Mrs. Griffin
The birds and the bees…

Humans and most other animals reproduce
sexually.

This means that two cells fuse to make a new
“baby” organism.

These specialized cells are called gametes or
sex cells

Sperm and egg cells are human gametes
Meiosis

What is it?

Who does it?
Making sex cells (sperm and egg)
All sexually reproducing organisms
In males- testes
In females- ovaries

Where does it happen?

Why does it happen?

Why is a different process needed?
To all organisms to
reproduce
1) Chromosome # needs to be cut in half
2) Creates genetic diversity
Meiosis
MEIOSIS
Haploid
gametes
Diploid
adult
MITOSIS
Diploid
zygote
Remember, humans have 23
pairs of chromosomes
Pairs 1-22 are said to be
Homologous chromosomes
(they go together because
they have same traits but are
not identical)
Gene for eye color
Gene for hair color
Meiosis
Meiosis is a special kind of cell division
that halves the number of chromosomes
in gametes. This insures that the correct
number of chromosomes are passed on.
The goal is to make
haploid sex cells
Father
Mother
First division
Second division
Polar Bodies
(not functional)
What would happen if two
cells with 46 chromosomes fused?
46 chromosomes
+
46 chromosomes
92 chromosomes….
Nondisjunction =
chromosomes fail to “dis-join” or
separate:

Trisomy=gamete
with 3 of one type

Monosomy=gamete
with 1 of one type

Tripoidy= zygote
with three of one
type
Down Syndrome= 3 of #21
Karyotype
Klinefelter’s = XXY
Some symptoms…











Delayed puberty
Loss of libido - reduced interest in sexual activity
Impotence
Reduced facial hair - reduction in need to shave
Tiredness
Reduced muscle power and stamina
Change in body shape - increased fat and breast
development
Infertility
Osteoporosis - brittle bones resulting in fractures
Depression and sometimes dissocial behavior
Early heart disease
Turner’s Syndrome

XO sex chromosome

Instead of XX female
or XY for male
Meiosis has two stages:
Meiosis 1
and Meiosis 2
Meiosis 1: This is the stage where the
chromosome number is halved
 Includes 4 stages:

 Prophase 1
 Metaphase 1
 Anaphase 1
 Telophase 1
Meiosis 1

Prophase 1 is similar
in some ways to
prophase in mitosis.
 Chromosomes
condense
 Spindle fibers appear
 Nucleus and nucleolus
disappear
UNLIKE in mitosis, homologous
chromosomes line up next to each
other during prophase
•This process is
called synapsing
•Lined up
homologues are
called tetrads
When homologous chromosomes
hang out so close to each other,
they sometimes swap parts.
This is called “crossing over”
Synapsis and Crossing
Over in Action
Crossing over increases
genetic diversity
Why is this important
for a species’
long-term survival?
During metaphase 1, tetrads
line up on a metaphase plate
Then spindle fibers pull the
homologous chromosomes apart in
anaphase 1
In telophase 1, two daughter
cells are formed. They are NOT
identical! (Why?)
After Meiosis 1…
 Homologous chromosomes have
been separated.
 Two non-identical daughter cells
have been formed.
 The chromosome number has been
cut in half.
The big picture
Chromosomes are NOT
duplicated again
between
Meiosis 1 and Meiosis 2
Why not?
Meiosis 2 is similar to
mitosis

Two identical daughter cells are formed
from each of the cells created in Meiosis 1

Includes 4 Stages:
 Prophase 2
 Metaphase 2
 Anaphase 2
 Telophase 2
Meiosis II
Meiosis 2

Prophase 2: spindle reforms and chromosomes
move toward the metaphase plate

Metaphase 2: sister chromatids lined up on the
metaphase plate

Anaphase 2: sister chromatids are separated and
pulled toward opposite poles of the cell

Telophase 2 and Cytokinesis: nuclei form at either
pole and each cell is finally divided into two
identical daughter cells
All together now…
Let’s compare mitosis
with meiosis…