The Structure of Eukaryotic more,if the strand of DNA from a single chromosome were laid out in a straight line, it would be about 5 cen chromosomes timeters(2 inches)long. Fitting such a strand into a nu- In the century since discovery of chromosomes, we have cleus is like cramming a string the length of a football field learned a great deal about their structure and composition into a baseball-and thats only 1 of 46 chromosomes! In the cell, however, the DNA is coiled, allowing it to fit into a much smaller space than would otherwise be possible Composition of Chromatin Chromosomes are composed of chromatin, a complex of DNA and protein; most are about 40% DNA and 60% Chromosome coilin protein. A significant amount of RNA is also associated How can this long DNA fiber coil so tightly? If we gently with chromosomes because chromosomes are the sites of disrupt a eukaryotic nucleus and examine the DNA with RNA synthesis. The DNA of a chromosome is one very electron microscope, we find that it resembles a string of long, double-stranded fiber that extends unbroken through beads(figure 11.5). Every 200 nucleotides, the dNa du the entire length of the chromosome. a typical human plex is coiled around a core of eight histone proteins, form- chromosome contains about 140 million(1.4x 10%)nu- omplex known as a nucleosome. Unlike most cleotides in its DNA. The amount of information one proteins, which have an overall negative charge, histones chromosome contains would fill about 280 printed books of are positively charged, due to an abundance of the basic 1000 pages each, if each nucleotide corresponded to a amino acids arginine and lysine. They are thus strongly at "word"and each page had about 500 words on it. Further- tracted to the negatively charged phosphate groups of the within chromosome supercoil Levels of eukaryotic Chromatin hromosomal Nucleotides assemble into strano rands require further Chromatin fiber packaging to fit into the cell nucleus. The DNA duplex is tightly bound to and wound around proteins called bistones The DNA-wrapped histones are called CentralFNucleosome nucleosomes. The coalesce into chromatin fibers, ultimately coiling around into super coils that make up the form of DNA DNA double helix(duplex) 210 Part IV Reproduction and Heredity
The Structure of Eukaryotic Chromosomes In the century since discovery of chromosomes, we have learned a great deal about their structure and composition. Composition of Chromatin Chromosomes are composed of chromatin, a complex of DNA and protein; most are about 40% DNA and 60% protein. A significant amount of RNA is also associated with chromosomes because chromosomes are the sites of RNA synthesis. The DNA of a chromosome is one very long, double-stranded fiber that extends unbroken through the entire length of the chromosome. A typical human chromosome contains about 140 million (1.4 × 108) nucleotides in its DNA. The amount of information one chromosome contains would fill about 280 printed books of 1000 pages each, if each nucleotide corresponded to a “word” and each page had about 500 words on it. Furthermore, if the strand of DNA from a single chromosome were laid out in a straight line, it would be about 5 centimeters (2 inches) long. Fitting such a strand into a nucleus is like cramming a string the length of a football field into a baseball—and that’s only 1 of 46 chromosomes! In the cell, however, the DNA is coiled, allowing it to fit into a much smaller space than would otherwise be possible. Chromosome Coiling How can this long DNA fiber coil so tightly? If we gently disrupt a eukaryotic nucleus and examine the DNA with an electron microscope, we find that it resembles a string of beads (figure 11.5). Every 200 nucleotides, the DNA duplex is coiled around a core of eight histone proteins, forming a complex known as a nucleosome. Unlike most proteins, which have an overall negative charge, histones are positively charged, due to an abundance of the basic amino acids arginine and lysine. They are thus strongly attracted to the negatively charged phosphate groups of the 210 Part IV Reproduction and Heredity Supercoil within chromosome Chromosomes Coiling within supercoil Chromatin Chromatin fiber Nucleosome DNA Central histone DNA DNA double helix (duplex) FIGURE 11.5 Levels of eukaryotic chromosomal organization. Nucleotides assemble into long double strands of DNA molecules. These strands require further packaging to fit into the cell nucleus. The DNA duplex is tightly bound to and wound around proteins called histones. The DNA-wrapped histones are called nucleosomes. The nucleosomes then coalesce into chromatin fibers, ultimately coiling around into supercoils that make up the form of DNA recognized as a chromosome
DNA. The histone cores thus act as"magnetic forms"that promote and guide the coiling of the DNA. Further coiling somes wraps up into ⅨKκ higher order coils called ighly condensed portions of the chromatin are called heterochromatin. Some of these portions remain perma K ! il nently condensed, so that their DNA is never expressed The remainder of the chromosome, called euchromatin, is condensed only during cell division, when compact packa ing facilitates the movement of the chromosomes. At all other times, euchromatin is present in an open configura tion, and its genes can be expressed. The way chromatin is packaged when the cell is not dividing is not well under 3 35 stood beyond the level of nucleosomes and is a topic of in- FIGURE 11.6 A human karyotype. The individual chromosomes that make up Chromosome Karyotypes the 23 pairs differ widely in size and in centromere position. In this preparation, the chromosomes have been specifically stained Chromosomes may differ widely in appearance. They vary to indicate further differences in their composition and to in size, staining properties, the location of the centromere(a tinguish them clearly from one another constriction found on all chromosomes), the relative length of the two arms on either side of the centromere. and the positions of constricted regions along the arms. The partic ular array of chromosomes that an individual possesses is called its karyotype(figure 11.6). Karyotypes show marked Centromere differences among species and sometimes even among indi- viduals of the same species chromatids To examine a human karyotype, investigators collect a cell sample from blood, amniotic fluid, or other tissue and Homologous add chemicals that induce the cells in the sample to di chromosomes vide. Later, they add other chemicals to stop cell division at a stage when the chromosomes are most condensed and thus most easily distinguished from one another. The cells are then broken open and their contents, including the chromosomes, spread out and stained. To facilitate the examination of the karyotype, the chromosomes are FIGURE 11.7 usually photographed, and the outlines of the chromo- The difference between homologous chromosomes and sister somes are cut out of the photograph and arranged in chromatids. Homologous chromosomes are a pair of the same chromosome-say, chromosome number 16. Sister chromatids order(see figure 11.6) are the two replicas of a single chromosome held together by the centromeres after DNA replication How Many Chromosomes Are in a Cell? With the exception of the gametes(eggs or sperm)and a few specialized tissues, every cell in a human body is luman body cell contains a total of 46 replicated chromo- diploid(2n). This means that the cell contains two nearly somes, each composed of two sister chromatids joined by identical copies of each of the 23 types of chromosomes, one centromere. The cell thus contains 46 centromeres and for a total of 46 chromosomes. The haploid(1n)gametes 92 chromatids(2 sister chromatids for each of 2 homo- contain only one copy of each of the 23 chromosome types, logues for each of 23 chromosomes). The cell is said to while certain tissues have unusual numbers of chromo ontain 46 chromosomes rather than 92 because by con somes-many liver cells, for example, have two nuclei, vention, the number of chromosomes is obtained by count while mature red blood cells have no nuclei at all. The two copies of each chromosome in body cells are called homol- gous chromosomes, or homologues(Greek bomm Eukaryotic genomes are larger and more complex than agreement"). Before cell division, each homologue repli those of bacteria. Eukaryotic DNA is packaged tightly cates,producing two identical sister chromatids joined at into chromosomes, enabling it to fit inside cells the centromere, a condensed area found on all eukaryotic Haploid cells contain one set of chromosomes, while hromosomes(figure 11.7). Hence, as cell division begins, a diploid cells contain two sets Chapter 11 How Cells Divide 211
DNA. The histone cores thus act as “magnetic forms” that promote and guide the coiling of the DNA. Further coiling occurs when the string of nucleosomes wraps up into higher order coils called supercoils. Highly condensed portions of the chromatin are called heterochromatin. Some of these portions remain permanently condensed, so that their DNA is never expressed. The remainder of the chromosome, called euchromatin, is condensed only during cell division, when compact packaging facilitates the movement of the chromosomes. At all other times, euchromatin is present in an open configuration, and its genes can be expressed. The way chromatin is packaged when the cell is not dividing is not well understood beyond the level of nucleosomes and is a topic of intensive research. Chromosome Karyotypes Chromosomes may differ widely in appearance. They vary in size, staining properties, the location of the centromere (a constriction found on all chromosomes), the relative length of the two arms on either side of the centromere, and the positions of constricted regions along the arms. The particular array of chromosomes that an individual possesses is called its karyotype (figure 11.6). Karyotypes show marked differences among species and sometimes even among individuals of the same species. To examine a human karyotype, investigators collect a cell sample from blood, amniotic fluid, or other tissue and add chemicals that induce the cells in the sample to divide. Later, they add other chemicals to stop cell division at a stage when the chromosomes are most condensed and thus most easily distinguished from one another. The cells are then broken open and their contents, including the chromosomes, spread out and stained. To facilitate the examination of the karyotype, the chromosomes are usually photographed, and the outlines of the chromosomes are cut out of the photograph and arranged in order (see figure 11.6). How Many Chromosomes Are in a Cell? With the exception of the gametes (eggs or sperm) and a few specialized tissues, every cell in a human body is diploid (2n). This means that the cell contains two nearly identical copies of each of the 23 types of chromosomes, for a total of 46 chromosomes. The haploid (1n) gametes contain only one copy of each of the 23 chromosome types, while certain tissues have unusual numbers of chromosomes—many liver cells, for example, have two nuclei, while mature red blood cells have no nuclei at all. The two copies of each chromosome in body cells are called homologous chromosomes, or homologues (Greek homologia, “agreement”). Before cell division, each homologue replicates, producing two identical sister chromatids joined at the centromere, a condensed area found on all eukaryotic chromosomes (figure 11.7). Hence, as cell division begins, a human body cell contains a total of 46 replicated chromosomes, each composed of two sister chromatids joined by one centromere. The cell thus contains 46 centromeres and 92 chromatids (2 sister chromatids for each of 2 homologues for each of 23 chromosomes). The cell is said to contain 46 chromosomes rather than 92 because, by convention, the number of chromosomes is obtained by counting centromeres. Eukaryotic genomes are larger and more complex than those of bacteria. Eukaryotic DNA is packaged tightly into chromosomes, enabling it to fit inside cells. Haploid cells contain one set of chromosomes, while diploid cells contain two sets. Chapter 11 How Cells Divide 211 FIGURE 11.6 A human karyotype. The individual chromosomes that make up the 23 pairs differ widely in size and in centromere position. In this preparation, the chromosomes have been specifically stained to indicate further differences in their composition and to distinguish them clearly from one another. Sister chromatids Homologous chromosomes Centromere FIGURE 11.7 The difference between homologous chromosomes and sister chromatids. Homologous chromosomes are a pair of the same chromosome—say, chromosome number 16. Sister chromatids are the two replicas of a single chromosome held together by the centromeres after DNA replication
11.3 Mitosis is a key phase of the cell cycle Phases of the Cell Cycle The increased size and more complex organization of eu- Metaphase Anaphase karyotic genomes over those of bacteria required radical Telophase hanges in the process by which the two replicas of the are partitioned into the daughter cells during cell n. This division process is diagrammed as a cell consisting of five phases(figure 11. 8) The five phases D Interphase(G,,S,G2 phase GI is the primary growth phase of the cell. For many or- 口 Mitosis(M) ganisms, this encompasses the major portion of the cells ife span. S is the phase in which the cell synthesizes a eplica of the genome. G is the second growth phase, in which preparations are made for genomic separation During this phase, mitochondria and other organelles replicate, chromosomes condense, and microtubules begin to assemble at a spindle. Gl, S, and G2 together constitute interphase, the portion of the cell cycle be tween cell divisions FIGURE 11.8 M is the phase of the cell cycle in which the microtubu- The cell cycle. Each wedge represents one hour of the 22-hour lar apparatus assembles, binds to the chromosomes, and cell cycle in human cells growing in culture. GI represents the moves the sister chromatids apart. Called mitosis, this primary growth phase of the cell cycle, S the phase during which a process is the essential step in the separation of the two eplica of the genome is synthesized, and Gy the second growth daughter genomes. We will discuss mitosis as it occurs in animals and plants, where the process does not vary much (it is somewhat different among fungi and some protists) 24 hours. but some cells. like certain cells in the human Although mitosis is a continuous process, it is traditionally liver, have cell cycles lasting more than a year. During subdivided into four stages: prophase, metaphase, anaphase and telophase he cycle, growth occurs throughout the GI and g2 C is the phase of the cell cycle when the cytoplasm di phases(referred to as"gap"phases, as they separate S vides, creating two daughter cells. This phase is called from M), as well as during the s phase. The M phase cytokinesis. In animal cells, the microtubule spindle takes only about an hour, a small fraction of the entire helps position a contracting ring of actin that constricts cycle like a drawstring to pinch the cell in two. In cells with a Most of the variation in the length of the cell cycle cell wall, such as plant cells, a plate forms between the di from one organism or tissue to the next occurs in the phase Cells often pause in Gi before DNA replication and enter a resting state called Go phase; they may re- Duration of the Cell Cycle main in this phase for days to years before resuming cell division. At any given time, most of the cells in an ani The time it takes to complete a cell cycle varies greatly mal's body are in Go phase. Some, such as muscle and nerve cells, remain there permanently; others, such as among organisms. Cells in growing embryos can com- liver cells, can resume Gi phase in response to factors re- plete their cell cycle in under 20 minutes; the shortest known animal nuclear division cycles occur in fruit fly leased during injury embryos(8 minutes). Cells such as these simply divide their nuclei as quickly as they can replicate their DNA, Most eukaryotic cells repeat a process of growth and ell th. half of the The cycle can vary half by m, and essentially none by gi or G2. Because ma- length from a few minutes to several years ture cells require time to grow, most of their cycles are much longer than those of embryonic tissue. Typically, a dividing mammalian cell completes its cell cycle in about 212 Part IV Reproduction and Heredity
Phases of the Cell Cycle The increased size and more complex organization of eukaryotic genomes over those of bacteria required radical changes in the process by which the two replicas of the genome are partitioned into the daughter cells during cell division. This division process is diagrammed as a cell cycle, consisting of five phases (figure 11.8). The Five Phases G1 is the primary growth phase of the cell. For many organisms, this encompasses the major portion of the cell’s life span. S is the phase in which the cell synthesizes a replica of the genome. G2 is the second growth phase, in which preparations are made for genomic separation. During this phase, mitochondria and other organelles replicate, chromosomes condense, and microtubules begin to assemble at a spindle. G1, S, and G2 together constitute interphase, the portion of the cell cycle between cell divisions. M is the phase of the cell cycle in which the microtubular apparatus assembles, binds to the chromosomes, and moves the sister chromatids apart. Called mitosis, this process is the essential step in the separation of the two daughter genomes. We will discuss mitosis as it occurs in animals and plants, where the process does not vary much (it is somewhat different among fungi and some protists). Although mitosis is a continuous process, it is traditionally subdivided into four stages: prophase, metaphase, anaphase, and telophase. C is the phase of the cell cycle when the cytoplasm divides, creating two daughter cells. This phase is called cytokinesis. In animal cells, the microtubule spindle helps position a contracting ring of actin that constricts like a drawstring to pinch the cell in two. In cells with a cell wall, such as plant cells, a plate forms between the dividing cells. Duration of the Cell Cycle The time it takes to complete a cell cycle varies greatly among organisms. Cells in growing embryos can complete their cell cycle in under 20 minutes; the shortest known animal nuclear division cycles occur in fruit fly embryos (8 minutes). Cells such as these simply divide their nuclei as quickly as they can replicate their DNA, without cell growth. Half of the cycle is taken up by S, half by M, and essentially none by G1 or G2. Because mature cells require time to grow, most of their cycles are much longer than those of embryonic tissue. Typically, a dividing mammalian cell completes its cell cycle in about 24 hours, but some cells, like certain cells in the human liver, have cell cycles lasting more than a year. During the cycle, growth occurs throughout the G1 and G2 phases (referred to as “gap” phases, as they separate S from M), as well as during the S phase. The M phase takes only about an hour, a small fraction of the entire cycle. Most of the variation in the length of the cell cycle from one organism or tissue to the next occurs in the G1 phase. Cells often pause in G1 before DNA replication and enter a resting state called G0 phase; they may remain in this phase for days to years before resuming cell division. At any given time, most of the cells in an animal’s body are in G0 phase. Some, such as muscle and nerve cells, remain there permanently; others, such as liver cells, can resume G1 phase in response to factors released during injury. Most eukaryotic cells repeat a process of growth and division referred to as the cell cycle. The cycle can vary in length from a few minutes to several years. 212 Part IV Reproduction and Heredity 11.3 Mitosis is a key phase of the cell cycle. G2 S G1 C Metaphase Prophase Anaphase Telophase M Interphase (G1, S, G2 phases) Mitosis (M) Cytokinesis (C) FIGURE 11.8 The cell cycle. Each wedge represents one hour of the 22-hour cell cycle in human cells growing in culture. G1 represents the primary growth phase of the cell cycle, S the phase during which a replica of the genome is synthesized, and G2 the second growth phase
Interphase: Preparing for Mitosis Chromatid The events that occur during interphase, made up of the g S, and g phases, are very important for the successful con pletion of mitosis. During Gl, cells undergo the major por tion of their growth. During the S phase, each chromosome replicates to produce two sister chromatids, which remain at- tached to each other at the centromere. The centromere is Kinetochore a point of constriction on the chromosome, containing specific DNA sequence to which is bound a disk of protein called a kinetochore. This disk functions as an attachment site for fibers that assist in cell division(figure 11.9). Each Centromere chromosome's centromere is located at a characteristic site The cell grows throughout interphase. The GI and gz segments of interphase are periods of active growth, when proteins are synthesized and cell organelles produced. The cells DNA replicates only during the S phase of the cell cycle After the chromosomes have replicated in S phase, they chromosome remain fully extended and uncoiled. This makes them invis- ible under the light microscope. In G] phase, they begin the FIGURE 11.9 long process of condensation, coiling ever more tightly Kinetochores. In a metaphase chromosome, kinetochore involved in the rapid final conden- microtubules are anchored to proteins at the centromere. sation of the chromosomes that occurs early in mitosis. also during G phase, the cells begin to assemble the machinery they will later use to move the chromosomes to opposite Interphase is that portion of the cell cycle in which the poles of the cell. In animal cells, a pair of microtubule- chromosomes are invisible under the light microscope organizing centers called centrioles replicate. All eukary- because they are not yet condensed. It includes the Gl otic cells undertake an extensive synthesis of tubulin, the S, and G2 phases. In the G2 phase, the cell mobilizes its rotein of which microtubules are formed resources for cell division A Vocabulary of chromatin The complex of DNA and kinetochore A disk of protein bound to proteins of which eukaryotic chromosomes the centromere and attached to micro- Cell Division are composed tubules during mitosis, linking each chro- chromosome The structure within cells matid to the spindle apparatus. that contains the genes. In eukaryotes, it microtubule A hollow cylinder, about 25 consists of a single linear DNA molecule as- nanometers in diameter, composed of sub binary fission Asexual reproduction of a sociated with proteins. The DNA is repli- units of the protein tubulin. Microtubules cell by division into two equal or nearly cated during S phase, and the replicas sepa- lengthen by the addition of tubulin subunits equal parts. Bacteria divide by binary rated during M phase to their end(s) and shorten by the removal cytokinesis Division of the cytoplasm of a of subunits centromere A constricted region of a cell after nuclear division. mitosis Nuclear division in which repl length, composed of highly repeated DNA some that is extended except during cell di- genetically identical daughter nuclei. When equences(satellite DNA). During mitosis, vision and from which rna is transcribed. accompanied by cytokinesis, it produces matids and is the site to which the kineto- heterochromatin The portion of a chro- two identical daughter cells the centromere joins the two sister chro chores are attached mosome that remains permanently con- nucleosome The basic packaging unit of densed and therefore is not transcribed eukaryotic chromosomes, in which the chromatid One of the two copies of a into RNA. Most centromere regions are DNA molecule is wound around a cluster of replicated chromosome, joined by a single heterochromatic histone proteins. Chromatin is composed of centromere to the other strand homologues Homologous chromosomes: long strings of nucleosomes that resemble of ch somes that carry equivalent genes Chapter 11 How Cells Divide 213
Interphase: Preparing for Mitosis The events that occur during interphase, made up of the G1, S, and G2 phases, are very important for the successful completion of mitosis. During G1, cells undergo the major portion of their growth. During the S phase, each chromosome replicates to produce two sister chromatids, which remain attached to each other at the centromere. The centromere is a point of constriction on the chromosome, containing a specific DNA sequence to which is bound a disk of protein called a kinetochore. This disk functions as an attachment site for fibers that assist in cell division (figure 11.9). Each chromosome’s centromere is located at a characteristic site. The cell grows throughout interphase. The G1 and G2 segments of interphase are periods of active growth, when proteins are synthesized and cell organelles produced. The cell’s DNA replicates only during the S phase of the cell cycle. After the chromosomes have replicated in S phase, they remain fully extended and uncoiled. This makes them invisible under the light microscope. In G2 phase, they begin the long process of condensation, coiling ever more tightly. Special motor proteins are involved in the rapid final condensation of the chromosomes that occurs early in mitosis. Also during G2 phase, the cells begin to assemble the machinery they will later use to move the chromosomes to opposite poles of the cell. In animal cells, a pair of microtubuleorganizing centers called centrioles replicate. All eukaryotic cells undertake an extensive synthesis of tubulin, the protein of which microtubules are formed. Interphase is that portion of the cell cycle in which the chromosomes are invisible under the light microscope because they are not yet condensed. It includes the G1, S, and G2 phases. In the G2 phase, the cell mobilizes its resources for cell division. Chapter 11 How Cells Divide 213 Metaphase chromosome Kinetochore Kinetochore microtubules Centromere region of chromosome Chromatid FIGURE 11.9 Kinetochores. In a metaphase chromosome, kinetochore microtubules are anchored to proteins at the centromere. A Vocabulary of Cell Division chromatin The complex of DNA and proteins of which eukaryotic chromosomes are composed. chromosome The structure within cells that contains the genes. In eukaryotes, it consists of a single linear DNA molecule associated with proteins. The DNA is replicated during S phase, and the replicas separated during M phase. cytokinesis Division of the cytoplasm of a cell after nuclear division. euchromatin The portion of a chromosome that is extended except during cell division, and from which RNA is transcribed. heterochromatin The portion of a chromosome that remains permanently condensed and, therefore, is not transcribed into RNA. Most centromere regions are heterochromatic. homologues Homologous chromosomes; in diploid cells, one of a pair of chromosomes that carry equivalent genes. kinetochore A disk of protein bound to the centromere and attached to microtubules during mitosis, linking each chromatid to the spindle apparatus. microtubule A hollow cylinder, about 25 nanometers in diameter, composed of subunits of the protein tubulin. Microtubules lengthen by the addition of tubulin subunits to their end(s) and shorten by the removal of subunits. mitosis Nuclear division in which replicated chromosomes separate to form two genetically identical daughter nuclei. When accompanied by cytokinesis, it produces two identical daughter cells. nucleosome The basic packaging unit of eukaryotic chromosomes, in which the DNA molecule is wound around a cluster of histone proteins. Chromatin is composed of long strings of nucleosomes that resemble beads on a string. binary fission Asexual reproduction of a cell by division into two equal or nearly equal parts. Bacteria divide by binary fission. centromere A constricted region of a chromosome about 220 nucleotides in length, composed of highly repeated DNA sequences (satellite DNA). During mitosis, the centromere joins the two sister chromatids and is the site to which the kinetochores are attached. chromatid One of the two copies of a replicated chromosome, joined by a single centromere to the other strand
Mitosis Chromosome Prophase: Formation of the Mitotic Apparatus When the chromosome condensation initiated in G2 phase reaches the point at which individual condensed chromo- omes first become visible with the light microscope rst stage of mitosis, prophase, has begun. The condensa tion process continues throughout prophase; consequently some chromosomes that start prophase as minute threads appear quite bulky before its conclusion. Ribosomal RNA synthesis ceases when the portion of the chromosome bear Metaphase ing the rRNA genes is condensed Assembling the Spindle Apparatus. The assembly of the microtubular apparatus that will later separate the sister chromatids also continues during prophase. In ani- mal cells, the two centriole pairs formed during G2 phase begin to move apart early in prophase, forming between them an axis of microtubules referred to as spindle fibers By the time the centrioles reach the opposite poles of the cell, they have established a bridge of microtubules called the spindle apparatus between them. In plant cells,a similar bridge of microtubular fibers forms between op posite poles of the cell, although centrioles are absent in plant cells During the formation of the spindle apparatus, the nu clear envelope breaks down and the endoplasmic reticulum IGURE 11.10 reabsorbs its components. At this point, then, the micro- Metaphase In metaphase, the chromosomes array themselves in tubular spindle fibers extend completely across the cell, a circle around the spindle midpoint. from one pole to the other. Their orientation determines he plane in which the cell will subsequently divide hrough the center of the cell at right angles to the spindle apparatus. trous. The attachment of the two sides of a centromere In animal cell mitosis, the centrioles extend a radial to the same pole, for example, leads to a failure of the array of microtubules toward the plasma membrane when ter chromatids to separate, so that they end up in the they reach the poles of the cell. This arrangement of mi- same daughter cell crotubules is called an aster. Although the aster's func- tion is not fully understood, it probably braces the centri oles against the membrane and stiffens the point of Metaphase: Alignment of the Centromeres microtubular attachment during the retraction of the The second stage of mitosis, metaphase, is the phase spindle. Plant cells, which have rigid cell walls, do not where the chromosomes align in the center of the cell form asters When viewed with a light microscope, the chromosomes appear to array themselves in a circle along the inner cir Linking Sister Chromatids to Opposite Poles. Each cumference of the cell, as the equator girdles the earth(fig- chromosome possesses two kinetochores, one attached to ure 11.10). An imaginary plane perpendicular to the axis of the centromere region of each sister chromatid(see fig- the spindle that passes through this circle is called the ure 11.9). As prophase continues, a second group of mi- metaphase plate. The metaphase plate is not an actual struc- crotubules appears to grow from the poles of the cell to- ture, but rather an indication of the future axis of cell divi ward the centromeres. These microtubules connect the ion. Positioned by the microtubules attached to the kine- kinetochores on each pair of sister chromatids to the two tochores of their centromeres, all of the chromosomes line poles of the spindle. Because microtubules extending up on the metaphase plate(figure 11. 11). At this point, from the two poles attach to opposite sides of the cen- which marks the end of metaphase, their centromeres are tromere,they attach one sister chromatid to one pole and neatly arrayed in a circle, equidistant from the two poles of the other sister chromatid to the other pole. This the cell, with microtubules extending back towards the arrangement is absolutely critical to the process of mito- posite poles of the cell in an arrangement called a spi sis; any mistakes in microtubule positioning can be disas because of its shape 214 Part IV Reproduction and Heredity
Mitosis Prophase: Formation of the Mitotic Apparatus When the chromosome condensation initiated in G2 phase reaches the point at which individual condensed chromosomes first become visible with the light microscope, the first stage of mitosis, prophase, has begun. The condensation process continues throughout prophase; consequently, some chromosomes that start prophase as minute threads appear quite bulky before its conclusion. Ribosomal RNA synthesis ceases when the portion of the chromosome bearing the rRNA genes is condensed. Assembling the Spindle Apparatus. The assembly of the microtubular apparatus that will later separate the sister chromatids also continues during prophase. In animal cells, the two centriole pairs formed during G2 phase begin to move apart early in prophase, forming between them an axis of microtubules referred to as spindle fibers. By the time the centrioles reach the opposite poles of the cell, they have established a bridge of microtubules called the spindle apparatus between them. In plant cells, a similar bridge of microtubular fibers forms between opposite poles of the cell, although centrioles are absent in plant cells. During the formation of the spindle apparatus, the nuclear envelope breaks down and the endoplasmic reticulum reabsorbs its components. At this point, then, the microtubular spindle fibers extend completely across the cell, from one pole to the other. Their orientation determines the plane in which the cell will subsequently divide, through the center of the cell at right angles to the spindle apparatus. In animal cell mitosis, the centrioles extend a radial array of microtubules toward the plasma membrane when they reach the poles of the cell. This arrangement of microtubules is called an aster. Although the aster’s function is not fully understood, it probably braces the centrioles against the membrane and stiffens the point of microtubular attachment during the retraction of the spindle. Plant cells, which have rigid cell walls, do not form asters. Linking Sister Chromatids to Opposite Poles. Each chromosome possesses two kinetochores, one attached to the centromere region of each sister chromatid (see figure 11.9). As prophase continues, a second group of microtubules appears to grow from the poles of the cell toward the centromeres. These microtubules connect the kinetochores on each pair of sister chromatids to the two poles of the spindle. Because microtubules extending from the two poles attach to opposite sides of the centromere, they attach one sister chromatid to one pole and the other sister chromatid to the other pole. This arrangement is absolutely critical to the process of mitosis; any mistakes in microtubule positioning can be disastrous. The attachment of the two sides of a centromere to the same pole, for example, leads to a failure of the sister chromatids to separate, so that they end up in the same daughter cell. Metaphase: Alignment of the Centromeres The second stage of mitosis, metaphase, is the phase where the chromosomes align in the center of the cell. When viewed with a light microscope, the chromosomes appear to array themselves in a circle along the inner circumference of the cell, as the equator girdles the earth (figure 11.10). An imaginary plane perpendicular to the axis of the spindle that passes through this circle is called the metaphase plate. The metaphase plate is not an actual structure, but rather an indication of the future axis of cell division. Positioned by the microtubules attached to the kinetochores of their centromeres, all of the chromosomes line up on the metaphase plate (figure 11.11). At this point, which marks the end of metaphase, their centromeres are neatly arrayed in a circle, equidistant from the two poles of the cell, with microtubules extending back towards the opposite poles of the cell in an arrangement called a spindle because of its shape. 214 Part IV Reproduction and Heredity Chromosome Centrioles Metaphase plate Aster Spindle fibers FIGURE 11.10 Metaphase. In metaphase, the chromosomes array themselves in a circle around the spindle midpoint