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Mitosis
Cell Size Limits
• Why aren’t we just one giant cell??
3 limitations to cell size
1. Diffusion- is fast and efficient over short distances but
slow and inefficient over long distances
- cells would die long before nutrients could
reach the organelles
3 limitations to cell size
2. DNA limits cell size
- cells need A LOT of proteins to perform critical
functions BUT there is a limit to how quickly
DNA can be made into proteins.
3 limitations to cell size
3. Surface area-to volume ratio
- as a cell’s size increases, its volume increases
much faster than its surface area
Reasons for cell division
1. maintaining optimum cell size
2. growth in multicellular organisms
3. asexual reproduction of single-celled
organisms
4. to replace dead cells
Chromatin vs. Chromosome
• Chromatin = long strands of DNA wrapped
around proteins (tangled up spaghetti)
• Chromosome = coiled strands of DNA that
look more like rope or X’s; becomes like this
right before cell division
Cell cycle
• G1→G2 = interphase
• S phase = DNA replication
• G1 & G2 = growth 1 & 2, cell is growing and
preparing for division
• Which phase does a cell spend most of its
time in?
Mitosis
• Mitosis = process in which the nucleus of a
cell is divided into 2 nuclei, each with the
same number and kinds of chromosomes as
the parent cell (also called asexual
reproduction)
Interphase
• First step of the cell
cycle.
• Interphase is the
preparation phase.
• Three stages: G1,
Interphase and G2.
• What happens during
interphase?
Mitosis
• Mitosis is the second step of the cell cycle
(asexual reproduction)
• It is broken into four phases: prophase,
metaphase, anaphase and telophase.
Prophase
• First stage of mitosis.
• Nuclear envelope
begins to dissappear
• Chromatin coils up and
becomes X-shaped
chromosomes
Metaphase
• Chromosomes (X’s) line
up in the middle of the
cell.
• Spindle fibers attach to
the centromere on the
chromosome.
Anaphase
• Spindle fibers begin to
pull the chromosomes
(X’s) apart.
• Chromatid (V’s) now
move toward opposite
ends
Telophase
• Chromatid (V’s) reach the
poles.
• Nuclear envelope begins to
reappear around the
chromatid
• In plant cells a cell plate
appears down the middle to
divide the cell.
• In animal cells a cleavage
furrow appears to separate
the cell.
Cytokinesis
• Last stage of the cell
cycle.
• Cell has separated
creating two new
identical daughter cells.
Cytokinesis
• Cytokinesis= the process by which the
cytoplasm divides, thus forming 2 distinct cells
Results of Mitosis
• 2 identical daughter cells
• In unicellular organisms that is how it
reproduces
• In multicellular organisms, cell growth and
division results in a group of cells that work
together (all human cells undergo mitosis
except for our sex cells). Skin cells undergo
mitosis the most.
cells  tissue  organ  organ system 
ORGANISM
Cancer
• Enzymes control each phase of the cell cycle.
• Cancer = result of uncontrolled cell division
• Extra cells form masses called tumors
• Causes of cancer are both genetic and
environmental (smoke, exposure to UV rays, viral
infections)