meiosis dr. debbie payne, ruth h. liddell, shirley k. scarbrough alabama state university, math,...

MEIOSIS

Dr. Debbie Payne, Ruth H. Liddell, Shirley K. ScarbroughAlabama State University, Math, Science Partnership, Summer 2013

CROSSING OVER

When does it occur?Why is it important?

Meiosis in LilyIn 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.

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.

Lily Anther Cell: Meiosis I

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.

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.

Lily Anther Cell: Meiosis II

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.

Early Prophase I: The DNA was replicated shortly before division. The chromosomes appear longer and thinner than in mitosis. The nucleus and nucleolus enlarge.

Lily Ovulary Cell: Meiosis I

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.

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.

Lily Ovulary Cell: Meiosis II

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:

Name the following Stages and describe what is happening in each.

A BC D

EF

G

H

I

ANSWERS: LET’S REVIEW MEIOSIS

A. Prophase IB. Metaphase IC. Anaphase ID. Telophase IE. Metaphase IIF. Anaphase IIG.Telophase IIH. Meiosis II. Meiosis II