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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

99

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

22

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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

90

Spermatogenesis in the Testes

SpermatidSpermatid

9191

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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