MEIOSIS IS A SPECIAL
FORM OF CELL DIVISION
• MEIOSIS IS NECESSARY FOR
SEXUAL REPRODUCTION
• CELLS DIVIDE TWICE DURING
MEIOSIS.
– Before meiosis starts, the chromosomes of the
parent cell are copied. (so a cell ready to
divide has twice as many chromosomes as
usual)
• MEISOIS & MITOSIS DIFFER IN SOME
IMPORTANT WAYS
SEXUAL REPRODUCTION
• Human body cells have 46 chromosomes, its
“diploid” number or 2n (the full number of
chromosomes for the species- the 2n for a fruit fly is
8)
• GAMETES: cells with ½ the usual number of
chromosomes (one from each pair). Are 1n or
haploid cells (sex cells). Human gametes have 23
chromosomes.
• EGG is gamete formed in reproductive organ of a
female; SPERM is gamete formed in male
reproductive organs. FERTILIZATION: process
where a sperm (1n) and an egg (1n) combine to form
one new cell (2n).
Sexual Reproduction
• In humans the egg has 23 chromosomes, the sperm also has
23 chromosomes, and they form a fertilized egg with 46
chromosomes (the diploid number or 2n).
• MEIOSIS is a special kind of cell division that makes
haploid (1n) cells. During meiosis a single cell (2n) goes
through 2 cell divisions: meiosis I and meiosis II. This only
occurs in reproductive tissue of an organism.
2 CELL DIVISIONS
• Before meiosis begins, the chromosomes of
parent cell are copied (interphase); so twice as
many chromosomes as normal (4n)
• At the beginning of meiosis I, the cell has 2
copies of each homolog. These get separated not
once, but twice during meiosis to make 4 haploid
(1n) cells.
• Each haploid cell has one UNPAIRED set of
chromosomes
•
Meiosis
I
Prophase I: chromosomes pair
up with partners; 2 sets of
chromosome pairs in parent
cell now (doubled homologs
or 4n)
• Metaphase I: each set of
chromosome pairs line up
along center of cell.
• Anaphase I: 2 copies of one
homolog are pulled apart from
2 copies of other homolog (to
opposite ends of parent cell)
This is most critical time.
• Telophase I: a new cell
membrane forms at the center
of cell, dividing parent cell
into 2 daughter cells (each 2n
now)
Meiosis II
• Prophase II: in each daughter cell
are 2 copies each of n
chromosomes (2n) still attached to
each other.
• Metaphase II: chromosomes line
up in center of cell.
• Anaphase II: the 2 copies of each
chromosome separate & pull to
opposite ends of the cell.
• Telophase II: new cell membrane
forms in center of cell, so each cell
divides into 2 (1n) daughter cells,
making a total of 4 (1n) cells.
• In humans (& other species) only
1 of the 4 daughter cells made by a
female becomes an egg. The rest
dissolve or are never formed.
•
Meiosis
&
Mitosis
Differ
Only cells becoming gametes go through meiosis. All other
cells go through mitosis
• A cell dividing by meiosis goes through 2 cell divisions, but
chromosomes aren’t copied before the second division. In
mitosis, chromosomes are always copied before division.
• Meioisis daughter cells are haploid (1n) and only have ½ of
parent cells genetic material, while mitosis daughter cells are
diploid (2n) and have exactly the same genetic material as
the parent cell.