Cell Division
Human Life Cycle Involves
1) Growth
Mitosis- a nuclear division of a parental cell that produces two daughter cells
with the same chromosome number as the parental cell. (2n= 46)
2) Reproduction
Meiosis- two nuclear divisions of a parental cell that produces four daughter
cells with half the chromosome number as the parental cell. (n= 23)
Mitosis
The cell cycle is made of 2 phases: Interphase & cell division (mitosis).
I. Interphase: is the period between two divisions.
It is the period of growth where chromosomes are not visible but can duplicate.
Chromosomes are called chromatin material.
Importance of Interphase: Duplication of chromosomic material.
By: Hanadi Al-Shafee
Mitosis: is a process of cell division which results in the production of two
daughter cells from a single parent cell. The daughter cells are identical to one
another and to the original parent cell.
In a typical animal cell, mitosis can be divided into four stages:
1. Prophase:
- Chromatin condenses and becomes chromosomes (Each chromosome is
made of 2 chromatids attached at
the centromere).
- Centrosomes duplicate &
transform into 2 asters.
- Nuclear membrane and the
nucleolus disappear
- .
- Spindle fibers (achromatic spindle)
form between the 2 asters that start
to move apart).
-
2. Metaphase:
- Chromosomes are aligned at the “equator” forming the “equatorial plate”.
By: Hanadi Al-Shafee
- Asters moves to the poles
3. Anaphase:
- The 2 sister chromatids of each chromosome separate after the division of
the centromeres.
- “Polar Ascension”, also called “Pole Formation”: Chromatids migrate
toward
opposite
poles
of the cell.
4. Telophase:
-
Chromosomes uncoil and change back to chromatin.
Nuclear membrane reappears.
Spindle fibers disappear.
Asters change back to centrosome.
The cell divide into 2 daughter cells: each contains “2n” chromosomes as
in the mother cell, nut each chromosome is made of 1 chromatid.
By: Hanadi Al-Shafee
- Then, the 2 daughter cells enter interphase to duplicate chromatin, and the
cycle repeats.
II.
The difference between animal & plant cell division:
 In plant cells the spindle fibers forms from the cytoplasm because plant
cells lack the presence of centrosome.
 In plant cell: no aster formation and has cell wall and cell membrane.
 In animal cell: Asters present and has cell membrane only
III.
Example: Animal Cell:
 If 2n: 4 chromosomes. (where n= number of pairs).
 At the beginning of mitosis, the cell is made of 2n = 4 chromosomes.
Each chromosome is made of 2 sister chromatids.
 By the end of Mitosis, each cell contains 2n=4 chromosomes same
number of mother's cell chromosomes, but each chromosome is made of
1 sister chromatid.
Mitosis in an animal cells
2n=4
By: Hanadi Al-Shafee
Mitosis of Plant cell
2n=4
Chromosomic variation during interphase and mitosis:
By: Hanadi Al-Shafee
Meiosis
Meiosis is a type of cell division that reduces the number of chromosomes in
the parent cell by half and produces four gamete cells. This process is required
to produce egg and sperm cells (gametes) for sexual reproduction.
All human cells possess 46 chromosomes (2n) with 2 copies of each
chromosome, except for gametes that contain 23 chromosomes (n).
Sperm is the gamete of male: contains 22 chromosomes (from autosomes) + X
or 22 chromosomes (from autosomes) + Y
Sperm: 22+X or 22+Y
Ovum is the female gamete: it contains 22 chromosomes (from autosomes) + X
Ovum: 22+X
Property
Mitosis
Meiosis
DNA replication
Occurs during interphase
Occurs during interphase
before mitosis begins.
before meiosis begins.
One division
Two successive divisions:
Number of divisions
Meiosis I (reductional
division), Meiosis II
(equational division)
Number of daughter
Two daughter (2n) cells
Four daughter cells (n),
cells
identical genetically to the each contains half the
mother cell.
mother chromosomes.
Kind of cells undergo
Somatic cells (body cells)
Sex cells (gametes)
Role in the body (uses)
Produces cells for:
Produces male and female
- Growth
gametes.
- Wound healing
- Cell repair
By: Hanadi Al-Shafee
I. The Reductional Division (Meiosis I):
1- Prophase 1:
-
Nuclear membrane disappears.
Centrosome changes into asters that migrate to opposite poles.
Spindle fibers appear between the 2 asters.
Chromatin condenses into chromosomes.
Homologous chromosomes pair up to form tetrads.
2- Metaphase:
- Homologous chromosomes pair & line up at the equator, forming the
“equatorial plate”.
3- Anaphase 1:
- “Polar Ascension”: Homologous chromosomes separate & each
chromosome migrates toward opposite poles.
By: Hanadi Al-Shafee
4- Telophase 1:
- Two daughter cells are formed. Each cell contains 91n) chromosomes i.e.
half the number of chromosomes of the mother cell.
- Each chromosome is still made up of 2 sister chromatids.
II. The equational Division (Meiosis II):
5- Prophase 2: Same events as in Meiosis I, but each cell contains (1n)
chromosomes.
6- Metaphase 2: Same events as in Meiosis I, but each cell contains (1n(
chromosomes.
7- Anaphase 2: Sister chromatids separate & migrate toward opposite poles.
8- Telophase 2: 4 daughter cells are formed: Each cell contains “n”
chromosomes, and each chromosome is made of 1 chromatid.
By: Hanadi Al-Shafee
III. Fertilization:
In sexual reproduction, the sperm (the male gamete) and
the ovum (the female gamete) unite. It results in the
formation of the zygote which is the first cell of a new living organism.
It is the male who determines the gender (sex) of the newborn. This depends on
whether his gamete contains an X or Y chromosome.
By: Hanadi Al-Shafee