1. Introduction
The continuity of life from one cell to another is based on the reproduction of cells via cell division.
This division process occurs as part of the cell cycle (the life of a cell from its origin in the division of a parent cell until its own division into two).
The division of a unicellular وحيد الخلية organism (e.g. Amoeba) reproduces an entire organism, increasing the population.
Cell division is also central to the development of a multicellular عديد الخلية organism that begins as a fertilized egg or zygote.
Fig. 12.1, Page 216
Figs. 12.1, Page 216

2. Division is differ among cells:.
Multicellular organisms also use cell division to repair and renew cells that die normally or by accidents (blood cells from bone marrow).
Cell division distributes the genetic material (DNA) to two daughter cells.
Division is differ among cells:.
Skin cells divide frequently.
Liver cells divide when needed (damage repair).
Nerve cells and muscle cells do not divide at all.

3. 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 individualorganism (LM).
(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).

4. 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

5. DNA Genetic information - genome Packaged into chromosomes Figure 12.3

6. 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 prokaryotic cell

7. 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.
23 nearly-identical pairs

8. 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
2n = 6
Two sister chromatids
of one replicated
chromosome
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)
Centromere

9. Homologues Homologous chromosomes: 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

10. Chromosome Duplication
In preparation for cell division, DNA is replicated and the chromosomes condense
Each duplicated chromosome has two sister chromatids, which separate during cell division
0.5 µm
Chromosome duplication (including DNA synthesis)
Centromere
Separation of sister chromatids
Sister chromatids
Centrometers
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.

11. Chromosome Duplication
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:
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Two unduplicated
chromosomes
Centromere
Sister
chromatids
Duplication
Non-sister chromatids
Two duplicated chromosomes

12. DNA duplication during interphase
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

13. Cell division distributes identical sets of chromosomes to daughter cells
A cell’s genetic information (genome البنك الـﭽينى) is packaged as DNA.
In prokaryotes, the genome is often a single long DNA molecule.
In eukaryotes, the genome consists of several DNA molecules.
A human cell must duplicate about 3 m of DNA and separate the two copies such that each daughter cell ends up with a complete genome.
DNA molecules are packaged into chromosomes.
Every eukaryotic species has a characteristic number of chromosomes in the nucleus.
Human somatic cells (body cells) have chromosomes.
Human gametes أمشاج (sperm or eggs) have chromosomes, half the number in a somatic cell الخلية الجسدية.
Each eukaryotic chromosome consists of a long, linear DNA molecule.

14. Each chromosome has hundreds or thousands of genes (the units that specify an organism’s inherited characters الصفات الوراثية).
This DNA-protein complex (chromatin) is organized into a long thin fiber.
After the DNA duplication, chromatin condenses form (chromosome).
Each duplicated chromosome consists of two sister chromatids which contain identical copies of the chromosome’s DNA.
The narrow region where the chromosomal strands connect is the called centromere.
Later, the sister chromatids are pulled apart and repackaged into two new nuclei at opposite ends of the parent cell during cell division.
The process of the formation of the two daughter nuclei called (mitosis) and is usually followed by division of the cytoplasm (cytokinesis الإنشطار الخلوى ). It occurs in somatic cells الخلايا الجسدية

15. These processes continue every day to replace dead and damaged cell.
In the gonads المناسل, cells undergo a meiosis division, which yields four daughter cells, each with half the chromosomes number of the parent cell.
In humans, meiosis reduces the number of chromosomes from 46 to 23.
Each of us inherited 23 chromosomes from each parent: one set in an egg and one set in a sperm during meiosis.
gametes الأمشاج(eggs or sperm) are produced only in gonads المناسل (ovaries or testes).
The fertilized egg undergoes trillions of cycles of mitosis and cytokinesis to produce a fully developed multicellular human.
These processes continue every day to replace dead and damaged cell.
Fertilization fuses two gametes together and doubles the number of chromosomes to 46 again.

16. Definitions
Genes: The units that specify an organism’s inherited characters.
Chromatin: A DNA-protein complex which is organized into a long thin fiber
Chromosome: The package that formed from a condensed, coiled and folded chromatin.
Chromatids: Two sister arms (chromatids) formed from each duplicated chromosome. They contain identical copies of the chromosome’s DNA
Centromere: The narrow region at which the chromosomal strands are connect togreher.
Mitosis: Is the division process which forms two daughter nuclei
Cytokinesis: الإنشطار الخلوى Is the division stage of the cytoplasm which usually follow the mitosis.
Meiosis: A division process that occurs In the gonads المناسل, and yields four daughter cells, each with half the chromosomes of the parent.

17. Mitotic Division of an Animal Cell
G2 OF 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
Nonkinetochore microtubules

18. Mitotic Division of an Animal Cell
METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS
Spindle
Metaphase plate
Nucleolus forming
Cleavage furrow
Nuclear envelope forming
Centrosome at one spindle pole
Daughter chromosomes

19. 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.
G2 OF INTERPHASE
Centrosomes (with centriole pairs)
Chromatin (duplicated)
Nucleolus
Nuclear envelope
Plasma membrane

20. 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”).
The centrosomes move away from each other, apparently propelled by the lengthening microtubules between them.
PROPHASE
Early mitotic spindle
Aster
Centromere
Chromosome, consisting of two sister chromatids

21. Metaphase The centrosomes are now at opposite ends of the cell.
Spindle
Metaphase plate
Centrosome at one spindle pole
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.

22. 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

23. The Mitotic Spindle
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
1 µm

24. 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 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 nonkinetochore microtubules lengthen.
By the end of anaphase, the two ends of the cell have equivalent—and complete—collections of chromosomes.
ANAPHASE
Daughter chromosomes

25. TELOPHASE AND CYTOKINESIS
Nucleolus forming
Cleavage furrow
Nuclear envelope forming
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.

26. Cytokinesis Cleavage of cell into two halves Animal cells
Constriction belt of actin filaments
Plant cells
Cell plate
Fungi and protists
Mitosis occurs within the nucleus

27. Cytokinesis In Animal And Plant 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
(a) Cleavage of an animal cell (SEM)
(b) Cell plate formation in a plant cell (SEM)

28. 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 next
Variation
Shows that offspring differ somewhat in appearance from parents and siblings

29. 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 hereditary variation

30. 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 unique combinations of genes inherited from the two parents - sexual reproduction

31. 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
Zygote
Haploid
Mitosis
(a) Animals

32. 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 produce haploid gametes by meiosis

33. Sexual Reproduction - The Human Life Cycle
Haploid (n)
Diploid (2n)
Haploid gametes (n = 23)
Ovum (n)
Sperm
Cell (n)
MEIOSIS
FERTILIZATION
Ovary
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)
During fertilization, sperm and ovum fuse forming a diploid zygote
The zygote develops into an adult organism

34. 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
that differs from that of either parent)

35. 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.

36. Meiosis
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
separate
Haploid cells with
replicated chromosomes
Sister chromatids
Haploid cells with unreplicated chromosomes
Meiosis I
Meiosis II
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

37. 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 because the DNA is already duplicated.

38. 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
Chiasma,
site of
crossing
over

39. 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
Pairs of homologous
chromosomes split up
Tetrads line up

40. 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
Pairs of homologous
chromosomes split up
Tetrads line up

41. 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 end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated

42. 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 AND
Sister chromatids
separate
Haploid daughter cells
forming

43. 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
Cleavage
furrow
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE I AND
CYTOKINESIS
TELOPHASE II AND
Sister chromatids
separate
Haploid daughter cells
forming

44. 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
Cleavage
furrow
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE I AND
CYTOKINESIS
TELOPHASE II AND
Sister chromatids
separate
Haploid daughter cells
forming

45. 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
Cleavage
furrow
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE I AND
CYTOKINESIS
TELOPHASE II AND
Sister chromatids
separate
Haploid daughter cells
forming

46. Crossing over Occurs during prophase I.
The two homologous chromosomes joint together 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”.
It is important in genetic variation in sexual life cycle.

47. 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 virtually identical in meiosis II and mitosis

48. A Comparison of Mitosis and Meiosis
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

49. A Comparison Of Mitosis And 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
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
Sister chromatids separate during anaphase II
Anaphase
Telophase
Sister chromatids
separate during
anaphase 2n
Daughter cells
of mitosis
2n = 6

50. Comparison Mitosis Meiosis 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 genetically identical to original cell (except for mutation)
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

52. Sexual life cycles produce genetic variation among offspring
Three mechanisms contribute to genetic variation الإختلافات الوراثية:
independent assortment الإنتقال الحر للكروموسومات
crossing over العبور
random fertilization التلقيح العشوائى
1)- Independent assortment: of chromosomes contributes to genetic variability due to the random orientation of tetrads at the metaphase plate.
There is a fifty-fifty chance that a particular daughter cell of meiosis I will get the maternal chromosome of a certain homologous pair and a fifty-fifty chance that it will receive the paternal chromosome.

53. Independent assortment alone would find each individual chromosome in a gamete that would be exclusively maternal or paternal in origin.
3)- Crossing over:
Homologous portions أجزاء متماثلة of two non-sister chromatids exchange places, producing recombinant chromosomes which combine genes inherited from each parent.
2- The random fertilization: it adds to the genetic variation arising from meiosis.
Any sperm can fuse with any egg.