Download Meiosis II

MEIOSIS
Meiosis is a specialized type of cell division that occurs in the formation of
gametes such as egg and sperm. Although meiosis appears much more
complicated than mitosis, it is really just two divisions in sequence, each
one of which has strong similarities to mitosis. The illustrations used in the
discussion which follows were modified from Campbell, Biology, 1996.
Meiosis I
Meiosis I, the first of the two divisions, is often called reduction division,
since it is here that the chromosome compliment is reduced from 2N
(diploid) to 1N (haploid).
Interphase
Interphase in meiosis is identical to interphase in mitosis and there is no
way, by simply observing the cell, to determine what type of division the
cell will undergo when it does divide. Meiotic division will only occur in
cells associated with male or female sex organs.
Prophase I
Prophase I is virtually identical to prophase in mitosis, involving the
appearance of the chromosomes, the development of the spindle apparatus,
and the breakdown of the nuclear membrane (envelope). Although this
drawing from the text shows the chromosomes pairs in close association,
including the chiasmata or points of contact of the arms of adjacent
chromosomes, this is characteristic only of very late Prophase I and is more
common in Metaphase I.
Metaphase I
Here is where the critical difference occurs between Metaphase I in meiosis
and metaphase in mitosis. In the latter, all the chromosomes line up on the
metaphase plate in no particular order. In Metaphase I, the chromosome
pairs are aligned on either side of the metaphase plate. It is during this
alignment that chromatid arms may overlap and temporarily fuse
(chiasmata), resulting in crossovers
Anaphase I
During Anaphase I the kinetochore spindle fibers contract, pulling the
homologous pairs away from each other and toward each pole of the cell.
Telophase I
A cleavage furrow typically forms at this point, followed by cytokinesis, but
the nuclear membrane (envelope) usually is not reformed and the
chromosomes do not disappear. At the end of Telophase I, each daughter
cell has a single set of chromosomes, half the total number in the original
cell where the chromosomes were present in pairs. While the original cell
was diploid, the daughter cells are now haploid. This is why Meiosis I is
often called reduction division.
Meiosis II
Meiosis II is quite simple in that it is simply a mitotic division of each of the
haploid cells produced in Meiosis I. There is no Interphase between Meiosis
I and Meiosis II and the latter begins with:
Prophase II
A new set of spindle fibers forms and the chromosomes begin to move
toward the equator of the cell.
Metaphase II
All the chromosomes in the two cells align with the metaphase plate.
Anaphase II
The centromeres split and the kinetochore spindle fibers shorten, drawing
the chromosomes toward each pole of the cell.
Telophase II
A cleavage furrow develops, followed by cytokinesis and the formation of
the nuclear membrane (envelope). The chromosomes begin to fade, replaced
by the granular chromatin characteristic of interphase. When Meiosis II is
complete, there will be a total of four daughter cells, each with half the total
number of chromosomes as the original cell.
In the case of male structures, all four cells will eventually develop into
typical sperm cells. In the case of female life cycles in higher organisms,
three of the cells will typically abort, leaving a single cell to develop into an
egg cell which is usually much larger than a typical sperm cell.