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bnit 15. asexual reproduction ACTIVITY 15-1. 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 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 chromosomes of the parent cell double, and 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 one-celled 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.

Question 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 interrup-tion between one phase and the next.

• Interphase is the stage between successive mitotic cell divisions. Dur- interphase ing this period, the chromosomes, which are within the nucleus, are in the form of a fine network of threads called chromatin. The nucleus is sur-rounded 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 rep-licate. The centrioles also double.

Questions 1. Label the structures indicated in the diagram below.

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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 cen-trioles. Astral rays radiate from each centriole. The chromosomes shorten and thicken. Each pair of identical chromosomes is connected at the cen-tromere. 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 chromatid of a pair is attached to one pole, and its sister chromatid is attached to the opposite pole. The nuclear membrane and nucleoli disappear during prophase.

Questions 1. Label the parts indicated in the diagram below.

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

During metaphase, the pairs of chromosomes line up along the cell metaphase 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.

Question What happens to the chromosomes during metaphase?

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anaphase During anaphase, the separated chromatids migrate to opposite poles of the cell. The actual mechanism of chromosome movement is not known, but it is thought to involve both a shortening of the spindle fibers and an outward movement of each of the poles.

Question What happens to the chromosomes during anaphase?

telophase

Questions

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. In the space below, make a diagram showing a cell in late telophase.

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 the cell and between the poles and the chromosomes.

Cleavage in plant cells differs from cleavage in animal cells because of the presence of the cell wall. Instead of the cleavage furrow of animal cells, a division plate forms through the equator of the plant cell. The division plate is a cellulose wall that divides the cell in two.

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ACTIVITY 15-2. TYPES OF ASEXUAL REPRODUCTION

binary fission Binary fission is a form of mitotic cell division that results in the production of two daughter cells of approximately equal size. Amebas divide by bi-nary fission:4n amebas, the nucleus undergoes mitotic division, producing an identical second nucleus that migrates to the opposite side of the cell. At that point, division of the cytoplasm occurs, producing two identical daughter cells. In binary fission in the paramecium, the micronucleus, which contains the hereditary material, divides by mitosis. The macronu-cleus simply enlarges and splits in two. The cell cytoplasm pinches in, forming two daughter cells, each with a micronucleus and a macronucleus. The other cell organelles, such as contractile vacuoles and oral grooves, regenerate in each daughter cell.

Questions I. Both amebas and paramecia reproduce asexually by a form of cell division called

2. In binary fission, is the division of the cytoplasm between the two daughter cells equal or unequal?

3. What factor is thought to trigger cell division in amebas?

buddin Budding is a form of asexual reproduction in which the nucleus divides by g 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 reproduce by budding. A bump forms on the parent organikn, enlarges, and forms a mouth and tentacles. Hydras also reproduce sexually.

Questions 1. How does budding differ from binary fission?

2. How can colonies of organisms be produced by budding?

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Spores are highly resistant cells produced asexually by specialized parent spores cells. Each spore is surrounded by a thick protective wall that allows it tO

withstand extreme drying, heat, and cold. Bread molds produce spores in spore cases, or sporangia, which form at the ends of certain hyphae. Each sporangium produces thousands of spores. Eventually, the sporaggium bursts, releasing the spores. Those that land in favorable environments germinate, producing new bread mold organisms.

Questions 1. In bread molds, spores are produced in specializ.ed structures called

or

2. A spore is surrounded by a thick, protective

3. Within the spore are several and a small amount of

4. Under what types of conditions would the capacity to produce spores be important?

Regeneration is the ability to replace body parts. For example, when a regeneration 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 oc-curs by mitosis.

Questions 1. What is regeneration?

2. How could the ability of sponges to regenerate be of value commercially?

3. To what extent can regeneration occur in humans?

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REVIEW EXERCISES: UNIT 15 A. Fill in the blanks in the statements below.

1. The phase of mitosis in which replication of the chromosomes occurs is

2. The phase of mitosis in which doubling of the centrioles occurs is

3. The phase of mitosis in which the chromosomes are in the form of a chromatin network is

4. The phase of mitosis in which the centrioles move to opposite poles of the cell is

5. The two pairs of centrioles are connected by fibers.

6. Each pair of chromatids is connected together at the

7. Each individual strand of a doubled chromosome is called a

8. The nuclear membrane and nucleolus disappear during the phase of mitosis called

9. The phase of mitosis in which the chromosomes line up along the cell equator is

10. The phase of mitosis in which pairs of identical chromosomes separate is

• - H. The phase of mitosis in which chromosomes move to opposite poles of the cell is

12. The phase of mitosis in which a nucleus forms at each end of the cell is

• 13. The phase of mitosis in which the nuclear membrane and nucleoli reappear is

14. The phase of mitosis in which division of the cell cytoplasm occurs is

• 15. In plant cells, no asters form during mitosis because plant cells do not contain

16. In plant cells, division of the cell cytoplasm during mitosis involves the formation of a

• 17. A form of mitotic cell division that serves as a means of reproduction in amebas and various

other protists is 18. A form of asexual reproduction in which a new individual grows out from the body of the

parent is 19. A form of reproductive cell that is surrounded by a thick protective wall and that can

withstand bad conditions is a

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