MEIOTIC CELL DIVISION

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