11–4 Meiosis
The making of sex cells!
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Making Sex Cells

Meiosis is the
process of making
sex cells
(gametes)

Gametes are sex
cells

Ex: sperm, egg,
pollen, etc…
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Female Gametes - Eggs
Human Eggs
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Male Gametes - Sperm
Human Sperm
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Chromosomes

A body cell in an
adult fruit fly has 8
chromosomes, as
shown in the
drawing on the
right.

Four of the
chromosomes came
from the fruit fly's
male parent, and 4
came from its
female parent.
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Homologous Chromosomes
 chromosomes
that each have a
corresponding chromosome from
the opposite-sex parent

Example: Fruit-Fly (Drosophila) Chromosomes 8 chromosomes (4 from mom and 4 from dad)
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Different Types of Cells
A
cell that contains both sets of
homologous chromosomes is said to
be diploid (“two sets”)
 a diploid cell is sometimes
represented by the symbol 2N
 for
Drosophila, the diploid number is
8, which can be written 2N = 8
 Diploid
cells)
cells = body cells (somatic
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 Haploid
Cells = a cell that contains
only a single set of chromosomes
 Therefore
only a single set of genes
a
haploid cell is sometimes
represented by the symbol N
 for
Drosophila, the haploid number is
4, which can be written N = 4

Haploid cells = sex cells (gametes)
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How are haploid (N) gamete cells
produced from diploid (2N) cells?
Meiosis
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Meiosis
a
process of reduction
division in which the number
of chromosomes per cell is
cut in half through the
separation of homologous
chromosomes in a diploid
cell.
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Phases of Meiosis
 two
distinct divisions, called
meiosis I and meiosis II
 By
the end of meiosis II, the
diploid cell that entered meiosis
has become 4 haploid cells.
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What is the diploid
number for these
cells?
2
What is the
haploid number
for these cells?
1
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Meiosis I
 Before
meiosis I, each chromosome is
replicated.
 Division looks similar to mitosis
 Prophase I, Metaphase I, Anaphase I,
Telophase I
 prophase of meiosis I - each
chromosome pairs with its
corresponding homologous
chromosome to form a structure called
a tetrad
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Tetrads
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Crossing Over
 As
homologous chromosomes pair up
and form tetrads in meiosis I, they
exchange portions of their chromatids
in a process called crossing-over
 results in the exchange of alleles
between homologous chromosomes
and produces new combinations of
alleles
 Alleles = alternative forms of the
same gene (ex: blue eyes vs. brown
eyes)
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After Crossing Over…
homologous chromosomes separate
 two new cells are formed
 New cells:

each pair of homologous chromosomes
was separated
 neither of the daughter cells has the two
complete sets of chromosomes (they
have been shuffled and sorted)
 The new cells are DIFFERENT from each
other

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Meiosis II
 two
cells produced by meiosis I
now enter a second meiotic division
 NO DNA replication before Meiosis
II
 Prophase II, Metaphase II, Anaphase
II, Telophase II
 paired chromatids separate
 Produces: haploid cells (N)

Haploid cells are DIFFERENT from each
other
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Gamete Formation

Males gametes = sperm (pollen in plants)



All sperm the same size at the end of Meiosis
4 sperm produced for each round of meiosis
Female gametes = eggs



One egg produced and 3 polar bodies (egg is
MUCH larger in size)
The one egg receives the most cytoplasm
One egg produced for each round of meiosis
(and 3 polar bodies which can’t be fertilized)
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Fertilization of a
human egg by
sperm.
Notice the HUGE
size difference.
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Comparing Mitosis and Meiosis
 Mitosis
and Meiosis sound alike but
are VERY different!
 Mitosis produces two genetically
identical diploid somatic cells
 Meiosis produces four genetically
different haploid sex cells
 Animated comparison
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Mitosis
 Makes
body cells (somatic cells)
 diploid cell  two diploid (2N)
daughter cells.
 Daughter cells are identical to each
other & the original parent cell
 Purpose: Allows an organism's body
to grow and replace cells. In
eukaryotic unicellular organisms, it is
a form of reproduction.
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Meiosis
 Makes
sex cells (gametes)
 a diploid cell  four haploid (N)
cells
 These cells are genetically different
from the diploid cell and from one
another.
 Purpose: Sexual reproduction.
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