cells? - Fort Bend ISD

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Transcript cells? - Fort Bend ISD

11–4 Meiosis
The making of sex cells!
Making Sex Cells
Meiosis is the
process of making
sex cells
Gametes are sex
Ex: sperm, egg,
pollen, etc…
Female Gametes - Eggs
Human Eggs
Male Gametes - Sperm
Human Sperm
A body cell in an
adult fruit fly has 8
chromosomes, as
shown in the
drawing below.
Four of the
chromosomes came
from the fruit fly's
male parent, and 4
came from its
female parent.
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)
Different Types of Cells
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 = body cells (somatic
 Haploid
Cells = a cell that contains
only a single set of chromosomes
 Therefore
only a single set of genes
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)
How are haploid (N) gamete cells
produced from diploid (2N) cells?
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
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.
What is the diploid
number for these
What is the
haploid number
for these cells?
Meiosis I
 Before
meiosis I, each chromosome is
 Division looks similar to mitosis
 Prophase I, Metaphase I, Anaphase I,
Telophase I
 prophase of meiosis I, however, each
chromosome pairs with its
corresponding homologous
chromosome to form a structure called
a tetrad
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 = alternative forms of the
same gene (ex: blue eyes vs. brown
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
Meiosis II
 two
cells produced by meiosis I
now enter a second meiotic division
 NO DNA replication before Meiosis
 Prophase II, Metaphase II, Anaphase
II, Telophase II
 paired chromatids separate
 Produces: haploid cells (N)
Haploid cells are DIFFERENT from each
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)
Fertilization of a
human egg by
Notice the HUGE
size difference.
Comparing Mitosis and Meiosis
 Mitosis
and Meiosis sound alike but
are VERY different!
 Mitosis results in the production of
two genetically identical diploid
 Meiosis produces four genetically
different haploid cells
 Makes
body cells (somatic cells)
 diploid cell  two diploid (2N)
daughter cells.
 Daughter cells are identical to each
other & the original parent cell
 Mitosis allows an organism's body to
grow and replace cells.
 Makes
sex cells (gametes)
diploid cell  four haploid (N)
 These
cells are genetically different
from the diploid cell and from one