Meiosis
I Definition:
*The formation of Gametes(sex cells) from
GERM CELLS that separates the two sets
of genes so that each gamete ends up with
just one set. *Each organism must inherit a
single copy of every gene from each of its
“parents.”-increases genetic variation
Called reduction division as the
chromosome # in the resultant gametes is ½
the normal somatic cell # after two mitotic
divisions.
A germ cell is any biological cell that gives rise to
the gametes of an organism that reproduces
sexually. In many animals, the germ cells originate
near the gut of an embryo and migrate to the
developing gonads. There, they undergo cell
division of two types, mitosis and meiosis, followed
by cellular differentiation into mature gametes,
either eggs or sperm. Unlike animals, plants do not
have germ cells set aside in early development.
Instead, germ cells can come from somatic cells in
the adult
II Chromosome Number
All organisms have different
numbers of chromosomes.
These sets of chromosomes are
homologous.
Each of the chromosomes that
came from the male parent has a
corresponding chromosome from
the female parent.
• Common Name--Diploid Chromosome #--Do
not copy this down FYI
• Buffalo—60
• Fruit fly--8
• Cat--38
• Cattle-60
• Dog--78
• Donkey--62
• Goat--60
• Horse--64
• Human--46
• Pig--38
• Sheep--54
What are the minimum and
maximum number of chromosomes?
Do not copy down, just FYI
• The record for minimum number of
chromosomes belongs to a subspecies of
the ant Myrmecia pilosula, in which females
have a single pair of chromosomes. This species
reproduces by a process called haplodiploidy, in
which fertilized eggs (diploid) become females,
while unfertilized eggs (haploid) develop into
males. Hence, the males of this group of ants
have, in each of their cells, a single
chromosome.
• The record for maximum number of
chromosomes is found in found in the
fern family. Polyploidy is a common
conduction in plants, but seemingly taken
to its limits in the Ophioglossum
reticulatum. This fern has roughly 630
pairs of chromosomes or 1260
chromosomes per cell. The fact that these
cells can accurately segregate these
enormous numbers of chromosomes
during mitosis is truly remarkable
Chromosome Number
A cell that contains both sets of homologous
chromosomes is said to be diploid=2N
The number of chromosomes in a diploid
cell is sometimes represented by the symbol
2N.
Ex. Fruit Fly, the diploid number is 8, which
can be written as 2N=8.
Chromosome Number
The gametes of sexually reproducing
organisms contain only a single set of
chromosomes, and therefore only a single
set of genes-HAPLOID=N
These cells are haploid. Haploid cells are
represented by the symbol N.
Ex. Fruit FLy, the haploid number is 4, which
can be written as N=4.
Phases of Meiosis
III Phases of Meiosis
Meiosis is 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.
Phases of Meiosis
Meiosis involves two divisions, meiosis I
and meiosis II.
By the end of meiosis II, the diploid cell
that entered meiosis has become 4
haploid cells.
Phases of Meiosis
Meiosis I
Interphase I
Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I
and
Cytokinesis
First division of meiosis
• Prophase 1:
• Each chromosome duplicates and remains closely
associated.
• These are called sister chromatids. Crossing-over can
occur during the latter
• part of this stage.
• Metaphase 1:
• Homologous chromosomes align at the equatorial plate.
• Anaphase 1:
• Homologous pairs separate with sister chromatids
remaining
• together.
• Telophase 1:
• Two daughter cells are formed with each daughter
containing
• only one chromosome of the homologous pair.
Phases of Meiosis
Interphase I
Cells undergo a
round of DNA
replication, forming
duplicate
chromosomes.
Phases of Meiosis
Prophase I
Each chromosome
pairs with its
corresponding
homologous
chromosome to form
a tetrad.
There are 4
chromatids in a
tetrad.
MEIOSIS I
Prophase I
Phases of Meiosis
When homologous chromosomes form tetrads in
PROPHASEs I, they exchange portions of their
chromatids in a process called crossing over.
Crossing-over produces new combinations of alleles.
Nondisjunction may also occur, chromosomes fail to
separate properly, causing extra or missing
crhomosomes within the gametes.
Phases of Meiosis
MEIOSIS I
Metaphase I
Spindle fibers attach
to the chromosomes.
Phases of Meiosis
MEIOSIS I
Anaphase I
The fibers pull the
homologous
chromosomes
toward opposite
ends of the cell.
Phases of Meiosis
Nuclear membranes
form.
The cell separates into
two cells.
The two cells produced
by meiosis I have
chromosomes and
alleles that are different
from each other and
from the diploid cell that
entered meiosis I.
MEIOSIS I
Telophase I and
Cytokinesis
Phases of Meiosis
Meiosis II
The two cells produced by meiosis I now enter a
second meiotic division.
Unlike meiosis I, neither cell goes through
chromosome replication.
Each of the cell’s chromosomes has 2 chromatids.
Phases of Meiosis
Meiosis II
Telophase I and
Cytokinesis I
Meiosis II
Prophase II
Metaphase II
Anaphase II Telophase II
and
Cytokinesis
Phases of Meiosis
MEIOSIS II
Prophase II
Meiosis I results in
two haploid (N)
daughter cells, each
with half the number
of chromosomes as
the original cell.
Phases of Meiosis
MEIOSIS II
Metaphase II
The chromosomes
line up in the center
of cell.
Phases of Meiosis
MEIOSIS II
Anaphase II
Centromeres divide
and sister chromatids
separate and move
toward opposite ends
of the cell.
Phases of Meiosis
MEIOSIS II
Telophase II and Cytokinesis
Meiosis II results in four
haploid (N) daughter
cells.
IV Gamete Formation
In male animals, meiosis results in four equalsized gametes called sperm.
Spermatogenesis
In many female animals, only one egg
results from meiosis. The other three cells,
called polar bodies, are usually not
involved in reproduction. Oogenesis
V Comparing Mitosis and Meiosis
Mitosis results in the production of two
genetically identical diploid cells. Meiosis
produces four genetically different haploid
cells.
Comparing Mitosis and Meiosis
Mitosis
• Cells produced by mitosis have the same number
of chromosomes and alleles as the original cell.
• Mitosis allows an organism to grow and replace
cells.
• Some organisms reproduce asexually by mitosis.
Comparing Mitosis and Meiosis
Meiosis
• Cells produced by meiosis have half the number of
chromosomes as the parent cell.
• These cells are genetically different from the
diploid cell and from each other.
• Meiosis is how sexually-reproducing organisms
produce gametes.
Comparing Mitosis and Meiosis
Mitosis
Meiosis
Diploid(2N)
(Body,somatic)
Diploid(2N)
(germ)
ONCE
ONCE
ONE
TWO
Number of daughter cells
produced
TWO
FOUR
Chromosome number of
daughter cells
Diploid (2n)
Haploid (n)
Purpose
Growth and repair
Production of
gametes or spores
Chromosome number of
parent cells
Number of DNA replications
Number of divisions
END OF SECTION