SEXUAL
REPRODUCTION &
MEIOSIS
SEXUAL REPRODUCTION
 During
Sexual Reproduction, two sex
cells (sperm & egg) come together
Sperm are formed in male reproductive
organs
 Eggs are formed in female reproductive
organs

 The
joining of egg and sperm is
called Fertilization
 The new cell that is formed is called
a Zygote
DIPLOID CELLS
 Your

A
body forms two types of cells
Body Cells & Sex Cells
human body cell has 46
chromosomes
 Each chromosome has a “mate” that is
similar in size and has similar DNA
 The “mates” are paired into 23 pairs
 When cells have pairs of similar
chromosomes they are called diploid.
HAPLOID CELLS
Sex
cells do NOT have
pairs of chromosomes
Human sex cells have 23
chromosomes
They are called haploid
MEIOSIS AND SEX CELLS
A
process called meiosis
produces haploid sex cells
During meiosis, two
divisions of the nucleus
occur
 Meiosis
I
 Meiosis II
MEIOSIS
MEIOSIS I
 Before
the start of
meiosis I, the
chromosomes are
duplicated just
like mitosis

Each set of
duplicated
chromosomes is
called a
chromatid
MEIOSIS I
In
Prophase I the chromatids
are tightly coiled together,
each duplicated chromosome
comes near its mate.
What are chromatids?
MEIOSIS I
In
Metaphase I, the pairs of
duplicated chromosomes, or
chromatids, line up at the
equator. Each chromatid
pair attaches to a _spindle at
the centromere.
 What
is a centromere?
Homologous
Chromosomes
MEIOSIS I
In
Anaphase I the
chromosome pairs are
pulled to _opposite_sides
of the cell. Each
duplicated chromosome
still has two _chromatids_.
MEIOSIS I
In
Telophase I the
_cytoplasm divides and
two new cells form. Each
cell has one duplicated
chromosome from each
pair.
MEIOSIS II
NO
chromosome
duplication occurs before
meiosis II
The two cells formed
during meiosis I will begin
meiosis II
MEIOSIS II
 In
Prophase II the __chromosomes__ and
__spindle reappear in each of the cells
 In Metaphase II the chromatid pairs move
to the equator.
 Where
is the equator?
 However, each centromere
attaches to the
spindleindividually.
MEIOSIS II
In
Anaphase II the
__centromere divides and the
_chromatids___ separate and
move to opposite sides of the
cell. Each _chromatid is now
an individual chromosome.
MEIOSIS II
In
Telophase II the
spindle disappears and
the nuclear membrane
forms. The cytoplasm
divides and produces
__4_ cells.
SUMMARY OF MEIOSIS
 During
meiosis I, two cells formed
 During meiosis II, both of these cells
formed two cells
 Four sex cells were formed
 Each sex cell has ½ the number of
chromosomes that was in the
original nucleus
= 4 sex cells with 23 unpaired
chromosomes
ZDONK
Donkey
(62 chromosomes) +
Zebra(46 chromosomes) =
Zonkey
 How
many chromosomes does
the Zdonk have?
 What happens when meiosis
occurs in the Zdonk?
MISTAKES IN MEIOSIS
Mistakes
in meiosis can produce
sex cells with too many or too
few chromosomes
If the zygote survives, every cells
in that organism will have the
wrong number of chromosomes
Down Syndrome (Trisomy 21)