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Meiosis
Need For Reproduction
Reproduction is the life process in which
living things produce other living things of
the same species.
Reproduction is not necessary for the life
of one individual organism.
It is necessary for the continued survival
of a particular group of organisms.
Cell Division
All cells arise from other cells by cell
division.
When cells grow to a certain size they
must either divide or die.
If a cell continued to grow without dividing,
the surface area of the cell would become
too small to hold the cell’s contents.
Cell Division
 Cell division is a complex series of changes in
the nucleus of a cell that leads to the production
of two new cells.
 The new cells are called daughter cells.
 The nuclei of the daughter cells are usually
identical to each other and to that of the parent
cell.
 The daughter cells grow and increase in size
until they divide and produce two more daughter
cells.
 This process continues, resulting in organism
growth and reproduction.
Mitosis
All cells in the body (except sex cells) are
produced by the process of mitotic cell
division.
Mitosis involves a complex series of
changes in the nuclei of body cells that
produce identical (same) daughter cells.
They have the same number and type of
chromosomes as the parent cells.
The Cell Cycle – Stages of Mitosis
Although the events of mitosis are an
ongoing process, they are generally
described in terms of separate phases,
or stages.
Interphase
Prophase
Metaphase
Anaphase
Telophase
Meiosis
The process of meiosis involves two cell
divisions and produces cells that are
different from the parent cell.
Meiosis produces cells that have one-half the
number of chromosomes as the parent cells.
If meiosis did not take place, the fertilized egg
would have double the amount of chromosomes it
needs.
Meiosis takes place during sexual
reproduction when sex cells, called the egg
and sperm, are produced in sex organs.
The Cell Cycle- Stages of Meiosis
 During meiosis, two divisions of the nucleus
occur. These divisions are called meiosis I and
meiosis II.
 Interphase
 Prophase I
 Metaphase I
 Anaphase I
 Telophase I
 Prophase II
 Metaphase II
 Anaphase II
 Telophase II
Meiosis
One-half the number of chromosomes is
called the monoploid, (or n) number.
In humans, the monoploid number (n) is 23.
When the egg and sperm unite during
fertilization, the species normal
chromosome number called the diploid or
(2n) number is restored (brought back).
The diploid number in humans is 46.
n (sperm) + n (egg) = 2n
Summary of Meiosis
 Two cells form during meiosis I.
 In meiosis II, both of these cells form two cells.
 The result is four sex cells.
 Each sex cell has one-half the number of
chromosomes in its nucleus that was in the
original nucleus.
 From a human cell with 46 paired
chromosomes, meiosis produces four sex cells
each with 23 unpaired chromosomes.
Comparison of Mitosis and Meiosis
Characteristic
Mitosis
Meiosis
Number of daughter cells
2
4
Number of cell division
1
2
Daughter cells are
diploid or haploid
Diploid
Haploid
Daughter cells are
identical or different
Identical
Different
Diploid
Diploid
Parent cell is
diploid or haploid