AP Biology
Notes: Mitosis
Mitosis:
* Unicellular organisms, the division of one cell to form two
reproduces an entire organism
* In multicellular organisms, cell division allows:
Growth and development from the
fertilized egg
Replacement of damaged or dead cells
Cell division
* Precisely replicates its DNA
* Allocates the two copies of DNA to opposite ends of the cell
* Separates into two daughter cells containing identical
hereditary information
* Usually proceeds in to sequential steps: nuclear division
(mitosis) and division
of the cytoplasm (cytokinesis). Not all cells undergo
cytokinesis following mitosis
M phase, the shortest part of the cell cycle and the phase during which the
cell
divides, includes:
1. Mitosis-Division of the nucleus
2. Cytokinesis-Division of the cytoplasm
Interphase, the nondividing phase includes most of the a cell's growth
and
metabolic activates
* Is about 90% of the cell cycle
* Is a period of intense biochemical activity during which the
cell grows and
copies its chromosomes in preparation for cell division
Consists of three periods
1. G1
phase= First growth phase (G stands for gap
2. S
phase-synthesis phase occurs when DNA is synthesized as
chromosomes are duplicated (S stands for synthesis)
3. G2
phase- second growth phase
Mitosis is a continuous process, but for ease of description, mitosis is usually
divided
into five stages: prophase, prometaphase, metaphase, anaphase, and
telophase
G2 of interphase:
A well-defined nucleus bounded by a nuclear envelope
One or more nucleoli
Two centrosomes adjacent to the nucleus (formed earlier by
replication of a single
centrosome)
In animals, a pair of centrioles in each centrosome
In animals, a radial microtubular arraay (Aster) around each
pair of centrioles
Duplicated chromosomes that cannot be distinguished
individually due to loosely
packed chromatin fibers (Chromosomes were duplicated
earlier in S phase).
Prophase
In the nucleus:
Nucleoli disappear
Chromatin fibers condense into discrete, observable
chromosomes composed
of two identical sister chromatids joined at the centromere
In the cytoplasm:
Mitotic spindle forms. It is composed of microtubules
between the two
centrosomes or microtubule-organizing centers
Centrosomes move apart, apparently propelled along the
nuclear surface by
lengthening of the microtubule bundles between them.
Prometaphase
Nuclear envelope fragments, which allows microtubules to
interact with the
highly condensed chromosomes
Spindle fibers (bundles of microtubules) extend from each
pole towards the
cell's equator.
Each chromatid now has a specialized structure, the kinetochore,
located at
the centromere region.
Kinetochore microtubules become attached to the
kinetochores and put the
chromosomes into agitated motion
Nonkinetochore 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
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 form opposite ends of there parent cell
Entire structure formed by nonkinetochore microtubules plus
kinetochore microtubules
is called the spindle.
Anaphase
Anaphase is characterized by movement It begins when parried
centromeres of
each chromosome move apart.
*Sister chromatids split apart into
separate chromosomes and move towards
opposite poles of the cell
*Because kinetochore firs 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
Telophase and Cytokinesis
Nonkinetochore microtubules further elongate
the cell
Daughter nuclei begin to form at the
two poles
Nuclear envelopes form around the
chromosomes from fragments of the parent
cell's nuclear envelope and portions
of the endomembrane system.
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 two genetically identical nuclei,
is complete Cytokinesis has begun and
the appearance of two separate
daughter cells occurs shortly after
mitosis is complotted.
The mitotic spindle distributes chromosomes to daughter cells
Many of the events of mitosis depend on the formation of a mitotic
spindle.
The mitotic spindle forms in the cytoplasm form microtubules and
associated proteins.
*Microtubules of the cytoskeleton are
partially disassembled during spindle formation
*Spindle
microtubules are aggregates of tow proteins, α and β-Tubulin
*The assembly of spindle
microtubules begins in the centrosome or microtubule
organizing center.
The chronology of mitotic spindle formation is as follows:
Interphase: The centrosome replicates to form two
centrosomes located just outside
the nucleus
Prophase: The two centrosomes move farther apart.
Spindle microtubules radiate form the
centrosomes, elongating at the
end ways from their centrosome.
Kinetochore microtubules from one pole may attach to a kinetochore,
moving the
chromosome toward that pole. This movement is checked when microtubules
form the
opposite pole attach to the chromosome's other kinetochore.
The chromosome oscillates back and forth until it stabilizes and alights at the
cell's midline
Microtubules can remain attached to a kinetochore only if there is
opposing tension
from the other side. It is this opposing tension that
stabilizes the microtubule-
kinetochore connection and allows the proper alignment and movement
of
chromosomes at the cell's midline.
Metaphase. All the duplicated chromosomes align on the cell's
midline, or
metaphase plate
Anaphase. The chromosome's centromeres split and the sister chromatids
move
as separate chromosomes toward opposite ends of the cell. The
kinetochore
and nonkinetochore microtubules direct the segregation
of the chromosomes The kinetochore microtubules shorten during anaphase
by depolyerizing at
their kinetochore ends; pulling the chromosomes poleward.
The mechanism of this interaction between kinetochores and microtubules
may
involve microtubule-walking proteins similar to dynein that "walk" a
chromosome
along the shortening microtubules
Cytokinesis
First a cleavage furrow forms as a shallow groove
in the cell surface near the
old metaphase
A contractile ring of actin microfilamnts forms on the
cytoplasmic side of the furrow
this ring contracts until it pinches the parent cell in two.
Finally the reaming mitotic spindle breaks the the two cells
become completely separate.
Regulation of the Cell cycle
Human skin cells dived frequently
Liver cells only dived in appropriate situations, such as wound repair
Nerve, muscle and other specialized cells do not dived in mature humans.
The cell-cycle control system has checkpoints in the G1, G2,
and M phases of
the cell cycle.
For many cells the G1 checkpoint (known as
the "restriction point" in the
mammalian cells) is the most important
* A go-ahead signal usually
indicates that the cell will complete the cycle and divide
* In the absence of a go-ahead
signal, the cell may exit the cell cycle switching to
the nondividng state called G0
phase.
* Many cells other the human
body are the G0 phase. Muscle and nerve cells will
remain in G0 until they
die. Liver cells may be recruited back to the cell cycle
under certain cues, such as growth
factors.
Kinase Control:
*Protein kinases are enzymes that catalyze the transfer of a phosphate group
from
ATP to a target protein.
*Phosphorylation, in turn, induces a conformational change that either activates
or
inactivates a large protein.
*These regulatory proteins are named cyclins, because their concentrations
change
cyclically during the cell cycle.
*Protein kinases that regulate cell cycles are cyclin-dependent kinases (Cdks);
they
are active only when attached to a particular cyclin
*Even though the Cdk concentration stays the same throughout he cell cycle,
it
activity changes in response to the changes in cyclin concentration
*Cyclin's rhythmic changes in concentration regulate MPF, so as cyclin concentration
rises and falls, the amount of active MPF changes in a similar way
MPF
Cyclin is produced at a
uniform rate throughout the cell cycle, and it 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 initiates the
following process:
*Chromosome condensation during prophase
*Nuclear envelope dispersion during prometaphase
*MPF
activates proteolytic enzymes
*The
proteolytic enzymes 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.