Meiosis is the process of cell division that produces gametes (sex cells) like sperm and eggs, which have half the number of chromosomes as regular body cells. During meiosis, homologous chromosomes pair up and may exchange sections through crossing over, introducing genetic variation. This results in four daughter cells from two divisions for males and one viable egg cell for females. Meiosis reduces the chromosome number so fertilization restores the diploid number, maintaining genetic diversity between generations.

1. Cell Division – Meiosis
Chromosomes are DNA!
• Chromosomes contain genetic information

2. Cell Division – Mitosis (Review)
– Division of a somatic cell that results in 2 genetically
identical daughter cells
• Cells must divide for growth, repair of tissues, and
asexual reproduction
• Cell division begins in Interphase when the
chromosomes duplicate

3. Parent cell
Chromosomes
duplicate 2 new daughter
cells identical to
parent cell
• Daughter cells are genetically identical to parent cell –
same kind and number of chromosomes
• Mitosis occurs in somatic or body cells
Ex: liver, heart, skin, stomach
• Every organism has its own unique number of
chromosomes. Humans have 46.
This is called its diploid number or the total number of
chromosomes in a somatic cell.
Diploid means “2 sets” and is written as “2N”.

4. • Body cells of adult organisms have 2 sets of
homologous (matching) chromosomes – 1 set from
female parent and 1 set from male parent

5. Cell Division –Meiosis
– the process in which the number of chromosomes in the
original cell is reduced by HALF through the separation
of homologous chromosomes
• Meiosis occurs in sex organs only
• Males (XY) – sex organs are the testes in humans
• Females (XX) – sex organs are the ovaries in humans
• Meiosis also occurs in the sex organs of other
animals, plants, fungi, etc…

7. Parent
cell
Chromosomes
duplicate
Division 1
Division 2
Daughter cells
have half as
many
chromosomes
as parent cell

8. Meiosis produces sex cells –
cells with ½ the number of
chromosomes as the original
cell
• Males – meiosis produces
4 sperm
• Females – meiosis
produces 1 (viable) egg
The other 3 cells are
called polar bodies – they
give up their cytoplasm to
nourish the 1 good egg.
• Egg and sperm (sex cells)
are also called gametes

9. • Gametes have ½ the number of chromosomes as
somatic (body) cells. We call this the haploid number.
Haploid means “1 set” and is written as “N”.
If human diploid number is 46, what is its haploid
number? 23
Diploid # of a dog – 78 Haploid # of a dog – 39
Diploid # of a fly – 8 Haploid # of a fly – 4

10. • When does meiosis occur in humans?
1. Males beginning at puberty
2. Females before birth – all eggs are produced before
birth and at puberty eggs mature

11. Chromosome Number
• Remember, chromosome number is unique to each
kind of organism and all cells (except sex cells) in an
organism have the same kind and number of
chromosomes.
Ex: All humans have 46 chromosomes and all cells in
the human body (except sperm and egg) have 46
chromosomes.
• This is why the chromosome number in sex cells must
be reduced in half by meiosis
Ex: Humans have 46 chromosomes in their somatic
cells, but 23 chromosomes in their sex cells
(egg and sperm)

13. 23
23
46
Zygote develops into
embryo and finally adult
organism by mitosis
Fertilization – process by which an egg and sperm unite
Zygote – fertilized egg
Embryo – organism in early stage of development
Fertilized egg –
zygote

14. • Without meiosis …………
Fertilized egg –
zygote
46
46
92

15. Unique events in Meiosis
• Homologous (matching) chromosomes pair up before
1st cell division
Homologous chromosomes:
-look alike
-code for same traits
-receive one from each parent

16. • During 1st division, homologous chromosomes exchange
genes during process called “crossing over”
• These homologous chromosomes separate during 2nd
division of meiosis – so chromosomes in gametes are
different from each other due to crossing over
• Crossing over increases genetic variation and is the
reason why siblings look different

17. No crossing over
– daughter cells
are identical to
parent cells
Crossing over
occurs –causes
genetic variation
(Daughter cells
are NOT identical
to parent cell)

18. Comparing Mitosis and Meiosis

20. Mitosis Meiosis
What kind of cells? Somatic cells
Male (XY) = Sperm
Female (XX) = Egg
When does this occur? Any time
Male (XY) = puberty
Female (XX) = before birth
# of Divisions
(Draw picture)
1 2
# of Daughter cells 2
Male (XY) = 4 sperm
Female (XX) = 1 viable egg
# of Chromosomes
Same as parent cell
diploid or 2N
In humans 46
Half as many as parent cell
haploid or N
In humans 23
Type of Reproduction Asexual Sexual
Genetic Composition
Daughter cells
identical / not identical
to parent cell
Daughter cells
identical / not identical
to parent cell
Genetic variation
Pairing of Homologous
Chromosomes
YES / NO
YES / NO
Crossing over of genes
Function/Importance
Growth, repair; development of adult
from zygote
Production of gametes:
eggs and sperm
Sex Cells