Celldivision (1)

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Phases of the Cell Cycle

• The cell cycle consists of

– Interphase – normal cell activity – The mitotic phase – cell divsion

INTERPHASE

Growth

G ₁ (DNA synthesis)

Growth G₂

Cel l Div

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Functions of Cell Division

20 µm

100 µm 200 µm

(a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM).

(b) Growth and development. This micrograph shows a

sand dollar embryo shortly after the fertilized egg divided, forming two 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

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DNA

•

Genetic information - genome

•

Packaged into chromosomes

50 µm

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DNA And Chromosomes

•

An average eukaryotic cell has about 1,000

times more DNA then an average

prokaryotic cell.

•

The DNA in a eukaryotic cell is organized

into several linear chromosomes, whose

organization is much more complex than the

single, circular DNA molecule in a

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Chromosomes

•

All eukaryotic cells store genetic information

in chromosomes.

–

Most eukaryotes have between 10 and 50

chromosomes in their body cells.

–

Human cells have 46 chromosomes.

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Structure of Chromosomes

•

Chromosomes are composed of a

complex of DNA and protein called

chromatin

that condenses during cell

division

•

DNA exists as a single, long,

double-stranded fiber extending chromosome’s

entire length.

•

Each unduplicated chromosome contains

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 Every 200 nucleotide pairs, the DNA wraps twice around a

group of 8 histone proteins to form a nucleosome.

 Higher order coiling and supercoiling also help condense

and package the chromatin inside the nucleus:

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

The degree of coiling can vary in different

regions of the chromatin:



Heterochromatin

refers to highly coiled

regions where genes aren’t expressed.



Euchromatin

refers to loosely coiled regions

where genes can be expressed.

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•

Prior to cell division each

chromosome duplicates

itself.

•

During this time, only the

heterochromatin is visible, as

dense granules inside the

nucleus.

•

There is also a dense area of

RNA production called the

nucleolus:

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12 5 µm

Pair of homologous chromosomes

Centromere

Sister chromatids

Karyotype

• An ordered, visual representation of the chromosomes in a cell

• Chromosomes are photographed when they are highly condensed, then photos of the individual chromosomes are arranged in order of decreasing size:

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Chromosomes

• Non-homologous chromosomes

– Look different

– Control different traits

• Sex chromosomes

– Are distinct from each other in their

characteristics

– Are represented as X and Y

– Determine the sex of the individual, XX being

female, XY being male

• In a diploid cell, the chromosomes occur in pairs.

The 2 members of each pair are called

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Chromosomes

• A diploid cell has two sets of each of its chromosomes

• A human has 46 chromosomes (2n = 46)

• In a cell in which DNA synthesis has occurred all the chromosomes are

duplicated and thus each consists of two identical sister chromatids

Maternal set of

chromosomes (n = 3)

Paternal set of

chromosomes (n = 3)

2n = 6

Two sister chromatids of one replicated chromosome

Two nonsister chromatids in a homologous pair

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Homologues

• Homologous chromosomes: • Look the same

• Control the same traits

• May code for different forms of each trait

• Independent origin - each one was inherited

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Chromosome Duplication

0.5 µm Chromosome duplication (including DNA synthesis) Centromere Separation of sister chromatids Sister chromatids

Centrometers Sister chromatids

A eukaryotic cell has multiple chromosomes, one of which is represented here. Before duplication, each chromosome has a single DNA molecule.

Once duplicated, a chromosome consists of two sister chromatids connected at the centromere. Each chromatid 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

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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

Two unduplicated chromosomes

Centromere

Sister

chromatids chromatids Sister Duplication

Non-sister chromatids

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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:

Metaphase chromosome

Kinetochore Kinetochore

microtubules

Centromere region of

chromosome

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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

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Phases of the Cell Cycle

• Interphase – G

1 - primary growth

– S - genome replicated

– G

2 - secondary growth

• M - mitosis

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Interphase

•

G

1

-

Cells undergo majority of growth

•

S

-

Each chromosome replicates (Synthesizes) to

produce sister chromatids

– Attached at centromere

– Contains attachment site (kinetochore)

•

G

2

-

Chromosomes condense - Assemble

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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

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Mitotic Division of an Animal Cell

G₂OF INTERPHASE PROPHASE PROMETAPHASE

Centrosomes

(with centriole pairs) Chromatin (duplicated)

Early mitotic spindle

Aster

Centromere

Fragments of nuclear envelope

Kinetochore

Nucleolus Nuclear envelope

Plasma membrane

Chromosome, consisting of two sister chromatids

Kinetochore microtubule

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METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

Spindle

Metaphase

plate _Nucleolus

forming Cleavage

furrow

Nuclear envelope forming Centrosome at

one spindle pole

Daughter chromosomes

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G₂ of Interphase

• 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 chromosomes in its somatic cells (chromosomes appear blue, microtubules green, intermediate filaments red). For simplicity, the drawings show only four chromosomes.

G₂OF INTERPHASE

Centrosomes

(with centriole pairs) Chromatin (duplicated)

Nucleolus Nuclear envelope

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Prophase

• 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”).

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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

Metaphase plate

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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)

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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 Chromosomes Sister chromatids Aster Centrosome Metaphase plate Kineto-chores Kinetochore microtubules 0.5 µm Overlapping nonkinetochore microtubules 1 µm Centrosome

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Anaphase

• 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 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

nonkinetochore microtubules lengthen. • By the end of anaphase, the two ends of the cell have equivalent—and

complete—collections of chromosomes.

ANAPHASE

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Telophase

• 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

Nucleolus forming Cleavage

furrow

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Mitosis in a plant cell

1 Prophase.

The chromatin is 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 discrete chromosomes; each consists of two identical sister chromatids. Later in prometaphase, the nuclear envelop will fragment.

Metaphase. The spindle is complete, and the chromosomes, attached to microtubules at their kinetochores, are all at the metaphase plate.

Anaphase. The chromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of cell as their

kinetochore

microtubles shorten.

Telophase. Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divided the cytoplasm in two, is growing toward the perimeter of the parent cell.

2 3 4 5

Nucleus

Nucleolus

Chromosome Chromatine

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Cytokinesis

• Cleavage of cell into two

halves

– Animal cells

 Constriction belt of

actin filaments

– Plant cells

 Cell plate

– Fungi and protists

 Mitosis occurs

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Cytokinesis In Animal And Plant Cells

Daughter cells

Cleavage furrow

Contractile ring of microfilaments

Daughter cells

100 µm

1 µm

Vesicles forming cell plate

Wall of

patent cell Cell plate

New cell wall

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Meiosis and Sexual Life Cycles

• Living organisms are distinguished by their ability to

reproduce their own kind

• Heredity

– Is the transmission of traits from one generation to the

– Shows that offspring differ somewhat in appearance

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Inheritance of Genes

• Genes are segments of DNA, units of heredity • Offspring acquire genes from parents by

inheriting chromosomes

• Genetics is the scientific study of heredity and

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Inheritance of Genes

•

Each gene in an organism’s DNA has a

specific locus on a certain chromosome

•

We inherit one set of chromosomes from our

mother and one set from our father

•

Two parents give rise to offspring that have

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Asexual Reproduction

•

In asexual reproduction, one parent

produces genetically identical offspring by

mitosis

Figure 13.2

Parent Bud

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Sexual Reproduction

• Fertilization and meiosis alternate in sexual life cycles • A life cycle is the generation-to-generation sequence of

stages in the reproductive history of an organism

Gametes

Diploid multicellular

organism

Key

MEIOSIS FERTILIZATION

n

2n

2n Zygote

Haploid Diploid

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Sex Cells - Gametes

•

Unlike somatic cells, sperm and egg cells

are haploid cells, containing only one set of

chromosomes

•

At sexual maturity the ovaries and testes

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Sexual Reproduction - The Human Life Cycle

• During fertilization,

sperm and ovum fuse forming a diploid

zygote

• The zygote develops

into an adult organism

Haploid (n) Diploid (2n)

Haploid gametes (n = 23)

Ovum (n)

Sperm Cell (n)

Ovary _Testis Diploid

zygote (2n = 46)

Mitosis and development

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Meiosis

•

Reduces the chromosome number such that

each daughter

•

Cell has a haploid set of chromosomes

•

Ensures that the next generation will have:

–

Diploid number of chromosome

–

Exchange of genetic information

(combination of traits

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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.

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Meiosis

• Meiosis reduces the

number of chromosome sets from diploid to

haploid

• Meiosis takes place in

two sets of divisions

– Meiosis I reduces the

number of chromosomes from diploid to haploid

– Meiosis II produces four

haploid daughter cells

Figure 13.7

Interphase

Homologous pair of chromosomes in diploid parent cell

Chromosomes replicate

Homologous pair of replicated chromosomes

Sister

chromatids Diploid cell with

replicated chromosomes 1 2 Homologous chromosomes separate

Haploid cells with replicated chromosomes

Sister chromatids separate

Haploid cells with unreplicated chromosomes

Meiosis I

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Meiosis Phases

• Meiosis involves the same four phases seen in

mitosis

 prophase

 metaphase

 anaphase

 telophase

• They are repeated during both meiosis I and

meiosis II.

• The period of time between meiosis I and meiosis

II is called interkinesis.

• No replication of DNA occurs during interkinesis

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Prophase I

• Prophase I occupies more than 90% of the time required for meiosis

• Chromosomes begin to condense

• In synapsis, the 2 members of each homologous pair of chromosomes line

up side-by-side, aligned gene by gene, to form a tetrad consisting of 4 chromatids

• During synapsis, sometimes there is an exchange of homologous parts

between non-sister chromatids. This exchange is called crossing over

• Each tetrad usually has one or more chiasmata, X-shaped regions where

crossing over occurred

Prophase I of meiosis

Tetrad

Nonsister chromatids

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Metaphase I

• At metaphase I, tetrads line up at the metaphase plate, with one

chromosome facing each pole

• Microtubules from one pole are attached to the kinetochore of one

chromosome of each tetrad

• Microtubules from the other pole are attached to the kinetochore of the

other chromosome Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Microtubule attached to kinetochore Tetrad

PROPHASE I METAPHASE I ANAPHASE I

Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example

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Anaphase I

• In anaphase I, pairs of homologous chromosomes separate • One chromosome moves toward each pole, guided by the

spindle apparatus

• Sister chromatids remain attached at the centromere and

move as one unit toward the pole

Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Microtubule attached to kinetochore Tetrad

PROPHASE I METAPHASE I ANAPHASE I

Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example

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Telophase I and Cytokinesis

• In the beginning of telophase I, each half of the

cell has a haploid set of chromosomes; each

chromosome still consists of two sister chromatids

• Cytokinesis usually occurs simultaneously,

forming two haploid daughter cells

• In animal cells, a cleavage furrow forms; in plant

cells, a cell plate forms

• No chromosome replication occurs between the

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Prophase II

• Meiosis II is very similar to mitosis

• In prophase II, a spindle apparatus forms

• In late prophase II, chromosomes (each still composed of

two chromatids) move toward the metaphase plate

Cleavage furrow

PROPHASE II METAPHASE II ANAPHASE II

TELOPHASE I AND

CYTOKINESIS TELOPHASE II ANDCYTOKINESIS

Sister chromatids separate

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Metaphase II

• At metaphase II, the sister chromatids are at the metaphase plate

• Because of crossing over in meiosis I, the two sister chromatids of each

chromosome are no longer genetically identical

• The kinetochores of sister chromatids attach to microtubules extending

from opposite poles

TELOPHASE I AND

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Anaphase II

• At anaphase II, the sister chromatids separate

• The sister chromatids of each chromosome now move as

two newly individual chromosomes toward opposite poles

TELOPHASE I AND

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Telophase II and Cytokinesis

• In telophase II, the chromosomes arrive at opposite poles

• Nuclei form, and the chromosomes begin decondensing

• Cytokinesis separates the cytoplasm

• At the end of meiosis, there are four daughter cells, each with a

haploid set of unreplicated chromosomes

• Each daughter cell is genetically distinct from the others and from the

parent cell

TELOPHASE I AND

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A Comparison of Mitosis and Meiosis

• Mitosis conserves the number of chromosome

sets, producing cells that are genetically identical to the parent cell

• Meiosis reduces the number of chromosomes sets

from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell

• The mechanism for separating sister chromatids is

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• Three events are unique to meiosis, and all three

occur in meiosis l:

– Synapsis and crossing over in prophase I:

Homologous chromosomes physically connect and exchange genetic information

– At the metaphase plate, there are paired homologous

chromosomes (tetrads), instead of individual replicated chromosomes

– At anaphase I of meiosis, homologous pairs move

toward opposite poles of the cell. In anaphase II of meiosis, the sister chromatids separate

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MITOSIS MEIOSIS

Prophase

Duplicated chromosome (two sister chromatids)

Chromosome replication

Parent cell

(before chromosome replication)

Chiasma (site of

crossing over) MEIOSIS I

Prophase I

Tetrad formed by synapsis of homologous chromosomes

Metaphase

Chromosomes positioned at the metaphase plate

Tetrads

positioned at the metaphase plate

Metaphase I

Anaphase I Telophase I

Haploid n = 3

MEIOSIS II Daughter cells of meiosis I Homologues separate during anaphase I; sister chromatids remain together

Daughter cells of meiosis II

n n n n

Sister chromatids separate during anaphase II

Anaphase Telophase Sister chromatids separate during anaphase 2n 2n Daughter cells of mitosis

2n = 6

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Comparison

• Meiosis

• DNA duplication followed by 2 cell divisions

• Sysnapsis

• Crossing-over

• One diploid cell produces 4 haploid cells • Each new cell

has a unique combination of genes

• Mitosis

• Homologous

chromosomes do not pair up

• No genetic exchange between homologous chromosomes

• One diploid cell produces 2 diploid cells or one haploid cell produces 2 haploid cells • New cells are

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Sexual Reproduction - The Human Life Cycle

• During fertilization,

sperm and ovum fuse forming a diploid

zygote

• The zygote develops

into an adult organism

Haploid (n) Diploid (2n)

Haploid gametes (n = 23)

Ovum (n)

Sperm Cell (n)

Ovary _Testis Diploid

zygote (2n = 46)

Mitosis and development

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Spermatocytes to Spermatids

• Primary spermatocytes undergo meiosis I, forming

two haploid cells called secondary spermatocytes

• Secondary spermatocytes undergo meiosis II and

their daughter cells are called spermatids

• Spermatids are small round cells seen close to the

lumen of the tubule

• Late in spermatogenesis, spermatids are nonmotile

• Spermiogenesis – spermatids lose excess

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Spermatogenesis

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Oogenesis

• Production of female sex cells by meiosis

• In the fetal period, oogonia (2n ovarian stem cells) multiply by

mitosis and store nutrients

• Primordial follicles appear as oogonia are transformed into

primary oocytes

• Primary oocytes begin meiosis but stall in prophase I

• From puberty, each month one activated primary oocyte

completes meiosis one to produce two haploid cells

– The first polar body

– The secondary oocyte

• The secondary oocyte arrests in metaphase II and is ovulated

• If penetrated by sperm the second oocyte completes meiosis

II, yielding:

– One large ovum (the functional gamete)

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