Mitosis
Ch. 10
Why Divide?
• Growth of organism
• Repairs
• Reproduction
• Genetic Variation
2 Types:
Mitosis:
– Parent cell produces
genetically identical
diploid (2n) daughter
cells
Meiosis:
– Parent cell produces
genetically varied
haploid (n) daughter
cells
Follow the Chromosomes
• Chromosome compact
DNA; easier for transporting
How many in humans?
– 23
• Homologous
chromosomes pairs of
chromosomes in 2n cells;
one copy from each parent
How many in humans?
– 46 (2n= 2x23)
Word for when too many
chromosomes are present;
47XXY?
– Ploidy; deadly in animals but
helpful in plants
• Sister chromatids copies of
chromosomes during mitosis
The Cell Cycle and Mitosis
• What is the growth and
development phase of a cell
called?
– Interphase
• 3 Phases in interphase:
1) G1 Phase normal cell
growth (Growth 1)
– Various in length between cell
types
2) S Phase DNA is replicated
and proteins for making
chromosomes (Synthesis)
– 10-12 hours long
3) G2 Phase cell growth and
preparation for division
(Growth 2)
– 4-6 hours long
No Growth Phase
• G0 Phase cell continues
to function but no longer
grows to do division
– Cells can be permanently
be in G0 or pushed back
into G1 by signals
• What cells never leave
G0?
– Most human nerve cells
• What signals start up
growth again?
– Growth factors,
hormones, and other
external signal molecules
M Phase
• M Phase mitosis;
completed in 5 steps
1) Prophase
2) Prometaphase
3) Metaphase
4) Anaphase
5) Telophase
What is the last step to
division?
– Cytokinesis; splitting of
the cytoplasm
Prophase
1) Chromosomes begins
to form as chromatin is
condensed
– 2m of DNA changes to 23
pairs of chromosomes
2) Nucleolus shrinks and
disappears
3) Spindle fibers formed
between two
centromeres
4) Centromeres move to
opposite sides of the
cell (spindle poles)
Late Prophase
1) Nuclear envelope
disappears
2) Spindle fibers extend
and attach sister
chromosomes at the
centromeres
3) Spindle fibers meet up
with other spindle
fibers across the cell
Metaphase
1) Spindle fibers
lengthen and shorten
to move
chromosomes to the
middle of the cell
(metaphase plate)
Anaphase
1) Spindle fibers shorten
and pull sister
chromatids to
opposite spindle poles
2) Sister chromatids are
now called “daughter
chromatids”
Telophase
1) Opposite of prophase
2) Spindle fibers
disassemble
3) Chromosomes
decondense
4) Nuclear envelope and
nucleolus reappear
Cytokinesis
In animals, protists, and
many fungi:
1) Furrow forms and cuts
cells in half
2) Organelles that were
copied are moved to
each cell
In plants:
1) Cell plate forms as
guide for new cell wall
2) Cell wall forms slowly
and divides cell in two
The Powerful Mitotic Spindle Fibers
• All movement of
chromosomes and
cytokinesis depend on
spindle fibers
• Spindle fiber:
– complex made of
microtubules and motor
proteins
– Grow in all directions
from centrioles creating
the centrosome or
Microtubule Organizing
Center (MTOC)
Two Spindle Types
1) Kinetochore
microtubules:
– Bind chromosomes to
spindle poles
– Chromosomes “walk” along
tube
– Tube is broken down as
chromosome moves
2) Nonkinetochore
microtubules:
– Overlap cross the cell and
push against each other
– Lengthening tubes pushes
cells apart
Cell Cycle Regulation
• 3 Checkpoints
• Each ensures the major step is
completed and the cell is ready
to commit to the next big step
1) G1/S checkpoint: cell must
either commit to whole
division cycle or not
– May require an external factor
(growth hormone)
2) G2/M checkpoint: commits
cell to mitosis or not
– Wont continue if DNA is to
damaged
3) Metaphase checkpoint: cell
wont finish division unless
chromosomes are lined up
properly
Cdks Levels in the Cycle
• Checkpoint regulated
internally by cyclin
proteins and cyclindependent kinase
enzymes (Cdks)
• Cdks need cyclins to be
active and they start a
phosphorylation
cascade leading to a
target protein
• What does this mean
for cyclin levels in the
cycle?
– They fluctuate through
divison but Cdks levels
are constant
Physical Inhibition
• Growth factors typically cause
the levels of cyclin/Cdk
activity to increase while
inhibitors lower it
• What might inhibit cell growth
after a wound has healed?
– Physical contact of over cells;
receptors recognize cells of the
same type and stop growing
• Contact Inhibition keeps
cells in check inside organs
and systems inside complex
organism
– Also limits bacterial growth in
cultures
– Pushes cells in G0 phase until
contact is broken
The Big “C”
• What happens if contact
inhibition is inhibited?
– Cells continue to grow and form
large masses (tumors)
• Most cancers are caused by a
shutdown of contact inhibition
or an over stimulation of
cyclin/Cdk complexes
• Tumor mass of cells
– Benign local or no growth; may
not need to be removed
– Metastasis growing and
spreading through the body; must
be treated
• Why does cancer hurt the body?
– Pressure from tumors, stealing
blood and nutrition, over
production of signals, etc…
What Causes Cancer?
• Most cancers are the
result of damaged DNA
caused by age, chemicals
(carcinogens), or naturally
occurring mutations in the
DNA (heredity)
• Oncogenes mutated
genes causing the cancer
• What type of oncogenes
would you see in most
cancers?
– Cyclin/Cdk regulation
– Growth hormone
receptors
Cell Cycle in Prokaryotes
• Prokaryotes go through a
similar G1, S, and G2 cycle,
however they divide by binary
fission
• Their single circular
chromosome is replicated and
then each is pulled to
opposite sides of the cell
• The cell, once large enough,
divides in half by cytokinesis
• Process is very short and
simple, which is why bacteria
can divide so quickly; most of
the time is spent coping the
DNA