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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Completion of Telophase and Cytokinesis.
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.
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.
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.
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.
Late Anaphase: Cytokinesis actually begins during late anaphase. In animal
cells, the cell membrane begins to pinch inward.
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.
Intermediate Stage of Telophase: At this point, the nuclear membranes have
formed and the spindle is beginning to break down.
Late Telophase: As in the plant cell, the chromosomes again take the form of
chromatin. Cytokinesis is nearly complete.
Completion of Telophase and Cytokinesis.
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.
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.
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.
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.
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.
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: 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.
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.
Anaphase II: During anaphase II, the sister centromeres 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 in Prophase I: The DNA was replicated shortly before division. The
chromosomes appear longer and thinner than in mitosis. The nucleus and nucleolus
enlarge.
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.
Metaphase I: The paired homologs move together onto the spindle at the
equatorial plane.
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.
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. Notice
also 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 of the parent cell.
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.