44200_01_p1-26 3/2/04 4:01 PM Page 1
1
GENETICS
AND THE ORGANISM
KEY QUESTIONS
• What is the hereditary material?
• What is the chemical and physical structure
of DNA?
• How is DNA copied in the formation of new
cells and in the gametes that will give rise to
the offspring of an individual?
• What are the functional units of DNA that
carry information about development and
physiology?
• What molecules are the main determinants
of the basic structural and physiological
properties of an organism?
• What are the steps in translating the
information in DNA into protein?
• What determines the differences between
species in their physiology and structure?
• What are the causes of variation between
individuals within species?
Genetic variation in the color of corn kernels. Each kernel • What is the basis of variation in
represents a separate individual with a distinct genetic
makeup. The photograph symbolizes the history of humanity’s
populations?
interest in heredity. Humans were breeding corn thousands of
years before the advent of the modern discipline of genetics. OUTLINE
Extending this heritage, corn today is one of the main
research organisms in classical and molecular genetics. 1.1 Genes as determinants of the inherent
[William Sheridan, University of North Dakota; photograph by Travis properties of species
Amos.]
1.2 Genetic variation
1.3 Methodologies used in genetics
1.4 Model organisms
1.5 Genes, the environment, and the organism
1
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2 Chapter 1 • Genetics and the Organism 1.1 Genes as determinants of the inherent properties of species 3
CHAPTER OVERVIEW features are clearly familial; for example, animals of The next sections of this chapter show how genes
Testes Ovaries
a certain unique color often have offspring with the influence the inherent properties of a species and how
hy study genetics? There are two basic reasons. same color, and in human families, certain features, allelic variation contributes to variation within a species.
W
DNA DNA
REPLICATION REPLICATION
First, genetics occupies a pivotal position in the such as the shape of the nose, definitely “run in the These sections are an overview; most of the details will
entire subject of biology. Therefore, for any serious stu- family.” Hence we might suspect that a hereditary be presented in later chapters.
dent of plant, animal, or microbial life, an understanding component explains at least some of the variation
Division of gonad Division of gonad
of genetics is essential. Second, genetics, like no other within a species. cells cells
scientific discipline, is central to numerous aspects of hu- 1.1 Genes as determinants
man affairs. It touches our humanity in many different The answer to the first question is that genes dictate
ways. Indeed, genetic issues seem to surface daily in our the inherent properties of a species. The products of of the inherent properties Sperm Egg
lives, and no thinking person can afford to be ignorant of most genes are specific proteins. Proteins are the main of species
its discoveries. In this chapter, we take an overview of macromolecules of an organism. When you look at an Zygote
the science of genetics, showing how it has come to oc- organism, what you see is either a protein or something What is the nature of genes, and how do they perform
cupy its crucial position. In addition, we provide a per- that has been made by a protein. The amino acid se- their biological roles? Three fundamental properties
are required of genes and the DNA of which they are DNA
spective from which to view the subsequent chapters. quence of a protein is encoded in a gene. The timing and REPLICATION
First, we need to define what genetics is. Some define rate of production of proteins and other cellular compo- composed.
it as the “study of heredity,” but hereditary phenomena nents are a function both of the genes within the cells
were of interest to humans long before biology or genet- and of the environment in which the organism is devel- 1. Replication. Hereditary molecules must be capable Division of
ics existed as the scientific disciplines that we know to- oping and functioning. of being copied at two key stages of the life cycle asexual (body)
(Figure 1-2). The first stage is the production of the cells
day. Ancient peoples were improving plant crops and The answer to the second question is that any one
domesticated animals by selecting desirable individuals gene can exist in several forms that differ from one an- cell type that will ensure the continuation of a
for breeding. They also must have puzzled about the in- other, generally in small ways. These forms of a gene are species from one generation to the next. In plants Repeat
heritance of individuality in humans and asked such called alleles. Allelic variation causes hereditary variation and animals, these cells are the gametes: egg and divisions
questions as “Why do children resemble their parents?” within a species. At the protein level, allelic variation be- sperm. The other stage is when the first cell of a
and “How can various diseases run in families?” But comes protein variation. new organism undergoes multiple rounds of division
these people could not be called “geneticists.” Genetics to produce a multicellular organism. In plants and
as a set of principles and analytical procedures did not animals, this is the stage at which the fertilized egg,
begin until the 1860s, when an Augustinian monk the zygote, divides repeatedly to produce the
named Gregor Mendel (Figure 1-1) performed a set of complex organismal appearance that we recognize.
Figure 1-2 DNA replication is the basis of the perpetuation of
experiments that pointed to the existence of biological 2. Generation of form. The working structures that life through time.
elements that we now call genes. The word genetics make up an organism can be thought of as form or
comes from the word “gene,” and genes are the focus of substance, and DNA has the essential “information”
the subject. Whether geneticists study at the molecular, needed to create form. cell’s production of proteins. Each chromosome in the
cellular, organismal, family, population, or evolutionary 3. Mutation. A gene that has changed from one allelic genome carries a different array of genes. In diploid cells,
level, genes are always central in their studies. Simply form into another has undergone mutation — an each chromosome and its component genes are present
stated, genetics is the study of genes. event that happens rarely but regularly. Mutation is twice. For example, human somatic cells contain two
What is a gene? A gene is a section of a threadlike not only a basis for variation within a species, but sets of 23 chromosomes, for a total of 46 chromosomes.
double-helical molecule called deoxyribonucleic acid, also, over the long term, the raw material for Two chromosomes with the same gene array are said to
abbreviated DNA. The discovery of genes and the un- evolution. be homologous. When a cell divides, all its chromosomes
derstanding of their molecular structure and function (its one or two copies of the genome) are replicated and
have been sources of profound insight into two of the We will examine replication and the generation of then separated, so that each daughter cell receives the
biggest mysteries of biology: form in this section and mutation in the next. full complement of chromosomes.
To understand replication, we need to understand
1. What makes a species what it is? We know that cats the basic nature of DNA. DNA is a linear, double-helical
always have kittens and people always have babies. DNA and its replication structure that looks rather like a molecular spiral stair-
This commonsense observation naturally leads to An organism’s basic complement of DNA is called its case. The double helix is composed of two intertwined
questions about the determination of the properties genome. The somatic cells of most plants and animals chains made up of building blocks called nucleotides.
of a species. The determination must be hereditary contain two copies of their genome (Figure 1-3); these Each nucleotide consists of a phosphate group, a
because, for example, the ability to have kittens is organisms are diploid. The cells of most fungi, algae, and deoxyribose sugar molecule, and one of four different
inherited by every generation of cats. bacteria contain just one copy of the genome; these or- nitrogenous bases: adenine, guanine, cytosine, or
2. What causes variation within a species? We can ganisms are haploid. The genome itself is made up of thymine. Each of the four nucleotides is usually desig-
distinguish one another as well as our own pet cat one or more extremely long molecules of DNA that are nated by the first letter of the base it contains: A, G, C,
from other cats. Such differences within a species organized into chromosomes. Genes are simply the re- or T. Each nucleotide chain is held together by bonds
require explanation. Some of these distinguishing Figure 1-1 Gregor Mendel. [Moravian Museum, Brno.] gions of chromosomal DNA that are involved in the between the sugar and phosphate portions of the
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4 Chapter 1 • Genetics and the Organism 1.1 Genes as determinants of the inherent properties of species 5
Figure 1-3 Successive Figure 1-5 DNA T S P P P
enlargements bringing the genetic gene replication in process. S S
S P S P S P S P S T G
material of an organism into sharper gene Blue nucleotides of P S P P P
focus. the original double helix; A T G C C T P
P
S
gene
gold new nucleotides
A
P P
P
T A C G G S Original DNA double helix
being polymerized A
P
to form daughter chains. Identical P S P S P S P S P S P A S P S P S P S P S P
S
Organism Each cell Each S sugar; daughter
DNA is a C C T G A
(human) nucleus One specific chromosome DNA DNA
A human body contains an is one long DNA double helix.
P phosphate group. double
Direction
polymerase
chromosome
is made up identical molecule, and helices G G A C T
pair P
of trillions complement of genes are forming P S
S P S P S P S P S P S P S P S P S P S
T
of cells. chromosomes functional regions T
S
P
in two copies. of this DNA. A T G C C P
T
P
P
Each copy
A P
P
is a genome. T A C G G S
P P
S
S P S P
P S P S P S P S P
C
P
S Free nucleotides
A
consecutive nucleotides, which form the “backbone” of bases that form base pairs are said to be complementary. P
the chain. The two intertwined chains are held together Hence a short segment of DNA drawn with arbitrary A S
by weak bonds between bases on opposite chains (Fig- nucleotide sequence might be
ure 1-4). There is a “lock-and-key” fit between the bases
· · · ·CAGT· · · · proteins an organism can synthesize, as well as the tim- contains uracil (U), which like thymine, pairs with ade-
on the opposite strands, such that adenine pairs only
· · · ·GTCA· · · · ing and amount of production of each protein, is an ex- nine. Hence the RNA bases are A, G, C, and U. The tran-
with thymine and guanine pairs only with cytosine. The
tremely important determinant of the structure and scription process, which occurs in the cell nucleus, is
MESSAGE DNA is composed of two nucleotide chains
physiology of organisms. A protein generally has one of very similar to the process for replication of DNA be-
held together by complementary pairing of A with T and G two basic functions, depending on the gene. First, the cause the DNA strand serves as the template for making
with C. protein may be a structural component, contributing to the RNA copy, which is called a transcript. The RNA
the physical properties of cells or organisms. Examples transcript, which in many species undergoes some struc-
For replication of DNA to take place, the two of structural proteins are microtubule, muscle, and hair tural modifications, becomes a “working copy” of the in-
strands of the double helix must come apart, rather like proteins. Second, the protein may be an active agent in formation in the gene, a kind of “message” molecule
the opening of a zipper. The two exposed nucleotide cellular processes — such as an active-transport protein called messenger RNA (mRNA). The mRNA then en-
chains then act as alignment guides, or templates, for the or an enzyme that catalyzes one of the chemical reac- ters the cytoplasm, where it is used by the cellular
deposition of free nucleotides, which are then joined to- tions of the cell. machinery to direct the manufacture of a protein. Fig-
gether by the enzyme DNA polymerase to form a new The primary structure of a protein is a linear chain ure 1-6 summarizes the process of transcription.
strand. The crucial point illustrated in Figure 1-5 is that of amino acids, called a polypeptide. The sequence of
because of base complementarity, the two daughter amino acids in the primary chain is specified by the se-
MESSAGE During transcription, one of the DNA strands
DNA molecules are identical with each other and with quence of nucleotides in the gene. The completed pri- of a gene acts as a template for the synthesis of a
the original molecule. mary chain is coiled and folded — and in some cases, as- complementary RNA molecule.
sociated with other chains or small molecules — to form
a functional protein. A given amino acid sequence may
MESSAGE DNA is replicated by the unwinding of the two fold in a large number of stable ways. The final folded TRANSLATION The process of producing a chain of
strands of the double helix and the building up of a new amino acids based on the sequence of nucleotides in the
complementary strand on each of the separated strands of the
state of a protein depends both on the sequence of
amino acids specified by its gene and on the physiology mRNA is called translation. The nucleotide sequence of
original double helix.
of the cell during folding. an mRNA molecule is “read” from one end of the
mRNA to the other, in groups of three successive bases.
Generation of form These groups of three are called codons.
MESSAGE The sequence of nucleotides in a gene
If DNA represents information, what constitutes form at specifies the sequence of amino acids that is put together AUU CCG UAC GUA AAU UUG
the cellular level? The simple answer is “protein” be- by the cell to produce a polypeptide. This polypeptide then
codon codon codon codon codon codon
folds under the influence of its amino acid sequence and other
cause the great majority of structures in a cell are pro-
molecular conditions in the cell to form a protein.
tein or have been made by protein. In this section, we Because there are four different nucleotides, there are
trace the steps through which information becomes 4 4 4 64 different codons possible, each one
form. TRANSCRIPTION The first step taken by the cell to coding for an amino acid or a signal to terminate transla-
The biological role of most genes is to carry infor- make a protein is to copy, or transcribe, the nucleotide tion. Because only 20 kinds of amino acids are used in
mation specifying the chemical composition of proteins sequence in one strand of the gene into a complemen- the polypeptides that make up proteins, more than one
or the regulatory signals that will govern their produc- tary single-stranded molecule called ribonucleic acid codon may correspond to the same amino acid. For in-
tion by the cell. This information is encoded by the se- (RNA). Like DNA, RNA is composed of nucleotides, stance, AUU, AUC, and AUA all encode isoleucine, while
Figure 1-4 Ribbon representation of the DNA double helix. quence of nucleotides. A typical gene contains the infor- but these nucleotides contain the sugar ribose instead of UUU and UUC code for phenylalanine, and UAG is a
Blue sugar-phosphate backbone; brown paired bases. mation for one specific protein. The collection of deoxyribose. Furthermore, in place of thymine, RNA translation termination (“stop”) codon.
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6 Chapter 1 • Genetics and the Organism 1.1 Genes as determinants of the inherent properties of species 7
Polypeptide aa1 unmasks a gene may come from outside the cell, for ex- Figure 1-9 Simplified view of gene action in a eukaryotic cell.
aa2 The basic flow of genetic information is from DNA to RNA to
ample, from a steroid hormone or a nutrient. Alterna-
aa3 protein. Four types of genes are shown. Gene 1 responds to
aa8
tively, the signal may come from within the cell as the
DNA aa4 external regulatory signals and makes a protein for export;
Nucleus result of the reading of other genes. In either case,
tRNA aa5 gene 2 responds to internal signals and makes a protein for
special regulatory sequences in the DNA are directly
use in the cytoplasm; gene 3 makes a protein to be
aa6 aa7 affected by the signal, and they in turn affect the
Primary transported into an organelle; gene 4 is part of the organelle
Transcription RNA transcription of the protein-encoding gene. The regu- DNA and makes a protein for use inside its own organelle.
transcript latory substances that serve as signals bind to the regula- Most eukaryotic genes contain introns, regions (generally
tory region of the target gene to control the synthesis of noncoding) that are cut out in the preparation of functional
transcripts. messenger RNA. Note that many organelle genes have introns
RNA processing Figure 1-9 illustrates the essentials of gene action in a
Codon Codon Codon Codon Codon Codon Codon and that an RNA-synthesizing enzyme is needed for organelle
4 5 6 7 8 9 10 generalized eukaryotic cell. Outside the nucleus of the cell mRNA synthesis. These details have been omitted from the
is a complex array of membranous structures, including diagram of the organelle for clarity. (Introns will be
Mature mRNA explained in detail in subsequent chapters.)
Amino acid Figure 1-7 Translation. An amino acid (aa) is added to a
chain Transport to growing polypeptide chain in the translation of mRNA.
cytoplasm External Nuclear chromosomes
Gene 1
signal
mRNA GENE REGULATION Let’s take a closer look at the
structure of a gene, which determines the final form of
Translation the RNA “working copy” as well as the timing of tran- Internal signal
Gene 2
Ribosome scription in a particular tissue. Figure 1-8 shows the gen-
eral structure of a gene. At one end, there is a regulatory
region to which various proteins involved in the regula- Intron
Figure 1-6 Transcription and translation in a eukaryotic cell. tion of the gene’s transcription bind, causing the gene to removal
The mRNA is processed in the nucleus, then transported to Nuclear Gene 3
be transcribed at the right time and in the right amount.
the cytoplasm for translation into a polypeptide chain. membrane
A region at the other end of the gene signals the end mRNA 1
point of the gene’s transcription. Between these two end
Protein synthesis takes place on cytoplasmic organelles
regions lies the DNA sequence that will be transcribed
called ribosomes. A ribosome attaches to one end of an
to specify the amino acid sequence of a polypeptide. mRNA 2
mRNA molecule and moves along the mRNA, catalyzing mRNA 3
Gene structure is more complex in eukaryotes than
the assembly of the string of amino acids that will consti-
in prokaryotes. Eukaryotes, which include all the multi-
tute the primary polypeptide chain of the protein. Each
cellular plants and animals, are those organisms whose
kind of amino acid is brought to the assembly process by a
cells have a membrane-bound nucleus. Prokaryotes are
small RNA molecule called transfer RNA (tRNA), which
organisms with a simpler cellular structure lacking a nu-
is complementary to the mRNA codon that is being read
cleus, such as bacteria. In the genes of many eukaryotes,
by the ribosome at that point in the assembly.
the protein-encoding sequence is interrupted by one or
Trains of ribosomes pass along an mRNA molecule, Endoplasmic reticulum
more stretches of DNA called introns. The origin and
each member of a train making the same type of polypep-
functions of introns are still unclear. They are excised
tide. At the end of the mRNA, a termination codon causes
from the primary transcript during the formation of
the ribosome to detach and recycle to another mRNA.
mRNA. The segments of coding sequence between the
The process of translation is shown in Figure 1-7.
introns are called exons. mRNA 4
Some protein-encoding genes are transcribed more
Golgi apparatus
MESSAGE The information in genes is used by the cell in or less constantly; these are the “housekeeping” genes
two steps of information transfer: DNA is transcribed into Gene 4
that are always needed for basic reactions. Other genes
mRNA, which is then translated into the amino acid sequence
of a polypeptide. The flow of information from DNA to RNA to
may be rendered unreadable or readable to suit the
Circular organelle chromosome
protein is a central focus of modern biology. functions of the organism at particular times and under
particular external conditions. The signal that masks or Mitochondrion or chloroplast
Gene
Cell membrane Key
Transcribed region Protein-coding region of DNA RNA polymerase
Exon Intron Exon Intron Exon Intron Exon Noncoding region Secreted protein
Protein-coding region of RNA Proteins used in cell
Figure 1-8 Generalized structure
of a eukaryotic gene. This Noncoding region Protein encoded by
mitochondrion or chloroplast
Regulation of Termination example has three introns and Promoter Amino acid chain
initiation of of transcription four exons.
transcription Regulatory proteins Ribosome
Protein-encoding sequence
1
GENETICS
AND THE ORGANISM
KEY QUESTIONS
• What is the hereditary material?
• What is the chemical and physical structure
of DNA?
• How is DNA copied in the formation of new
cells and in the gametes that will give rise to
the offspring of an individual?
• What are the functional units of DNA that
carry information about development and
physiology?
• What molecules are the main determinants
of the basic structural and physiological
properties of an organism?
• What are the steps in translating the
information in DNA into protein?
• What determines the differences between
species in their physiology and structure?
• What are the causes of variation between
individuals within species?
Genetic variation in the color of corn kernels. Each kernel • What is the basis of variation in
represents a separate individual with a distinct genetic
makeup. The photograph symbolizes the history of humanity’s
populations?
interest in heredity. Humans were breeding corn thousands of
years before the advent of the modern discipline of genetics. OUTLINE
Extending this heritage, corn today is one of the main
research organisms in classical and molecular genetics. 1.1 Genes as determinants of the inherent
[William Sheridan, University of North Dakota; photograph by Travis properties of species
Amos.]
1.2 Genetic variation
1.3 Methodologies used in genetics
1.4 Model organisms
1.5 Genes, the environment, and the organism
1
,44200_01_p1-26 3/2/04 4:01 PM Page 2 44200_01_p1-26 3/2/04 4:01 PM Page 3
2 Chapter 1 • Genetics and the Organism 1.1 Genes as determinants of the inherent properties of species 3
CHAPTER OVERVIEW features are clearly familial; for example, animals of The next sections of this chapter show how genes
Testes Ovaries
a certain unique color often have offspring with the influence the inherent properties of a species and how
hy study genetics? There are two basic reasons. same color, and in human families, certain features, allelic variation contributes to variation within a species.
W
DNA DNA
REPLICATION REPLICATION
First, genetics occupies a pivotal position in the such as the shape of the nose, definitely “run in the These sections are an overview; most of the details will
entire subject of biology. Therefore, for any serious stu- family.” Hence we might suspect that a hereditary be presented in later chapters.
dent of plant, animal, or microbial life, an understanding component explains at least some of the variation
Division of gonad Division of gonad
of genetics is essential. Second, genetics, like no other within a species. cells cells
scientific discipline, is central to numerous aspects of hu- 1.1 Genes as determinants
man affairs. It touches our humanity in many different The answer to the first question is that genes dictate
ways. Indeed, genetic issues seem to surface daily in our the inherent properties of a species. The products of of the inherent properties Sperm Egg
lives, and no thinking person can afford to be ignorant of most genes are specific proteins. Proteins are the main of species
its discoveries. In this chapter, we take an overview of macromolecules of an organism. When you look at an Zygote
the science of genetics, showing how it has come to oc- organism, what you see is either a protein or something What is the nature of genes, and how do they perform
cupy its crucial position. In addition, we provide a per- that has been made by a protein. The amino acid se- their biological roles? Three fundamental properties
are required of genes and the DNA of which they are DNA
spective from which to view the subsequent chapters. quence of a protein is encoded in a gene. The timing and REPLICATION
First, we need to define what genetics is. Some define rate of production of proteins and other cellular compo- composed.
it as the “study of heredity,” but hereditary phenomena nents are a function both of the genes within the cells
were of interest to humans long before biology or genet- and of the environment in which the organism is devel- 1. Replication. Hereditary molecules must be capable Division of
ics existed as the scientific disciplines that we know to- oping and functioning. of being copied at two key stages of the life cycle asexual (body)
(Figure 1-2). The first stage is the production of the cells
day. Ancient peoples were improving plant crops and The answer to the second question is that any one
domesticated animals by selecting desirable individuals gene can exist in several forms that differ from one an- cell type that will ensure the continuation of a
for breeding. They also must have puzzled about the in- other, generally in small ways. These forms of a gene are species from one generation to the next. In plants Repeat
heritance of individuality in humans and asked such called alleles. Allelic variation causes hereditary variation and animals, these cells are the gametes: egg and divisions
questions as “Why do children resemble their parents?” within a species. At the protein level, allelic variation be- sperm. The other stage is when the first cell of a
and “How can various diseases run in families?” But comes protein variation. new organism undergoes multiple rounds of division
these people could not be called “geneticists.” Genetics to produce a multicellular organism. In plants and
as a set of principles and analytical procedures did not animals, this is the stage at which the fertilized egg,
begin until the 1860s, when an Augustinian monk the zygote, divides repeatedly to produce the
named Gregor Mendel (Figure 1-1) performed a set of complex organismal appearance that we recognize.
Figure 1-2 DNA replication is the basis of the perpetuation of
experiments that pointed to the existence of biological 2. Generation of form. The working structures that life through time.
elements that we now call genes. The word genetics make up an organism can be thought of as form or
comes from the word “gene,” and genes are the focus of substance, and DNA has the essential “information”
the subject. Whether geneticists study at the molecular, needed to create form. cell’s production of proteins. Each chromosome in the
cellular, organismal, family, population, or evolutionary 3. Mutation. A gene that has changed from one allelic genome carries a different array of genes. In diploid cells,
level, genes are always central in their studies. Simply form into another has undergone mutation — an each chromosome and its component genes are present
stated, genetics is the study of genes. event that happens rarely but regularly. Mutation is twice. For example, human somatic cells contain two
What is a gene? A gene is a section of a threadlike not only a basis for variation within a species, but sets of 23 chromosomes, for a total of 46 chromosomes.
double-helical molecule called deoxyribonucleic acid, also, over the long term, the raw material for Two chromosomes with the same gene array are said to
abbreviated DNA. The discovery of genes and the un- evolution. be homologous. When a cell divides, all its chromosomes
derstanding of their molecular structure and function (its one or two copies of the genome) are replicated and
have been sources of profound insight into two of the We will examine replication and the generation of then separated, so that each daughter cell receives the
biggest mysteries of biology: form in this section and mutation in the next. full complement of chromosomes.
To understand replication, we need to understand
1. What makes a species what it is? We know that cats the basic nature of DNA. DNA is a linear, double-helical
always have kittens and people always have babies. DNA and its replication structure that looks rather like a molecular spiral stair-
This commonsense observation naturally leads to An organism’s basic complement of DNA is called its case. The double helix is composed of two intertwined
questions about the determination of the properties genome. The somatic cells of most plants and animals chains made up of building blocks called nucleotides.
of a species. The determination must be hereditary contain two copies of their genome (Figure 1-3); these Each nucleotide consists of a phosphate group, a
because, for example, the ability to have kittens is organisms are diploid. The cells of most fungi, algae, and deoxyribose sugar molecule, and one of four different
inherited by every generation of cats. bacteria contain just one copy of the genome; these or- nitrogenous bases: adenine, guanine, cytosine, or
2. What causes variation within a species? We can ganisms are haploid. The genome itself is made up of thymine. Each of the four nucleotides is usually desig-
distinguish one another as well as our own pet cat one or more extremely long molecules of DNA that are nated by the first letter of the base it contains: A, G, C,
from other cats. Such differences within a species organized into chromosomes. Genes are simply the re- or T. Each nucleotide chain is held together by bonds
require explanation. Some of these distinguishing Figure 1-1 Gregor Mendel. [Moravian Museum, Brno.] gions of chromosomal DNA that are involved in the between the sugar and phosphate portions of the
,44200_01_p1-26 3/2/04 4:01 PM Page 4 44200_01_p1-26 3/2/04 4:01 PM Page 5
4 Chapter 1 • Genetics and the Organism 1.1 Genes as determinants of the inherent properties of species 5
Figure 1-3 Successive Figure 1-5 DNA T S P P P
enlargements bringing the genetic gene replication in process. S S
S P S P S P S P S T G
material of an organism into sharper gene Blue nucleotides of P S P P P
focus. the original double helix; A T G C C T P
P
S
gene
gold new nucleotides
A
P P
P
T A C G G S Original DNA double helix
being polymerized A
P
to form daughter chains. Identical P S P S P S P S P S P A S P S P S P S P S P
S
Organism Each cell Each S sugar; daughter
DNA is a C C T G A
(human) nucleus One specific chromosome DNA DNA
A human body contains an is one long DNA double helix.
P phosphate group. double
Direction
polymerase
chromosome
is made up identical molecule, and helices G G A C T
pair P
of trillions complement of genes are forming P S
S P S P S P S P S P S P S P S P S P S
T
of cells. chromosomes functional regions T
S
P
in two copies. of this DNA. A T G C C P
T
P
P
Each copy
A P
P
is a genome. T A C G G S
P P
S
S P S P
P S P S P S P S P
C
P
S Free nucleotides
A
consecutive nucleotides, which form the “backbone” of bases that form base pairs are said to be complementary. P
the chain. The two intertwined chains are held together Hence a short segment of DNA drawn with arbitrary A S
by weak bonds between bases on opposite chains (Fig- nucleotide sequence might be
ure 1-4). There is a “lock-and-key” fit between the bases
· · · ·CAGT· · · · proteins an organism can synthesize, as well as the tim- contains uracil (U), which like thymine, pairs with ade-
on the opposite strands, such that adenine pairs only
· · · ·GTCA· · · · ing and amount of production of each protein, is an ex- nine. Hence the RNA bases are A, G, C, and U. The tran-
with thymine and guanine pairs only with cytosine. The
tremely important determinant of the structure and scription process, which occurs in the cell nucleus, is
MESSAGE DNA is composed of two nucleotide chains
physiology of organisms. A protein generally has one of very similar to the process for replication of DNA be-
held together by complementary pairing of A with T and G two basic functions, depending on the gene. First, the cause the DNA strand serves as the template for making
with C. protein may be a structural component, contributing to the RNA copy, which is called a transcript. The RNA
the physical properties of cells or organisms. Examples transcript, which in many species undergoes some struc-
For replication of DNA to take place, the two of structural proteins are microtubule, muscle, and hair tural modifications, becomes a “working copy” of the in-
strands of the double helix must come apart, rather like proteins. Second, the protein may be an active agent in formation in the gene, a kind of “message” molecule
the opening of a zipper. The two exposed nucleotide cellular processes — such as an active-transport protein called messenger RNA (mRNA). The mRNA then en-
chains then act as alignment guides, or templates, for the or an enzyme that catalyzes one of the chemical reac- ters the cytoplasm, where it is used by the cellular
deposition of free nucleotides, which are then joined to- tions of the cell. machinery to direct the manufacture of a protein. Fig-
gether by the enzyme DNA polymerase to form a new The primary structure of a protein is a linear chain ure 1-6 summarizes the process of transcription.
strand. The crucial point illustrated in Figure 1-5 is that of amino acids, called a polypeptide. The sequence of
because of base complementarity, the two daughter amino acids in the primary chain is specified by the se-
MESSAGE During transcription, one of the DNA strands
DNA molecules are identical with each other and with quence of nucleotides in the gene. The completed pri- of a gene acts as a template for the synthesis of a
the original molecule. mary chain is coiled and folded — and in some cases, as- complementary RNA molecule.
sociated with other chains or small molecules — to form
a functional protein. A given amino acid sequence may
MESSAGE DNA is replicated by the unwinding of the two fold in a large number of stable ways. The final folded TRANSLATION The process of producing a chain of
strands of the double helix and the building up of a new amino acids based on the sequence of nucleotides in the
complementary strand on each of the separated strands of the
state of a protein depends both on the sequence of
amino acids specified by its gene and on the physiology mRNA is called translation. The nucleotide sequence of
original double helix.
of the cell during folding. an mRNA molecule is “read” from one end of the
mRNA to the other, in groups of three successive bases.
Generation of form These groups of three are called codons.
MESSAGE The sequence of nucleotides in a gene
If DNA represents information, what constitutes form at specifies the sequence of amino acids that is put together AUU CCG UAC GUA AAU UUG
the cellular level? The simple answer is “protein” be- by the cell to produce a polypeptide. This polypeptide then
codon codon codon codon codon codon
folds under the influence of its amino acid sequence and other
cause the great majority of structures in a cell are pro-
molecular conditions in the cell to form a protein.
tein or have been made by protein. In this section, we Because there are four different nucleotides, there are
trace the steps through which information becomes 4 4 4 64 different codons possible, each one
form. TRANSCRIPTION The first step taken by the cell to coding for an amino acid or a signal to terminate transla-
The biological role of most genes is to carry infor- make a protein is to copy, or transcribe, the nucleotide tion. Because only 20 kinds of amino acids are used in
mation specifying the chemical composition of proteins sequence in one strand of the gene into a complemen- the polypeptides that make up proteins, more than one
or the regulatory signals that will govern their produc- tary single-stranded molecule called ribonucleic acid codon may correspond to the same amino acid. For in-
tion by the cell. This information is encoded by the se- (RNA). Like DNA, RNA is composed of nucleotides, stance, AUU, AUC, and AUA all encode isoleucine, while
Figure 1-4 Ribbon representation of the DNA double helix. quence of nucleotides. A typical gene contains the infor- but these nucleotides contain the sugar ribose instead of UUU and UUC code for phenylalanine, and UAG is a
Blue sugar-phosphate backbone; brown paired bases. mation for one specific protein. The collection of deoxyribose. Furthermore, in place of thymine, RNA translation termination (“stop”) codon.
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6 Chapter 1 • Genetics and the Organism 1.1 Genes as determinants of the inherent properties of species 7
Polypeptide aa1 unmasks a gene may come from outside the cell, for ex- Figure 1-9 Simplified view of gene action in a eukaryotic cell.
aa2 The basic flow of genetic information is from DNA to RNA to
ample, from a steroid hormone or a nutrient. Alterna-
aa3 protein. Four types of genes are shown. Gene 1 responds to
aa8
tively, the signal may come from within the cell as the
DNA aa4 external regulatory signals and makes a protein for export;
Nucleus result of the reading of other genes. In either case,
tRNA aa5 gene 2 responds to internal signals and makes a protein for
special regulatory sequences in the DNA are directly
use in the cytoplasm; gene 3 makes a protein to be
aa6 aa7 affected by the signal, and they in turn affect the
Primary transported into an organelle; gene 4 is part of the organelle
Transcription RNA transcription of the protein-encoding gene. The regu- DNA and makes a protein for use inside its own organelle.
transcript latory substances that serve as signals bind to the regula- Most eukaryotic genes contain introns, regions (generally
tory region of the target gene to control the synthesis of noncoding) that are cut out in the preparation of functional
transcripts. messenger RNA. Note that many organelle genes have introns
RNA processing Figure 1-9 illustrates the essentials of gene action in a
Codon Codon Codon Codon Codon Codon Codon and that an RNA-synthesizing enzyme is needed for organelle
4 5 6 7 8 9 10 generalized eukaryotic cell. Outside the nucleus of the cell mRNA synthesis. These details have been omitted from the
is a complex array of membranous structures, including diagram of the organelle for clarity. (Introns will be
Mature mRNA explained in detail in subsequent chapters.)
Amino acid Figure 1-7 Translation. An amino acid (aa) is added to a
chain Transport to growing polypeptide chain in the translation of mRNA.
cytoplasm External Nuclear chromosomes
Gene 1
signal
mRNA GENE REGULATION Let’s take a closer look at the
structure of a gene, which determines the final form of
Translation the RNA “working copy” as well as the timing of tran- Internal signal
Gene 2
Ribosome scription in a particular tissue. Figure 1-8 shows the gen-
eral structure of a gene. At one end, there is a regulatory
region to which various proteins involved in the regula- Intron
Figure 1-6 Transcription and translation in a eukaryotic cell. tion of the gene’s transcription bind, causing the gene to removal
The mRNA is processed in the nucleus, then transported to Nuclear Gene 3
be transcribed at the right time and in the right amount.
the cytoplasm for translation into a polypeptide chain. membrane
A region at the other end of the gene signals the end mRNA 1
point of the gene’s transcription. Between these two end
Protein synthesis takes place on cytoplasmic organelles
regions lies the DNA sequence that will be transcribed
called ribosomes. A ribosome attaches to one end of an
to specify the amino acid sequence of a polypeptide. mRNA 2
mRNA molecule and moves along the mRNA, catalyzing mRNA 3
Gene structure is more complex in eukaryotes than
the assembly of the string of amino acids that will consti-
in prokaryotes. Eukaryotes, which include all the multi-
tute the primary polypeptide chain of the protein. Each
cellular plants and animals, are those organisms whose
kind of amino acid is brought to the assembly process by a
cells have a membrane-bound nucleus. Prokaryotes are
small RNA molecule called transfer RNA (tRNA), which
organisms with a simpler cellular structure lacking a nu-
is complementary to the mRNA codon that is being read
cleus, such as bacteria. In the genes of many eukaryotes,
by the ribosome at that point in the assembly.
the protein-encoding sequence is interrupted by one or
Trains of ribosomes pass along an mRNA molecule, Endoplasmic reticulum
more stretches of DNA called introns. The origin and
each member of a train making the same type of polypep-
functions of introns are still unclear. They are excised
tide. At the end of the mRNA, a termination codon causes
from the primary transcript during the formation of
the ribosome to detach and recycle to another mRNA.
mRNA. The segments of coding sequence between the
The process of translation is shown in Figure 1-7.
introns are called exons. mRNA 4
Some protein-encoding genes are transcribed more
Golgi apparatus
MESSAGE The information in genes is used by the cell in or less constantly; these are the “housekeeping” genes
two steps of information transfer: DNA is transcribed into Gene 4
that are always needed for basic reactions. Other genes
mRNA, which is then translated into the amino acid sequence
of a polypeptide. The flow of information from DNA to RNA to
may be rendered unreadable or readable to suit the
Circular organelle chromosome
protein is a central focus of modern biology. functions of the organism at particular times and under
particular external conditions. The signal that masks or Mitochondrion or chloroplast
Gene
Cell membrane Key
Transcribed region Protein-coding region of DNA RNA polymerase
Exon Intron Exon Intron Exon Intron Exon Noncoding region Secreted protein
Protein-coding region of RNA Proteins used in cell
Figure 1-8 Generalized structure
of a eukaryotic gene. This Noncoding region Protein encoded by
mitochondrion or chloroplast
Regulation of Termination example has three introns and Promoter Amino acid chain
initiation of of transcription four exons.
transcription Regulatory proteins Ribosome
Protein-encoding sequence
