12 Mitosis
12.1 Most cell division results in genetically identical daughter cells
Key roles of cell division
- Prokaryotes/unicellular eukaryotes: cell division gives rise to new organisms
- Multicellular eukaryotes: cell division enables organisms to develop from the fertilized
egg + function in renewal and repair of cells that die
- Most crucial function of cell division: the distribution of identical genetic material to
two daughter cells
Cellular organization of the genetic material
- Genome: a cell’s DNA, its genetic information -> each daughter cell needs to end up
with the same, complete genome
- DNA molecules (with a lot of genes: the units of information that specify an organism’s
inherited traits) are packaged into chromosomes (one long, linear DNA molecule)
- Chromatin: the entire complex of DNA and proteins that is the building material of
chromosomes
- Somatic cells (all body cells except for reproduction cells): 46 chromosomes; gametes
(reproductive cells: sperm or eggs): 23 chromosomes
Distribution of chromosomes during eukaryotic cell division
After DNA replication, the chromosomes condense -> each chromatin fiber becomes coiled
and folded -> each duplicated chromosome consists of two sister chromatids (joined copies
of the original chromosome), which are attached in a sister chromatid cohesion by cohesins
-> two sister chromatids separate and move into two new nuclei (mitosis) -> cytokinesis: the
division of cytoplasm
-> centromere: region made up of repetitive sequences in the chromosomal DNA where the
chromatid is attached most closely to its sister chromatid; mediated by proteins.
-> arm: portion of a chromatid to either side of the centromere
12.2 The mitotic phase alternates with interphase in the cell cycle
Phases of the cell cycle
Cell cycle: the life of a cell from the time it is first formed during division of a parent cell until
its own division into two daughter cells
- Mitotic (M) phase: mitosis (distribution of chromosomes into two daughter nuclei)+
cytokinesis (division of cytoplasm, producing two daughter cells)
- Interphase
- G1 phase: metabolic activity and growth -> duration is most variable in organisms
(some cells divide infrequently or not at all)
, - S phase: duplication of chromosomes (DNA synthesis), metabolic activity, growth
- G2 phase: preparation for cell division, metabolic activity and growth
Mitosis in the animal cell
G2/Interph Nuclear Nucleus Two Chromosomes
envelope contains centrosomes cannot be seen
ase encloses nucleoli have formed
nucleus (largest by
structure in duplication→
nucleus that organize
produces the microtubules of
ribosomes) spindle
Prophase Chromatin Nucleoli Each Mitotic spindle Centrosomes
fibers become disappear duplicated forms move away
tightly coiled chromosome from each
appears as two other
sister
chromatids
Prometap Nuclear Microtubules Chromosomes Kinetochore Microtubules Nonkinetochore
envelope invade nuclear more has formed at attach to microtubules
hase fragments area condensed centromere of kinetochores interact with
each the opposite
chromatid pole,
lengthening the
cell
Metaphas Centrosomes Chromosomes Kinetochores
at opposite have arrived at are attached to
e poles of cell metaphase kinetochore
plate microtubules
(centromeres) coming from
opposite poles
Anaphase Shortest stage Cohesin Daughter Cell elongates Two ends of
proteins chromosomes as cell have
cleaved -> move to nonkinetochore identical and
sister opposite sides microtubules complete
chromatids of cell; lengthen collections of
separate centromeres chromosomes
are pulled
Telophase Two daughter Nucleoli Chromosomes Remaining Mitosis is
nuclei form; reappear become less spindle complete
nuclear condensed microtubules
envelope forms are
depolymerized
Cytokinesi Daughter cells Cleavage
appear furrow pinches
s because of cell in two
division of
cytoplasm
Centrosomes: regions in animal cells that organize the microtubules of the spindle ->
contains two centrioles
Mitotic spindle: centrosomes + microtubules that extend from them
,Metaphase plate: a plane that is equidistant between the spindle’s two poles
The mitotic spindle: a closer look
In order for mitosis to occur, a mitotic spindle, a structure that consists of fibers made of
microtubules and associated proteins, needs to form. Assembly of spindle microtubules
starts in centrosomes: a single centrosome duplicates during interphase -> centrosomes
move apart until they are at opposite ends of the cell. Asters (a radial array of short
microtubules) extend from each centrosome.
→ spindle includes: centrosomes, spindle microtubules and the asters
Each of the two sister chromatids has a kinetochore (structure made up of proteins that have
assembled on specific sections of DNA at each centromere) → attach to spindle
microtubules -> tug-of-war (motor proteins) → aligned on metaphase plate → separase
separates the sister chromatids -> chromosomes move toward opposite ends of cell →
cytokinesis
Cytokinesis: a closer look
- In animal cells: Cleavage: the process by which cytokinesis occurs → first sign:
cleavage furrow: shallow groove in the cell surface near the old metaphase plate ->
actin microfilaments interact with myosin molecules -> contractile ring of actin
microfilaments contracts -> parent cell is pinched in two
- In plant cells: a cell plate is created during telophase from vesicles that move along
microtubules to the middle of the cell → cell plate enlarges until there are two
daughter cells
Binary fission in bacteria
Binary fission: a type of reproduction in which the cell grows to twice its size and then divides
into two cells + asexual reproduction in single-celled prokaryotes → process initiated by
replication of specific place on the chromosome (origin of replication) → cell elongates →
proteins cause pinching inward → two daughter cells
The evolution of mitosis
Some of the proteins involved in bacterial binary fission are related to eukaryotic proteins that
function in mitosis → The process of binary fission gave rise to mitosis. Some methods of cell
division by modern-day species could represent intermediate stages from binary fission to
mitosis.
12.3 The eukaryotic cell cycle is regulated by a molecular control system
The cell cycle control system
Cell cycle control system: a cyclically operating set of molecules in the cell that both triggers
and coordinates key events in the cell cycle -> cell cycle is regulated by both internal and
, external signals; checkpoint: control point where stop and go-ahead signals can regulate the
cycle (G1, G2, M)
- The cell cycle clock: regulatory molecules:
- Kinases: activate/inactive other proteins by phosphorylation -> to be activated, need
to be attached to cyclin (cyclin-dependent kinases/cdks)
- Cyclins: cyclically fluctuating concentration in the cell
- Cyclin-cdk complex: MPF (M phase promoting factor)
The presence of MPF initiates mitosis by phosphorylation; MPF contributes to other
processes as well (chromosome condensation; spindle formation) → MPF switches
itself off by initiating a process that causes the destruction of its own cyclin -> cdk is
inactive until it can combine again with a cyclin → fluctuation of cyclin-cdk complexes
control the stages of the cell cycle
- Stop and go signs: internal and external signals at the checkpoints: Most important
checkpoint is the one at G1: if this checkpoint isn’t passed → G0 state (non dividing
state). Example of internal cue, M checkpoint: only when kinetochores of all the
chromosomes are properly attached to the spindle -> regulatory protein complex
activated. External cues: Essential nutrients and growth factors need to be there to
stimulate cell division
Density-dependent inhibition: a phenomenon in which crowded cells stop dividing
Anchorage dependence: to divide, cells must be attached to something
→ signal is sent that inhibits or promotes cellular division
Loss of cell cycle controls in cancer cells
Possible explanations for the uncontrolled growth of cancer cells:
- They do not need growth factors to grow and divide
- They make the required growth factor themselves
- Abnormal cell cycle control system
Cells in culture that acquire the ability to divide indefinitely have undergone a process called
transformation, causing them to behave like cancer cells. If and when cancer cells stop
dividing, they do so at random points in the cycle. Cancerous cells also prevent apoptosis
from happening when a mistake has occurred when replicating DNA. Benign tumors do not
cause serious issues, whereas malign tumors do (this tissue is able to spread in the entire
body). Metastasis is the spread of cancer to locations distant from their original site.
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