Cell division occurs through mitosis and meiosis. Mitosis produces somatic cells with identical chromosomes to the parent cell, while meiosis produces gametes with half the number of chromosomes. Meiosis has two divisions: meiosis I separates homologous chromosomes and reduces the number to haploid, and meiosis II separates sister chromatids to produce four haploid daughter cells each with a single set of chromosomes. This ensures genetic variation and maintains chromosome number from generation to generation.

1. CELL DIVISION
(meiosis I &II)
DEPARTMENT OF BIOLOGICAL SCIENCES
FEDERAL UNIVERSITY DUTSIN-MA
BIO 231(Cell biology & Histology)
BY RUQAYYA ADAM

2. CELL DIVISION
• Cell division is a means of reproduction; whereby parent cells
divide in to two or more daughter cells thereby transmitting
traits from parents to offsprings (progeny).
• Chromosomes play an important role in determining the
characteristics of a cell and its progeny
• It is important that the chromosomes are correctly distributed to
the daughter cells.
• Cells normally have a fixed number of chromosomes which occur
in pairs called Diploid condition
• There are two types of cell division based on the behavior of
chromosomes
• Mitotic cell division
• Meiotic cell division

3. Diploid vs. Haploid
• Diploid – a cell that contains homologous chromosomes
(one from each parent)
represented by the symbol 2n
– Found in somatic or body cells (ex. Skin, digestive
tract)
• Example : Humans  2n = 46
• Haploid – a cell that contains only a single set of
chromosomes (one from either parent, not both);
represented by the symbol n
– Found in gametes or sex cells – sperm & egg
• Example: Humans  n = 23

4. MITOTIC CELL DIVISION
• Takes place during growth of an organism, occurs in somatic
cells, and daughter cells ends up exactly with the same number
of chromosomes as the parents.
• It is primarily concerned with equal distribution of chromosomes
into daughter cells
• Note; not all somatic cells undergo mitosis e.g Mammalian RBC
(which has no nucleus) does not divide neither does a neuron (a
specialized cell
• Most cells that divide mitotically are found in the regions of
growth; plant root tips, skin cells, cells of developing embryo
e.t.c.

5. MEIOTIC CELL DIVISION
• Is otherwise known as reduction division
• is a special type of cell division of germ cells in sexually-
reproducing organisms used to produce the gametes
• In more complex plants, it occurs in the formation of spores
• Daughter cells ends up with half the total number of
chromosomes present in parent cells
• The daughter cells are said to be in the haploid condition with
(n) number of chromosomes
• Meiosis consists of two successive division
• The 1st Meiotic division
• The 2nd Meiotic division

6. Chromosome Numbers
• Somatic cells: (diploid = 2n = 46 chromosomes in humans)
• Gametes: (haploid = n = 23 chromosomes in humans)
MAKING OF SPERM
MAKING OF EGG

7. Chromosome numbers of some common
Organisms

8. MEIOTIC CELL DIVISON
• The 1st meiotic division is regarded as reduction division in
which the parent cells split into two
• And the homologous chromosomes get separated from each
other going into different cells.
• The 2nd meiotic division is where the product of 1st meiotic
division divides again to give a total of four(4) daughter cells.
• It is concerned with separating the chromatids.
• The process of meiosis is divided into series of stages (P,M,A,T)
just like in mitosis but each stage is designed with I or II to
indicate it either belongs to first or second division of meiosis

9. MEIOSIS I
• INTERPHASE I
• Stage before prophase I
• Contains: centrioles and
Chromatin
Made of stages:
• G1 – basic cell growth
• S – replication and repair of DNA
• G2 – final preparation for cell
division
Centrioles
Nucleus
(with chromatin)

10. MEIOSIS I
• PROPHASE I
• Nucleolus disappears, centrioles become arranged at
opposite ends of the nucleus
• Chromosomes condense
• The difference between this prophase I and prophase of
mitosis is that the homologous chromosomes come to lie side
by side a process known as synapsis
• The chromosomes become coiled round each other, and when
they move apart slightly, the chromatids of the two
homologous chromosomes make contact at a point known as
chiasmata
• The chiasmata may be followed by chromosomes breakage
with subsequent exchange of the broken segment between
maternal and paternal chromatids, a process known as
crossing over

11. Centrioles
PROPHASE I

12. Crossing over happens when exchange of
chromosomal segment or genetic materials occur
between non-sister homologous chromosomes by
breakage and union

13. MEIOSIS I
• METAPHASE I
• The chromosomes move to the
equator of the spindle
• One important difference from
what is obtained in mitosis is
the homologous chromosomes
together behave as a unit
• At this stage the cell is now
ready for the separation of the
homologous chromosomes

14. MEIOSIS I
• ANAPHASE I
• The homologous chromosomes
each consisting of a pair of
chromatids join at the
centromere region
• and moves to the opposite poles
of the spindle
• The sister chromatids gets
separated from each other
• If the centrioles do not properly
attach to the spindle fiber, then
non disjunction will occur
• Which causes gametes to have an
unequal number of chromosomes
Centrioles
Spindle
fibers
Homologous
Chromosomes

15. MEIOSIS I
• TELOPHASE I
• After the chromosomes have
reach the end of the poles,
Cytokinesis occur.
• The cell creates a temporary
nucleus around the homologous
chromosomes
• Usually the daughter cells
undergo a short interphase but
sometimes the chromosomes
remain condense and the
daughter cell move straight into
the prophase of the 2nd meiotic
division
Centrioles
Homologous
Chromosomes
Nuclear
Membrane

17. MEIOSIS II
• The main aim of the 2nd
meiotic division is to
separate the chromatids
from one another
• Each of the Meiosis II
Stages run in 2 cells at the
same time
• PROPHASE II
• A new spindle fiber is
formed and the centriole
have replicated

18. MEIOSIS II
• METAPHASE II
• The chromosomes
move to the
equator of the
spindle

19. MEIOSIS II
• ANAPHASE II
• At this phase the
chromosomes
separate
• and move apart
from each other
• and they later
become the
chromosomes of the
daughter cells

20. Meiosis II
• TELOPHASE II
• After reaching the poles,
cytokinesis occur in the usual
way
• The spindle fiber disappears
• The nucleolus and nuclear
membrane are reformed
• The chromosomes uncoil and
regain their threadlike form
• Meiosis is now complete
resulting in the formation of
four daughter cells with each
having haploid (n) number of
chromosomes
Nuclear membrane

22. SIGNIFICANCE OF MEIOSIS
• It is responsible for the formation of sex cells gametes e.g
sperm, ovum/egg, spores
• Production of variation: causes variation in descendants so
that individuals differ in resemblance with their parents
• Provides constancy of chromosome number from generation
to generation by reducing the number from diploid to haploid,
thereby producing haploid gametes