Meiosis
Meiosis
Functions to decrease the # of chromosomes
by half
Human = 46x (diploid number)
After Meiosis = 23x (haploid number)
Occurs in gametes (sperm/eggs)
Overview of Meiosis
Two successive
nuclear division
Produces 4 haploid
cells
Similar to mitosis
Prophase I
1. Homologues (like) chromosomes pair up.
 They make TETRAD that is composed of two
chromatids from each chromosome.
2. Crossing-over may occur at this point.
 Chromatids break and may be reattached to a
different homologous chromosome.
Crossing over
Metaphase I
Tetrads line-up along
the equator of the
nuclei
Spindle fibers attach
to the centromere
Anaphase I
Tetrads separate
Independent assortment of
homologous chrms.
(random separation)
Drawn to opposite poles
by the spindle fibers
Centromere in anaphase I
remain intact
Telophase I
One complete set of
chromosomes is in
each “future cell”.
Prophase II
Nuclear envelopes (if
they formed during
telophase I) dissolve
Chromosome
condense again
Spindle fibers reform
Metaphase II
spindles moving
chromosomes into the
middle and attaching
to the opposite sides of
the centromeres
Anaphase II
duplicated
Telophase II
Cytokinesis
Cytoplasm
divides into
the 4 haploid
cells
Formation of Gametes
In animals, meiosis produces haploid
reproductive cells called gametes
Occurs in the reproductive organs
Testes and Ovaries
Spermatogenesis
Production of sperm cells
Diploid cell divides into 4 sperm cells
Oogenesis
Production of an egg cell
Diploid cell divides to form 1 mature egg
(receives most of cytoplasm) and 3 eggs that
won’t survive called polar bodies
Chromosome Numbers in
Various Species
Carrot
18
Cat
32
Dog
78
Chimpanzee
48
Earthworm
36
Fruit fly
8
Garden pea
20
Horse
64
Lettuce
18
Haploid
Numbers?
Critical Thinking
Develop graphs to illustrate the mass of DNA and
chromosome number in each phase of meiosis.
Your graphs should be similar to the format
shown for mitosis. For telophase, consider only
one of the offspring cells at the end of telophase.
Let the number 1 represent the mass of DNA that
is found in a human ovum. Let the number 1
equal the number of chromosomes found in a
human ovum.
Mass and Chromosome # in
Mitosis
4
3.5
3
2.5
DNA Mass Units
Chromosome #
2
1.5
1
0.5
0
Inter Pro Met Ana Telo Cyto