The cell cycle

• Division is different according to the type of cell: *Skin cells = frequently. *Liver cells = when needed *Nerve cells and muscles = don’t divide at all• in prokaryotes the genome is single long DNA molecule• in eukaryotes the genome consists of several DNA molecules• somatic cells have 46 chromosomes• sex cells have 23 chromosomes

Division is differ among cells

The key roles of cell division• The continuity of life from one cell to another is based on the

reproduction of cells via cell division.• This process is a part of the cell cycle • It occurs in both unicellular and multicellular organisms.• The development of multicellular organisms need cell division

to form a fertilized egg or zygote. It also uses it to repair and renew cells.

Cell division distributes identical sets of chromosomes to daughter cells:

• A human must duplicate 3m of DNA and separate the two copies so the two daughter cells can have complete genome.

• DNA molecules are packaged in chromosomes.• Human somatic cells (body cells) have 46

chromosomes in its nucleus.• Human gametes (sperms and eggs) have half the

number in somatic cells which is 23 chromosomes.• Each eukaryotic chromosome consists of a long linear

DNA molecule.

A characteristic A characteristic number of chromosomes in the number of chromosomes in the human’s nucleushuman’s nucleus

• somatic cells (body cells) 46 chromosomes

• gametes (sperm or eggs) 23 chromosomes

• Each chromosome has a large number of inherited characters (genes)

• The chromatin is a long thin fiber, after the DNA duplicates the chromatin condenses to form chromosome.

• Each duplicated chromosome has 2 sister chromatids which contains two identical copies of the chromosomes DNA.

• Centromere is the narrow region where the strands connect.

• Then the two sister chromatids are separated and are repackaged onto two new nuclei at opposite ends of the parent cell during the division.

• This process is called Mitosis and is usually followed by the division of the cytoplasm called Cytokinesis which occurs in somatic cells.

1 chromosome hundreds or 1 chromosome hundreds or thousands thousands of of genes genes

Has

1 chromosome two 1 chromosome two sistersisterchromatidschromatids which contain which contain identical identical copies of the copies of the chromosome’s DNA.chromosome’s DNA.

consists of

“ “ two sister two sister chromatids chromatids is connected by is connected by centromere “centromere “

ChromosomesThey are the condensed form of Chromatin,

which consists of protein + DNA

Duplicated chromosome(sister chromatids)

One chromatid

mitosis mitosis : :

• The process of the formation of the two daughter The process of the formation of the two daughter

nuclei callednuclei called(each with same the chromosomes number of the parent (each with same the chromosomes number of the parent

cell “ 46 chromosomes “ ) cell “ 46 chromosomes “ )

•division of the cytoplasm (division of the cytoplasm (cytokinesiscytokinesis).).

•It occurs in somatic cellsIt occurs in somatic cells

Cell division It happens in somatic cells and produce two daughter cells with 2n

the nuclear envelope

disappear.Microtubules

from each pole attach with one

kinetochores

centromeres divide, sister

chromatids are separated.

Each is pulled towards a pole

via spindle fibers.

• It is the division of the cytoplasm, and it follows mitosis and meiosis.

• How does it happen?Contraction of the cell pinches the cell

into two new cells

Cytokinesis

Meiosis :Meiosis :

• yields four daughter cellsyields four daughter cells(each with half the chromosomes number of the parent (each with half the chromosomes number of the parent

cell “ 23 chromosomes “ ) cell “ 23 chromosomes “ )

• It occurs in the gonadsIt occurs in the gonads

• gametes gametes (eggs or sperm) (eggs or sperm) are produced only in are produced only in gonads gonads (ovaries or testes).(ovaries or testes).

• In the Gonads the cells under go a Meiosis division that produce 4 daughter cells each carrying half the chromosomes from the parent cell.

• You inherit 23 chromosomes from each parent. 1 set found in the egg and 1 set found in the sperm.

• The fertilized egg under goes trillions of mitosis divisions and cytokinesis to produce a developed multicellular human.

• Fertilization fuses two gametes together and doubles the number back to 46 chromosomes.

They are chromosomes that are paired during meiosis. They consists of four chromatids. One pair comes from the father and the other from the mother. They are the same size, they have the same number of genes.

It is a display of an individual’s chromosomes that arranged according to size and shapes

A normal somatic cell has 46 pairs of chromosomes

Homologous, one parental & one maternal

This 22 pairs are called autosomes

23th is called sex chromosome.

The female has XXThe male has XY

Mitotic cell cycleThe mitotic (M) phase alternates with much longer interphase.•The M phase includes Mitosis and cytokinesis•This accounts for 90% of the cells cycle.During the interphase the cell prepares for division by producing cytoplasmic organelles and copying its chromosomes.

The Mitotic Cell CycleThe Mitotic Cell CycleThe mitotic phase includes mitosis and cytokinesis.

• During interphase ( interphase takes for 90% of the cell cycle) the cell prepares for

division by producing cytoplasmic organelles and copying its chromosomes.A. Interphase has three subphases:•The G1 phase (“first gap”): the cell grows.•The S phase (“synthesis”): the chromosomes are copied. •The G2 phase (“second gap”): the cell completes preparations for cell division.

2323

2323

o The fertilized egg undergoes trillions of cycles of mitosis and cytokinesis to

produce a fully developed multicellular human .

o Fertilization fuses two gametes together and doubles the number of chromosomes

to 46 again.

 Meiosis Division Occurs in two steps:A)Meiosis I • Separate homologous chromosomes. • Results in 2 haploid cells with replicated chromosomesOccurs in many steps: 1) interphase2) Prophase I3) Metaphase I4) Anaphase I5) Telophase I

Cytokinesis:•by the same mechanisms as mitosis usually occurs simultaneously.•In some species, nucleimay reform, but there is no further replication of chromosomes.

1) Interphase :-Chromosomes Chromosomes sister chromatids. sister chromatids.2) Prophase I :-Chromosomes condense & homologous chromosomes pair up tetrads.Chromatids of homologous chromosomes are crossed at (chiasmata).3) Metaphase I :-The tetrads arranged at metaphase plate, Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad.4) Anaphase I :-Homologous chromosomes separate and are pulled toward opposite poles.5) Telophase I :-Homologous chromosomes keep moving until there is a haploid Homologous chromosomes keep moving until there is a haploid set at each pole.set at each pole. Each chromosome consists of linked sister chromatids.

B) Meiosis II• No further replication of chromosomes.•Occurs in the newly resulting cells from Meiosis I.•(4 haploid cells)Occurs in many steps: 1) Prophase II 2) Metaphase II3) Anaphase II4) Telophase II

Cytokinesis:• separates the cytoplasm.•At the end of meiosis, there are four haploid daughter cells.

Mitosis

• Mitosis usually consists of 5 sub phases.a.Prophaseb.Prometaphasec. Metaphased.Anaphasee.Telophase• The chromosomes have been duplicated and are loosely

packed in the last stage of the interphase.

Lets know the stages of Mitosis !

Mitosis:Mitosis: is usually include five sub-phases is usually include five sub-phases

1- Prophase1- Prophase 2- Prometaphase, 2- Prometaphase,

3- Metaphase3- Metaphase 4- Anaphase4- Anaphase 5- Telophase5- Telophase

1- Prophase1- Prophase

the chromosomes are tightly coiled, with sister chromatids joined together, The nucleoli disappear. The mitotic spindle begins to form and appears to push the centrosomes away from each other towards opposite ends (poles) of the cell.

2- Prometaphase 2- Prometaphase

the nuclear envelope fragments and microtubules from one pole attach to one of two kinetochores (special regions of the centromere) while microtubules from the other pole

attach to the other kinetochore.

3- Metaphase3- Metaphase

the spindle fibers push the sister chromatids until they are all arranged at the imaginary plane equidistant between

the poles, defining metaphase

4- Anaphase4- Anaphase

the centromeres divide, result in separating the sister chromatids. Each is then pulled toward the pole to which it is attached by spindle fibers. By the end, the two poles have

equivalent collections of chromosomes.

5- Telophase5- Telophase

the cell continues to elongate as free spindle fibers from each centrosome push off each other

CytokinesisCytokinesis, begins as the division of the cytoplasm , begins as the division of the cytoplasm occursoccurs

CytokinesisCytokinesisdivides the cytoplasmdivides the cytoplasm

Contraction of the cell pinches the cell into two new cells

Cytokinesis

• Cytokinesis is the division of the cytoplasm that typically follows the mitosis division.

• The contraction of the cell pinches the cell into two new cells.

Meiosis :•DNA replication: occurs once, during the interphase before meiosis 1 begins.•Number of divisions: two, each include prophase, metaphase, anaphase and telophase.•Number of daughter cells and genetic composition:•4, each haploid(n) half the parent cell and not identical to the parent cell or to each other.•Role: produces gametes, reduces chromosome number by half and introduces genetic variability among the gamets.

The role of Meiosis in sexual life cycles:• In humans, each somatic cell has 46 chromosomes,

these homologous chromosome pairs carry genes that control the same inherited characters.

• A Karyotype display of the 46 chromosomes shows 23 pairs of chromosomes that have the same length, centromere position and staining pattern. Which means it’s the display of the chromosomes according to size and shape.

Chromosomes (sex & autosomes)

• An exception to the rule of the homologous chromosomes is found in the sex chromosomes the X and Y.

• The pattern of inheritance determine the individual’s sex:1.Females have a homologous pair of X chromosomes (XX)2.Males have a homologous pair of X and Y chromosomes (XY)• The other 22 pairs of chromosomes are called Autosomes.

• Sperm cells or ova have only one set of chromosomes, 22 autosomes and an X or Y.

• A cell with a single chromosome set is Haploid. The Haploid number of chromosomes in humans is 23 chromosomes.

• A haploid sperm fuses with a haploid ovum resulting in fertilization.

• The fertilized egg ( zygote) has a diploid set of chromosomes from the maternal and paternal family lines.

• As an organism develops from a zygote to a mature adult the zygotes genes passes on to all the somatic cells my Mitosis.

• Gametes are NOT produced by Mitosis instead it undergoes a process of Meioses.

Meiosis (reduction division)• This division reduces the chromosome number from

diploid to haploid.• The meiosis division resembles the step/stages in the

mitosis. And they are both preceded by the replication of chromosomes.

• In meiosis the chromosomes replicate only ONCE followed by two cell divisions called Meiosis I and Meiosis II which produces four daughter cells.

• Each final daughter cell has half the chromosome number, Haploid.

The Meiosis division reduces the chromosome number by copying the chromosome only once and dividing twice.

•Meiosis I is the first division that separates the homologous chromosomes.

•Meiosis II is the second division that separates the sister chromatids.

A. Meiosis I : its very similar to the Mitosis division.

1. Interphase : the chromosomes are replicated to form the chromatids.

2. Prophase I : the chromosomes condense and the homologous chromosomes pair up to form Tetrads. When the two homologous chromosomes attach together it is called Synapsis. The chromatids are crossed at chiasmata and segments of the chromosomes are exchanged ( Crossing over ).

3. Metaphase I : the tetrads are arranged at the metaphase plate. The microtubules from one pole is attached to the kinetochore of one of the chromosomes of each tetrad while those from the other pole are attached to the other.

4.Anaphase I : the homologous chromosomes separate and are pulled in opposite directions towards the poles.

5. Telophase I : the movement of the homologous chromosomes continue until there is a haploid set at each end. Each chromosome consists of linked sister chromatids.By the same mechanisms as the Mitosis division Cytokinesis usually occurs simultaneously.In some species the nuclei may reform but there are no more chromosome replications.

B. Meiosis II :1.Prophase II : a spindle apparatus forms and attaches to the kinetochores of each sister chromatid and moves them around.2.Metaphase II : the sister chromatids are arranged at the metaphase plate.3.Anaphase II : the centromeres of the sister chromatids are separated and then pulled to opposite poles.4.Telophase II : the separated sister chromatids arrive at opposite poles and the nuclei is formed around the chromatids.Cytokinesis it separates the cytoplasm. At the end of the Meiosis division there are 4 haploid daughter cells.

Crossing over• Crossing over occurs during Prophase I where the two

homologous chromosomes are joint very closely.• Two non-sister chromatids of the homologous

chromosomes are crossed over at a chiasma point and exchange corresponding segments.

• The resulting chromosomes are called Recombinant chromosomes.

• This is very important in genetic variation in sexual life cycles.

Sexual life cycles produce genetic variations among offspring:

• There are three mechanisms that contribute to genetic variation:

1.Independent assortment2.Crossing over3.Random fertilization.A.Independent assortment: contributes to genetic

variability due to the random orientation of tetrads at the metaphase plate.

B. crossing over: homologous portions of two non-sister chromatids exchange places producing recombinant chromosomes which combines genes inherited from both parents.

c. the random fertilization: it adds to the genetic variation arising from the Meiosis division. Any sperm can fuse with any egg.

Mitosis produces 2 identical daughter cells while Meiosis produces 4 very different daughter cells.

the nuclear envelope

disappear.Microtubules

from each pole attach with one kinetochores of

the tetrads.

tetrads divide, homologous

chromosomesare separated.

Each is pulled towards a pole

via spindle fibers.

the nuclear envelope

disappear.Microtubules

from each pole attach with one

kinetochore.

Sister chromosomes

divide, and they separate.

Each is pulled towards a pole

via spindle fibers.

Summery of Meiosis I & II

• PLZ,OPEN THE VIDEODifference between mitosis ans meiosis

http://www.youtube.com/watch?v=Ba9LXKH2ztU

• PLZ,OPEN THE VIDEO

Cell Cycle

http://www.youtube.com/watch?v=lf9rcqifx34

• PLZ,OPEN THE VIDEO

mitosis

http://www.youtube.com/watch?v=cvlpmmvB_m4

• PLZ,OPEN THE VIDEO

meiosis

http://www.youtube.com/watch?v=R_LUJSqeSrI

• Q1: Write whether each of the following statements is True (T) or False (F):

• • 1-( ) Mitosis (M) is the longest phase of the cell cycle.• 2- ( )Only interphase is not a stage of mitosis.• 3- ( )Genes are short segments of DNA. • 4-( )Random fertilization means Any sperm can fuse with

any egg.• 5- ( )All duplicated chromosomes are aligned at equator in

Anaphase.•

• Q 2: Choose the correct answer (one answer only): • 1-The correct sequence of the cell cycle is:

• a)- G1 – M – S – G2 b)- G1 – S – G2 - M• c)- M – G2 – S – G1 d)- G2 – S – G1 - M

• • • 2-Which of the following is true about homologous chromosomes during prophaseI of meiosis I:

•a)- they are lined the equator of the cell•b)- they are seen as tetrads

•c)- they are separated and carried to opposite poles d)- both a and b

• 3-Which of the following cells does not divide at all:• a)- white blood cell b)- skin cell• c)- liver cell d)- nerve cell

• • 4-During cell division, crossing over results in the formation of:

• a)- homologous chromosomes b)- recombinant chromosomes• c)- sex chromosomes d)- non of them

• • 5-A human cell containing 22 pairs of chromosomes + X X chromosomes is called:

• a)- a male somatic cell b)- an ovum• c)- a female somatic cell d)- a sperm

• 6-During cell cycle, Prophase includes which of the following stages:• a)- G1 b)- S

• c)- G2. d)- none of them• 

• 7-Crossing-over occurs between:• a)- two sister chromatides b)- two non-sister chromatides

• c)- both a and b d)- none of them• 8-In the human body, a sperm and ovum are called:

• a)- somatic cell b)- gametes• c)- diploid cell d)- all a, b and c

•9-Meiosis ------------------------the parental chromosome number.•a-Doubles b-Reduces

•c-Corrupts d-Maintains•10-The interphase(the non-dividing phase)in the cell cycle is divided into;

•a-G1 phase b-G2 phase

•c-S phase d-All of the above.