MEIOSIS
Dr. Debbie Payne, Ruth H. Liddell, Shirley K. Scarbrough
Alabama State University, Math, Science Partnership, Summer 2013
CROSSING OVER
When does it occur?
Why is it important?
Meiosis in Lily
In Mitosis the cell’s chromosomes are replicated so that when nuclear
division occurs followed by cytoplasmic division the two new daughter
cells are identical. In Meiosis, however, there are two consecutive
nuclear and cytoplasmic divisions with only one chromosome
replication which results in four new nuclei rather than two nuclei.
Both nuclei resulting from mitosis are diploid and have the same
number of chromosomes as the parent. Meiotic division results in
haploid nuclei which contain half the number of chromosomes as the
parent. In the lily, each germ cell found in the male organs, or anthers,
undergoes meiosis to produce four haploid microspores. The lily
ovulary contains six rows of ovules, which produce egg cells by
meiosis. Meiosis involves two consecutive divisions, Meiosis I and
Meiosis II. The first meiotic division separates the two members of
each pair of chromosomes; the second meiotic division separates the
identical chromatids of each chromosome.
Lily Anther Cell: Meiosis I
Early Prophase I: Crossing-over of genetic material may occur between
homologous chromatids, resulting in the potential for genetic variation. A pair of
homologous chromosomes consist of one which is contributed by the mother
and one which is contributed by the father. The chromosomes become visible.
Later Prophase I: The chromosomes continue to shorten and thicken. The
nuclear membrane begins to disappear.
Metaphase I: In Metaphase I the nuclear membrane has completely
disappeared and the spindle forms. The centromeres become attached to the
spindle fibers. The chromosomes line up at the equatorial plane.
Anaphase I: During anaphase I meiosis, pairs of homologous chromosomes
separate, unlike anaphase of mitosis where sister chromatids separate.
Telophase I: Meiosis I ends with Telophase I. In the anther, cytokinesis
separates the two new nuclei into two haploid sister cells.
Lily Anther Cell: Meiosis II
Prophase II: There is no replication of DNA prior to Prophase II.The second
meiotic division begins with prophase II. The chromatids of each
chromosome are not wound tightly around each other as in mitosis, but are
held together only at their centromeres.
Anaphase II: During anaphase II, the centromeres split and the sister
chromatids separate.
Telophase II: The anther cell ends meiosis II with telophase II and
cytokinesis. The result is four separate haploid microspores.
Lily Ovulary Cell: Meiosis I
Early Prophase I: The DNA was replicated shortly before division. The
chromosomes appear longer and thinner than in mitosis. The nucleus and
nucleolus enlarge.
Later Prophase I: The two chromatids of each chromosome are not yet
visible. They become visible only later in prophase when the paired
homologs move slightly apart. During this phase, the nuclear membrane
disintegrates.
Metaphase I: The paired homologs move together onto the spindle at
the equatorial plane.
Anaphase I: During anaphase I of meiosis I, the pole to which a member
of a homologous pair migrants is entirely random. Therefore there is
some reshuffling of genetic possibilities.
Telophase I: Telophase I in the ovulary does not result in the formation of
two new cells. Rather, the two haploid sister nuclei remain in the same
cell. The nuclei do not enter interphase between the two meiotic divisions,
and there is no replication of the DNA.
Lily Ovulary Cell: Meiosis II
Prophase II: Remember that during Meiosis II, there are half as many
chromosomes as in Meiosis I. Rather than two pairs of chromatids for
each chromosome, there is only one pair.
Metaphase II: Notice that both nuclei are contained in one ovulary cell.
Also, notice that the spindles form at right angles to the plane of the first
meiotic spindle.
Anaphase II: There are now four sets of genetic material in this ovulary
cell, each with half the number of chromosomes as the parent cell.
Telophase II: In the ovulary cell, meiosis ends in telophase II, with the
formation of four haploid megaspore nuclei. They are not separated by
cell walls in lilies.
GAMETE FORMATION IN HUMANS
FATHER: Male
MOTHER: Female
- Sperm
- Fetus
HUMAN INHERITIBLE TRAITS
HOW a KAROTYPE is MADE
LET’S REVIEW MEIOSIS:
E
H
C
A
I
F
G
D
B
Name the following Stages and describe what is happening in each.
ANSWERS: LET’S REVIEW MEIOSIS
A. Prophase I
B. Metaphase I
C. Anaphase I
D. Telophase I
E. Metaphase II
F. Anaphase II
G.Telophase II
H. Meiosis I
I. Meiosis II