Meiosis
Production of sex cells for sexual reproduction
Meiosis - A process of reduction division in
which the number of chromosomes per cell
is cut in half through the separation of
homologous chromosomes in a diploid cell.
Meiosis Terminology
Diploid – 2 sets of chromosomes (2N)
Haploid – 1 set of chromosomes (N)
Homologous – chromosomes that each have
a corresponding chromosome from the
opposite sex parent
Tetrad – 4 chromatids in a homologous pair
Synapsis – the pairing of 2 homologous
chromosomes (during prophase I)
Gametes – sex cell (egg and sperm)
Crossing-over - when chromosomes overlap
and exchange portions of their chromatids
(exchanging some of their genes)
Meiosis Stages
 Meiosis usually involves 2 distinct stages
 Meiosis I
 Meiosis II
Prophase I
Prophase I
 Each chromosome pairs with its corresponding
homologous chromosome to form a tetrad.
 Crossing-over may occur here
(pieces of chromosomes or genes
are exchanged causing more
genetic variation)
Metaphase I
 Spindle fibers attach to the homologous
chromosomes
 Homologous chromosomes (tetrads) line up
at the center
Anaphase I
 The spindle fibers pull each one of the
homologous chromosomes toward opposite
ends of the cell.
Telophase I & Cytokinesis
 Nuclear membranes
form around each
set of chromosomes
 The cell separates
into 2 cells
Prophase II
 Meiosis I results in two haploid (N) cells.
 Each cell has half the number of
chromosomes as the original cell.
Metaphase II
 The chromosomes line up at the center
(similar to metaphase in mitosis).
Anaphase II
 Sister chromatids separate and move to
opposite ends of the cell.
Telophase II & Cytokinesis
 Nuclear membrane reappears
 The cytoplasm separates
 Meiosis II results in 4 haploid cells
Gamete Formation
 In males, meiosis results in 4 sperm cells
 In females, meiosis results in 1 egg cell
and three polar bodies, which are not
used in reproduction.
Mitosis vs Meiosis
Mitosis
Results in
(chromosome #)
Does a synapsis
occur?
# of divisions
# of daughter cells
Cells are genetically
identical or different?
Occurs in …
When does it
occur?
Role it plays in the
body
Meiosis
Mitosis vs Meiosis
Mitosis
Meiosis
Results in
(chromosome #)
2 Diploid Cells (2N)
Same as parent
4 Haploid Cells (N)
Half of parent
Does a synapsis
occur
no
yes
# of divisions
1
2
# of daughter cells 2
4
Cells are genetically
identical or different?
Genetically Identical
Genetically Different
Occurs in…
Somatic (body) Cells Sex (gametes) Cells
When does it
occur
Throughout life
Role it plays in the Growth and repair
body
Sexual maturity
Produce gametes for
sexual reproduction