Cellular Reproduction,
Part 1: Mitosis
Lecture 10
Fall 2008
Cell Theory
Cell theory:
• All organisms are made of cells
• All cells arise from preexisting cells
How do new cells arise?
• Cell division
– the reproduction of cells
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Cell Division
What is cell division used for?
• Growth
• Wound repair
• Reproduction
Both sexual and asexual
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Cell Division
Fig. 8.1
• Multicellular organisms start out life as a single cell
• A series of cell divisions occur that give rise to all the cells
in an organisms body
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Cell Division
Two components
• Cytokinesis (“cell movement”)
– Division of the cytoplasm into two daughter
cells
• Division of the nucleus (and of the DNA)
Cell Division
Two different types of nuclear division possible
Mitosis
• Form of cell division that leads to identical
daughter cells with the full complement of DNA
• Occurs in somatic cells
– Cells of body that are not reproductive cells
Meiosis
• Form of cell division that leads to non-identical
daughter cells with one-half the complement of
DNA
• Forms gametes
– Reproductive cells (sperm cells & egg cells)
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Chromosomes
• Genome
– the complete complement of an organisms genetic
material, DNA
• Gene
– a discrete unit of hereditary information consisting of
a specific nucleotide sequence in DNA
• Prokaryotes have one long DNA molecule
• Eukaryotes have many DNA molecules =
chromosomes
– Chromosome: a thread-like, gene-carrying
molecule located in the nucleus
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Chromosomes
• Chromosomes in the nucleus contain
most of an organisms genes
– Some DNA in mitochondria and chloroplasts
• Number of chromosomes vary depending
on species
– Somatic cells of humans = 46 chromosomes
• Two sets of 23 chromosomes
• 23 from egg and 23 from sperm
– Gametes of humans = 23 chromosomes
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Chromosomes
• Chromosomes made of chromatin
– Complex of DNA and protein molecules
– Role of protein molecules
• Organize chromatin
• Help control activity of gene
– Mass of long fibers spread out in the nucleus
• Chromatin fibers form compact chromosomes only
during cell division
Fig. 12.3
• Each chromosome contains
one very long, linear DNA molecule
that carries several hundred to a
few thousand genes
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DNA Packing
Chromosomes condense to
structures visible with a light
microscope
1. Histones attach to DNA
– Histones – small proteins that
assist with DNA packing
– Creates an area called a
nucleosome
• DNA wrapped around histones
• “beads on a string”
2. Tight helical fiber
3. Supercoil
4. More looping & folding
Fig.
Replication of Chromosomes
DNA needs to be duplicated prior
to cell division
• Duplicated chromosome =
sister chromatids
– Held together by adhesive
proteins (cohesins)
– Centromere
• Specialized region where sister
chromatids most closely attached
• During mitosis, sister
chromatids separate, one each
to the two daughter cells
• Once separated, they are now
individual chromosomes
Fig. 12.4
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The Cell Cycle
Cell cycle
• Life cycle of the cell
• Starts with origination of a cell and ends with cell
division
• Two main phases
– Interphase
• ~90% of time
– Mitotic phase
• ~10% of time
Fig. 12.5
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The Cell Cycle
Interphase
• Normally activity of cell
• Cell grows as it produces more contents of
cell cytoplasm
• Chromosome duplication occurs
Fig. 12.5
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The Cell Cycle
Interphase
– Cell growth occurs in all three sub-phases
• G1 (first gap)
• S phase (synthesis)
– Chromosome replication
• Sister chromatids
• G2 (second gap)
– Cell preparing to divide
Fig. 12.5
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Mitosis
Mitotic phases
• Prophase
• Prometaphase
• Metaphase
• Anaphase
• Telophase
• Cytokinesis occurs during telophase
• Activity
G2 of Interphase
• Chromosomes (chromatin)
duplicated
• Chromosomes still
uncondensed
– Not clearly distinguishable as
separate chromosomes
• Centrosome
– Non-membrane bound organelle
– Organizes the cell’s microtubules
– Centrosome replicates into 2
• Contains centrioles
– Structures made of microtubules
– Found only in animals
– Microtubules will form even if
centrioles destroyed
Fig. 12.6
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Mitosis: Prophase
• Chromosomes condense
– Each chromosome appears
as two identical sister
chromatids joined together
• Formation of mitotic
spindle
– Centrosomes move towards
poles of cell
– Microtubules extend from
centrosomes
– Asters – short microtubules
extending from centrosome
– Will guide the separation of
the daughter chromosomes
Fig. 12.6
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Mitosis: Prometaphase
• Breakdown of nuclear
envelope
• Microtubules of mitotic
spindle start to attach to
chromosomes
– Centromere contains special
attachment proteins
• kinetochore
– Movement of chromosomes
towards center of cell
• Non-kinetochore
microtubules interact with
others from opposite pole
Fig. 12.6
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Mitosis: Metaphase
• Centrosomes at opposite
poles
• Mitotic spindle fully formed
• Chromosomes lined up at
equator of cell (metaphase
plate)
– Each sister chromatid is
attached to a kinetochore
microtubule
– Other non-kinetochore
microtubules meet with each
other from opposite ends of the
cell
Fig. 12.6
Mitosis: Anaphase
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• Sister chromatids of each chromosome
separate
– Cohesion proteins cleaved
– Now a “daughter” chromosome
• Chromosomes move away from center,
towards poles
– Kinetochore microtubules shorten & bring
chromosomes with them towards each pole
– Read Fig. 12.8 Inquiry
• Cell elongates
– Non-kinetochore microtubules lengthen
– Pushes poles of cell further apart
• End of Anaphase
– Both poles have equivalent and complete
set of chromosomes
Fig. 12.6
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Mitosis: Telophase
• Chromosomes at poles of
cell
• Two daughter nuclei begin
to form in the cell
– Nuclear envelope forms
around each one
• Chromosomes become
less condensed
• Mitotic spindle
deconstructed
Fig. 12.6
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Cytokinesis
Cytokinesis in animals
• Cleavage
• Cleavage furrow
– Shallow groove at equator
of cell
– Parent cell “pinched” in
two by contraction of ring
of microfilaments
Fig. 12.9
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Cytokinesis
Cytokinesis in plants
• Cell wall material bound
in vesicles
• Vesicles line up, fuse to
form cell plate
• Grows until it fuses with
plasma membrane
• Cell wall contents bind
with cell wall
Fig. 12.9
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Asexual Reproduction
• Mitosis also used to
reproduce entire organisms
• Asexual reproduction
– Reproduction involving only
one parent that produces
genetically identical (clone)
offspring
– Does not involve sperm and
egg (no meiosis)
Fig. 8.2
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Asexual Reproduction
Japanese knotweed broken stem
sprouts shoots and roots
Aspen root
“suckers” – produce
new shoots and
stems
Asexual Reproduction
Binary fission
• Prokaryotes
• Not mitosis
• Single large circular chromosome
• Chromosome duplicated
– Starts at origin of replication
– Copy of origin site moves to other
pole of cell
• Cell elongates
• Plasma membrane grows inward
to form two daughter cells
Fig. 12.11
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Evolution of Mitosis
• Hypothesis: mitosis evolved from binary fission in
prokaryotes
– Some similar proteins
• Possible intermediate stages still seen other organisms
Fig. 12.12
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