1 cell cycle, cell division and mitosis. 2 phases of the cell cycle the cell cycle consists of –...
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Cell Cycle, Cell Division and Mitosis
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Phases of the Cell Cycle
The cell cycle consists of– Interphase – normal cell activity– The mitotic phase – cell division
INTERPHASE
Growth
G 1(DNA synthesis)
Growth
G2
Cel
l Div
sion
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Cell Division All cells are derived from pre-
existing cells New cells are produced for
reproduction, growth and to replace damaged or old cells
Differs in prokaryotes (bacteria) and eukaryotes (protists, fungi, plants, & animals)
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Functions of Cell Division(a)Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism
(b)Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM)
(c)Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM).
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Cell Division
• An integral part of the cell cycle• Results in genetically identical
daughter cells• Cells duplicate their genetic
material– Before they divide, ensuring that each daughter cell receives an exact copy of the genetic material, DNA
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Cell Devision
DNA must be copied or replicated before cell division
Each new cell will then have an identical copy of the DNA
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Identical Daughter Cells
Parent Cell
Two identical daughter cells
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Prokaryotic Chromosome
The DNA of prokaryotes (bacteria) is one, circular chromosome attached to the inside of the cell membrane
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Eukaryotic Chromosomes All eukaryotic cells store
genetic information in chromosomes
Each chromosome is Each chromosome is composed of a single, composed of a single, tightly coiled DNA tightly coiled DNA moleculemolecule
Chromosomes can’t be Chromosomes can’t be seen when cells aren’t seen when cells aren’t dividing and are called dividing and are called chromatinchromatin
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Chromatids
Duplicated chromosomes are called chromatids & are held together by the centromere
Called Sister Chromatids
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ChromosomesMaternal set ofchromosomes (n = 3)
Paternal set ofchromosomes (n = 3)
2n = 6
Two sister chromatidsof one replicatedchromosome
Two nonsisterchromatids ina homologous pair
Pair of homologouschromosomes(one from each set)
Centromere
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Homologues Chromosome• Look the same• Control the same traits• May code for different forms of each trait• Independent origin - each one was
inherited from a different parent
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Chromosome Duplication
0.5 µm
Chromosomeduplication(including DNA synthesis)
Centromere
Separation of sister
chromatids
Sisterchromatids
Centrometers Sister chromatids
A eukaryotic cell has multiplechromosomes, one of which is
represented here. Before duplication, each chromosome
has a single DNA molecule.
Once duplicated, a chromosomeconsists of two sister chromatids
connected at the centromere. Eachchromatid contains a copy of the
DNA molecule.
Mechanical processes separate the sister chromatids into two chromosomes and distribute
them to two daughter cells.
• In preparation for cell division, DNA is replicated and the chromosomes condense
• Each duplicated chromosome has two sister chromatids, which separate during cell division
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• Because of duplication, each condensed chromosome consists of 2 identical chromatids joined by a centromere.
• Each duplicated chromosome contains 2 identical DNA molecules (unless a mutation occurred), one in each chromatid:
Chromosome Duplication
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Two unduplicatedchromosomes
Centromere
Sisterchromatids
Sisterchromatids
Duplication
Non-sisterchromatids
Two duplicated chromosomes
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Structure of Chromosomes• The centromere is a constricted region of the chromosome
containing a specific DNA sequence, to which is bound 2 discs of protein called kinetochores.
• Kinetochores serve as points of attachment for microtubules that move the chromosomes during cell division:
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Metaphase chromosome
Kinetochore
Kinetochoremicrotubules
Centromereregion ofchromosome
Sister Chromatids
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Structure of Chromosomes– Diploid - A cell possessing two copies of each
chromosome (human body cells).Homologous chromosomes are made up of sister chromatids joined at the centromere.
– Haploid - A cell possessing a single copy of each chromosome (human sex cells).
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Types of Cell Reproduction Asexual reproduction involves a single
cell dividing to make 2 new, identical daughter cells
Mitosis & binary fission are examples of asexual reproduction
Sexual reproduction involves two cells (egg & sperm) joining to make a new cell (zygote) that is NOT identical to the original cells
Meiosis is an example of sexual reproduction
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Cell Division in Prokaryotes Prokaryotes such as
bacteria divide into 2 identical cells by the process of binary fission
Single chromosome makes a copy of itself
Cell wall forms between the chromosomes dividing the cell
Parent Parent cellcell
2 identical daughter cells
Chromosome Chromosome doublesdoubles
Cell splitsCell splits
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Phases of the Cell Cycle• Interphase
– G1 - primary growth – S - genome replicated (DNA replication)– G2 - secondary growth
• M - mitosis• C - cytokinesis
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Phases of the Cell Cycle
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Interphase• G1 - Cells undergo majority of growth• S - Each chromosome replicates (synthesizes) to produce sister chromatids (DNA replication)
– Attached at centromere– Contains attachment site (kinetochore)
• G2 - Chromosomes condense - Assemble
machinery for division such as centrioles
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Interphase - G1 Stage
1st growth stage after cell division
Cells mature by making more cytoplasm & organelles
Cell carries on its normal metabolic activities
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Interphase – S Stage Synthesis stage DNA is copied or replicated
Two identical copies of DNA
Original DNA
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Interphase – G2 Stage 2nd Growth Stage Occurs after DNA has been copied All cell structures needed for
division are made (e.g. centrioles) Both organelles & proteins are
synthesized
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Daughter Cells
DNA CopiedDNA Copied
Cells MatureCells MatureCells prepare for Cells prepare for DivisionDivision
Cell Divides into Identical Cell Divides into Identical cellscells
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Mitosis Some haploid & diploid cells divide by mitosis. Each new cell receives one copy of every
chromosome that was present in the original cell.
Produces 2 new cells that are both genetically identical to the original cell.
DNA duplication
during interphase Mitosis
Diploid Cell
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Mitosis Division of the
nucleus Also called
karyokinesis Only occurs in
eukaryotes four stages Doesn’t occur in
some cells such as brain cells
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Four Mitotic Stages
Prophase Metaphase Anaphase Telophase
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Early Prophase Chromatin in nucleus condenses to
form visible chromosomes Mitotic spindle forms from fibers in
cytoskeleton or centrioles (animal)
Chromosomes
Nucleolus Cytoplasm
Nuclear Membranee
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Late Prophase Nuclear membrane & nucleolus
are broken down Chromosomes continue
condensing & are clearly visible Spindle fibers called kinetochores
attach to the centromere of each chromosome
Spindle finishes forming between the poles of the cell
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Late Prophase
Nucleus & Nucleolus have disintegrated
Chromosomes
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Spindle Fiber attached to Chromosome
Kinetochore FiberKinetochore Fiber
ChromosomeChromosome
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Review of Prophase
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Spindle Fibers The mitotic spindle form from the
microtubules in plants and centrioles in animal cells
Polar fibers extend from one pole of the cell to the opposite pole
Kinetochore fibers extend from the pole to the centromere of the chromosome to which they attach
Asters are short fibers radiating from centrioles
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The Spindle
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Metaphase Chromosomes, attached to the kinetochore
fibers, move to the center of the cell Chromosomes are now lined up at the equator
Pole of the Cell
Equator of Cell
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Metaphase
Chromosomes Chromosomes lined at the lined at the EquatorEquator
Asters at the Asters at the polespoles
Spindle Spindle FibersFibers
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Metaphase
AsterAster
Chromosomes at Equator
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Metaphase
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Anaphase
Occurs rapidly Sister
chromatids are pulled apart to opposite poles of the cell by kinetochore fibers
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Anaphase
Sister Sister Chromatids Chromatids being being separatedseparated
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Anaphase
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Telophase Sister chromatids at opposite
poles Spindle disassembles Nuclear envelope forms around
each set of sister chromatids Nucleolus reappears CYTOKINESIS occurs Chromosomes reappear as
chromatin
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Comparison of Anaphase & Telophase
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Mitosis in a plant cell
1 Prophase. The chromatinis condensing. The nucleolus is beginning to disappear.Although not yet visible in the micrograph, the mitotic spindle is staring to from.
Prometaphase.We now see discretechromosomes; each consists of two identical sister chromatids. Laterin prometaphase, the nuclear envelop will fragment.
Metaphase. The spindle is complete,and the chromosomes,attached to microtubulesat their kinetochores, are all at the metaphase plate.
Anaphase. Thechromatids of each chromosome have separated, and the daughter chromosomesare moving to the ends of cell as their kinetochoremicrotubles shorten.
Telophase. Daughternuclei are forming. Meanwhile, cytokinesishas started: The cellplate, which will divided the cytoplasm in two, is growing toward the perimeter of the parent cell.
2 3 4 5
NucleusNucleolus
ChromosomeChromatinecondensing
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Cytokinesissis Means division of the cytoplasm Division of cell into two,
identical halves called daughter cells
In plant cells, cell plate forms at the equator to divide cell
In animal cells, cleavage furrow forms to split cell
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Cytokinesis In Animal And Plant Cells
Daughter cells
Cleavage furrow
Contractile ring of microfilaments
Daughter cells
100 µm1 µmVesicles
forming cell plate
Wall of patent cell Cell plate
New cell wall
(a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (SEM)
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Cytokinesis
Cleavage furrow in animal cell
Cell plate in animal cell
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Daughter Cells of Mitosis Have the same number of
chromosomes as each other and as the parent cell from which they were formed
Identical to each other, but smaller than parent cell
Must grow in size to become mature cells (G1 of Interphase)
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Identical Daughter Cells
Chromosome number the same, but cells smaller than parent cell
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Mitotic Division of an Animal Cell
G2 OF INTERPHASE PROPHASE PROMETAPHASE
Centrosomes(with centriole pairs) Chromatin
(duplicated)
Early mitoticspindle
Aster
CentromereFragmentsof nuclearenvelope
Kinetochore
Nucleolus Nuclearenvelope
Plasmamembrane
Chromosome, consistingof two sister chromatids
Kinetochore microtubule
Nonkinetochoremicrotubules
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METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS
Spindle
Metaphaseplate Nucleolus
forming
Cleavagefurrow
Nuclear envelopeformingCentrosome at
one spindle poleDaughter chromosomes
Mitotic Division of an Animal Cell
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A nuclear envelope bounds the nucleus.
The nucleus contains one or more nucleoli (singular, nucleolus).
Two centrosomes have formed by replication of a single centrosome.
In animal cells, each centrosome features two centrioles.
Chromosomes, duplicated during S phase, cannot be seen individually because they have not yet condensed.
The light micrographs show dividing lung cells from a newt, which has 22 hromosomes in its somatic cells (chromosomes appear blue, microtubules green, intermediate filaments red). For simplicity, the drawings show only four chromosomes.
G2 OF INTERPHASECentrosomes(with centriole pairs) Chromatin
(duplicated)
Nucleolus Nuclearenvelope
Plasmamembrane
G2 of Interphase
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• The chromatin fibers become more tightly coiled, condensing into discrete chromosomes observable with a light microscope.
• The nucleoli disappear.• Each duplicated chromosome
appears as two identical sister chromatids joined together.
• The mitotic spindle begins to form. It is composed of the centrosomes and the microtubules that extend from them. The radial arrays of shorter microtubules that extend from the centrosomes are called asters (“stars”).
• The centrosomes move away from each other, apparently propelled by the lengthening microtubules between them.
PROPHASE
Early mitoticspindle
AsterCentromere
Chromosome, consistingof two sister chromatids
Prophase
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Metaphase• Metaphase is the longest stage of mitosis, lasting about 20 minutes.• The centrosomes are now at opposite ends of the cell. •The chromosomes convene on the metaphase plate, an imaginary plane that is equidistant between the spindle’s two poles. The chromosomes’ centromeres lie on the metaphase plate. • For each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles. • The entire apparatus of
microtubules is called the spindle because of its shape.
METAPHASE
Spindle
Metaphaseplate
Centrosome at one spindle pole
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The Mitotic Spindle• The spindle includes the centrosomes, the
spindle microtubules, and the asters• The apparatus of microtubules controls
chromosome movement during mitosis• The centrosome replicates, forming two
centrosomes that migrate to opposite ends of the cell
• Assembly of spindle microtubules begins in the centrosome, the microtubule organizing center
• An aster (a radial array of short microtubules) extends from each centrosome
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• Some spindle microtubules attach to the kinetochores of chromosomes and move the chromosomes to the metaphase plate
• In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell
Microtubules ChromosomesSisterchromatids
AsterCentrosome
Metaphaseplate
Kineto-chores
Kinetochoremicrotubules
0.5 µm
Overlappingnonkinetochoremicrotubules
1 µmCentrosome
The Mitotic Spindle
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Anaphase is the shortest stage of mitosis, lasting only a few minutes.
Anaphase begins when the two sister chromatids of each pair suddenly part. Each chromatid thus becomes a full- fledged chromosome.
The two liberated chromosomes begin moving toward opposite ends of the cell, as their kinetochore microtubules shorten. Because these microtubules are attached at the centromere region, the chromosomes move centromere first (at about 1 µm/min).
The cell elongates as the non kinetochore microtubules lengthen.
By the end of anaphase, the two ends of the cell have equivalent—and complete—collections of chromosomes.
ANAPHASE
Daughter chromosomes
Anaphase
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• Two daughter nuclei begin to form in the cell.
• Nuclear envelopes arise from the fragments of the parent cell’s nuclear envelope and other portions of the endomembrane system.
• The chromosomes become less condensed.
• Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete.
TELOPHASE AND CYTOKINESIS
Nucleolusforming
Cleavagefurrow
Nuclear envelopeforming
Telophase
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MeiosisFormation of Gametes
(Eggs & Sperm)
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Meiosis1. Reduction in amount of genetic material in
the daughter cell2. A type of cell division that results in four
daughter cells each with half number of chromosome of the parent cell, as in the production of gametes and plant spores
3. The process by which the nucleus divides in all sexually reproducing organisms during the production of spores and gametes
4. The process by which the cell divides producing daughter cell that have a single set of chromosomes and are called haploid as opposed to diploid cells with two sets of chromosome
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Facts About Meiosis Preceded by interphase which
includes chromosome replication Two meiotic divisions:
1. Meiosis I 2. Meiosis II
(Reduction- division) Original cell is diploid (2n) Four daughter cells produced
that are monoploid (1 n)
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Facts About Meiosis• Only diploid cells can divide by
meiosis.• Prior to meiosis I, DNA
replication occurs.• During meiosis, there will be two
nuclear divisions, and the result will be four haploid nuclei.
• No replication of DNA occurs between meiosis I and meiosis II.
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Facts About Meiosis Daughter cells contain half the
number of chromosomes as the original cell
Produces gametes (eggs & sperm) Occurs in the testes in males
(Spermatogenesis) Occurs in the ovaries in females
(Oogenesis))
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Meiosis It is the fundamental basis of
sexual reproduction Two haploid (1n) gametes are
brought together through fertilization to form a diploid (2n) zygote
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Fertilization
1n =31n =3
2n = 62n = 6
“Putting it all together””
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Stages of Meiosis
Meiosis I • Prophase I• Metaphase I• Anaphase I• Telophase I
Meiosis II• Prophase IIProphase II• Metaphase IIMetaphase II• Anaphase IIAnaphase II• Telophase IITelophase II
Begins with Interphase
Chromosomes replicate
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Replication of Chromosomes Replication is
the process of duplicating a chromosome
Occurs prior to division
Replicated copies are called sister chromatids
Held together at centromere
Occurs in Occurs in InterphaseInterphase
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A Replicated Chromosome
Homologs (same genes, different alleles)
SisterSisterChromatidsChromatids(same genes,(same genes,same alleles)same alleles)
Gene X
Homologs separate in meiosis I and therefore different alleles separate.
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Meiosis Forms Haploid GametesMeiosis must reduce the chromosome
number by halfFertilization then restores the 2n number
from mom from dad child
meiosis reducesgenetic content
toomuch!
The right number!
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Meiosis: Two Part Cell Division
Homologsseparate
Sister chromatids
separate
Haploid
Meiosis I
MeiosisII
Diploid
Haploid
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Meiosis I: Reduction Division
Nucleus Spindlefibers Nuclear
envelope
Early Prophase I(Chromosome number doubled)
Late Prophas
e I
Metaphase I Anaphas
e I
Telophase I
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Prophase I
Early prophaseEarly prophaseHomolog’s pair.Homolog’s pair.Crossing over Crossing over occursoccurs.
Late prophaseChromosomes condense.Spindle forms.Nuclear envelope fragments..
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Tetrads Form in Prophase I
Homologous chromosomes(each with sister chromatids)
Join to form a TETRAD
Called Synapsis
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Crossing-Over Homologous
chromosomes in a tetrad cross over each other
Pieces of chromosomes or genes are exchanged
Produces Genetic recombination in the offspring
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Homologous Chromosomes During Crossing-Over
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Crossing-over multiplies the already huge number of different gamete types produced by independent assortment
Crossing-Over
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Metaphase I
Homologous pairs of chromosomes align along the equator of the cell
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Anaphase I
Homolog’s separate and move to opposite poles.
Sister chromatid’s remain attached at their centromers.
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Telophase I
Nuclear envelopes Nuclear envelopes reassemble.reassemble.
Spindle disappears.Spindle disappears.
Cytokinesis divides cell Cytokinesis divides cell into two.into two.
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Meiosis IIOnly one homolog of Only one homolog of each chromosome is each chromosome is present in the cellpresent in the cell.
Meiosis II produces gametes with one copy of each chromosome and thus
one copy of each gene.
Sister chromatids carry identical genetic
information.
Gene X
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Meiosis II
Prophase II Metaphas
e IIAnaphase II
Telophase II 4
Identical haploid cells
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Prophase IIProphase II
Nuclear envelope fragments.
Spindle forms.
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Metaphase II
Chromosomes align along equator of cell.
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Anaphase II
Sister Sister chromatids chromatids separate and separate and move to opposite move to opposite polespoles.
Equator
Pole
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Telophase II
Nuclear envelope assembles.
Chromosomes decondense.
Spindle disappears.
Cytokinesis divides cell into two.into two.
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Results of Meiosis
Gametes (egg & sperm) form
Four haploid cells with one copy of each chromosome
One allele of each gene
Different combinations of alleles for different genes along the chromosome
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GametogenesisOogenesis or
Spermatogenesis
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Spermatogenesis Occurs in the
testes Two divisions
produce 4 spermatids
Spermatids mature into sperm
Men produce about 250,000,000 sperm per daysperm per day
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Spermatogenesis in the Testes
SpermatidSpermatid
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Oogenesis Occurs in the ovaries Two divisions produce 3 polar
bodies that die and 1 egg Polar bodies die because of
unequal division of cytoplasm Immature egg called oocyte Starting at puberty, one oocyte
matures into an ovum (egg) every 28 days
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Oogenesis in the Ovaries
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Oogenesis
Oogonium(diplloid)
Mitosis
Primaryoocyte(diploid)
Meiosis I
Secondaryoocyte(haploid)
Meiosis II(if fertilizationoccurs)
First polar bodymay divide (haploid)
Polarbodiesdie
Ovum (egg)
Secondpolar body(haploid)
a
A
X
X
a
X
A X
a
X
a
X
Matureegg
A
X
A
X
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Mitosis• Functions
– Asexual reproduction
– Growth, repair • Occurs
throughout plant• Produces clones• Diploid parents
and offspring
Mitosis & Meiosis
Meiosis• Function
– Sexual reproduction
• Occurs only in cells that give rise to sperm and eggs
• Produces variable offspring
• Diploid parents, haploid offspring
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Mitosis MeiosisNumber of divisions 1 2
Number of daughter cells
2 4
Genetically identical? Yes No
Chromosome numberSame as parent
Half of parent
Where Somatic cells Germ cells
WhenThroughout
lifeAt sexual maturity
RoleGrowth and
repairSexual
reproduction
Mitosis & Meiosis
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Mitosis and Meiosis
• Mitosis
– Two diploid cells produced
– Each identical to parent
• Meiosis
– Four haploid cells produced
– Differ from parent and one another