Cell Division - Meiosis
Meiosis
• Process of cell
division that reduces
the number of
chromosomes in new
cells to half the
number in the original
cell.
• The haploid cells
produced are called
gametes.
Meiosis
• Cells preparing to
divide by meiosis
undergo the G1, S
and G2 phases of
interphase.
Meiosis
• Cells will then begin
meiosis with a
duplicate set of
chromosomes.
• Meiosis is divided into
two stages called
Meiosis I and Meiosis
II.
Meiosis I
• Prophase I
• DNA coils tightly into
chromosomes,
spindles appear, and
the nuclear
membrane
disassembles.
Prophase I
• Each chromosome
lines up next to its
homologue. This is
called synapsis.
• Each pair of
homologous
chromosomes is
called a tetrad.
Prophase I
• During this stage,
crossing-over occurs.
• This is when genetic
material breaks off
and attaches to the
adjacent chromatids.
• The result is genetic
recombination.
Metaphase I
• Tetrads line up
randomly along the
midline of the dividing
cell.
• Spindles attach to the
centromeres of the
homologous
chromosomes.
Anaphase I
• Each homologous
chromosome moves
to an opposite pole.
• The chromosomes
are separated
randomly. This is
called independent
assortment.
Telophase I
• Chromosomes reach
the opposite ends of
the cell and
cytokinesis begins
Meiosis II
• Meiosis II is not started by the copying of
DNA.
• There may be a short resting stage but
usually there is no true interphase.
Prophase II
• Spindle fibers form
and begin to move
the chromosomes
toward the midline of
the cell
Metaphase II
• Chromosomes move
to the midline of the
cell.
Anaphase II
• The chromatids
separate and move
toward opposite poles
of the cell.
Telophase II
• Cytokinesis occurs
and produces 4 cells
• A nuclear membrane
forms around the
chromosomes in each
of the 4 new cells.
Formation of Gametes
• In males, meiosis
occurs in the testes.
This is called
spermatogenesis.
• Four sperm cells
result from meiosis.
Formation of Gametes
• In females, meiosis
occurs in the ovaries.
This is called
Oogenesis.
Formation of Gametes
• During cytokinesis for
females, the
cytoplasm is
unequally divided.
• This results in only 1
egg cell being formed
and 3 polar bodies.
Asexual Reproduction
• Offspring from only
one parent
• Offspring are
genetically identical
• Occurs from binary
fission or mitosis
Sexual Reproduction
• Offspring from two
parents
• Offspring are
genetically different
• Meiosis