Cell Division

The Cell Cycle

-Cycle means the repetition of a sequence of events during a given time period.

-The cell cycle is divided into:

1. Mitosis: ordinary cell division.

2. Interphase.

DIVISION

M

MitosisG2

Seco

nd g

apG1

DNA synthesis

Fir

st

gap

S

INTERPHASE

The Cell Cycle

Cell Theory and Cell Division

Cell Theory

• Def: All organisms consist of cells and arise from pre-existing cells

• Cell cycle: predictable sequence of cell life events

– Chromosomes: thread-like bodies visible during cell division

The interphase is divided into three phases: 1-Gap I or G phase: The one following mitosis 2-Gap S phase: Synthesis phase where DNA is duplicated 3-Gap 2 or G 2 phase: -So, it is in the G1 or G2 phase where the cell is

performing its specialized function while in the S phase it is only directed to duplicate its DNA..

-In the interphase the cells become specialized working cells, so we may get prolonged G1 or rarely G2 phase.

Functions of the nucleus 1-It directs the cell division. 2-It carries all the genetic information and hereditary characters. 3-It controls all the cell functions including protein synthesis. 4-It is responsible for the formation of RNA.

The cells of the body may be categorized into 3 types:

1- Very highly specialized cells-They never divide i.e. we posses a certain

number of these cells since birth.

-If they die or wear out, they never regenerate. e.g. nerve cells, and cardiac muscle cells.

2- Highly specialized cells -They can keep their members from other cells of

the same family type from a stem or a mother cell.

e.g. cells lining the stomach and intestine.

3- Specialized cells -They can go into cycle only under extreme conditions. e.g. liver cells (hepatocytes). -In case of removal of up to 2/3 of the liver in

experimental animals, these cells are able to restore the normal size of the liver.

•

Cell Theory and Cell Division

Gap2 phase: a period of time between S and beginning of mitosis

– Synthesis of materials used in mitosis

– Microtubule proteins, ATP, other organelles

– 4-5 hours duration

• Duration times based upon one organism, others may differ

DIVISION

M

MitosisG2

Seco

nd g

apG1

DNA synthesis

Fir

st

gap

S

INTERPHASE

The Cell Cycle

Cell Division-In the human body, two types of

cell division are known:

I-Mitosis. II-Meiosis.

Cell Theory and Cell Division

Set of chromosomes

• Ploidy: a term referring to the numbers of sets of chromosomes in a given cell

• Def: one of each kind of chromosome that is characteristic of a species– Human set has 23 chromosomes

• Haploid (n): one set

• Diploid (2n): two sets

I-Mitosis -It is the process by which the cell divides. -In animal cells, the whole process takes

from 1-15 hours.

-Before going into mitosis, duplication of

the chromosomes occurs during the s-stage of the interphase.

-At the G2 stage of the interphase, we have 46 d-chromosomes (double-stranded chromosomes) and also double pairs of centrioles.

-During mitosis, the 46 d-chromosomes separate giving us 92 s-chormosomes, where each half of them is directed towards the corresponding pole of the cell.

-Thus, two nuclei with the typical46 s-chromosome arrangement of a nucleus in the g1 stage of the

interphase are formed, and two similar daughter cells result.

• -Another important feature of mitosis, is the formation of mitotic spindle.

• -The cells in the G1 stage have 2 centrioles which are perpendicular to each other. If this cell starts to go to the s-phase and G2 phase, these 2 centrioles become 4 in number.

Phases of Mitosis

• 1-Prophase• 2-Metaphase• 3-Anaphase• 4-Telephase

Mitosis (M Phase)

Prophase Events

• Chromosomes condense, become visible– Replicated chromosomes (=chromatids) joined

at the centromere

• Spindle apparatus forms and attaches to kinetochore (structure associated with the centromere)

• Nuclear membrane begins to disintegrate

1-Prophase:-Disapearance of nucleolus and nuclear

envelop. -Formation of the mitotic spindle. -Direction of each divided cenrtiole to

one of the cell pole. -Chromosomes condensed to 2

chromatid

Mitosis (M Phase)

Metaphase Events

• Chromosomes align along the equator of the cell

• Spindle appratus anchored (in animals) at the centriole (site of spindle formation)

PRIOR TO MITOSIS MITOSISChromosomes replicate,forming siblingchromatids

Sibling chromatids separate

CentrosomesCentrioles

Early mitotic spindleKinetochore

1. Chromosomesreplicate in parent cell.

2. Prophase: Chromosomescondense and mitotic spindle begins to form. Nuclear envelope breaks down.

3. Metaphase:Chromosomes migrate to middle of cell

Interphase Prophase Metaphase

Stages of Mitosis

2-Metaphase:-The chromosomes are aligned in the

equatorial plane.-The chromosomes are attached to the

spindles at the kinetochore

Mitosis (M Phase)

Anaphase Events

• Centromere divides allowing spindle apparatus to shorten and separate sibling chromatids

• Separated chromatids called chromosomes

• Chromosomes move to opposite poles of the cell

4. Anaphase: Sister chromatids separate. Chromosomes are pulled to opposite poles of the cell.

5. Telophase: The nuclear envelope reforms. 6. Cytokinesis: The cell divides.

Anaphase Telophase

Stages of Mitosis

4-Telophase:*Reappearance of the nuclear memb.

and constriction in the cytoplasm to form two new cells

* Formation of the chromatin network

Mitosis (M Phase)

Telophase and Cytokinesis Events

• Nuclear envelope begins to form around chromosomes

• Chromosomes become diffuse

• Spindle apparatus disappears

• Cytokinesis begins

Mitosis (M Phase)

• Cytokinesis in animals

characterized by a cleavage furrow

• Cytokinesis in plants

characterized by the formation of a new wall between newly forming nuclei

Mitosis in Progress

Interphase

17 m

Animal Cell Mitosis

Prophase

17 m

Animal Cell Mitosis

Metaphase

17 m

Animal Cell Mitosis

Anaphase

17 m

Animal Cell Mitosis

Telophase

17 m

Animal Cell Mitosis

Cytokinesis

17 m

Animal Cell Mitosis

MEIOSISMEIOSIS

Meiosis

• The form of cell divisioncell division by which gametes, gametes, with halfhalf the number of chromosomes,chromosomes, are produced.

• Diploid (2n) Diploid (2n) haploid (n)haploid (n)

• MeiosisMeiosis is sexual reproduction.sexual reproduction.

• Two divisionsTwo divisions (meiosis I meiosis I and meiosis IImeiosis II).

MeiosisMeiosis

• Sex cells Sex cells divide to produce gametesgametes (sperm or egg)(sperm or egg).

• GametesGametes have halfhalf the number of chromosomeschromosomes.

• Occurs only in gonads (testes or ovaries).Occurs only in gonads (testes or ovaries).

Male: spermatogenesisMale: spermatogenesisFemale: oogenesisFemale: oogenesis

• MeiosisMeiosis is similar to mitosismitosis with some chromosomal differences.

SpermatogenesisSpermatogenesis

2n=46

humansex cell

diploid (2n)

n=23

n=23

meiosis I

n=23

n=23

n=23

n=23

sperm

haploid (n)

meiosis II

II-Meiosis-It is the division which occur in the gamete

cells in the ovary and the seminiferous tubules.

-The mother cell in the female ovary is the primary oocyte.

-At the time of birth, the baby girl has two million primary oocytes in her two ovaries.

-The oocyte passes to the s-stage where it contains 46 d-chromosomes then through the G2 to start the division.

Meiosis I (four phases)Meiosis I (four phases)

• Cell division Cell division that reduces the chromosomechromosome number by one-half.one-half.

• four phasesfour phases:a.a. prophase Iprophase Ib.b. metaphase Imetaphase Ic.c. anaphase Ianaphase Id.d. telophase Itelophase I

Prophase IProphase I

• Longest and most complex phase (90%).Longest and most complex phase (90%).

• ChromosomesChromosomes condense.

• SynapsisSynapsis occurs: homologous chromosomes homologous chromosomes come together to form a tetradtetrad.

• TetradTetrad is two chromosomeschromosomes or four chromatidschromatids (sister and nonsister chromatids).

First meiotic division-It consists of prophase, metaphase, anaphase

and telophase. -In the prophase there are 5 stages:

1-Leptotene 2-Zygotene3- Pachytene4-Diplotene 5-Diakinesis

1-Leptotene The 46 d-chromosomes appear as ribbons or

in a thread-like form.

2-Zygotene Each member of the pair of d-chromosomes

lies beside each other .thus, a bivalent is formed, one of a maternal

origin and the other from a paternal origin.

3- Pachytene The chromosomes become thicker and

shorter.

4-Diplotene Each bivalent consists of 2 d-chromosomes and

every d-chromosome is composed of 2 strands or 2 s-chromosomes.

5-Diakinesis -The 2 arms of the bivalent may cross one

another and this is called a chiasma. -This causes detachment and the fragment

reunits with the other stump. -This results in the exchange of paternally

derived chromosome portions with parts of maternally derived chromosomes and vice-versa.

Prophase IProphase I - - SynapsisSynapsis

Homologous chromosomes

sister chromatids sister chromatidsTetrad

Homologous ChromosomesHomologous Chromosomes• Pair of chromosomeschromosomes (maternalmaternal and paternalpaternal) that are similar in shape

and size.

• Homologous pairs (tetrads) (tetrads) carry genes controlling the same inherited traits.

• Each locuslocus (position of a gene) (position of a gene) is in the same position on homologues.

• Humans have 23 pairs of homologous chromosomes.homologous chromosomes.

a. 22 pairs of autosomesautosomesb. 01 pair of sex chromosomessex chromosomes

Homologous ChromosomesHomologous Chromosomes

Paternal Maternal

eye color locus

eye color locus

hair color locus

hair color locus

Crossing OverCrossing Over

• Crossing over (variation) Crossing over (variation) may occur between nonsister chromatidschromatids at the chiasmatachiasmata.

• Crossing overCrossing over: segments of nonsister chromatidschromatids break and reattach to the other chromatidchromatid.

• Chiasmata (chiasma) Chiasmata (chiasma) are the sites of crossing overcrossing over.

Crossing Over - variation Crossing Over - variation

nonsister chromatids

chiasmata: site of crossing over

variation

Tetrad

Sex ChromosomesSex Chromosomes

XX chromosome - female XY chromosome - male

Interphase IInterphase I

• Similar to mitosismitosis interphase.

• ChromosomesChromosomes replicate (S phase).(S phase).

• Each duplicated chromosomechromosome consist of two identical sister chromatidschromatids attached at their centromerescentromeres.

• CentrioleCentriole pairs also replicate.

Interphase IInterphase I

• NucleusNucleus and nucleolusnucleolus visible.

nuclear membrane

nucleolus

cell membrane

chromatin

Prophase IProphase I

centriolesspindle fiber

asterfibers

Metaphase IMetaphase I• Shortest phaseShortest phase

• TetradsTetrads align on the metaphase platemetaphase plate.

• INDEPENDENT ASSORTMENT OCCURS:INDEPENDENT ASSORTMENT OCCURS:1. Orientation of homologous pair to poles is random.2. Variation3. Formula: 2n

Example:Example: 2n = 42n = 4then then n = 2 n = 2

thusthus 2 22 2 = 4 combinations= 4 combinations

Metaphase IMetaphase I

metaphase plate

OR

metaphase plate

Anaphase IAnaphase I

• Homologous chromosomes Homologous chromosomes separate and move towards the poles.

• Sister chromatids Sister chromatids remain attached at their centromerescentromeres.

Anaphase IAnaphase I

Telophase ITelophase I

Meiosis IIMeiosis II

• No interphase II No interphase II (or very short - no more DNA replicationDNA replication)

• Remember:Remember: Meiosis II Meiosis II is similar to mitosismitosis

Prophase IIProphase II

• same as prophaseprophase in mitosismitosis

Metaphase IIMetaphase II

• same as metaphasemetaphase in mitosismitosis

metaphase platemetaphase plate

Anaphase IIAnaphase II

• same as anaphaseanaphase in mitosismitosis• sister chromatids separatesister chromatids separate

Telophase IITelophase II

• Same as telophasetelophase in mitosismitosis.

• Nuclei form.

• CytokinesisCytokinesis occurs.

• Remember:Remember: four haploid daughter cells four haploid daughter cells produced.produced.

gametes = sperm or egggametes = sperm or egg

Telophase IITelophase II

MeiosisMeiosis

2n=4

sex cell

diploid (2n)

n=2

n=2

meiosis I

n=2

n=2

n=2

n=2

sperm

haploid (n)

meiosis II

KaryotypeKaryotype• A method of organizing the A method of organizing the chromosomes chromosomes of a cell in relation to of a cell in relation to

number, size, and type.number, size, and type.

FertilizationFertilization

• The fusion of a spermsperm and eggegg to form a zygotezygote.• A zygote is a fertilized egg

n=23egg

sperm n=23

2n=46zygote