Meiosis
Creating Gamete Cells
MEIOSIS: a
process by which
the number of
homologous
chromosomes per
cell is reduced by
half the number
and is referred to
as haploid cells (n).
46 chromosomes
Human female karyotype
23 pairs
Meiosis only happens in Gametes
( sex cells – egg or sperm)
46 chromosomes
23 pairs
Human male karyotype
What are homologous chromosomes?
a similar pair of
chromosomes, one
from the father &
one from the
mother
What is a Diploid Cell?
A cell that contains 2
complete sets of
homologous
chromosomes is said
to be diploid or 2n
Brain cells
Blood cells
Somatic (body cells)
are diploid cells.
Nerve cells
Liver cells
What is a Haploid Cell?
A cell that contains 1 complete set of
chromosomes.
These are called
gametes or sex cells
(egg or sperm)
Meiosis: production of gametes
• Alternating stages
– chromosome number
must be reduced
• diploid  haploid
• 2n  n
haploid
– humans: 46  23
• meiosis reduces
chromosome number
• makes gametes
– fertilization restores
chromosome number
• haploid  diploid
• n  2n
diploid
There are 2
distinct divisions
of meiosis:
Meiosis I
Interphase I
Prophase I
and
Metaphase I
Meiosis II
Anaphase I
resulting in 4 haploid
daughter cells with
“n” number of
chromosomes
Telophase I
Metaphase II
Telephase II and
Cytokinesis
Each chromosome
pairs with its
corresponding
homologous
chromosome to
form a structure
called a tetrad.
Meiosis I - Prophase I
There are 4 chromatids in a tetrad.
Overview of meiosis
I.P.M.A.T.P.M.A.T
2n = 4
interphase 1
prophase 1
metaphase 1
anaphase 1
n=2
prophase 2 metaphase 2 anaphase 2 telophase 2
n=2
telophase 1
As homologous
chromosomes pair
up, they exchange
genetic information
in a process called
crossing over
producing new
combinations of
alleles – accounting
for variation among
species.
Sexual reproduction creates variability
Sexual reproduction allows us to maintain both
genetic similarity & differences.
Jonas
Brothers
Baldwin brothers
Martin & Charlie Sheen, Emilio Estevez
Gamete Formation in Meiosis
2n
2n
2n
n
2n
2n
n
n
Sperm - male
n
2n
n
Polar Bodies
Egg- female
Human Sperm + Human Egg Fertilization
Human Zygote
23
23
46
N
N
2N
Mitosis
Cells
Examples
Meiosis
Somatic (body)
Gametes (sex)
Skin, Blood
Sperm, Egg
2n
Chromosomes
Found ?
All over body
Cytokinesis
Once
n
Testes, Ovaries
Twice
MITOSIS
MEIOSIS
2n
2n
2n
2n
2n
n
2n
n
n
n