bio 203w - the cell cycle

what is the most basic function of the cell cycle?

to replicate accurately the vast amount of DNA in the chromosomes and then segregate the DNA into genetically identical daughter cells so that each cell receives a complete copy of the entire genome

cells reproduce by _______ their contents and _______ in two in a process called the ________.

duplicating, dividing, cell cycle

early frog embryo cells have a cell cycle time of

30 minutes

yeast cells have a cell cycle time of

1.5-3 hours

intestinal epithelial cells have a cell cycle time of

about 12 hours

mammalian fibroblasts in culture have a cell cycle time of 

about 20 hours

human liver cells have a cell cycle time of

about 1 year

what are the four phases of the eukaryotic cell cycle?

m phase, g1 phase, s phase, g2 phase

the period between one m phase and the next is called ________.

interphase

_______ encompasses the remaining _____ phases of the cell cycle.

interphase, three

what happens during the s phase?

the cell replicates its DNA

the s phase is split into two gap phases called

g1 phase and g2 phase

what happens during the g1 phase and the g2 phase?

the cell continues to grow

at particular points in g1 and g2, the cell decides whether to _______ to the next phase or _____ to allow more time to prepare.

proceed, pause

what happens in the m phase?

the nucleus divides in a process called mitosis, then the cytoplasm divides in a process called cytokinesis

what generally happens during interphase?

a cell continues to transcribe genes, synthesize proteins, and grow in mass.

Together with S phase, G1 and G2 provide the time needed for the cell to _______ and to _____ its ________ organelles.

enlarge, duplicate, cytoplasmic

The cell-cycle control system plays a central part in the ______ of cell numbers in the tissues of the body; if the control system _________ such that cell division is excessive, ______ can result.

regulation, malfunctions, cancer

________ and _______ help to configure duplicated chromosomes for segregation.

cohesions, condensins

_________ tie together the two adjacent sister chromatids in each duplicated chromosome, forming large ______ that surround the sister chromatids, preventing them from coming apart, until the rings are broken late in ______.

cohesins, protein rings, mitosis

________ help coil each sister chromatid into smaller, more compact structure that can be more easily _______ during mitosis.

condensins, segregated

_______ act by forming ______ in the DNA.

condensins, loops

the _______ pulls the sister chromatids apart towards the _____ of mitosis.

mitotic spindle, end

after the duplicated chromosomes have ______, two complex _______ machines assemble in sequence to carry out the major mechanical processes that occur in the ________.

condensed, cytoskeletal, m phase

the mitotic spindle carries out ______ division, and in animal cells, the _______ divides the entire cell in two.

nuclear, contractile ring

two transient _____ structures mediate m phase in __________.

cytoskeletal, animal cells

the contractile ring consists mainly of ______ and _____ filaments arranged in a _____ around the _______ of a cell.

actin, myosin, ring, equator

as the contractile ring _______, it _____ the membrane ________, splitting the cell in two.

contracts, pulls, inward

what are the first five stages of the m phase?

prophase, prometaphase, metaphase, anaphase, telophase

before _______ or _____ begins, each chromosome has been _______ and consists of two identical sister chromatids that are held together along their length by _____ proteins.

nuclear division, mitosis, duplicated, cohesin

during mitosis, the cohesin proteins are ______, the sister chromatids split apart, and the chromosomes are pulled to opposite poles of the cell by the ________.

removed, mitotic spindle 

sister chromatids ____ at the beginning of ______, then the ________ pulls the separated sisters to opposite poles of the cell.

split, anaphase, mitotic spindle

duplication is necessary for the ________ to be able to help form the two ____ of the mitotic spindle, and it allows each daughter cell to receive its own centrosome.

centrosome, poles

centrosome duplication begins at the same time as ___________, and the process is triggered by the same ____ that initiate DNA replication.

DNA replication, Cdks

when the centrosome duplicates, both copies remain together as a _________ on one side of the _________.

single complex, nucleus

as mitosis begins, the two centrosomes separate, and each ______ a radial array of microtubules called ____. the two asters move to opposite sides of the nucleus to form the two poles of the mitotic spindle.

nucleates, asters

centrosome duplication begins at the start of ___ phase and is complete by the end of __.

s, g2

The centrosomes move ____, and the microtubules that interact between the two asters __________ preferentially to form a _____ mitotic spindle, with an ______ at each pole. When the nuclear envelope breaks down, the spindle microtubules are able to interact with the duplicated chromosomes.

apart, elongate, bipolar, aster

kinetochore

a protein complex associated with each sister chromatid

kinetochore microtubules are central players in ________. 

chromosome separation

what three main types of microtubules does a mitotic spindle contain?

astral microtubules, kinetochore microtubules, and non-kinetochore microtubules

what happens during prophase?

the duplicated chromosomes consisting of two closely associated sister chromatids, condense. outside the nucleus, the mitotic spindle assembles between the two centrosomes, which have begun to mover apart.

what happens during prometaphase?

the breakdown of the nuclear envelope, chromosomes now attaching to spindle microtubules via their kinetochores and undergo active movement

what happens during metaphase?

the chromosomes are aligned at the equator of the spindle, midway between the spindle poles. the kinetochore microtubules on each sister chromatid attach to opposite poles of the spindle.

what happens during anaphase?

the sister chromatids synchronously separate and are pulled slowly toward the spindle pole to which they are attached. the kinetochore microtubules get shorter, and the spindle poles also move apart, both contributing to chromosome segregation

what happens during telophase?

the two sets of chromosomes arrive at the poles of the spindle. a new nuclear envelope reassembles around each set, completing the formation of two nuclei and marking the end of mitosis. the division of the cytoplasm begins with the assembly of the contractile ring.

what happens during cytokinesis?

the cytoplasm is divided in two by a contractile ring of actin and myosin filaments, which pinches the cell into two daughters, each with one nucleus.

kinetochores assemble at the ___________ sequence.

centromere DNA

each _________ kinetochore binds _________ microtubules.

mammalian, 20-40

_________ proteins and __________ can direct the assembly of a functional ______ spindle in the absence of ________.

motor, chromosomes, bipolar, centrosomes

plant cells lack centrosomes, but

they have proteins that serve the equivalent of centrosomes

_________ triggers sister chromatid separation at ________.

proteolysis, anaphase

the cohesin linkage is destroyed by a _____ called _______.

protease, separase

Before _________ begins, this protease is held in an inactive state by an inhibitory protein called ______. At the beginning of anaphase, securin is targeted for destruction by ______ —the same protein complex, discussed earlier, that marks M-cyclin for degradation.

anaphase, securin, APC/C

once ______ has been destroyed, _______ is free to sever the ______ linkages.

securin, separase, cohesin

APC/C indirectly triggers the cleavage of the cohesins that hold sister chromatids together. It catalyzes the __________ and destruction of an inhibitory protein called securin, which blocks the activation of a proteolytic enzyme called separase.

ubiquitylation

When freed from securin, separase ________ the cohesin complexes, allowing the mitotic spindle to pull the sister chromatids apart. ______ was originally called the anaphase-promoting complex because of its central role in this process.

cleaves, APC/C

in ______, the sister chromatids are pulled toward opposite poles as the kinetochore microtubules depolymerize. the force driving this movement is generated mainly at the kinetochore.

anaphase a

in ________, the two spindle poles move apart as the result of two seperate forces: the elongation and sliding of non-kinetochore microtubules past one another pushes the two poles apart , and forces exerted on the outward pointing astral microtubules at each spindle pole pull the poles away from each other, toward the cell cortex.

anaphase b

what is the first visible sign of cytokinesis in animal cells?

a puckering and furrowing of the plasma membrane that occurs during anaphase

the _________________ is formed by the action of the contractile ring underneath the plasma membrane. 

cleavage furrow

how does the mitotic spindle dictate the position of the cleavage furrow?

during anaphase, the overlapping microtubules that form the central spindle recruit and activate proteins that signal to the cell cortex to initiate the assembly of the contractile ring at a position midway between the spindle poles

the contractile ring of animal cells is made of ______ and ________ filaments.

actin, myosin

Cell division in many animal cells is accompanied by large changes in cell shape and a decrease in the _______ of the cell to its neighbors, to the _______, or both.

adherence, extracellular matrix

cytokinesis in plant cells involves the

formation of a new cell wall

The assembly process is guided by a structure called the _______, which is formed by the remains of the microtubules at the equator of the old mitotic spindle.

phragmoplast

Small membrane-enclosed vesicles, largely derived from the ___________ and filled with _______ and ________- required for the cell wall matrix, are transported along the microtubules to the __________.

Golgi apparatus, polysaccharides, glycoproteins, phragmoplast

telomere specific cohesins are removed by

tankyrase

how do divisions during cleavage differ?

blastomeres don’t grow after cleavage, so cells are progressively smaller and cell divisions are much faster, taking less than one hour

why are cell divisions faster during cleavage?

due to the shortening or removal of the resting phases, g1 and g2

early cleavage

occurs quickly: stockpile of maternal effect gene products, mRNA and proteins

no embryonic transcription/translation

eventually, maternal reserves are depleted

mid blastula transition

the point at which the embryo begins to rely on its own synthesis processes

late cleavage

cell cycle slows down

growth/gap phases (g1 and g2) are added or elongated

synthesis of gene products needed