cell cycle events is synchronized by rhythmic changes in teha ctivity of certain protein kinases - these are enzymes that catalyze the transfer of a phosphate group from ATP to a target protein - phosphorylation, then induces a conformational change that either activates or inactivates a target protein - changes in target protein affect the progression through the cell cycle

anchorage dependence

- to dicide, normal cells must adhere to a surface - anchorage is signaled to the cell cycle control system via pathways involving membrane proteins and elements of the cytoskeleton that are linked to them - abnormal cancer cell so not exhibit density or anchorage dependence

G2 cell

- well defined nucleus bounded by nuclear envelope - one or more nucleoli - 2 centrosomes adjacent to nucleus (formed earlier by replication of a single centrosome) - in animals, pair of centrioles in each chromosome - in animals, a radial microtubular array (aster) around each pair of centrioles - duplicated chromosomes that cannot be distinguished individually due to loosely packed chromatin fibers

cell cycle check points

G1, G2, M

interphase chromosome

loosely folded

mitotic phase chromosome

tightly folded

chromatin

very coiled