Transcript cell division: mitosis - College of the Atlantic

cell division: mitosis
biology 1
• Why cells divide and what they have to
do to accomplish this
• Division in prokaryotes
• Division in eukaryotes
– Chromosone structure
– Phases of the cell cycle
• Cell growth factors
– Cancerous growths
Why divide?
• Ensures persistence of genome
– Precisely replicates DNA
– Equally distributes DNA to opposite end of cell
– Seperates into two identical daughter cells
• Strategy to counter loss of SA:Vol ratio as cell
grows larger
• Permits growth and development of a
multicellular organism
• Allows replacement of damaged or dead cells
Genome = total endowment of DNA unique to each species
• In prokaryotes, cell division is through binary
fission
• In eukaryotes, division is by mitosis. For
example:
The human life cycle
Individual inherits 46
chromosones, 23
from each parent
Mitosis produces
genetically identical
daughter cells.
Process is
responsible for
growth, development
and repair
Meiosis in the gonads
halves the chromosone
number
Sperm cell
(23 chromosones)
Zygote
(46 chromosones)
Fertilization restores the chromosone
number to 46
Ovum
(23 chromosones)
Prokaryotes divide by binary fission
• Most genetic material incorporated into a single circular
chromosone made of double stranded DNA and
associated proteins
• Contains 1/1000 of eukaryote dna: still, highly folded and
packed into cell
• Binary fission
– Chromosone replicates into identical loops, each attached to the
plasma membrane at adjacent sites
– Between attachment sites the membrane grows and separates
the two copies of the chromosone
– Bacterium grows to twice its size and cleavage furrow develops
– Cell wall develops across bacterium between the two
chromosones
Cell division in eukaryotes
• Chromosones consist of a DNA-protein
complex called chromatin. Proteins include
histones that aid in coiling of nucleic
material in dense, visible chromatids
• Mitosis duplicates chromosones to pairs of
sister chromatids (duplication is very
precise: only 1 error in 100,000) and
sends each replication to opposite poles of
the cell
• Mitosis may be followed by cytokinesis
The cell cycle
Interphase
• 90% of time spent in
Interphase
– G1 = first growth phrase
– S = duplication of DNA
– G2 = second growth phase
S-phase
G1
G2
• 10% in dividing, or M
phase
– Mitosis - division of nucleus
– Cytokinesis
Mitosis
M-phase
• Characterized by
G2 Interphase
– Well defined nucleus bound by nuclear
envelope
– Nucleoli
– A pair of centrosomes adjacent to nucleus
(division occurs in S-phase
• (in animals) a pair of centrioles in each
centrosome
• (in animals) a microtubular array (aster) around
each centrosome
– Duplicated chromosones cannot be
distinguished (chromatin too loosely
packed)
Prophase
• Characterized by
– Disappearance of nucleoli
– Chromatin fibers condense into discrete
chromosones composed of two identical
sister chromatids joined at a centromere
– Formation of mitotic spindle, composed of
microtubules between the two
centrosomes
– Centrosomes move apart, migrating across
surface of nuclear envelope
Prometaphase
• Characterized by
– Nuclear envelope fragments and dissolves
– Spindle fibers extend from each pole
towards the cell equator
– Each chromatid has a specialized center
called the kinetochore, located at the
centromere
– Kinetochore microtubules attach to the
kinetochores
– Non-kinetochore microtubules from each
centrosome overlap each other
Metaphase
• Characterized by
– Centrosomes are at opposite poles of the cell
– Chromosones migrate to the metaphase plate
– Centromeres of all chromosones are aligned on
metaphase plate
– Kinetochores of sister chromatids face opposite
poles, so that identical chromatids are attached
to kinetochore fibers radiating from opposite
ends of the parent cells
– Entire structure formed by kinetochore and
non-kinetochore fibers is termed the spindle
Anaphase
• Characterized by
– Sister chromatids split apart into separate
chromosones, moving to opposite ends of the cell
(depolymerization of microtubules at kinetochore
end)
– Chromosones move centromere-first, moving
chromatids in a v-shape
– Poles of cell move further apart, elongating cell
At the end of anaphase, the two poles of the
cell have identical collections of chromosones
Telophase
• Characterized by
– Non-kinetochore microtubules further
elongate the cell
– Daughter nuclei begin to form at the two poles
– Nuclear envelope forms around chromosones
from fragments of parent cell’s nuclear
envelope
– Nucleoli reappear
– Chromatin protein uncoils and chromosones
become less distinct
Cytokinesis
• (in animal cells) Cleavage furrow forms and
pinches off the two daughter cells
– Done by a contractile ring of microtubules on
cytoplasm side of membrane
• (in plant cells) a cell plate (old metaphase
plate) grows between the two daughter cells.
– Cell membrane grows either side along length of
cell plate, which becomes the cell wall
By end of cytokinesis
– Two separate daughter cells with genetically
identical nuclei
Control of cell division
• Cues include
– Growth factors may bind with membrane receptors
to promote division
– Density dependent inhibition (affected by nutrients
and space)
– G1 phase has a ‘point of no return’, termed the
restriction point, passed which a cell must go into
the s-phase
• If a cell doesn’t go past restriction point, goes into G0
phase - stasis
– Cell size: ratio of cytoplasm to genome ration must
be high enough to allow cell to go past restriction
point
The mitotic clock
• Still mostly unknown process: the onset of Sphase appears to commit cell to G2 and m-phase
• Cell cycle events are synchronized by rhythmic
changes in regulatory proteins called protein
kinases that catalyze the phosphorylation of a
target protein by ATP
• Phosphorylation activates or inhibits the target
protein’s activity
• Cyclical changes in kinase activity are controlled
by a second group of regulatory proteins called
cyclins
• Protein kinases that regulate cell cycles
are cyclin-dependent kinases (Cdks)
• Cdk concentration stay the same
throughout the cell cycle. However, its
activity varies
• For example active MPF (maturation
promoting factor)
– Phosphorylates chromatin proteins,
causing chromosones to condense in
prophase
– Phosphorylation of nuclear envelope in
prometaphase promotes fragmentation
• Cyclin, produced throughout the cell cycle,
accululates during interphase
• Cyclin combines with cdk to make active
MPF, which peaks in concentration with the
peak in cyclin
• MPF initiates mitosis
• At the end of mitosis, MPF activates an
enzyme that destroys cyclin
mitosis
Interphase
Relative
concentration
time
mitosis
Interphase
mitosis
Cancers - cells lacking division control
• Cancerous cells do not respond to standard cellular
controls, growing and dividing until nutrients are
exhausted
• Cells that become cancerous are said to have
transformed
• Immune system usually destroys such cells, but if
not, cancerous cells coalesce to tumors
• If cells remain at the tumor, then the tumor is benign
• If cells spread out, then tumor is malignant - spread is
called metastasis
• Cause of cancerous cells is probably caused by
alteration of genes that control cell division