Meiosis:
Making haploid reproductive cells
Goal of Meiosis
The purpose is to make haploid cells so that when 2 haploid
cells combine you have a new diploid offspring that has an
original combination of genes.
+
Haploid cell
(1n)
=
+
Haploid cell = Diploid cell (2n)
(1n)
that turns into a
muticellular offspring
Meiosis Overview
o Makes 4 haploid cells
o Takes place in sex cells (gametes) such as egg &
sperm cells
o Location: Testes (male) and ovaries (female)
o The Goal:
o To make 4 haploid daughter cells for reproduction that
are genetically different from the parent cells.
o Each haploid daughter cell will contain a mixture of the
parents chromosomes, so that the 4 daughter cells
should be different.
Meiosis
• Involves 2 cell divisions
• The cell will go through interphase (G1,
S, G2)…but then will divide twice 
resulting in 4 haploid cells
• There are 2 parts in Meiosis:
–1. Meiosis 1 and Meiosis 2
Meiosis I and Meiosis II
• Both go through each of the
phases of mitosis (PMAT)
• After each one cytokinesis
occurs
• The goal is to reduce
chromosome number to
haploid so that 2 cells can
combine to make a diploid
organism
Meiosis 1: Prophase 1
• DNA coils into
chromatids
• Spindle fibers appear
• Nucleus/nucleolus
disappear
• *Homologous
chromosomes pair up
into tetrads of 4
chromatids, called
synapsis
• *Crossing over may
occur causing genetic
recombination
What is crossing over?
When a homologous pair bumps into each other
during prophase I and they exchange portions one
chromatid  leads to genetic recombination
Metaphase I & Anaphase I
 Metaphase I:
 Anaphase I:
Tetrads line up
Homologous pairs split
randomly in the center.
randomly, called
Spindle fibers attach to
independent assortment
each end of the
homologous pair
Independent Assortment
The random lining up of homologous pairs during metaphase
I. This allows for different gametes to be possible and also
leads toward genetic recombination
More independent assortment
What is genetic recombination?
• …when the combination of genes in an offspring are
different from the gene combination found in the
parents
• Occurs in 2 ways in meiosis
1. Crossing over where homologous pairs exchange portions of
one chromatid
2. Independent assortment where the homologous pairs line
up randomly or without influences like which parent they
came from or whether they are dominant or recessive.
Telophase I and Cytokinesis I
• Chromosomes reach poles
• Cytokinesis occurs forming 2
haploid cells with 2 copies of
their chromosomes
• Nuclear membrane may or
may not reform before
meiosis II
Meiosis II
(2nd cell division, similar to mitosis steps, PMAT)
•
•
•
•
•
Prophase II: Spindle fibers form, centrioles appear, nucleolus disappears
Metaphase II: Chromosomes move to center (along equator)
Anaphase II: Chromatids separate toward the poles
Telophase II: Nuclear membrane reforms. Spindle fibers disassemble.
Cytokinesis occurs forming 4 haploid cells with 1 copy of each chromosome.
How does Meiosis differ in males vs. females?
Since meiosis is producing sex cells (sperm & eggs) it takes place in the
testes (males) & the ovaries (female)
• In males:
– Spermatogenesis - the formation of 4 spermatids
(haploid cells) which will turn into spermatozoa (sperm
cells).
• In females:
– Oogenesis - the formation of 1 egg cell (ova) with more
cytoplasm than the other 3 cells. The remaining 3 cells
are called polar bodies and will disintegrate
The
phases
of
Meiosis
Diagrams
Let us compare
Mitosis VS. Meiosis
• Mitosis makes IDENTCIAL
cells to the parent
• Mitosis make DIPLOID
cells
• Mitosis occurs in BODY
cells
• Mitosis makes 2
DAUGHTER CELLS
• Meiosis make DIFFERENT
cells from the parent
• Meiosis makes HAPLOID
cells
• Meiosis occurs in
REPRODUCTIVE cells
• Meiosis makes 4
DAUGHTER CELLS
Animations
Johnkyrk.com
http://www.lewport.wnyric.org/jwanamaker/
animations/meiosis.html
http://www.stolaf.edu/people/giannini/