What is Meiosis?
Meiosis is a cell division process that results in the formation of sex cells.
-gametes in animals
-spores in plants
Meiosis involves two divisions producing a total of four
daughter cells.
Summary of the Phases of Meiosis
A cell undergoing meiosis will divide two times; the first division is meiosis 1 and the
second is meiosis 2. Each has four phases which will be covered in this presentation.
meiosis 1: prophase 1, metaphase 1, anaphase 1, and telophase 1
meiosis 2: prophase 2, metaphase 2, anaphase 2, and telophase 2
Location:
Purpose:
Meiosis only occurs in
sexually reproducing
organisms such as
eukaryotic plant and
animal diploid cells.
Diploid cells are cells
which have two sets of
chromosomes.
The purpose of
meiosis to reduce
the normal diploid
cells (2n) to
haploid cells (n),
“n” being the
chromosome
number.
Prophase I:
•
•
•
•
•
•
•
Already copied
chromosomes start to
thicken and shorten.
Chromosomes pair
together.
Crossing-over occurs.
Two homologous pairs join
together to form tetrads.
Nucleolus disappears.
Meiotic spindle forms
between two pairs of
centrioles.
Spindle enters nucleus.
Metaphase I:
• Centrioles at opposite poles
of cell.
• Tetrads become situated on
metaphase plate.
• Spindle fibers from one pole
attach to one chromosome of
each pair; spindle fibers from
other pole attach to other
chromosomes.
Metaphase Plate
Meiotic Spindle
Anaphase I:
• Two chromosomes of each
tetrad separate, move
toward opposite poles.
• Note: Sister chromatids
remain attached at
centromeres, move together
toward poles.
Telophase I:
• Homologous chromosome
pairs complete migration to
poles; now haploid set of
chromosomes at each pole.
• Nuclear envelope forms
around each chromosome
set.
• Spindle disappears.
• Cytokinesis occurs.
Begins with the two
sister cells formed
in Meiosis I
The four
phases of
Meiosis II are:
1. Prophase 2
2. Metaphase 2
3. Anaphase2
4. Telophase2
The results are four haploid
daughter cells, each with
half the chromosomes as
the original diploid cell
Step 1: Prophase 2
- Begins without duplicating DNA.
-Nucleolus and nuclear envelope disappear
-Centriole duplicates and they move to opposite poles of the cell
Step 2: Metaphase 2
-The spindle fibers from the centrioles attach to the chromosomes
- Chromosomes line up on the equatorial plate.
Step 3: Anaphase 2
- Centromeres divide and separated sister
chromatids move to each pole.
Step 4: Telophase 2
- Four haploid daughter cells are produced, and
cell division is complete.
What is special about
the end product?
The daughter cells are genetically
different from the parent cell, and have
half the number of chromosomes.
Prophase 1
•Nucleolus disappears, and meiotic spindle
forms between 2 centrioles
•Nuclear envelope disappears, meiotic
spindle enters
Metaphase1
•Centrioles move to opposite poles of cell
•The chromosomes are arranged on the
metaphase plate
•Spindle fibers from each pole attach to a
chromosome from each pair of sister chromatids
Anaphase 1
•Chromosomes of each bivalent separate and
are pulled by the spindle to opposite poles
Telophase 1
•The chromosomes continue to move until there
is a haploid set of chromosomes at each pole
•A nuclear envelope forms around each
chromosome set
•The spindle disappears and cytokinesis follows
Prophase 2
•Nucleolus disappears and meiotic spindle
forms between 2 centrioles
•The nuclear envelope disappears and the
spindle apparatus forms
Metaphase 2
•Centrioles move to opposite poles of cell
•Chromosomes are arranged on the
metaphase plate
Anaphase 2
•Centromeres are pulled apart
•Separated chromatids are moved to
opposite poles of the cell by the meiotic
spindle
Telophase 2
•Nuclear envelope forms around chromosomes
•Cytokinesis produces four daughter cells, each
having a set of haploid chromosomes
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<http://www.phschool.com/science/biology_place/biocoach/mei
osis/teloii.html>.
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<http://biology.clc.uc.edu/courses/bio104/meiosis.htm>.
Foster, Gerry. "Section 3: Cells and the Cell Cycle - Part c." Meiosis.
2006. 11 Oct 2006
<http://www.dvbiology.org/biologyweb/bcell2.htm#part%20d>.
"The Biology Project: Cell Biology." Meiosis I & II. Aug. 2004. Department
of Biochemistry and Molecular Biophysics . 11 Oct 2006
<http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/page3.
html>.