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Mitosis and Asexual Reproduction
MITOTIC CELL DIVISION
Functions of Mitosis
There are two basic types of reproduction – asexual and sexual. In asexual reproduction there is
only one parent, and the genetic material of the offspring is identical to that of the parent. In sexual
reproduction there is a combination of hereditary material from the two individuals and the offspring
produced are not identical to either parent.
Mitotic cell division is an asexual process – the two daughter cells produced are genetically
identical to the parent cell. In the course of mitosis, the process by which the nucleus divides, the
chromosomes of the parent cell double. When the cell divides in half, each new cell contains a set of
chromosomes identical to that of the parent cell.
Mitotic cell division is a means of reproduction in many unicellular organisms. In some
multicellular organisms it may also be a means of reproduction, but more often it is involved only in
growth, regeneration, and healing (tissue repair).
1.
What is the basic difference between asexual and sexual reproduction?
MITOSIS IN ANIMAL CELLS
For convenience, the process of mitosis is described as occurring in stages, or phases. However, it
is a continuous process, with no interruption between one phase and the next.
Interphase
Interphase is the stage between successive mitotic cell divisions. During this period, the
chromosomes, which are within the nucleus, are in the form of a fine network of threads called chromatin.
The nucleus is surrounded by a membrane, and contains one or more nucleoli. Outside the nucleus is a
small region called the centrosome, which contains a pair of centrioles. Toward the end of interphase the
chromosomes double or replicate. The centrioles also double.
1.
Label the structures indicated in the diagram of mitosis.
2.
In what form are the chromosomes during interphase?
3.
What happens to the chromosomes during interphase?
Prophase
At the beginning of prophase the pairs of centrioles separate and move to opposite poles of the
cell. Spindle fibers form, connecting the pairs of centrioles. Astral rays radiate from each centriole. The
chromosomes shorten and thicken. Each pair of identical chromosomes is connected at the centromere.
Each strand of the pair is called a chromatid. The chromatids of a chromosome pair are attached by
spindle fibers to opposite poles. That is, one chromatids of a pair are attached to one pole, and its sister
chromatid is attached to the opposite pole. The nuclear membrane and nucleoli disappear during prophase.
1.
Label the parts indicated in the diagram of mitosis.
2.
What happens to the chromosomes during prophase?
3.
What happens to the nucleus during prophase?
4.
The fibers that form between the opposite pairs of centrioles are called _________
Metaphase
During metaphase, the pairs of chromosomes line up along the equator – that is, they lie in a plane
midway between the two poles and at right angles to the line connecting the poles. Once they are lined up
at the equator, the sister chromatids of each pair separate.
1.
Label the parts indicated in the diagram of mitosis.
2.
Where are the chromosomes located during metaphase?
Anaphase
During anaphase, the daughter chromosomes migrate to opposite poles of the cell. The actual
mechanism of chromosome movement involves both a shortening of the spindle fibers and an outward
movement of each of the poles.
1.
Label the parts indicated in the diagram of mitosis.
2.
What happens to the chromosomes during anaphase?
Telophase
Telophase is the final stage of mitotic cell division. Once the chromosomes have reached
opposite poles of the dividing cell, a nucleus forms at each end of the cell. A nuclear membrane forms
around each new nucleus, nucleoli appear, and centrosomes appear outside each new nucleus. Within each
nucleus, the chromosomes lengthen and form the fine chromatin network characteristic of the interphase
nucleus.
At the center of the cell, the cytoplasm pinches inward ( the cleavage furrow), dividing the
original cell in two. Each daughter cell contains a nucleus and approximately half the cytoplasm and
organelles of the original cell.
1.
Label the parts indicated in the diagram of mitosis.
2.
What happens to the chromosomes during telophase?
3.
How does cytoplasmic division occur in animal cells?
MITOSIS IN PLANT CELLS
Basically, mitosis in plant cells is the same as mitosis in animal cells. However, there are
structural differences in the two cell types that cause differences in the mitotic process. There are no
centrosomes or centrioles in plant cells. Spindles with spindle fibers form between the two poles of cell
and between the poles and the chromosomes.
Cleavage in plant cells differ from cleavage in animal cells because of the presence of the cell
wall. Instead of the cleavage furrow of animal cells, a cell plate forms through the equator of the plant cell.
The cell plate is a cellulose wall that divides the cell in two.
PLANT CELLS
ANIMAL CELLS
Daughter cells contain the same number of
chromosomes as parent cell.
No centrioles present; spindle forms
Daughter cells have about half the cytoplasm of
parent cell.
Cytoplasmic division occurs by formation a
cleavage furrow
1.
Compare the rate of cell division in unicellular organisms to the rate in multicellular
organisms.
2.
What mechanism is thought to stimulate cell division in simple organism such as the
paramecium?
TYPES OF ASEXUAL REPRODUCTION
Binary Fission
Binary fission is a form of mitotic cell division that results in the production of two daughter cells.
Amebas divide by binary fission. In amebas, the nucleus undergoes mitotic division, producing an
identical second nucleus that migrates to the opposite sides of the cell. At that point, division of the
cytoplasm occurs, producing two identical daughter cells.
1. Both bacteria and protozoa reproduce asexually by a form of cell division called.
2.Using numbers 1 – 5 indicate the proper sequence of binary fission in the diagrams of the
ameba.
Budding
Budding is a form of asexual reproduction in which the nucleus divides by mitosis, but the
cytoplasm divides unequally. In yeasts, the nucleus undergoes mitotic division. One nucleus remains in
the parent cell, while the other enters a small bulge in the cytoplasm. This bulge, which is the bud, grows,
and a membrane forms between the bud and the parent cell. Eventually, the bud breaks off.
Hydras. Which are multicellular animals, also produce by budding. A bump forms on the parent
organism, enlarges, and forms a mouth and tentacles. Hydras also reproduce sexually.
1.
How does budding differ from binary fission?
2.
How can colonies of organisms be produced by budding?
Spore Formation
Spores are single, specialized cells that are produced by certain organisms. Spores can be formed
sexually or asexually. When released, spores germinate and grow to form new individuals. Many spores
are surrounded by thick protective walls that allow them to withstand extreme heat, drying, and cold.
Bread molds produce spores in spore cases on specialized stalks that grow upward from the
surface. Each spore case produces thousands of spores. Eventually, the spore cases burst, releasing the
spores. Those that land in favorable environments germinate, producing new bread mold organisms.
1.
In bread molds, spores are produced in specialized structures called
2.
List some organisms that produce asexually-formed spores.
3.
Under what types of conditions might the capacity to form spores be advantageous to an
organism?
Regeneration
Regeneration is the ability to replace body parts. For example, when a flatworm is cut in half, the
head end will grow a new tail, and the tail end will grow a new head. Starfish also have the ability to
regenerate lost parts. There are usually limitations on the size or portion of an organism that can be
regenerated. In humans and other vertebrates, small wounds heal by regeneration of new tissues to replace
damaged tissues. Regeneration occurs by mitosis.
1.
What is regeneration?
2.
To what extent can regeneration occur in humans? Why?
VEGETATIVE PROPAGATION
In some species of plants, new individuals can be produced asexually from the roots, stems, and
leaves of existing plants. Vegetative propagation occurs naturally in some plants, and in others it may be
artificially induced by human activities.
Natural Vegetative Propagation
The production of new plants from the vegetative portions of a parent plant occurs naturally in
several ways.
1.
Label each type of vegetative propagation shown in the diagram using the terms – bulb,
rhizome, corm, stolon, and tuber.
Artificial Vegetative Propagation
Vegetative propagation may be artificially induced by using special techniques on parts of stems,
leaves, or roots. These techniques include layering, cutting, and grafting.
1.
Why do humans do this?