Chromosomes. and Cell Reproduction

CHAPTER 6 Chromosomes and Cell Reproduction Quick Review Looking Ahead Answer the following without referring to earlier sections of your book. 1...
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CHAPTER

6

Chromosomes and Cell Reproduction

Quick Review

Looking Ahead

Answer the following without referring to earlier sections of your book. 1. Define the term mutation. (Chapter 1, 6C Section 1) 2. Describe the structure of proteins and of DNA. (Chapter 2, Section 3) 9A 3. Summarize the function and structure of the nucleus and of microtubules. (Chapter 3, Section 2) 4A Did you have difficulty? For help, review the sections indicated.

Section 1 Chromosomes Formation of New Cells by Cell Division How Chromosome Number and Structure Affect Development

Section 2 The Cell Cycle The Life of a Eukaryotic Cell Control of the Cell Cycle

Section 3 Mitosis and Cytokinesis

Reading Activity Copy the following statements in your notebook: • Chromosomes from females determine the sex of humans.

Chromatid Separation in Mitosis Mitosis and Cytokinesis

• Every human cell contains 46 chromosomes. • Healthy cells cannot become cancerous cells. Before you read the chapter, write down if you agree with each statement. After you have finished reading the chapter, decide whether you still agree with your first response. www.scilinks.org This cluster of cells is smaller than the head of a pin, but over the next 17 days, they will divide repeatedly to form a new mouse. Chromosomes inside each cell carry the instructions for growth and development of an individual.

National Science Teachers Association sci LINKS Internet resources are located throughout this chapter.

CHAPTER 6 Chromosomes and Cell Reproduction Copyright © by Holt, Rinehart and Winston. All rights reserved.

117

Section 1

Chromosomes

Objectives ● Identify four examples of cell division in eukaryotes and one example in prokaryotes. 4A 4B 6E ● Differentiate between a gene, a DNA molecule, a chromosome, and a chromatid. 6A 6E ● Differentiate between homologous chromosomes, autosomes, and sex chromosomes. 6A 6E ● Compare haploid and 6E diploid cells. ● Predict how changes in chromosome number or structure can affect 6C 6F development.

Key Terms gamete binary fission gene chromosome chromatid centromere homologous chromosome diploid haploid zygote autosome sex chromosome karyotype

Formation of New Cells by Cell Division About 2 trillion cells are produced by an adult human body every day. This is about 25 million new cells per second! These new cells are formed when older cells divide. Cell division, also called cell reproduction, occurs in humans and other organisms at different times in their life. In Figure 1, the cells of the fawn that is growing and developing and the cells in the wound that is healing are undergoing cell division. The type of cell division differs depending on the organism and why the cell is dividing. For example, bacterial cells undergoing reproduction divide by one type of cell division. Eukaryotic organisms undergoing growth, development, repair, or asexual reproduction divide by a different type of cell division. And the formation of gametes involves yet a third type of cell division. Gametes are an organism’s reproductive cells, such as sperm or egg cells. Regardless of the type of cell division that occurs, all of the information stored in the molecule DNA (deoxyribonucleic acid) must be present in each of the resulting cells. Recall from Chapter 3 that DNA stores the information that tells cells which proteins to make and when to make them. This information directs a cell’s activities and determines its characteristics. Thus, when a cell divides, the DNA is first copied and then distributed. Each cell ends up with a complete set (copy) of the DNA. Figure 1 Cell division The cells of these organisms are undergoing some type of cell division.

Repair

118

Growth and development

CHAPTER 6 Chromosomes and Cell Reproduction Copyright © by Holt, Rinehart and Winston. All rights reserved.

Prokaryotic Cell Reproduction A prokaryote’s single DNA molecule is circular and is attached to the inner cell membrane. Prokaryotes reproduce by a type of cell division called binary fission. Binary fission is a form of asexual reproduction that produces identical offspring. In asexual reproduction, a single parent passes exact copies of all of its DNA to its offspring. Binary fission occurs in two stages: first, the DNA is copied (so that each new cell will have a copy of the genetic information), and then the cell divides. The prokaryote divides by adding a new cell membrane to a point on the membrane between the two DNA copies. As new material is added, the growing cell membrane pushes inward and the cell is constricted in the middle, like a long balloon being squeezed near the center. A new cell wall forms around the new membrane. Eventually the dividing prokaryote is pinched into two independent cells. Each cell contains one of the circles of DNA and is a complete functioning prokaryote.

Real Life Escherichia coli cells can produce 1 million new cells in less than 7 hours. A variety of E. coli known as O157:H7 is sometimes found in raw or undercooked meat. When such meat is eaten, this bacteria can cause life-threatening intestinal bleeding and kidney failure. Thorough cooking is necessary to destroy the bacteria. Finding Information Research outbreaks of E. coli O157:H7 in your community or state.

Eukaryotic Cell Reproduction The vast amount of information encoded in DNA is organized into units called genes. A gene is a segment of DNA that codes for a protein or RNA molecule. A single molecule of DNA has thousands of genes lined up like train cars. Genes play an important role in determining how a person’s body develops and functions. When genes are being used, the DNA is stretched out so that the information it contains can be used to direct the synthesis of proteins. As a eukaryotic cell prepares to divide, the DNA and the proteins associated with the DNA coil into a structure called a chromosome, as shown in Figure 2. Before the DNA coils up, however, the DNA is copied. The two exact copies of DNA that make up each chromosome are called chromatids (KROH muh tihdz). The two chromatids of a chromosome are attached at a point called a centromere. The chromatids, which become separated during cell division and placed into each new cell, ensure that each new cell will have the same genetic information as the original cell.

Figure 2 Chromosome structure. A chromosome consists of DNA tightly coiled around proteins. The chromosomes are formed as a cell prepares to divide.

Centromere

Chromosome (made of 2 chromatids)

Supercoil within chromosome

Further coiling within supercoil

DNA and proteins

DNA double helix

SECTION 1 Chromosomes Copyright © by Holt, Rinehart and Winston. All rights reserved.

119

How Chromosome Number and Structure Affect Development www.scilinks.org Topic: Chromosomes Keyword: HX4042

Each human somatic cell (any cell other than a sperm or egg cell) normally has two copies of 23 different chromosomes, for a total of 46 chromosomes. The 23 chromosomes differ in size, shape, and set of genes. Each chromosome contains thousands of genes that play important roles in determining how a person’s body develops and functions. For this reason, a complete set of all chromosomes is essential to survival.

Sets of Chromosomes Each of the 23 pairs of chromosomes consists of two homologous (hoh MAHL uh gus) chromosomes, or homologues (HOH muh logs). Homologous chromosomes are chromosomes that are similar in size, shape, and genetic content. Each homologue in a pair of homologous chromosomes comes from one of the two parents, as shown in Figure 3. Thus, the 46 chromosomes in human somatic cells are actually two sets of 23 chromosomes. One set comes from the mother, and one set comes from the father. A human chromosome is shown in Figure 4.

Figure 3 Fertilization When haploid gametes fuse, they produce a diploid zygote.

Egg cell n = 23

Sperm cell n = 23 Fertilization

Zygote 2n = 46

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CHAPTER 6 Chromosomes and Cell Reproduction Copyright © by Holt, Rinehart and Winston. All rights reserved.

All of the cells in the body, other than gametes, are somatic cells. When a cell, such as a somatic cell, contains two sets of chromosomes, it is said to be diploid (DIHP loyd). Unlike somatic cells, human gametes contain only one set of chromosomes (23 total). When a cell, such as a gamete, contains one set of chromosomes, it is said to be haploid (HAP loyd). Biologists use the symbol n to represent one set of chromosomes. The haploid number in a human gamete can be written as n  23. The diploid number in a somatic cell can be written as 2n  46. The fusion of two haploid gametes— a process called fertilization—forms a diploid zygote, as shown in Figure 3. A zygote (ZY goht) is a fertilized egg cell, the first cell of a new individual. As seen in Table 1, each organism has a characteristic number of chromosomes. The number of chromosomes in cells is constant within a species. Fruit flies, for example, have only eight chromosomes in each cell. Although most species have different numbers of chromosomes, some species by chance have the same number. For example, potatoes, plums, and chimpanzees all have 48 chromosomes in each cell. Many plants have far more chromosomes. Some ferns have more than 500. A few kinds of organisms— such as the Australian ant Myrmecia, the plant Haplopappus (a desert relative of the sunflower), and the fungus Penicillium (from which the antibiotic penicillin is obtained)—have only one pair of chromosomes.

Magnification: 12,542

Figure 4 Human chromosome. As many as 500 chromosomes lined up end to end would fit in a 0.2 cm space—about the thickness of a nickel. The chromosome above has replicated and consists of two identical chromatids.

Table 1 Chromosome Number of Various Organisms Organism

Number of chromosomes

Penicillium

1–4

Saccharomyces (yeast)

18

Mosquito

6

Housefly

12

Garden pea

14

Corn

20

Adder’s tongue fern

1,262

Frog

26

Human

46

Orangutan

48

Dog

78

SECTION 1 Chromosomes Copyright © by Holt, Rinehart and Winston. All rights reserved.

121

Sex Chromosomes The word chromosome is from the Greek chroma, meaning “color,” and soma, meaning “body.” Chromosomes were so named because they absorbed a colored dye that made them more visible under a microscope.

Of the 23 pairs of chromosomes in human somatic cells, 22 pairs are called autosomes. Autosomes are chromosomes that are not directly involved in determining the sex (gender) of an individual. The sex chromosomes, one of the 23 pairs of chromosomes in humans, contain genes that will determine the sex of the individual. In humans and many other organisms, the two sex chromosomes are referred to as the X and Y chromosomes. The genes that cause a fertilized egg to develop into a male are located on the Y chromosome. Thus, any individual with a Y chromosome is male, and any individual without a Y chromosome is female. For example, in human males, the sex chromosomes are made up of one X chromosome and one Y chromosome (XY). The sex chromosomes in human females consist of two X chromosomes (XX). Because a female can donate only an X chromosome to her offspring, the sex of an offspring is determined by the male, who can donate either an X or a Y. The structure and number of sex chromosomes vary in different organisms. In some insects, such as grasshoppers, there is no Y chromosome—the females are characterized as XX and the males are characterized as XO (the O indicates the absence of a chromosome). In birds, moths, and butterflies, the male has two X chromosomes and the female has only one.

Change in Chromosome Number Each of an individual’s 46 chromosomes has thousands of genes. Because genes play an important role in determining how a person’s body develops and functions, the presence of all 46 chromosomes is essential for normal developFigure 5 A human karyotype ment and function. A person must Karyotypes are used to examine an individual’s chromosomes. have the characteristic number of To prepare a karyotype, chromosomes in his or her cells. photographs of the Humans who are missing even one chromosomes are cut out, arranged in pairs from largest of the 46 chromosomes do not surto smallest, and numbered. vive. Humans with more than two copies of a chromosome, a condition called trisomy (TRY soh mee), will not develop properly. Abnormalities in chromosome number can be detected by analyzing a karyotype (KAR ee uh tiep), a photo of the chromosomes in a dividing cell that shows the chromosomes arranged by size. Figure 5 shows a typical karyotype. A portion of a karyotype from an individual with an extra copy of chromosome 21 is also shown in Figure 5. This condition is called People with Down syndrome have Down syndrome, or trisomy 21. three copies of chromosome 21 in Short stature, a round face with their karyotype.

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CHAPTER 6 Chromosomes and Cell Reproduction Copyright © by Holt, Rinehart and Winston. All rights reserved.

upper eyelids that cover the inner corners of the eyes, and varying degrees of mental retardation are characteristics of people with Down syndrome. In mothers younger than 30, Down syndrome occurs in about 1 in 1,500 births. In mothers 37 years old, the incidence doubles to 1 in 290 births. In mothers over 45, the risk is as high as 1 in 46 births. Older mothers are more likely to have a baby with Down syndrome because all the eggs a female will ever produce are present in her ovaries when she is born, unlike males who produce new sperm throughout adult life. As a female ages, her eggs can accumulate an increasing amount of damage. Because of this risk, a pregnant woman over the age of 35 may be advised to undergo prenatal testing that includes fetal karyotyping. What events can cause an individual to have an extra copy of a chromosome? When sperm and egg cells form, each chromosome and its homologue separate, an event called disjunction (dihs JUHNK shuhn). If one or more chromosomes fail to separate properly—an event called nondisjunction—one new gamete ends up receiving both chromosomes and the other gamete receives none. Trisomy occurs when the gamete with both chromosomes fuses with a normal gamete during fertilization, resulting in offspring with three copies of that chromosome instead of two. In Down syndrome, nondisjunction involves chromosome 21.

www.scilinks.org Topic: Genetic Disorders Research in Texas Keyword: HXX4008

On the Trail of a Chromosomal Deletion

O

ne of every 4,000 babies is born with a genetic disorder called DiGeorge syndrome. This disorder causes serious heart defects that must be surgically corrected within a few days after birth. Children born with DiGeorge syndrome can also have blood ailments, facial abnormalities, a deficient immune system, and other problems.

A Faulty Chromosome Karyotypes of people with DiGeorge syndrome show that they have one normal and one faulty 22nd chromosome. The faulty chromosome is missing a small region that contains 25 genes. To understand how this chromosomal deletion results in

DiGeorge syndrome, researchers at Baylor College of Medicine in Houston have been studying the disorder in mice. First, the researchers found that they could produce similar heart defects in mice by deleting a part of mouse chromosome 16. When these mice were bred with mice that had a duplication of the same part of chromosome 16, their offspring had no heart defects. Because the deleted part contained only 15 genes, the search for the cause of DiGeorge syndrome was narrowed from 25 genes to 15 genes.

Finding the Crucial Gene Using a technology called chromosome engineering, the Baylor

researchers eventually zeroed in on a gene called Tbx1. They showed that deleting Tbx1 on one chromosome 16 in mice causes the heart defects of DiGeorge syndrome. Tbx1 is also required for the development of other embryonic structures besides the heart. Thus, the results of research on DiGeorge syndrome may provide clues about the genetic causes of other birth defects.

SECTION 1 Chromosomes Copyright © by Holt, Rinehart and Winston. All rights reserved.

123

Change in Chromosome Structure Changes in an organism’s chromosome structure are called mutations. Breakage of a chromosome can lead to four types of mutations. In a deletion mutation, a piece of a chromosome breaks off completely. After cell division, the new cell will lack a certain set of genes. In many cases this proves fatal to the zygote. In a duplication mutation, a chromosome fragment attaches to its homologous chromosome, which will then carry two copies of a certain set of genes. A third type of mutation is an inversion mutation, in which the chromosome piece reattaches to the original chromosome but in a reverse orientation. If the piece reattaches to a nonhomologous chromosome, a translocation mutation results.

Modeling Chromosomal Mutations You can use paper and a pencil to model the ways in which chromosome structure can change.

1

6C

2

3

4

5

Materials Original chromosome

14 note-card pieces, pencils, tape Procedure

1

1. Write the numbers 1–8 on note-card pieces (one number per piece). Tape the pieces together in numerical order to model a chromosome with eight genes. 2. Use the “chromosome” you made to model the four alterations in chromosome structure discussed on this page and illustrated at right. For example, remove the number 3 and reconnect the remaining chromosome pieces to represent a deletion.

3. Reconstruct the original chromosome before modeling a duplication, an inversion, and a translocation. Use the extra note-card pieces to make the additional numbers you need.

2

4

5

Deletion 1

2

1

2

3

4

5

Duplication

1

Analysis Describe how a cell might be affected by each mutation if the cell were to receive a chromosome with that mutation.

2

4

3

5

Inversion

1

2

9

3

4

5

Translocation

Section 1 Review Summarize how prokaryotic cells divide by

Critical Thinking Evaluating Conclusions

binary fission.

Do you agree or disagree that homologous chro6A 6E mosomes are found in gametes. Explain.

4A 4B 6E

Identify the point in a eukaryotic cell cycle

at which DNA coils up to form 6A 6E chromosomes. Summarize the difference between a haploid

cell and a diploid cell.

124

6E

TAKS Test Prep

How does the karyotype of a person with Down syndrome differ from a normal 4B 6F karyotype? A It lacks a chromosome. B It has two sex chromosomes. C It has a damaged chromosome. D It has an extra copy of a chromosome.

CHAPTER 6 Chromosomes and Cell Reproduction Copyright © by Holt, Rinehart and Winston. All rights reserved.

The Cell Cycle

Section 2

The Life of a Eukaryotic Cell

Objectives

Cell division in eukaryotic cells is more complex than cell division in bacteria because it involves dividing both the cytoplasm and the chromosomes inside the nucleus. Many internal organelles must be correctly rearranged before the eukaryotic cell can properly divide and form two fully functioning cells.

The Cell Cycle The life of a eukaryotic cell is traditionally shown as a cycle, as illustrated in Figure 6. The cell cycle is a repeating sequence of cellular growth and division during the life of an organism. A cell spends 90 percent of its time in the first three phases of the cycle, which are collectively called interphase. A cell will enter the last two phases of the cell cycle only if it is about to divide. The five phases of the cell cycle are summarized below:

● Identify the major events that characterize each of the five phases of the cell cycle. 4B 6E ● Describe how the cell cycle is controlled in eukaryotic cells. 4B 6E ● Relate the role of the cell cycle to the onset 4B 6C of cancer.

Key Terms cell cycle interphase mitosis cytokinesis cancer

1. First growth (G1) phase. During the G1 phase, a cell grows rapidly and carries out its routine functions. For most organisms, this phase occupies the major portion of the cell’s life. Cells that are not dividing remain in the G1 phase. Some somatic cells, such as most muscle and nerve cells, never divide. Therefore, if these cells die, the body cannot replace them. 2. Synthesis (S) phase. A cell’s DNA is copied during this phase. At the end of this phase, each chromosome consists of two chromatids attached at the centromere. 3. Second growth (G2) phase. In the G2 phase, preparations are made for the nucleus to divide. Hollow protein fibers called microtubules are assembled. The microtubules are used to move the chromosomes during mitosis. 4. Mitosis. The process during cell division in which the nucleus of a cell is divided into two nuclei is called mitosis (mie TOH sihs). Each nucleus ends up with the same number and kinds of chromosomes as the original cell. 5. Cytokinesis. The process during cell division in which the cytoplasm divides is called cytokinesis (SIET oh kih nee sihs). Mitosis and cytokinesis produce new cells that are identical to the original cells and allow organisms to grow, replace damaged tissues, and, in some organisms, reproduce asexually.

Figure 6 The eukaryotic cell cycle. The cell cycle consists of phases of growth, DNA replication, preparation for cell division, and division of the nucleus and cytoplasm. INTE

RP HA S

E

S (DNA synthesis)

G1 (Cell growth)

G2 (Growth and preparation for mitosis)

Cytokinesis Mitosis

SECTION 2 The Cell Cycle Copyright © by Holt, Rinehart and Winston. All rights reserved.

125

Control of the Cell Cycle If a cell spends 90 percent of its time in interphase, how do cells “know” when to divide? How is the cycle controlled? Just as traffic lights control the flow of traffic, cells have a system that controls the phases of the cell cycle. Cells have a set of “red light–green light” switches that are regulated by feedback information from the cell. The cell cycle has key checkpoints (inspection points) at which feedback signals from the cell can trigger the next phase of the cell cycle (green light). Other feedback signals can delay the next phase to allow for completion of the current phase (yellow or red light). The cell cycle in eukaryotes is controlled by many proteins. Control occurs at three principal checkpoints, as shown in Figure 7.

Reviewing Information Learn the stages of interphase by reviewing the steps numbered 1–5 on the previous page. You can see in Figures 6 and 7 that the cell cycle is a repeating series of three steps followed by mitosis and cytokinesis.

1. Cell growth (G1) checkpoint. This checkpoint makes the decision of whether the cell will divide. If conditions are favorable for division and the cell is healthy and large enough, certain proteins will stimulate the cell to begin the synthesis (S) phase. During the S phase, the cell will copy its DNA. If conditions are not favorable, cells can typically stop the cell cycle at this checkpoint. The cell cycle will also stop at this checkpoint if the cell needs to pass into a resting period. Certain cells, such as some nerve and muscle cells, remain in this resting period permanently and never divide. 2. DNA synthesis (G2) checkpoint. DNA replication is checked at this point by DNA repair enzymes. If this checkpoint is passed, proteins help to trigger mitosis. The cell begins the many molecular processes that are needed to proceed into mitosis.

Figure 7 Control of the cell cycle. The cell cycle in eukaryotes is controlled at three inspection points, or checkpoints. Many proteins are involved in the control of the cell cycle. G1 checkpoint

3. Mitosis checkpoint. This checkpoint triggers the exit from mitosis. It signals the beginning of the G1 phase, the major growth period of the cell cycle.

When Control Is Lost: Cancer INTE RP HA S

E

S G1

G2 Cytokinesis Mitosis

Mitosis checkpoint

126

G2 checkpoint

Certain genes contain the information necessary to make the proteins that regulate cell growth and division. If one of these genes is mutated, the protein may not function, and regulation of cell growth and division can be disrupted. Cancer , the uncontrolled growth of cells, may result. Cancer is essentially a disorder of cell division. Cancer cells do not respond normally to the body’s control mechanisms. Some mutations cause cancer by overproducing growth-promoting molecules, thus speeding up the cell cycle. Others cause cancer by inactivating the control proteins that normally act to slow or stop the cell cycle.

CHAPTER 6 Chromosomes and Cell Reproduction Copyright © by Holt, Rinehart and Winston. All rights reserved.

Exploring Further Cancer Although all cancers are not curable, great progress has been made in cancer research over the last 30 years. We now know that cancer results from damage to a small set of genes that, in normal cells, limits the ability of cells to divide. What causes this damage? Certain environmental factors appear to be associated with cancer. For example, the incidence of cancer per thousand people is not uniform throughout the United States. Rather, it is higher in cities and in the Mississippi delta, suggesting that pollution and pesticide runoff may contribute to cancer. When pollutants, radiation, and other environmental factors associated with cancer are analyzed, a clear pattern emerges. Most cancer-causing agents are powerful mutagens—that is, they readily damage DNA. The conclusion that cancer is caused by mutation of a cell’s DNA is now supported by a very large body of evidence. How many mutations are required to produce cancer? Research in the last several years indicates that mutation of only a few genes can transform normal cells into cancerous ones. All of these cancer-causing genes are involved with regulating how fast cells grow and divide. How is cell division regulated? As a crude analogy, imagine a car parked on the side of a road. To get it going, you must step on the accelerator and release the brake.

Stepping on the Accelerator A cell divides when it receives a signal to do so. A “divide” signal is usually in the form of a chemical substance released by another cell. The substance is bound by a protein on the surface of the receiving cell. This binding activates a second protein inside the cell—relaying the signal from the outside of the cell to the inside. Here, a family of proteins then relay the signal inward to the

nucleus. One protein molecule passes the signal to the next like a baton in a relay race. The genes for these signal-carrying proteins are called oncogenes (onco is Greek for “mass” or “tumor.”). If oncogenes are changed by mutation to become more active, cancer can result. Like stepping on Melanoma cells the accelerator of a car, an increase in the activity of these proteins amplifies the “divide” signal. This causes the cell to divide more often.

Releasing the Brakes At the nucleus, the divide signal overrides a set of genes that act as “brakes.” These braking genes—called tumor supressor genes—prevent cell division from occurring too often. In cancer, these tumor suppressor genes are damaged. Like removing pressure from the brakes of a car increases a car’s speed, decreasing the activity of tumor suppressors speeds up cell division. Cells have three kinds of tumor suppressors, all of which must be disabled before cancer can occur. First, cells have proteins that inhibit DNA replication for limited periods. In cancer cells they are permanently inactivated. Second, cells have errorcorrecting proteins that detect damage to genes. In most cancers this error-detection has been disabled. Third, cancer cells rebuild the tips of their chromosomes. A little is lost from the ends of chromosomes at each replication, limiting the number of www.scilinks.org times a normal cell Topic: Cancer Cells can divide. Adding Keyword: HX4030 the deleted material back to the tips removes this limit to a cell’s life span.

Section 2 Review Differentiate between the G1, G2, and S phases

of the eukaryotic cell cycle.

4B 6E

Relate what occurs at each of the three principal

checkpoints in the cell cycle.

4B 6E

Critical Thinking Evaluating Information

Why are individual chromosomes more difficult to see during interphase than during mitosis? 4B 6E TAKS Test Prep

In the cell cyle of typical cancer cells, mutations have caused 4B 6C A slower growth. C uncontrolled growth. B a failure in mitosis. D a halt in cell division. SECTION 2 The Cell Cycle

Copyright © by Holt, Rinehart and Winston. All rights reserved.

127

Section 3

Mitosis and Cytokinesis

Objectives

Chromatid Separation in Mitosis

Every second about 2 million new red blood cells are produced in your body by cell divisions occurring in the bone marrow. These cells have received the signal to divide. The cells continue past the G2 phase and enter into the last two phases of the cell cycle—mitosis and ● Summarize the events cytokinesis. During mitosis the nucleus divides to form two nuclei, of the four stages each containing a complete set of the cell’s chromosomes. During of mitosis. 4B 6E cytokinesis the cytoplasm is divided between the two resulting cells. ● Differentiate cytokinesis in During mitosis, the chromatids on each chromosome are physi4B 6E animal and plant cells. cally moved to opposite sides of the dividing cell with the help of the spindle, shown in Figure 8. Spindles are cell structures made up Key Terms of both centrioles and individual microtubule fibers that are spindle involved in moving chromosomes during cell division.

● Describe the structure and function of the spindle during mitosis. 4B 6E

Forming the Spindle Animal cells usually have one pair of centrioles, with the centrioles at right angles to each other. During the G2 phase of the cell cycle, the centriole pair is replicated so that the cell has two pairs of centrioles as it enters the mitotic phase. When a cell enters the mitotic phase, the centriole pairs start to separate, moving toward opposite poles of the cell. As the centrioles move apart, the spindle begins to form. Centrioles and spindle fibers are both made of hollow tubes of protein called microtubules. Each spindle fiber is made of an individual microtubule. Each centriole, however, is made of nine triplets of

Figure 8 The spindle The spindle, made up of centrioles and spindle fibers, helps move chromosomes apart during mitosis.

Microtubule triplets

Centromere Cell

Chromatids

Spindle fibers Centrioles

128

Each centriole is composed of nine triplets of microtubules arranged in a circle.

CHAPTER 6 Chromosomes and Cell Reproduction Copyright © by Holt, Rinehart and Winston. All rights reserved.

microtubules arranged in a circle. Unlike animal cells, plant cells do not have centrioles, but they form a spindle that is almost identical to that of an animal cell.

Separation of Chromatids by Attaching Spindle Fibers Some of the microtubules in the spindle interact with each other. Others attach to a protein structure found on each side of the centromere. The two sets of microtubules extend out toward opposite poles of the cell. Once the microtubules attach to the centromeres and poles, the two chromatids in each chromosome can be separated. The chromatids are moved to each pole of the cell in a manner similar to bringing in a fish with a fishing rod and reel. When the microtubule “fishing line” is “reeled in,” the chromatids are dragged to opposite poles. The reeling in occurs because the ends of the spindle fibers are broken down bit by bit at each of the poles. As the fibers become shorter, the chromatids they are pulling move closer and closer to the poles. As soon as the chromatids separate from each other they are called chromosomes. When the chromosomes finally arrive, each pole has one complete set of chromosomes. 8

x 2+ 6x

0 2

5

-7-0

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