Cellular Reproduction CHAPTER OUTLINE Cells and Chromosomes Mitosis Meiosis Life Cycles of Some Model Genetic Organisms mother)with a cell from the udder of a Finn Dorset ewe(the genetic mother).The genetic material in the Blackface ewe's egg had been Dolly removed prior to fusing the egg with the udder cell.Subsequently.the Sheep have grazed on the hard-scrabble landscape of Scotland for newly endowed egg was stimulated to divide.It produced an embryo, which was implanted in the centuries.Finn Dorsets and Scottish Blackfaces are some of the uterus of another Blackface breeds raised by shepherds there.Every spring.the lambs that were ewe (the gestational or sur- conceived during the fall are born.They grow quickly and take their places in flocks-or in butcher shops.Early in 1997,a lamb unlike rogate mother).This embryo any other came into the world.This lamb,named Dolly.did not have grew and developed,and when the surrogate mother's a father,but she did have three mothers;furthermore,her genes were identical to those of one of her mothers.In a word.Dolly pregnancy came to term,Dolly was born. was a clone. Scientists at the Roslin Institute near Edinburgh,Scotland The technology that produced Dolly by fusing an egg from a Blackface ewe(the egg cell produced Dolly emerged from a century of basic research on the cellular basis of reproduc- tion.In the ordinary course of events,an egg cell from a female is fertilized by a sperm cell from a male,and the resulting zygote divides to produce genetically identical cells.These cells then divide The nuclei of three cells are inside many times to produce a mul- a long,thin micropipette.The ticellular organism.Within that topmost nucleus with its genetic organism,a particular group material is being injected into an of cells embarks on a different enucleated egg that is being held mode of division to produce in place by a wider pipette. specialized reproductive cells-either eggs or sperm.An egg from one such organism then unites with a sperm from another such organism to produce a new Dolly,the first cloned mammal.The photo on the right shows the offspring.The offspring grows up and the cycle continues,generation after generation.But Dolly,the first cloned mammal,was created by cloning process. sidestepping this entire process. 18
2 Cells and Chromosomes Mitosis Meiosis Life Cycles of Some Model Genetic Organisms Dolly Sheep have grazed on the hard-scrabble landscape of Scotland for centuries. Finn Dorsets and Scottish Blackfaces are some of the breeds raised by shepherds there. Every spring, the lambs that were conceived during the fall are born. They grow quickly and take their places in flocks—or in butcher shops. Early in 1997, a lamb unlike any other came into the world. This lamb, named Dolly, did not have a father, but she did have three mothers; furthermore, her genes were identical to those of one of her mothers. In a word, Dolly was a clone. Scientists at the Roslin Institute near Edinburgh, Scotland produced Dolly by fusing an egg from a Blackface ewe (the egg cell mother) with a cell from the udder of a Finn Dorset ewe (the genetic mother). The genetic material in the Blackface ewe’s egg had been removed prior to fusing the egg with the udder cell. Subsequently, the newly endowed egg was stimulated to divide. It produced an embryo, which was implanted in the uterus of another Blackface ewe (the gestational or surrogate mother). This embryo grew and developed, and when the surrogate mother’s pregnancy came to term, Dolly was born. The technology that produced Dolly emerged from a century of basic research on the cellular basis of reproduction. In the ordinary course of events, an egg cell from a female is fertilized by a sperm cell from a male, and the resulting zygote divides to produce genetically identical cells. These cells then divide many times to produce a multicellular organism. Within that organism, a particular group of cells embarks on a different mode of division to produce specialized reproductive cells—either eggs or sperm. An egg from one such organism then unites with a sperm from another such organism to produce a new offspring. The offspring grows up and the cycle continues, generation after generation. But Dolly, the first cloned mammal, was created by sidestepping this entire process. Cellular Reproduction Dolly, the first cloned mammal. The photo on the right shows the cloning process. The nuclei of three cells are inside a long, thin micropipette. The topmost nucleus with its genetic material is being injected into an enucleated egg that is being held in place by a wider pipette. 18 CHAPTER OUTLINE
Cells and Chromosomes 19 Cells and Chromosomes In the early part of the nineteenth century,a few In both prokaryotic and eukaryotic cells,the genetic decades before Gregor Mendel carried out his experi- ments with peas,biologists established the principle material is organized into chromosomes. that living things are composed of cells.Some organ- isms consist of just a single cell.Others consist of trillions of cells.Each cell is a complicated assemblage of molecules that can acquire materials,recruit and store energy,and carry out diverse activities,including reproduction.The simplest life forms,viruses,are not composed of cells.However,viruses must enter cells in order to function.Thus,all life has a cellular basis.As preparation for our journey through the science of genetics,we now review the biology of cells.We also discuss chromo- somes-the cellular structures in which genes reside. THE CELLULAR ENVIRONMENT Living cells are made of many different kinds of molecules.The most abundant is water.Small molecules-for example,salts,sugars,amino acids,and certain vitamins-readily dissolve in water,and some larger molecules interact favorably with it.All these sorts of substances are said to be hydrophilic.Other kinds of molecules do not interact well with water.They are said to be hydrophobic.The inside of a cell,called the cytoplasm,contains both hydrophilic and hydrophobic substances. The molecules that make up cells are diverse in structure and function.Carbo- hydrates such as starch and glycogen store chemical energy for work within cells. These molecules are composed of glucose,a simple sugar.The glucose subunits are attached one to another to form long chains,or polymers.Cells obtain energy when glucose molecules released from these chains are chemically degraded into simpler compounds-ultimately,to carbon dioxide and water.Cells also possess an assort- ment of lipids.These molecules are formed by chemical interactions between glycerol, a small organic compound,and larger organic compounds called fatty acids.Lipids are important constituents of many structures within cells.They also serve as energy sources.Proteins are the most diverse molecules within cells.Each protein consists of one or more polypeptides,which are chains of amino acids.Often a protein con- sists of two polypeptides-that is,it is a dimer;sometimes a protein consists of many polypeptides-that is,it is a multimer.Within cells,proteins are components of many different structures.They also catalyze chemical reactions.We call these catalytic pro- teins enzymes.Cells also contain nucleic acids-DNA and RNA,which,as already described in Chapter 1,are central to life. Cells are surrounded by a thin layer called a membrane.Many different types of molecules make up cell membranes;however,the primary constituents are lipids and proteins.Membranes are also present inside cells.These internal membranes may divide a cell into compartments,or they may help to form specialized structures called organelles.Membranes are fluid and flexible.Many of the molecules within a mem- brane are not rigidly held in place by strong chemical forces.Consequently,they are able to slip by one another in what amounts to an ever-changing molecular sea.Some kinds of cells are surrounded by tough,rigid walls,which are external to the mem- brane.Plant cell walls are composed of cellulose,a complex carbohydrate.Bacterial cell walls are composed of a different kind of material called murein. Walls and membranes separate the contents of a cell from the outside world. However,they do not seal it off.These structures are porous to some materials,and they selectively allow other materials to pass through them via channels and gates. The transport of materials in and through walls and membranes is an important activity of cells.Cell membranes also contain molecules that interact with materials in a cell's external environment.Such molecules provide a cell with vital informa- tion about conditions in the environment,and they also mediate important cellular activities
Cells and Chromosomes 19 In both prokaryotic and eukaryotic cells, the genetic material is organized into chromosomes. Cells and Chromosomes In the early part of the nineteenth century, a few decades before Gregor Mendel carried out his experiments with peas, biologists established the principle that living things are composed of cells. Some organisms consist of just a single cell. Others consist of trillions of cells. Each cell is a complicated assemblage of molecules that can acquire materials, recruit and store energy, and carry out diverse activities, including reproduction. The simplest life forms, viruses, are not composed of cells. However, viruses must enter cells in order to function. Thus, all life has a cellular basis. As preparation for our journey through the science of genetics, we now review the biology of cells. We also discuss chromosomes—the cellular structures in which genes reside. THE CELLULAR ENVIRONMENT Living cells are made of many different kinds of molecules. The most abundant is water. Small molecules—for example, salts, sugars, amino acids, and certain vitamins—readily dissolve in water, and some larger molecules interact favorably with it. All these sorts of substances are said to be hydrophilic. Other kinds of molecules do not interact well with water. They are said to be hydrophobic. The inside of a cell, called the cytoplasm, contains both hydrophilic and hydrophobic substances. The molecules that make up cells are diverse in structure and function. Carbohydrates such as starch and glycogen store chemical energy for work within cells. These molecules are composed of glucose, a simple sugar. The glucose subunits are attached one to another to form long chains, or polymers. Cells obtain energy when glucose molecules released from these chains are chemically degraded into simpler compounds—ultimately, to carbon dioxide and water. Cells also possess an assortment of lipids. These molecules are formed by chemical interactions between glycerol, a small organic compound, and larger organic compounds called fatty acids. Lipids are important constituents of many structures within cells. They also serve as energy sources. Proteins are the most diverse molecules within cells. Each protein consists of one or more polypeptides, which are chains of amino acids. Often a protein consists of two polypeptides—that is, it is a dimer; sometimes a protein consists of many polypeptides—that is, it is a multimer. Within cells, proteins are components of many different structures. They also catalyze chemical reactions. We call these catalytic proteins enzymes. Cells also contain nucleic acids—DNA and RNA, which, as already described in Chapter 1, are central to life. Cells are surrounded by a thin layer called a membrane. Many different types of molecules make up cell membranes; however, the primary constituents are lipids and proteins. Membranes are also present inside cells. These internal membranes may divide a cell into compartments, or they may help to form specialized structures called organelles. Membranes are fluid and flexible. Many of the molecules within a membrane are not rigidly held in place by strong chemical forces. Consequently, they are able to slip by one another in what amounts to an ever-changing molecular sea. Some kinds of cells are surrounded by tough, rigid walls, which are external to the membrane. Plant cell walls are composed of cellulose, a complex carbohydrate. Bacterial cell walls are composed of a different kind of material called murein. Walls and membranes separate the contents of a cell from the outside world. However, they do not seal it off. These structures are porous to some materials, and they selectively allow other materials to pass through them via channels and gates. The transport of materials in and through walls and membranes is an important activity of cells. Cell membranes also contain molecules that interact with materials in a cell’s external environment. Such molecules provide a cell with vital information about conditions in the environment, and they also mediate important cellular activities
20 Chapter 2 Cellular Reproduction PROKARYOTIC AND EUKARYOTIC CELLS When we survey the living world,we find two basic kinds of cells:prokaryotic and eukaryotic(Figure 2.1).Prokaryotic cells are usually less than a thousandth of a mil- limeter long,and they typically lack a complicated system of internal membranes and membranous organelles.Their hereditary material-that is,the DNA-is not isolated in a special subcellular compartment.Organisms with this kind of cellular organization are called prokaryotes.Examples include the bacteria,which are the most abundant life forms on Earth,and the archaea,which are found in extreme environments such as salt lakes,hot springs,and deep-sea volcanic vents.All other organisms-plants,animals,protists,and fungi-are eukaryotes. Eukaryotic cells are larger than prokaryotic cells,usually at least 10 times bigger, and they possess complicated systems of internal membranes,some of which are associated with conspicuous,well-organized organelles.For example,eukaryotic cells typically contain one or more mitochondria(singular,mitochondrion),which are ellip- soidal organelles dedicated to the recruitment of energy from foodstuffs.Algal and plant cells contain another kind of energy-recruiting organelle called the chtoroplast, which captures solar energy and converts it into chemical energy.Both mitochondria and chloroplasts are surrounded by membranes. The hallmark of all eukaryotic cells is that their hereditary material is contained within a large,membrane-bounded structure called the nucleus.The nuclei of eukary- otic cells provide a safe haven for the DNA,which is organized into discrete structures called chromosomes.Individual chromosomes become visible during cell division, when they condense and thicken.In prokaryotic cells,the DNA is usually not housed within a well-defined nucleus.We will investigate the ways in which chromosomal DNA is organized in prokaryotic and eukaryotic cells in Chapter 9.Some of the DNA within a eukaryotic cell is not situated within the nucleus.This extranuclear DNA is located in the mitochondria and chloroplasts.We will examine its structure and func- tion in Chapter 15. Both prokaryotic and eukaryotic cells possess numerous ribosomes,which are small organelles involved in the synthesis of proteins,a process that we will investigate in Chapter 12.Ribosomes are found throughout the cytoplasm.Although ribosomes are not composed of membranes,in eukaryotic cells they are often associated with a system of membranes called the endoplasmic reticulum.The reticulum may be con- nected to the Golgi complex,a set of membranous sacs and vesicles that are involved in the chemical modification and transport of substances within cells.Other small, membrane-bound organelles may also be found in eukaryotic cells.In animal cells, lysosomes are produced by the Golgi complex.These organelles contain different kinds of digestive enzymes that would harm the cell if they were released into the cytoplasm.Both plant and animal cells contain perioxisomes,which are small organ- elles dedicated to the metabolism of substances such as fats and amino acids.The internal membranes and oganelles of eukaryotic cells create a system of subcellular compartments that vary in chemical conditions such as pH and salt content.This variation provides cells with different internal environments that are adapted to the many processes that cells carry out. The shapes and activities of eukaryotic cells are influenced by a system of fila- ments,fibers,and associated molecules that collectively form the cytoskeleton.These materials give form to cells and enable some types of cells to move through their environment-a phenomenon referred to as cell motility.The cytoskeleton holds organelles in place,and it plays a major role in moving materials to specific locations within cells-a phenomenon called trafficking. CHROMOSOMES:WHERE GENES ARE LOCATED Each chromosome consists of one double-stranded DNA molecule plus an assort- ment of proteins;RNA may also be associated with chromosomes.Prokaryotic cells typically contain only one chromosome,although sometimes they also possess many smaller DNA molecules called plasmids.Most eukaryotic cells contain several
20 Chapter 2 Cellular Reproduction PROKARYOTIC AND EUKARYOTIC CELLS When we survey the living world, we find two basic kinds of cells: prokaryotic and eukaryotic ( Figure 2.1). Prokaryotic cells are usually less than a thousandth of a millimeter long, and they typically lack a complicated system of internal membranes and membranous organelles. Their hereditary material—that is, the DNA—is not isolated in a special subcellular compartment. Organisms with this kind of cellular organization are called prokaryotes. Examples include the bacteria, which are the most abundant life forms on Earth, and the archaea, which are found in extreme environments such as salt lakes, hot springs, and deep-sea volcanic vents. All other organisms—plants, animals, protists, and fungi—are eukaryotes. Eukaryotic cells are larger than prokaryotic cells, usually at least 10 times bigger, and they possess complicated systems of internal membranes, some of which are associated with conspicuous, well-organized organelles. For example, eukaryotic cells typically contain one or more mitochondria (singular, mitochondrion), which are ellipsoidal organelles dedicated to the recruitment of energy from foodstuffs. Algal and plant cells contain another kind of energy-recruiting organelle called the chloroplast, which captures solar energy and converts it into chemical energy. Both mitochondria and chloroplasts are surrounded by membranes. The hallmark of all eukaryotic cells is that their hereditary material is contained within a large, membrane-bounded structure called the nucleus. The nuclei of eukaryotic cells provide a safe haven for the DNA, which is organized into discrete structures called chromosomes. Individual chromosomes become visible during cell division, when they condense and thicken. In prokaryotic cells, the DNA is usually not housed within a well-defined nucleus. We will investigate the ways in which chromosomal DNA is organized in prokaryotic and eukaryotic cells in Chapter 9. Some of the DNA within a eukaryotic cell is not situated within the nucleus. This extranuclear DNA is located in the mitochondria and chloroplasts. We will examine its structure and function in Chapter 15. Both prokaryotic and eukaryotic cells possess numerous ribosomes, which are small organelles involved in the synthesis of proteins, a process that we will investigate in Chapter 12. Ribosomes are found throughout the cytoplasm. Although ribosomes are not composed of membranes, in eukaryotic cells they are often associated with a system of membranes called the endoplasmic reticulum. The reticulum may be connected to the Golgi complex, a set of membranous sacs and vesicles that are involved in the chemical modification and transport of substances within cells. Other small, membrane-bound organelles may also be found in eukaryotic cells. In animal cells, lysosomes are produced by the Golgi complex. These organelles contain different kinds of digestive enzymes that would harm the cell if they were released into the cytoplasm. Both plant and animal cells contain perioxisomes, which are small organelles dedicated to the metabolism of substances such as fats and amino acids. The internal membranes and oganelles of eukaryotic cells create a system of subcellular compartments that vary in chemical conditions such as pH and salt content. This variation provides cells with different internal environments that are adapted to the many processes that cells carry out. The shapes and activities of eukaryotic cells are influenced by a system of filaments, fibers, and associated molecules that collectively form the cytoskeleton. These materials give form to cells and enable some types of cells to move through their environment—a phenomenon referred to as cell motility. The cytoskeleton holds organelles in place, and it plays a major role in moving materials to specific locations within cells—a phenomenon called trafficking. CHROMOSOMES: WHERE GENES ARE LOCATED Each chromosome consists of one double-stranded DNA molecule plus an assortment of proteins; RNA may also be associated with chromosomes. Prokaryotic cells typically contain only one chromosome, although sometimes they also possess many smaller DNA molecules called plasmids. Most eukaryotic cells contain several
Cells and Chromosomes 21 Bacterial cell Capsule Outer membrane Cell wall Plasma membrane Genetic material 7hbhearit Ribosomes Flagellum Pilus (o) Animal cell -Nuclear pore -Nuclear envelope Free ribosomes Nucleus Mitochondrion Golgi apparatus Nucleolus Lysosome- Smooth endoplasmic Rough endoplasmic reticulum reticulum Microfilaments Ribosome 0 Cytoplasm Plasma membrane 0 Centrioles Cilia Microtubules 8 ( Plant cell Ribosome Nuclear pore Chloroplast Nucleus Nucleolus Rough endoplasmic Smooth endoplasmic reticulum reticulum Cytoplasm Plasma membrane Golgi apparatus Cell wall Mitochondrion -Vacuole Free ribosomes Vesicle Microtubules FIGURE 2.1 The structures of prokaryotic (c) (al and eukaryotic (b,c)cells
Cells and Chromosomes 21 FIGURE 2.1 The structures of prokaryotic (a) and eukaryotic (b, c) cells. Outer membrane Cell wall Plasma membrane Genetic material Ribosomes Pilus Flagellum Capsule (a) Bacterial cell Free ribosomes Animal cell Mitochondrion Golgi apparatus Lysosome Smooth endoplasmic reticulum Microfilaments Plasma membrane Cilia Nuclear pore Nuclear envelope Nucleus Nucleolus Rough endoplasmic reticulum Ribosome Cytoplasm Centrioles Microtubules (b) Ribosome Nuclear pore Nucleus Nucleolus Cytoplasm Plasma membrane Cell wall Mitochondrion Vacuole Golgi apparatus Smooth endoplasmic reticulum Chloroplast Microtubules Free ribosomes Vesicle Rough endoplasmic reticulum Plant cell (c)
22 Chapter 2 Cellular Reproduction different chromosomes-for example,human sperm cells have 23.The chromosomes of eukaryotic cells are also typically larger and more complex than those of prokary- otic cells.The DNA molecules in prokaryotic chromosomes and plasmids are circular, as are most of the DNA molecules found in the mitochondria and chloroplasts of eukaryotic cells.By contrast,the DNA molecules found in the chromosomes in the nuclei of eukaryotic cells are linear. Many eukaryotic cells possess two copies of each chromosome.This condition, referred to as the diploid state,is characteristic of the cells in the body of a eukaryote- that is,the somatic cells.By contrast,the sex cells or gametes usually possess only one copy of each chromosome,a condition referred to as the haploid state.Gametes are produced from diploid cells located in the germ line,which is the reproductive tissue of an organism.In some creatures,such as plants,the germ line produces both sperm and eggs.In other creatures,such as humans,it produces one kind of gamete or the other.When a male and a female gamete unite during fertilization,the diploid state is reestablished,and the resulting zygote develops into a new organism.During animal development,a small number of cells are set aside to form the germ line.All the gam- etes that will ever be produced are derived from these few cells.The remaining cells form the somatic tissues of the animal.In plants,development is less determinate. Tissues taken from part of a plant-for example,a stem or a leafcan be used to pro- duce a whole plant,including the reproductive organs.Thus,in plants the distinction between somatic tissues and germ tissues is not as clear-cut as it is in animals. Chromosomes can be examined by using a microscope.Prokaryotic chromo- somes can only be seen with the techniques of electron microscopy,whereas eukary- otic chromosomes can be seen with a light microscope Figure 2.2).Some eukaryotic chromosomes are large enough to be viewed with low magnification (20x);others require considerably more power(>500x). Eukaryotic chromosomes are most clearly seen during cell division when each chromosome condenses into a smaller volume.At this time the greater density of the chromosomes makes it possible to discern certain structural features.For example, each chromosome may appear to consist of two parallel rods held together at a com- mon point(Figure 2.2b).Each of the rods is an identical copy of the chromosome cre- ated during a duplication process that precedes condensation,and the common point, (a) 1 um (b) 10 um FIGURE 2.2 (a]Electron micrograph showing a bacterial chromosome extruded from a cell.(b)Light micrograph of human chromosomes during cell division.The constriction in each of the duplicated chromosomes is the centromere,the point at which spindle fibers attach to move the chromosome during cell division
22 Chapter 2 Cellular Reproduction different chromosomes—for example, human sperm cells have 23. The chromosomes of eukaryotic cells are also typically larger and more complex than those of prokaryotic cells. The DNA molecules in prokaryotic chromosomes and plasmids are circular, as are most of the DNA molecules found in the mitochondria and chloroplasts of eukaryotic cells. By contrast, the DNA molecules found in the chromosomes in the nuclei of eukaryotic cells are linear. Many eukaryotic cells possess two copies of each chromosome. This condition, referred to as the diploid state, is characteristic of the cells in the body of a eukaryote— that is, the somatic cells. By contrast, the sex cells or gametes usually possess only one copy of each chromosome, a condition referred to as the haploid state. Gametes are produced from diploid cells located in the germ line, which is the reproductive tissue of an organism. In some creatures, such as plants, the germ line produces both sperm and eggs. In other creatures, such as humans, it produces one kind of gamete or the other. When a male and a female gamete unite during fertilization, the diploid state is reestablished, and the resulting zygote develops into a new organism. During animal development, a small number of cells are set aside to form the germ line. All the gametes that will ever be produced are derived from these few cells. The remaining cells form the somatic tissues of the animal. In plants, development is less determinate. Tissues taken from part of a plant—for example, a stem or a leaf—can be used to produce a whole plant, including the reproductive organs. Thus, in plants the distinction between somatic tissues and germ tissues is not as clear-cut as it is in animals. Chromosomes can be examined by using a microscope. Prokaryotic chromosomes can only be seen with the techniques of electron microscopy, whereas eukaryotic chromosomes can be seen with a light microscope ( Figure 2.2). Some eukaryotic chromosomes are large enough to be viewed with low magnification (20); others require considerably more power (500). Eukaryotic chromosomes are most clearly seen during cell division when each chromosome condenses into a smaller volume. At this time the greater density of the chromosomes makes it possible to discern certain structural features. For example, each chromosome may appear to consist of two parallel rods held together at a common point (Figure 2.2b). Each of the rods is an identical copy of the chromosome created during a duplication process that precedes condensation, and the common point, FIGURE 2.2 (a) Electron micrograph showing a bacterial chromosome extruded from a cell. (b) Light micrograph of human chromosomes during cell division. The constriction in each of the duplicated chromosomes is the centromere, the point at which spindle fibers attach to move the chromosome during cell division. 1 μm (a) (b) 10 μm