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CHAPTER 5 Radiation Monitoring
87
9. Which of the following instruments should be used
to locate a lost radioactive source or detect low-level
radioactive contamination?
A. GM survey meter
B. Proportional counter
C. Ionization chamber–type survey meter (cutie pie)
D. Direct ion storage dosimeter
10. Which of the following instruments should be used
in an x-ray installation to measure the fluoroscopic
scatter radiation exposure rate?
A. Geiger detector
B. Cutie pie
C. Proportional counter
D. Direct ion storage dosimeter
6
Overview of Cell Biology
O B J E C T I V E S
After completing this chapter, the reader will be able to
perform the following:
• Define all key terms.
• State the purpose for acquiring a basic knowledge
of cell structure, composition, and cell function as
a foundation for understanding the effects of
radiation in biology.
• Identify and describe some important roles of the
major classes of organic and inorganic compounds
that exist in the cell.
• List the essential tasks of water in the human body.
C H A P T E R O U T L I N E
The Cell
Cell Chemical Composition
Protoplasm
Organic Compounds
Inorganic Compounds
Cell Structure
Cell Membrane
• Name and describe a landmark event pertaining to the human genome that occurred in 2001, and explain the progress that has been made since then as a result of this project.
• Describe the molecular structure of deoxyribonucleic acid, and explain the way it operates in the cell.
• Describe the structural differences between DNA
and RNA.
• List the various cellular components, and identify
their physical characteristics and functions.
• Distinguish between the two types of cell divisions,
mitosis and meiosis, and describe each process.
Cytoplasm
Cytoplasmic Organelles
Nucleus
Cell Division
Mitosis
Meiosis
Summary
K E Y T E R M S
amino acids
anaphase
carbohydrates
cell division
cell membrane
centrosome
chromosomes
cytoplasm
cytoplasmic organelles
deoxyribonucleic acid (DNA)
endoplasmic reticulum (ER)
genes
88
human genome
inorganic compounds
interphase
lipids
meiosis
messenger RNA (mRNA)
metaphase
mitochondria
mitosis (M)
nucleic acids
nucleus
organic compounds
osmosis
oxidation
prophase
proteins
protein synthesis
protoplasm
ribonucleic acid (RNA)
ribosomal RNA (rRNA)
ribosomes
saccharides
telophase
transfer RNA (tRNA)
CHAPTER 6 Overview of Cell Biology
89
Biology is a science that explores living things and
life processes. Cells are the basic units of all living matter. The cell is the fundamental element of structure,
development, growth, and life processes in the human
body. Before imaging professionals can comprehend the
effects of ionizing radiation on the human body, they
must acquire a basic knowledge of how cells are assembled, what they are made of, and how they operate. This
chapter is designed to provide an understanding of
cellular biology, which ultimately will help the learner
appreciate the effects of radiation in the body.
THE CELL
The human body is composed of trillions of cells.
These cells exist in a multitude of different forms and
perform many diverse functions for the body, such as
the following:
• Conduction of nerve impulses
• Contraction of muscles
• Support of various organs
• Transportation of body fluids, such as blood
Some cells are freely moving, independent units
(e.g., leukocytes), whereas others remain in one position
as part of the tissues of larger organisms throughout
their lifetimes (e.g., bone marrow cells). Every mature
human cell is highly specialized and has predetermined
tasks to perform in support of the body.
Cells:
• Move
• Grow
• React
• Protect themselves
• Repair damage
• Regulate life processes
• Reproduce
To ensure efficient cell operation, the body must have
food as a source of raw material for the release of energy,
be supplied with oxygen to help break down the food,
and have enough water to transport inorganic substances such as calcium and sodium into and out of
the cell. In turn, proper cell function enables the body
as a whole to maintain homeostasis or equilibrium,
which is the ability to operate in a normal manner
despite any changes the body may undergo due to outside influences, such as stress, exercise, injury, or disease.
In summary, cells are engaged in an ongoing process
of obtaining energy and converting it to support their
vital functions. Cells absorb molecular nutrients
through the cell membrane and use these nutrients to
produce energy and synthesize molecules. If exposure to
outside influences, such as ionizing radiation, damages
the components involved in molecular synthesis beyond
repair, then cells either behave abnormally or die.
CELL CHEMICAL COMPOSITION
Protoplasm
Cells are made of protoplasm, the living contents of a
cell surrounded by a plasma membrane. The protoplasm
is the chemical building material for all living things. This
substance carries on the:
• Complex process of metabolism
• Reception and processing of food and oxygen
• Elimination of waste products
Metabolism enables the cell to synthesize proteins
and produce energy. Protoplasm, which includes both
small and very large molecules, called macromolecules,
consists of:
• Organic compounds (those compounds that contain
carbon, hydrogen, and oxygen)
• Inorganic materials (compounds that do not contain
carbon)
These are either dissolved or suspended in water.
The biomolecules that constitute protoplasm are
formed from many elements, among which there are
four primary components:
• Carbon
• Hydrogen
• Oxygen
• Nitrogen
When combined with phosphorus and sulfur, they
comprise the essential major organic compounds:
• Proteins
• Carbohydrates
• Lipids
• Nucleic acids
These compounds are discussed later in this chapter.
The most important inorganic substances are:
• Water
• Mineral salts (electrolytes)
Water plays a fundamental role in sustaining life and
is the most abundant inorganic compound in the body.
The essential functions of water are listed in Box 6.1 and
are also discussed later in this chapter. Depending on
cell type, water normally accounts for 80% to 85% of
90
PROTOPLASM COMPOSITION
CHAPTER 6 Overview of Cell Biology
BOX 6.1 Life-Sustaining Role of Water in
the Human Body
• Acts as the medium in which acids, bases, and salts
are dissolved
• Functions as a solvent by dissolving chemical sub-
stances in the cell
• Functions as a transport vehicle for material the cell
uses or eliminates
• Maintains a constant body core temperature of
98.6°F (37°C)
• Provides a cushion for vital organs, such as the brain
and lungs
• Regulates concentration of dissolved substances
• Lubricates the digestive system
• Lubricates skeletal articulations (joints)
1% Nucleic acids
15% Protein
80% to 85%
Water
The Cell
Fig. 6.1 Depending on cell type, water normally accounts for
80% to 85% of protoplasm. (From Radiobiology and radiation
protection: Mosby’s radiographic instructional series, St. Louis,
1999, Elsevier.)
1% Carbohydrates
2% Lipids
protoplasm (Fig. 6.1). Mineral salts exist in smaller
quantities but are of vital importance in sustaining cell
life because they help produce energy and aid in the
conduction of nerve impulses. Mineral salts are also
instrumental in the prevention of muscle cramping.
Organic Compounds
The four major classes of organic compounds (proteins, carbohydrates, lipids [fats], and nucleic acids) all
contain carbon as a fundamental constituent (Box 6.2).
By combining with:
• Hydrogen
• Nitrogen
• Oxygen
carbon makes life possible. Of the four classes of
organic compounds, proteins contain the most carbon.
BOX 6.2 Major Classes of Organic
Compounds That Compose the Cell
Proteins Lipids (fats)
Carbohydrates Nucleic acid
Proteins. Proteins are the most elementary building
blocks of cells, and they make up approximately 15% of
cell content (see Fig 6.1). Proteins are essential for
growth, the construction of new body tissue (including
acellular tissue such as hair and nails), and the repair of
injured or debilitated tissue. Proteins are formed when
organic compounds called amino acids combine into
long, chainlike molecular complexes. Amino acids are
essentially composed of combinations of NH2 (called
amine) and COOH (carboxyl group) molecules. Thus
nitrogen, hydrogen, carbon, and oxygen are the key
constituents of amino acids, of which approximately
500 different types are currently known, although
humans require only 22 specific amino acids. To summarize, proteins are macromolecules made up of strings
of amino acids. When proteins are produced within a
cell, a process known as protein synthesis, the order of
arrangement of amino acids, determines the precise
function of each protein molecule, and the types
of proteins that any given cell contains determine the
characteristics, or genetics, of that cell. Key components
of genetic material, called chromosomes and genes, which
organize the amino acids into different orderings to
make different types of proteins, are discussed later in
this chapter (Fig. 6.2).
Structural and enzymatic proteins. Structural pro-
teins, such as those found in muscle, provide the body
with its shape and form and are a source of heat and
energy. Enzymatic proteins function as organic catalysts, that is, agents that affect the rate or speed of
chemical reactions without being altered themselves.
As a result of this, enzymatic proteins (commonly called
enzymes) moderate or control the cell’s various physiologic activities. Among other tasks, enzymes can cause
an increase in cellular activity that in turn causes biochemical reactions to occur more rapidly to meet the
needs of the cell in stressful situations. In general,
proper cell functioning depends heavily on enzymes.
Repair enzymes. Many of the proteins produced in
the cell, by necessity, are enzymes, initiating vital chemical reactions within the cell at the appropriate time.
CHAPTER 6 Overview of Cell Biology
91
Chromosomes Genes
Organize
22 different amino acids
INTO
Certain sequence
aa aa
Structural
proteins
Fig. 6.2 Chromosomes and genes organize the 22 different
amino acids into certain sequences to form the different structural and enzymatic proteins.
aa aa aa
TO FORM
Enzymatic
proteins
Some of the enzymes produced, called repair enzymes,
can also mend damaged molecules and are therefore
capable of helping the cell recover from a small amount
of radiation-induced damage. Both the catalytic (i.e.,
reaction facilitator) and repair capabilities of enzymes
are of vital importance to the survival of the cell.
Repair enzymes work effectively in radiation induced
cell injury associated with both diagnostic and therapeutic radiation energy ranges. However, if the radiation damage is excessive because of a large delivered
equivalent dose, the cellular harm will be too severe for
repair enzymes to have enough positive effect. Thus,
when ionizing radiation is used for therapeutic purposes to destroy malignant cells, a very significant effort
using the latest advances in imaging and treatment
planning algorithms is always made to minimize the
absorbed dose to healthy surrounding tissue. In radiation therapy, this concept is referred to as a therapeutic
ratio, wherein the intent is to deliver enough radiation
to kill cancerous cells in a tumor (i.e., damage them sufficiently so that they are irreparable) while delivering a
much less-than-cell-killing equivalent dose to any surrounding noncancerous tissue structure. This concept is
the foundation on which successful radiation therapy
rests.
Hormones and antibodies. In addition to provid-
ing structure and support for the body, proteins may
function as hormones and antibodies. Hormones are
chemical secretions manufactured by various endocrine glands (i.e., organs that secrete substances into
the blood stream) and carried by the bloodstream to
influence the activities of other parts of the body. For
example, hormones produced by the thyroid gland
located in the neck control metabolism throughout
the body. Hormones also regulate body functions, such
as growth and development.
Antibodies are protein molecules created by specialized cells in the bone marrow called B lymphocytes.
Lymphocytes are white blood cells involved in the
body’s immune reactions. Antibodies are produced
when other lymphocytes in the body, known as T lym-
phocytes, detect the presence of molecules that do not
belong to the body. These foreign objects (e.g., bacteria,
flu viruses) are called antigens. Although the skin of the
body is the initial barrier to any outside invasion by
pathogens or the like, once it has been penetrated, the
body’s primary defense mechanism against infection
and disease are the antibodies that chemically attack any
foreign invaders.
Carbohydrates. Carbohydrates, also referred to as
saccharides, make up approximately 1% of cell content
(see Fig. 6.1). They include starches and various sugars.
Carbohydrates range from simple to complex compounds (Box 6.3), even though they are composed
of only carbon, hydrogen, and oxygen. Simple sugars
such as glucose, fructose, and galactose, have six carbon
atoms and six molecules of water (e.g., glucose has the
chemical formula C6H12O6). Glucose is the primary
energy source for the cell. Because it is a simple sugar,
it is called a monosaccharide. Other sugars that have
two units of a simple sugar linked together are called
disaccharides. Sucrose (cane sugar) and lactose are
examples of disaccharides. Both monosaccharides and
disaccharides are relatively small molecules. Polysaccha-
rides contain several or many molecules of simple sugar.
BOX 6.3 Simple to Complex
Carbohydrates
Monosaccharides
C
6H12O6
Disaccharides
C
Polysaccharides
C
1 C6H12O
6H12O6
1 C6H12O6 1 C6H12O6 1 C6H12O6 1 .........
6H12O6
6
92
CHAPTER 6 Overview of Cell Biology
BOX 6.4 Lipid Formation
Fats or lipids
h
1 molecule of glycerin 1 3 molecules of fatty acid
h
Carbon, oxygen, hydrogen
Plant starches and animal glycogen* are the two most
important polysaccharides. Through the process of
metabolism, the body breaks these down into simpler
sugars for energy.
Carbohydrates, simply described as chains of sugar
molecules, function as short-term energy warehouses
for the body. Their primary purpose is to provide fuel
for cell metabolism. Although carbohydrates are found
throughout the human body, they are most abundant in
the liver and in muscle tissue. They also are important
structural parts of intercellular materials.
Lipids. Lipids can simply be defined as substances such
as fats and fatty acids, oil, or wax that dissolve in alcohol
but not in water. Lipids are organic macromolecules,
in general containing carbon, hydrogen, and oxygen. In
their simplest form, they are made up of a molecule of
glycerin† and three molecules of fatty acid** (Box 6.4).
Lipids are the structural parts of cell membranes, constituting approximately 2% of cell content (see Fig. 6.1).
Therefore, lipids are present in all body tissue. The functions they perform for the body are listed in Box 6.5.
Nucleic Acids. Nucleic acids are complex macromole-
cules comprising approximately 1% of the cell (see Fig. 6.1).
*Glycogen, also known as animal glycogen, is a polysaccharide
that is the main storage form for glucose in both animals and
humans. In humans, it is mainly concentrated in the liver,
comprising about 10% of the liver mass.
†
Glycerin is a clear, odorless, syrupy liquid that is a simple
sugar and alcohol compound. It has the chemical formula
C3H8O3.
**When glucose is broken down in the body during respiration, fats are among the generated intermediate products.
When a fat combines with an acidic group of atoms (e.g., the
carboxyl group, COOH), a fatty acid is formed. An example of
a fatty acid is CH3COOH, which is commonly known as acetic
acid
. Fatty acids are constituents of amino acids from which
proteins are built.
BOX 6.5 Functions That Lipids Perform
for the Body
1. Act as reservoirs for the long-term storage of energy
2. Insulate and guard the body against the environment
3. Support and protect organs such as the eyes and
kidneys
4. Provide essential substances necessary for growth
and development
5. Lubricate the joints
6. Assist in the digestive process
The much smaller structures that are the building blocks
of nucleic acids are called nucleotides. Each nucleotide is
a unit formed from a nitrogen-containing organic base,*
a five-carbon sugar molecule (deoxyribose), and a phos-
phate molecule.**
Deoxyribonucleic and ribonucleic acids. Cells
contain two types of nucleic acids that are of primary
importance to all life:
• Deoxyribonucleic acid (DNA)
• Ribonucleic acid (RNA)
The DNA macromolecule is composed of two long
sugar–phosphate chains, which twist around each other
in a double-helix (spiral) configuration and are linked
by pairs of nitrogenous organic bases*** (purines and
pyrimidines) at the sugar molecules of the chain to
*In general, a base is a substance that, among other characteristics, is slippery to the touch in aqueous solutions, tends to
accept protons (i.e., ionized hydrogen atoms) from any proton
donor, and reacts with acids to neutralize them, forming salts
in the process. If the base contains one or more carbon and
hydrogen bonded components, it may be classified as an
organic base.
**A phosphate molecule has the chemical description PO4 and
therefore consists of one phosphorus atom bonded to four
oxygen atoms. It is a negative ion carrying a charge of 23
produced by the dissolution of phosphoric acid H3PO4. Excluding the phosphate molecule, Fig. 6.3 displays the molecular structure of the sugars and the organic bases. The latter are
divided into two categories called purines and pyrimidines.
***If an organic base is covalently bonded (i.e., a chemical
union formed when electrons are shared between two atoms
wherein each atom typically contributes one electron to form
a pair of electrons that are shared by both atoms) to one or
more nitrogen atoms, this chemical combination is called a
nitrogenous base. A nitrogenous base may also contain oxygen
atoms that form bonds with carbon.
CHAPTER 6 Overview of Cell Biology
SUGARS
93
form a tightly coiled structure resembling a twisted
ladder or spiral staircase. The sugar–phosphate compounds are the rails, and the pairs of nitrogenous
bases, which consist of complementary chemicals, are
the steps, or rungs, of the DNA ladder-like structure
(Fig. 6.3). Hydrogen bonds attach the bases to each
other and join the two side rails of the DNA ladder.
A~T
A~T
P
C~G
S
P
S
P
S
P
S
P
S
P
S
P
S
P
S
P
S
A~T
T~A
G~C
T~A
C~G
C~G
P
S
T~A
C~G
A~T
G~C
T~A
P
A~T
C~G
P
S
P
S
P
S
P
S
H
T~A
S
P
S
P
S
Nitrogenous organic bases in DNA. The four
nitrogenous organic bases (Fig. 6.4) in DNA macromol-
ecules are as follows:
• Adenine (A)
• Cytosine (C)
• Guanine (G)
• Thymine (T)
H
C O
C
C
O
H
D-ribose
H
H
N
C
N C
C
H
H
H
C
C
C N
H
N
C
H
Cytosine (C) Thymine (T)
H
HH O
O
HH
C
C
HH
H
O
PURINES
H
N
C
C
N
N
N
O
H
PYRIMIDINES
H
H
O
O
C
N
C C
N
H
H
H
H
C
H
C O
C
HH
C
O
D-2-deoxyribose
O
N C
N
H
H
H
C
C
H
Guanine (G)Adenine (A)
H H
H
N
C
N
C
C
C
N
H
O
C
HH
C
HH
O
H
H
C
N
H
O
C
N
HH
C
O
Fig. 6.3 Diagram of a DNA macromolecule that illustrates its
twisted ladder-like or spiral staircase–like configuration. Alternating sugar and phosphate molecules form the side rails of
the ladder, and the nitrogenous organic bases, which consist
of the complementary chemicals adenine (A), thymine (T),
guanine (G), and cytosine (C), form the rungs, or steps. A
hydrogen bond joins the bases together.
Uracil (U)
Fig. 6.4 The components of nucleic acid (H, hydrogen; C,
carbon; N, nitrogen; O, oxygen). Sugars are strung together
with phosphate groups, and a base is attached to each sugar.
DNA uses d-2-deoxyribose sugar, and RNA uses d-ribose. Both
nucleic acids use the same two purines, but thymine (T) in
DNA is replaced by uracil (U) in RNA.
94
CHAPTER 6 Overview of Cell Biology
Adenine and guanine are purines, and the compounds cytosine and thymine are classified as pyrimi-
dines. As can be seen in the figure, a primary difference
between the two classes of compounds is the number of
carbon–nitrogen rings, with purines always having two
rings and pyrimidines only one. A unique characteristic
of these organic bases in DNA is that purines link with
pyrimidines only in certain specific combinations; more
precisely, adenine always bonds only with thymine,
and cytosine bonds only with guanine. This property
is the reason the two strands of DNA are described as
complementary.
DNA: the master chemical substance. DNA, which
is a very large conglomeration of complex molecules, is
regarded as the master chemical substance because it
contains all of the information a cell needs to function.
DNA carries the genetic information necessary for cell
replication and regulates all cellular activity needed to
direct protein synthesis. DNA establishes an individual’s
personal characteristics by regulating the ordering of
amino acids in the person’s constituent proteins during
the synthesis of these proteins. These arrangements
of amino acids are determined by the succession of
adenine–thymine and cytosine–guanine base pairs in
the DNA macromolecules. Therefore, the sequence
of nitrogenous base pairs in the DNA molecule constitutes a genetic code. Different sequences of amino acids
produce proteins with different functions. Protein characteristics determine cell characteristics, and cell characteristics ultimately determine the characteristics of
the entire individual. All of the information necessary to
construct and maintain a living organism is written in
the “genetic code book” of DNA—the letters, words, and
sentences are the arrangements and groupings of the
nitrogenous organic bases. Why is one person’s DNA dif-
ferent from another’s? Small differences in base pair
layouts are responsible for variations in human beings
because such slightly altered base pair configurations
lead to changes in the proteins produced, how much are
produced, and when they are produced.
Structural differences between DNA and RNA. RNA
is a long, single-stranded chain of cells that processes
protein. The nucleic acid polymer (a macromolecule
containing many repeated subunits) RNA plays an
essential part in the translation of genetic information
from DNA into protein products. To do this, RNA
functions as a messenger between DNA and the ribosomes, or “protein factories,” where synthesis occurs.
RNA differs structurally from DNA in several ways,
some of which are as follows:
• RNA is a single-strand macromolecular structure,
whereas DNA is a double-strand macromolecular
structure. Both have spiral ladder-like arrangements
of their bases.
•
RNA contains ribose,* whereas DNA contains
deoxyribose.*
• RNA has the nitrogenous base uracil (see Fig. 6.4) as
a component of its ladder steps, whereas DNA has
thymine instead in its ladder steps. Both also contain
the bases adenine, guanine, and cytosine as elements
of their spiral structure. Unlike with DNA, where
thymine forms a bond with adenine, for RNA, uracil
links with an adenine base.
• RNA performs many different biologic functions
(e.g., acts as an enzyme), but DNA carries the genetic
information.
• RNA has a much shorter chain of nucleotides than
DNA.
Messenger RNA. Because DNA is found mostly
in the cell nucleus, it cannot directly influence cellular
activity such as growth and differentiation, which occur
in the cytoplasm (the part of the cell that lies outside
the nucleus). Instead, DNA regulates cellular activity
indirectly, transmitting its genetic information outside
the cell nucleus by reproducing itself in the form of
messenger RNA (mRNA), which is able to leave the
cell nucleus. Once in the cytoplasm, mRNA directs the
process of making proteins from amino acids.
DNA serves as a prototype for mRNA, but mRNA
differs from DNA in two important ways:
1. mRNA contains in its backbone the sugar molecule, ribose, which differs only in the presence
of an extra O–H bond from the sugar molecule,
deoxyribose, found in the backbone or side rails
of DNA (see Fig. 6.3).
2. In mRNA, the pyrimidine base uracil (U) replaces
the thymine that is found in DNA (Fig. 6.5).
An mRNA macromolecule, as does any type of RNA,
resembles one half of a DNA macromolecule. RNA
*Ribose is an organic compound classified as a simple sugar.
Chemically, ribose is made up of a bonded pentagon-shaped
arrangement of 5 carbon atoms, 10 hydrogen atoms, and
5 oxygen atoms. If a ribose molecule should lose one of its
oxygen atoms, it is called deoxyribose.
CHAPTER 6 Overview of Cell Biology
Amino acid
aa
U
U
C
S
P
S
P
S
U
S
P
C
A
S
P
S
P
aa
aa
aa
mRNA
aa
aa
aa
Ribosome
aa
aa
95
aa
tRNA
G
A
A
G
G
U
P
S
P
S
P
S
G
C
Fig. 6.5 Messenger RNA (mRNA) resembles one half of a
DNA macromolecule. It appears as a single strand (one side
rail) of the DNA ladder-like configuration, with the ladder being
severed in half lengthwise. Uracil (U) replaces thymine (T) as
one of the nitrogenous organic bases in the mRNA molecule.
G
U
C
A
A
A
S
P
S
G
S
therefore appears as a single strand of the DNA ladderlike configuration, with the ladder being severed in
half lengthwise (see Fig. 6.5).
Transfer RNA. Macromolecules of mRNA carry
their genetic codes in their sequences of nitrogenous
organic bases (e.g., U, U, C, C, A, U, G, etc.) from the cell
Fig. 6.6 Ribosomes, the cell’s protein factories, travel along
the messenger RNA (mRNA) rails, linking transfer RNA (tRNA)
and its corresponding amino acids in the proper sequences to
produce the proteins appropriate for the needs of the cell.
nucleus to the ribosomes.* Proteins are manufactured
in the ribosomes. Within the ribosome, both mRNA
and transfer RNA (tRNA) macromolecules are present.
The mRNA delivers its genetic code to tRNA. This
encoded tRNA combines with individual amino acids
from different areas of the cell and attaches them
to the ribosomes, where the amino acids are subsequently arranged in specific orders to form chainlike
protein molecules. Each tRNA molecule is specifically
coded for a particular amino acid. Because each of the
22 different amino acids has an associated tRNA, at least
22 different types of tRNAs exist. The ribosomes travel
along the mRNA and link tRNA and its corresponding
amino acids in the correct order so that the proteins
necessary to provide for the needs of the cell are produced (Fig. 6.6).
Ribosomal RNA. Ribosomal RNA (rRNA) is yet
another type of RNA. Ribosomal RNA’s function is
to assist in the linking of mRNA to the ribosome to
facilitate protein synthesis.
Chromosomes and genes. Chromosomes are tiny,
rod-shaped bodies that under a microscope appear
to be long, threadlike structures that become visible
only in dividing cells (Fig. 6.7). Chromosomes are
composed of:
• Protein
• The genetic material DNA
*Ribosomes: small, spherical organelles (subunits of a cell that
perform a specific function) that are the assembly sites (similar to an auto assembly line) where mRNA and tRNA combine
amino acids into proteins. A further discussion of ribosomes
is provided later in this chapter.
96
A
Fig. 6.7 (A) A chromosome viewed under a microscope appears
rod shaped; (B) when further magnified, a chromosome appears
as a tightly wound spiral structure composed of hundreds of
genes—(C) a segment of the DNA macromolecule.
CHAPTER 6 Overview of Cell Biology
B
C
A normal human being has 46 different chromosomes composed of 23 pairs in each somatic (nonreproductive) cell. Individual male and female reproductive
cells, also known as germ cells, do not have this pairing.
Instead, each of these germ cells has only 23 chromosomes, which pair up to form a full set of 46 chromosomes when a sperm cell fertilizes an egg cell. The DNA
that makes up every chromosome is divided into many
hundreds of segments or subunits called genes. Each
gene, because of the ordering of its nitrogenous base
pairs, contains information responsible for or related to
one or more of the following:
• Directing cytoplasmic activities
• Controlling growth and development of the cell
• Transmitting various aspects of hereditary informa-
tion (e.g., hair color, blood type, general body char-
acteristics, etc.)
Thus, genes are the basic units of heredity. Taken as a
whole, they control the formation of proteins in every
cell through the intricate process of parentally shared
genetic coding.
The human genome. The total amount of genetic
material (DNA) contained within the chromosomes of
a human being is called the human genome. The human genome is the blueprint for each person’s body.
The process of locating and identifying the genes in the
genome is called mapping. A landmark event occurred
in 2001, when after years of intense effort two rival
groups succeeded in deciphering the human genome.1
Essentially, they uncovered the entire sequence of DNA
base pairs (i.e., all of the “rungs” of the DNA ladder
structure) on all 46 chromosomes. This major milestone in biology and medicine was accomplished by
Celera Genomics, a private company in Rockville,
Maryland, and the International Human Genome
Sequencing Consortium, a group of academic centers
funded mostly by the National Institutes of Health and
the Wellcome Trust of London.
2
The groups found that there are 2.9 billion base pairs
in the human genome and that these base pairs are
arranged into approximately 30,000 genes. It is estimated that these genes are capable of producing at least
90,000 different proteins.
According to the National Institutes of Health (2010),
the Human Genome Project has already led to the discovery of more than 1800 disease genes. In addition,
with the knowledge gained from the project, today’s
researchers can find a gene suspected of causing an
inherited disease in a matter of days, rather than the
years it took before the genome sequence was discovered. There are now more than 2000 genetic tests
for human conditions. These tests enable patients to
learn their genetic risks for disease and help health
care professionals diagnose disease. As of 2013 at least
350 biotechnology-based products resulting from the
Human Genome Project have been placed in clinical
trials. Possessing the complete sequence of the human
genome is similar to owning all the pages of a manual
needed to create the human body. The challenge now is
to determine how to read the contents of these pages
and understand how all of these many complex parts
function together in human health and disease.3 The
potential gain from this effort is to be able to eventually
both predict a person’s risk of disease and provide that
person with specific drugs for targeting and stopping
the disease.
Interpreting the map of the human genome is
similar to that of a building contractor discovering a
list of all the items that are needed to build a house
but not a blueprint that demonstrates how often or
in what order the steps should be performed. Presymptomatic testing, carrier screening, workplace genetic
screening, and testing by insurance companies pose
significant ethical issues. Second, the burgeoning
ability to manipulate human genes raises a number of
important ethical questions. Ethical, legal and social
issues raised by genomic research include: possible
discrimination by employers or health insurers, the
need for ethical standards for work with human
research subjects or tissues, and consideration of social,
cultural and religious perspectives on genetics and