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Mitosis and Meiosis Presentation and Image Set

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1 Mitosis and Meiosis Presentation and Image Set
This presentation describes the process of mitosis, in both plant (onion root tip) and animal (fish embryo) cells. It also describes the process of meiosis in the anther and ovulary of lily plants. The images provided are photomicrographs from different stages of mitosis and meiosis. We have also included labeled illustrations from three of Carolina’s Bioreview® sheets: Animal Cell Mitosis and Cytokinesis, Plant Cell Mitosis and Cytokinesis, and Lilium Life Cycle. The images that are used in the following slides are also contained in a folder on this CD. You may use them in your classroom or lab to create your own presentations or student assessments.

2 Mitosis in Onion Root Tip and Fish Blastula
Mitosis is the process of replication and division of the chromosomes and nucleus of a cell. Actively growing tissues, such as root tips, contain many cells undergoing mitosis. Mitosis is a continual process, in which the phases actually progress smoothly from one to the next with no distinct points between them. Mitosis, or nuclear division, is sometimes confused with cytokinesis, or cellular division. Cytokinesis is the division of the cytoplasm and separation of the two nuclei that result from mitosis—the making of two cells from one. This process is also essential for normal growth and development in most organisms.

3 Onion Root Tip Interphase: Interphase is best described as the usual state of the cell nucleus, and the longest period of the cell cycle. During interphase, the cell rapidly grows and produces organelles. DNA synthesis occurs within the nucleus of the cell, yielding two identical copies of each chromosome. The cell assembles the components needed for cellular division and readies itself for the process of mitosis. The nuclear membrane is present, and there is a distinct, visible nucleolus, composed of RNA and proteins.

4 Prophase: The first stage of mitosis is prophase
Prophase: The first stage of mitosis is prophase. During prophase, the nuclear membrane begins to disappear. The pairs of chromosomes coil around themselves and become visible as distinct, long threads. They are stained a dark purple in this image. Each chromosome is actually split lengthwise into two sister chromatids, which are the two copies made during interphase. The chromatids are twisted around each other and attached by a centromere. A system of microtubules is also forming that attaches to the centromeres and will later move sister chromatids to opposite poles.

5 Metaphase: Metaphase is the shortest phase of mitosis
Metaphase: Metaphase is the shortest phase of mitosis. During this phase, the chromosomes line up along the plane running through the equator of the cell, where the cell will divide later. The nuclear membrane has completely disappeared.

6 Anaphase: Anaphase begins when the chromosomes start to move away from the equatorial plane toward opposite poles. At this point, they are considered to be full-fledged daughter chromosomes. Thus, at this stage, the cell actually has twice the original number of chromosomes.

7 Late Anaphase: During late anaphase, the two groups of chromosomes are near their respective poles and have formed a tight group in each region. Each pole of the cell now has an equal and complete collection of daughter chromosomes.

8 Early Telophase: Early telophase and late anaphase are often difficult to distinguish. During telophase, the chromosomes begin to uncoil and return to their interphase form. Cytokinesis continues as well. Small membrane-bound vesicles appear and align along the equator of the plant cell. Cytokinesis proceeds from the inside outward in plant cells, as opposed to cytokinesis in animal cells, which occurs as the cell pinches from the outside inward to divide.

9 Intermediate Stage of Telophase: A nuclear membrane begins to form around each set of chromosomes. As the vesicles in the onion cell become aligned at the equatorial plane, they begin to fuse together, forming a double membrane across the cell. The resulting cell plate is a feature not found in animal cells.

10 Late Telophase: As the cell plate nears completion, the nuclear membranes become distinct. The chromosomes continue to uncoil and begin to take on their interphase appearance. The nucleoli are clearly visible in each nucleus.

11 Completion of Telophase and Cytokinesis.

12 Fish Embryo Cell Interphase: This fish embryo cell is in interphase. The cell is rapidly growing and producing organelles. The genetic material, or chromatin, is still very diffuse at this stage and is undergoing replication, producing two identical copies of each chromosome. Toward the end of interphase, the chromatin begins to condense into distinct chromosomes.

13 Prophase: During prophase, the nuclei shrink and, as in this image, disappear. The nuclear membrane breaks apart. Animal cells have spindle structures called asters. At the center of each aster is an organelle called a centriole. During prophase, centrioles migrate to opposite poles. Centrioles’ exact role in mitosis is unknown; their structure resembles the basal bodies of cilia and flagella.

14 Metaphase: The centromere of each chromosome is the structure aligned on the equator of the cell. When the centromeres divide during the next stage, each chromatid becomes a separate chromosome. In living cells, one can actually observe the chromatids uncoiling from each other as they line up.

15 Anaphase: One member of a pair of chromosomes moves toward one pole, and one moves toward the other. The movement of the daughter chromosomes is believed to be caused by the shortening of the spindle fibers. There is evidence that chromosomes cannot move during anaphase unless their centromeres are attached to spindle fibers.

16 Late Anaphase: Cytokinesis actually begins during late anaphase
Late Anaphase: Cytokinesis actually begins during late anaphase. In animal cells, the cell membrane begins to pinch inward.

17 Early Telophase: The cell membrane continues to pinch inward as the chromosomes uncoil further. The nuclear membranes begin to form during this stage of telophase. Small fragments of the membrane appear over the surface of the chromosome mass, fusing together to enclose the chromosomes completely in a new, small nucleus.

18 Intermediate Stage of Telophase: At this point, the nuclear membranes have formed and the spindle is beginning to break down.

19 Late Telophase: As in the plant cell, the chromosomes again take the form of chromatin. Cytokinesis is nearly complete.

20 Completion of Telophase and Cytokinesis.

21 Meiosis in Lily Mitosis is the duplication and subsequent equal division of a cell’s chromosomes to form two identical daughter cells. Meiosis, on the other hand, is a different kind of cell division, resulting in not two new nuclei, but four. While both nuclei resulting from mitosis are diploid and have the same number of chromosomes as the parent, each nucleus from meiotic division is haploid and contains 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 sequential divisions, usually called 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. Further details about megaspore and megagametophyte formation and lily fertilization can be found on the Lilium Life Cycle Bioreview® sheet.

22 Lily Anther Cell: Meiosis I
Early in Prophase I: During interphase, each chromosome has been replicated, exactly as in mitosis. Some crossing-over of genetic material may occur between homologous chromatids, resulting in the potential for genetic reassortment. The pairs of chromosomes, one maternal and one paternal in each pair, are called bivalents. Early in prophase I, as the chromosomes become visible, they still appear single-stranded.

23 Later in Prophase I: As prophase I progresses, the chromosomes continue to shorten and thicken. Prophase I ends with the disappearance of the nuclear membrane.

24 Metaphase I: Metaphase I is identified by the complete disappearance of the nuclear membrane and by the formation of the spindle apparatus. The centromeres become attached to the spindle fibers, and the chromosomes align on the equatorial plane.

25 Anaphase I: Anaphase I of meiosis has a different outcome than anaphase of mitosis. During anaphase I, double-stranded homologous chromosomes, or bivalents, move apart, while in anaphase of mitosis, sister chromatids move apart.

26 Telophase I: Meiosis I ends with telophase I
Telophase I: Meiosis I ends with telophase I. In the anther, cytokinesis separates the two new nuclei into two haploid sister cells.

27 Lily Anther Cell: Meiosis II
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.

28 Metaphase II: As in mitosis and in meiosis I, the chromosomes line up along the equatorial plane during metaphase II. Note that both sister cells are undergoing the process at the same time.

29 Anaphase II: During anaphase II, the sister centromeres and the sister chromatids separate.

30 Telophase II: The anther cell ends meiosis II with telophase II and cytokinesis. The result is four separate haploid microspores.

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

32 Later in Prophase I: The two chromatids of each chromosome are not yet visible. They become apparent only later in prophase when the paired homologs move slightly apart. During this phase, the nuclear membrane disintegrates.

33 Metaphase I: The paired homologs move together onto the spindle at the equatorial plane.

34 Anaphase I: During anaphase I, homologous chromosomes separate, but the pole to which a member of a homologous pair moves is a matter of chance. Thus, some genetic reshuffling often occurs at this point.

35 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.

36 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.

37 Metaphase II: Notice that both nuclei are contained in one ovulary cell. Notice also that the spindles form at right angles to the plane of the first meiotic spindle.

38 Anaphase II: There are now four sets of genetic material in this ovulary cell, each with half the number of chromosomes of the parent cell.

39 Telophase II: In the lily ovulary cell, meiosis ends in telophase II, with the formation of four haploid megaspore nuclei. They are not separated by cell walls.

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