5/21/2012
Mitosis
Genetics and Cellular Function
Mitosis: division of cells that results
in daughter cells with the same
the genetic information that the
original cell had.
• DNA replication and the cell cycle
• Mitosis
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46
46
Diploid 2n
Diploid 2 n
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27-2
Madre
Chromosomes are made of DNA
Padre
centromere
Almost all cells in body have
homologous pairs of
chromosomes
duplicated
chromosome
(2 DNA double
helices)
sister
chromatids
23 homologous pairs in
humans
Centromere
A replicated chromosome consists of two sister chromatids
i.e., 46 Total chromosomes
genes
Sister
chromatids
Chromatid – composed of DNA
Fig. 8-5
Cell Cycle
Cell Cycle
• Interphase is subdivided
into:
– G1 - cell performs
normal physiological
roles
– S - DNA is replicated in
preparation for division
– G2 - chromatin
condenses prior to
division
• Most cells of body are in interphase - the
non-dividing stage of life cycle
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Mitosis
• cell division in all body cells except the eggs and
sperm
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• Functions of mitosis
– growth of all tissues and organs after birth
– replacement of cells that die
– repair of damaged tissues
• 4 phases of mitosis
– prophase, metaphase, anaphase, telophase
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Mitosis
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Mitosis: Prophase
Prophase
Chromosomes condense
and nuclear envelope
breaks down. Spindle
fibers grow from centrioles.
Centrioles migrate to
opposite poles of cell.
• chromosomes shorten and thicken coiling into compact rods
• 46 chromosomes
– two chromatids per chromosome
– one molecule of DNA in each chromatid
Aster
2
Metaphase
Chromosomes lie along midline
of cell. Some spindle fibers
attach to kinetochores.
Fibers of aster attach
to plasma membrane.
Spindle fibers
Centriole
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Chromatids
Kinetochore
Cleavage furrow
• nuclear envelope disintegrates and releases chromosomes into the
cytosol
Anaphase
Centromeres divide in two.
Spindle fibers pull sister
chromatids to opposite poles
of cell. Each pole (future
daughter cell) now has an
identical set of genes.
Telophase
Chromosomes gather
4 at each pole of cell.
Chromatin decondenses.
New nuclear envelope
appears at each pole.
New nucleoli appear
in each nucleus.
Mitotic spindle
vanishes.
• centrioles sprout elongated microtubules – spindle fibers
– push centrioles apart as they grow
– pair of centrioles lie at each pole of the cell
• some spindle fibers grow toward chromosomes and attach to the
kinetochore on each side of the centromere
Daughter
cells in
interphase
• spindle fibers push chromosomes to line up along the midline of cell
Nuclear envelope
re-forming
Chromatin
Nucleolus
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Mitosis: Metaphase
Mitosis: Prophase
Aster
1 Prophase
Chromosomes condense
and nuclear envelope
breaks down. Spindle
fibers grow from centrioles. 1
Centrioles migrate to
opposite poles of cell.
2
2
Metaphase
Chromosomes lie along
midlineof cell. Some spindle fibers
attach to kinetochores.
Fibers of aster attach
to plasma membrane.
Spindle fibers
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© Ed Reschke
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Mitosis: Telophase
Mitosis: Anaphase
• chromatids cluster on each side of the cell
• rough ER produces new nuclear envelope
around each cluster
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• chromatids begin to uncoil and form chromatin
Anaphase
Centromeres divide in two.
Spindle fibers pull sister
chromatids to opposite poles
of cell. Each pole (future
daughter cell) now has an
identical set of genes.
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• mitotic spindle breaks up and vanishes
Chromatids
• each nucleus forms nucleoli
Kinetochore
– indicating it has already begun making RNA and
preparing for protein synthesis
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Figure 3.32: The stages of mitosis, p. 102.
Cytokinesis
• cytokinesis – the division of cytoplasm into two
cells
– telophase is the end of nuclear division but overlaps
cytokinesis
– early traces of cytokinesis visible in anaphase
Centrioles
(two pairs)
• achieved by motor protein myosin pulling on
microfilaments of actin in the terminal web of
cytoskeleton
Condensed Early mitotic
chromatin spindle
Fragments of
Pair of
centrioles nuclear
envelope
Polar
microtubules
Aster
• creates the cleavage furrow around the equator
of cell
• cell eventually pinches in two
Nucleolus
Nuclear
envelope
Interphase
Plasma
Chromosome, consisting
membrane of two sister chromatids
Early prophase
Kinetochore
Centromere
Kinetochore
microtubule
Spindle
pole
Late prophase
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Figure 3.32: The stages of mitosis (continued), p. 103.
Metaphase
plate
Spindle
Metaphase
Daughter
chromosomes
Anaphase
Contractile
ring at
cleavage
furrow
Nucleolus
forming
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Nuclear
envelope
forming
Telophase and cytokinesis
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Timing of Cell Division
Meiosis
Cells divide when:
• they have enough cytoplasm for two daughter cells
• they have replicated their DNA
• adequate supply of nutrients
• are stimulated by growth factor
– chemical signals secreted by blood platelets, kidney
cells, and other sources
• neighboring cells die, opening space in a tissue
Meiosis: division of cells that results
in daughter cells with one-half of
the genetic information that the
original cell had.
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Cells stop dividing when:
• when nutrients or growth factors are withdrawn
• contact inhibition – the cessation of cell division in
response to contact with other cells
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23
Diploid 2n
23
Haploid n
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Pa
Haploid
n
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Ma
Haploid
n
Meiosis
• Cell division occurring in ovaries and testes
to produce gametes (ova (egg) and sperm)
• Two divisional sequences
• Daughter cells have ½ the chromosomes the
original cell had
Junior = Zygote =
diploid organism=
2n
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Meiosis
Meiosis
• In 1st division:
– homologous chromosomes pair along equator
of cell rather than singly as in mitosis
– 1 member of homolog pair is pulled to each pole
– gives each daughter cell 23 different
chromosomes, consisting of 2 chromatids
• In 2nd division:
– each daughter divides; chromosomes split into
2 chromatids
– 1 goes to each new daughter cell
• Each daughter cell contains 23 chromosomes
– Orginal mother cell had 46
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Genetic diversity & Meiosis
Meiosis I (first division)
Early prophase I
Chromatin condenses to
form visible chromosomes;
each chromosome has 2
chromatids joined by a
centromere.
• Genetic recombination occurs in prophase I
1. Crossing-over: Parts of one homologous
chromosome are exchanged with its partner
homolog
Mid- to late prophase I
Homologous chromosomes
form pairs called tetrads.
Chromatids often break
and exchange segments
(crossing-over). Centrioles
produce spindle fibers.
Nuclear envelope
disintegrates.
Metaphase I
Tetrads align on
equatorial plane of
cell with centromeres
attached to spindle
fibers.
Meiosis II (second division)
Chromosome
Nucleus
Centromere
Centrioles
Tetrad
Telophase I
New nuclear
envelopes
form around
chromosomes;
cell undergoes
cytoplasmic
division
(cytokinesis).
Each cell is
now haploid.
Metaphase II
Chromosomes align on
equatorial plane.
Crossing-over
Centromere
Chromatid
Equatorial
plane
Anaphase II
Centromeres divide;
sister chromatids
migrate to opposite
poles of cell. Each
chr omatid now constitutes
a single-stranded
chromosome.
Telophase II
New nuclear envelopes
form around chromosomes;
chromosomes uncoil and
become less visible;
cytoplasm divides.
Anaphase I
Homologous
chromosomes
separate and
migrate to opposite
poles of the cell.
2. Independent assortment: the way
chromosomes line up during metaphase is
random
Prophase II
Nuclear envelopes
disintegrate again;
chromosomes still
consist of 2 chromatids.
New spindle forms.
Final product is 4
haploid cells with
single-stranded
chromosomes.
Cleavage furrow
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• Independent assortment
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The four possible chromosome arrangements at metaphase
of meiosis I
The eight possible sets of chromosomes after meiosis I
Fig. 8-13
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