Download meiosis!!! - Fort Bend ISD

MEIOSIS!!! (A.K.A
formation of gametes)
11.4
(p.275)
1.Genes are located on chromosomes
in the cell’s nucleus.
Mendel’s principles of genetics
require at least two things:
2.
*Each organism must inherit a single
copy of every gene from both its
“parents”
* When an organism produces its
own gametes, those two sets of
genes must be separated from
each other, so that each gamete
contains just one set of genes.
3. Using the fruit fly, Drosophila as an
example, we can see how this works.
* The body cells of the fruit fly have 8
chromosomes.
= maternal chromosomes
= paternal chromosomes
*It got 4 from its male (paternal) parent
and 4 from its female (maternal)
parent.
*These two sets of chromosomes are
homologous, meaning that each of
the 4 chromosomes that came from
the male parent has a corresponding
chromosome from the female
parent.
4. A cell that contains both sets of
homologous chromosomes is said to be
diploid. The number of chromosomes in a
diploid cell is sometimes represented by
the symbol 2N.
Diploid cells contain two complete sets of
chromosomes and two complete sets of
genes!!! Cells of an adult organism contain
two copies of each gene.
5. The GAMETES, however, contain only a
single set of chromosomes and therefore
a single set of genes. Such cells are said to
be haploid. This is usually written as N or
1N.
6. Haploid gametes are produced by the
process of Meiosis!!
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.
7. By the end of meiosis, the one diploid cell
has become 4 haploid cells!!
8. Prior to Meiosis I, each chromosome is
replicated.
Then the cells begin to divide in a way that
looks similar to mitosis.
BUT, during prophase of Meiosis I, each
chromosome pairs with its corresponding
homologous chromosome to form a
structure called a tetrad.
There are 4 chromatids in a tetrad.

An overview of meiosis: The Stages of
Meiosis
Interphase
Homologous pair
of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes
Meiosis I
1 Homologous
chromosomes
separate
Haploid cells with
replicated chromosomes
Meiosis II
2 Sister chromatids
separate
Figure 13.7
Haploid cells with unreplicated chromosomes
IMPORTANT!!! As homologous
chromosomes pair up and form tetrads
in Meiosis I, they exchange portions
of their chromatids in a process
called crossing-over!!
Crossing-over results in the exchange
of alleles between homologous
chromosomes and produces new
combinations of alleles.
Replication of homologous chromosomes
and formation of tetrad
www.synapses.co.uk
Crossing-Over
9.
What happens next? The
homologous chromosomes
separate and two new cells are
formed. Each cell has 4
chromatids, but SOMETHING is
different.
Because each pair of homologous
chromosomes was separated,
neither of the daughter cells has
the two complete sets of
chromosomes it would have in
a diploid cell.
Those two sets have been shuffled and
sorted like a deck of cards! THE TWO
CELLS PRODUCED BY MEIOSIS I HAVE
SETS OF CHROMOSOMES THAT ARE
DIFFERENT FROM EACH OTHER AND
FROM THE DIPLOID CELL THAT
ENTERED MEISOSIS I!!!!!
DRAW, COLOR, AND LABEL
MEIOSIS I on page 276…..be sure to
show how the chromatids are changed
after crossing over.
10.MEIOSIS II: the two cells produced
by Meiosis I now enter a second
meiotic division! NEITHER cell goes
through a round of chromosome
replication before entering Meiosis II.
Each of the cell’s chromosomes has 2
chromatids. During Metaphase II,
chromosomes line up in the center of
the cell. In Anaphase II, the paired
chromatids separate. The result is 4
daughter cells which contain the
haploid (N) number of chromosomes.
10.MEIOSIS II: the two cells
produced by Meiosis I now
enter a second meiotic division!
NEITHER cell goes through a
round of chromosome
replication before entering
Meiosis II.
Each of the cell’s chromosomes
has 2 chromatids. During
Metaphase II, chromosomes
line up in the center of the
cell.
In Anaphase II, the paired
chromatids separate. The
result is 4 daughter cells which
contain the haploid (N)
number of chromosomes.
www.contexo.info/DNA_Basics/Meiosis.htm

DRAW, LABEL, AND COLOR
MEIOSIS II on page 277
11. In male animals, the gametes are
called sperm.
In females, gametes are called an egg.
There are 4 haploid sperm produced in
meiosis.
In females, however, there is usually
only 1 egg produced!!
This occurs because of the
uneven division of the
cytoplasm at the end of
Meiosis I and II.
The other three “things”
produced by female meiosis
are called polar bodies. (they
don’t participate in
reproduction!)
12. Mitosis results in the formation of 2
diploid cells.
Mitosis produces identical cell!! Mitosis
allows an organism’s body to grow and
develop. It is used in asexual reproduction
Meiosis produces 4 haploid cells!!!
The cells are genetically different!!!! It
is how sexual reproducing organisms
produce gametes!!
Mitosis and Meiosis are VERY
DIFFERENT! (only their names are
similar)
www.brhs.ab.ca
A comparison of mitosis and meiosis
MITOSIS
MEIOSIS
Chiasma (site of
crossing over)
Parent cell
(before chromosome replication)
MEIOSIS I
Prophase I
Prophase
Chromosome
replication
Duplicated chromosome
(two sister chromatids)
Chromosome
replication
Tetrad formed by
synapsis of homologous
chromosomes
2n = 6
Chromosomes
positioned at the
metaphase plate
Metaphase
Sister chromatids
separate during
anaphase
Anaphase
Telophase
2n
Tetrads
positioned at the
metaphase plate
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Metaphase I
Anaphase I
Telophase I
Haploid
n=3
Daughter
cells of
meiosis I
2n
MEIOSIS II
Daughter cells
of mitosis
n
n
n
n
Daughter cells of meiosis II
Figure 13.9
Sister chromatids separate during anaphase II