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11-4 MEIOSIS PG 275
(May take a test on this section as
a stand alone test!)
Video Clip Intro
The sex cells or gametes
that carry the genetic
information are formed by
a special kind of cell
division known as
MEIOSIS.
The number of
chromosomes, the
structures that contain
the genetic information, is
reduced by half during
meiotic division.
By fertilization the full
number of chromosomes is
restored.
These 2 processes Meiosis
and fertilization- allow for
infinite variety in the
selection and
recombination of genetic
traits.
From Mendel's genetics
we know that an organism
inherits a single copy of
each gene from each of
their parents.
These 2 copies are
segregated from one
another during the
formation of gametes.
EX: fruit fly...each body
or somatic cell of a fruit
fly contains 8
chromosomes.
If chromosomes were not
separated the offspring
would have 16
chromosomes…their
offspring would have 32
and so on.
Drosophila melanogaster
CHROMOSOME NUMBER
PG 275
The chromosomes of the
fruit fly can be divided
into 2 sets…4
chromosomes from the
male parent and 4
chromosomes from the
female parent.
Each chromosome in the
male set has a
corresponding
chromosome in the female
set.
Video Clip-Homologous
Chromosomes
Will need to retrieve from f: drive….
•
(will need to retrieve from
f:drive)
Homologous Chromosomes Video clip Prentice Hall.wmv
HOMOLOGOUS- a
description of
chromosomes that occur
in pairs; having a
corresponding
structure…the
chromosomes are called
HOMOLOGS.
EX: Each of the
chromosomes from the
male parent have a
corresponding
chromosome from the
female parent.
A cell that contains both
sets of homologous
chromosomes are said to
be DIPLOID.
A diploid cell contains 2
complete sets of
chromosomes and 2
complete sets of
genes.The diploid number
is sometimes represented
by 2N…so for the
Drosophila 2N= 8.
DIPLOID- "two sets"…a
cell that contains both
sets of homologous
chromosomes.All of an
organisms cells (except
for sex cells) contain 2
alleles for a given trait.
Gametes or sex cells
contain only a single set
of genes because alleles
are separated during the
process of gamete
formation.
The GAMETES of
Sexually reproducing
organisms contain a single
set of chromosomes and
genes.
HAPLOID- a cell that
contains a single set of
chromosomes..represented
by the symbol N.Drosophila,
N =4…
“THE PHASES OF
MEIOSIS”
Video: Meiosis (2-3 min.)
Video Clip…
MEIOSIS- a process of
reduction division in which
the number of
chromosomes per cell is
cut in half and homologous
chromosomes that exist in
a diploid cell are
separated.
In most organisms meiosis
st
takes place in 2 stages..1
& 2nd meiotic divisions.
The phases of meiosis are
very different from the
phases of MITOSIS.
Video clip to retrieve from f: drive
Meiosis Overview video clip Prentice Hall.wmv
SEE FIG 11-15 page 276…
MEIOSIS I….
Special cells in
reproductive organs
undergo a round of DNA
replication…this
resembles mitosis but it is
not the same!
In Prophase 1 of meiosis,
each chromosome seeks
out its corresponding
homologous chromosome
to form a special
structure called a tetrad.
There are 4 chromatids
in a tetrad;
“CROSSING OVER”
homologous chromosomes
may exchange portions of
their chromatids, an
exchange of genes that
produces new
combinations of
genes…see figure 11-16
page 276
Video Clip: Crossing over
(Will need to retrieve from f: drive
Crossing Over Meiosis Prentice Hall Video Clip.wmv
METAPHASE 1
Metaphase I
Tetrads (rather than individual
chromosomes) line up in the
center of the cell.
ANAPHASE I
The homologous chromosomes
separate and (telophase 1) two
new cells are formed.
The 2 new cells have sets
of chromosomes that are
different from the parent
cells and different from
each other.
MEIOSIS II….
The 2 cells produced by
meiosis I enter meiosis
II.In the second meiotic
division the cells do not
under go DNA replication
so each cell’s
chromosomes contains 2
chromatids.
In METAPHASE II of
meiosis, : chromosomes
line up in the middle of
the cell. In ANAPHASE II
they separate…each of
the 4 daughter cells
receives 2 chromatids .
The daughter cells contain
the HAPLOID number = 2
chromosomes each.
*The amount of genetic
material has been
reduced and the
combinations of
chromosomes in each
gamete have been made
at random.
Video clip: has to be retrieved from f: drive Phases of meiosis
“MEIOSIS & GENETICS”
Meiosis I results in
segregation and
independent assortment.
GAMETE FORMATION…
In males the haploid
gametes produced by
meiosis are called
SPERM.Pollen grains
contain haploid sperm
cells.
The female gamete is
called an egg in animals
and an OVULE in higher
plants.
In females the cell
divisions are uneven, and
the egg or ovule gets most
of the cytoplasm…see
figure 11-17 pg 278;
3 other cells called polar
bodies are produced in
the female during
meiosis..they are called
polar bodies.
Video: Meiosis Square Dance
10 minute video
“COMPARING MITOSIS
AND MEIOSIS”
Mitosis results in the
production of 2
genetically identical cells.
A diploid cell divides and
gives rise to 2 diploid
daughter cells that are
genetically identical to
each other and identical
to the original parent cell.
Meiosis begins with a
diploid cell but produces
4 haploid cells that are
different from the
original diploid cell and
different from one
another.WHY?
Homologous chromosomes
are separated during the
st
1 meiotic division and
crossing over
occurs…giving new gene
combinations on the
chromosomes.
*MITOSIS results in the
production of 2
genetically identical
diploid cells.
*MEIOSIS results in the
production of 4
genetically DIFFERENT
haploid cells.
TEST CH 11