Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5196_Библиотеки_им_академика_М_И_Перельмана.pdf
CHAPTER 6 Overview of Cell Biology
97
health. Over the next few decades, the great tasks will be
to answer questions such as the following:
1. What determines when genes will produce proteins
and when genes will not?
2. In what order are various proteins produced during
development and throughout life?
3. What genes cause some individuals to be susceptible
to a certain disease?
4. Is it possible to learn how to deactivate those genes
and turn on other genes that provide resistance?
5. Are there genes that make some people more or less
sensitive to the effects of ionizing radiation?
6. Can the newly acquired insight into the human genome be used to both detect and properly correct the
defective genes that are the root of genetically transmitted disease?
Gene therapy raises another group of challenging
ethical issues. Gene therapy is an experimental technique that uses genes to treat or prevent disease. Bioethicists and researchers generally believe that human
genome editing for reproductive purposes should not be
attempted at this time, but that studies which would
make gene therapy safe and effective should continue.
Once the consent, safety, and scientific issues are
resolved, there is probably nothing ethically unique
about conducting somatic cell gene therapy or fetal
gene therapy to correct genetic diseases. In the future,
this technique may allow doctors to treat a disorder
by inserting a gene into a patient’s cells instead of using
drugs or surgery. Researchers are at this time testing
several approaches to gene therapy, including:
• Replacing a mutated gene that causes disease with a
healthy copy of the gene.
• Inactivating, or “knocking out,” a mutated gene that
is functioning improperly.
• Introducing a new gene into the body to help fight a
disease.
Although gene therapy is a promising treatment
option for a number of diseases (including inherited
disorders, some types of cancers, and certain viral infections), the technique remains risky and is still under
study to make sure that it will be safe and effective. Gene
therapy is currently being investigated only for diseases
that have no other cures.
4
Inorganic Compounds
Inorganic compounds are compounds that do not con-
tain carbon. The inorganic compounds found in the
body occur in nature independent of living things and
are made up of three categories:
• Inorganic acids
• Inorganic bases
• Salts (electrolytes)
Inorganic acids are hydrogen-containing compounds
such as HNO3 (nitric acid) that can attack and dissolve
metal. Inorganic bases are alkali or alkaline-earth (see
Appendix D, first 2 columns) OH compounds such as
Mg(OH)2 (otherwise known as milk of magnesia) that
can neutralize acids. Salts are chemical compounds
resulting from the action of an acid with a base. Salts
are sometimes referred to as electrolytes. Chemically,
they “are substances that become ions in solution and
thereby acquire the capacity to conduct electricity. Electrolytes are present throughout the human body, and
the balance of the electrolytes in our bodies is essential
for normal function of our cells and our organs.”5 A list
of some of the important electrolytes in the body may
be found in Box 6.6.
Water is the primary inorganic substance contained
in the human body; it comprises approximately 80% to
85% of the body’s weight (Fig. 6.8). If water content
within a cell is too low, the cell will collapse, resulting in
a lack of ability to continue normal biologic function.
Conversely, if water content is excessive, the cell most
likely will rupture. Therefore, it is imperative that the
correct amount of water in a cell be maintained.
6
Function of Water Within and Outside of the Cell. Within the cell, water is indispensable for meta-
bolic activities since it is the medium in which the
chemical reactions that are the basis of these activities
occur. Cellular water also acts as a solvent, keeping compounds dissolved so they can more easily interact and
their concentration be regulated. Outside the cell, water
functions as a transport vehicle for materials the cell
uses or eliminates. In addition, water is responsible for
maintaining a constant body core temperature of 98.6°F
BOX 6.6 Some of the Important
Electrolytes in the Body
Sodium (Na1) Chloride (Cl2)
Potassium (K
Calcium (Ca
Magnesium (Mg
1
) Bicarbonate (HCO
11
) Phosphate (HPO
11
) Sulfate (SO
2
)
3
2
)
4
22
)
4
98
Fig. 6.8 Water constitutes approximately 80% to 85% of the
body’s weight. (From Radiobiology and radiation protection:
Mosby’s radiographic instructional series, St. Louis, 1999, Elsevier.)
As a medium to dissolve and regulate acids, bases,
As a means of maintaining a constant body temperature
Fig. 6.9 Water’s role outside the cell. (From Radiobiology and
radiation protection: Mosby’s radiographic instructional series,
St. Louis, 1999, Elsevier.)
CHAPTER 6 Overview of Cell Biology
80% to 85% Water
As a transportation system to and from cells
and salts
98.6°F
in the cell. Because salts are inorganic (i.e., no carbon
is present) and have a crystalline atomic structure,
they are classified as mineral salts. Their presence is
vital for:
• Proper cell performance
• Creation of energy
• Conduction of impulses along nerves
Within an aqueous solution, these salts can be
broken down, with their constituents existing as ions
(particles carrying either a positive or negative electric
charge) in the cell. The resulting medium is called a
solute. The ions within the solute, via chemical reactions, cause materials to be altered, fragmented, and
recombined to form new substances. Potassium (K)
contributes most of the positive ions (K1 also known
as cations) present in cells, whereas phosphorus (P-)
contributes the majority of negative ions (anions).
Potassium is of primary importance in maintaining
adequate amounts of intracellular fluid. This is because
water, a solvent, will preferentially move across cell
surfaces or membranes into areas with a high concentration of ions, also known as high solute regions. This
motion is referred to as osmosis. Thus, by controlling
its concentration of potassium ions (as well as the everpresent sodium [Na] and chloride [Cl] ions resulting
from the intake of table salt), the cell regulates the
amount of water passing through its membrane and
consequently the amount of fluid it contains. Osmotic
pressure is the external pressure required to be applied
so that there is no net movement of solvent, typically
water, across the cell membrane. Retaining the correct
proportion of water in the cell causes osmotic pressure
to be maintained. Potassium also aids in maintaining
acid–base balance, a state of equilibrium, or stability,
between acids and bases.
(37°C) (Fig. 6.9) while at the same time serving to lubricate both the digestive system and skeletal articulations
(joints). Organs such as the brain and lungs are
also protected by a cushion of compounds composed
primarily of water.
Function of Mineral Salts Within the Cell. Salts
resulting from acid/base reactions, predominantly
involving sodium (Na) and/or potassium (K) (e.g.,
potassium chloride, sodium iodide, potassium nitrate,
and the like) preserve the correct proportion of water
CELL STRUCTURE
The normal cell has the following components
(Fig. 6.10):
1. Cell membrane
2. Cytoplasm
3. Cytoplasmic organelles (organelles are subcellular
structures)
a. Endoplasmic reticulum
b. Golgi apparatus or complex
c. Mitochondria
d. Lysosomes
CHAPTER 6 Overview of Cell Biology
99
Mitochondrion
Lysosome
Rough
endoplasmic
reticulum
Peroxisome
Cytoskeleton
Intermediate
filament
Smooth
endoplasmic
reticulum
Centrioles
Centrosome
Nuclear
envelope
Ribosomes
Nucleus
Mitochondria
Smooth endoplasmic
reticulum
Cilia
Free
ribosomes
Golgi
apparatus
Microvilli
Vesicle
Nucleolus
Microtubule
Fig. 6.10 Diagram of a typical cell, demonstrating its basic components. (From Thibodeau A: Anatomy and
physiology, ed 9, St. Louis, 2016, Elsevier.)
Microfilament
e. Ribosomes
f. Centrosomes
4. Nucleus
Cell Membrane—A “Plastic Storage Bag” to Contain the Cell
The cell membrane is a frail, semipermeable, flexible
structure encasing and surrounding the human cell.
The cell membrane, shown in blue in Fig. 6.10:
• Is made of lipids and proteins.
• Functions as a barricade to protect cellular contents
from the outside environment.
• Controls the passage of water and other materials
into and out of the cell.
Because the cell membrane allows penetration only
by certain types of substances and regulates the speed
at which these substances travel within the cell, it
plays a primary role in the cell’s transport system.
When a substance moves through the cell membrane
by osmosis, the transport system is classified as passive because the cell uses no energy to maintain the
concentration. When the movement of a substance
across a cell membrane is controlled more by the
properties and powers of the cell membrane than it is
by the relative concentrations of particles in fluid, the
transport system is classified as active. In active trans-
port, the cell must expend energy to pump substances
into and out of it.
Cytoplasm
Cytoplasm is the protoplasm that exists outside the
cell’s nucleus and is primarily composed of water, but
also contains:
• Proteins
• Carbohydrates
• Lipids
100
CHAPTER 6 Overview of Cell Biology
BOX 6.7 Major Tasks of the Cytoplasm
Behaving like a factory, the cytoplasm constituents
perform the following major tasks:
1. Accepts and builds up unrefined materials and from
these materials assembles new substances such as
carbohydrates, lipids, and proteins; the assembly of
larger molecules from smaller ones is known as
anabolism
2. Breaks down organic materials to produce energy
(catabolism)
3. Packages substances for distribution to other areas
of the cell or to various sites in the body through the
circulation
4. Eliminates waste products
• Salts
• Minerals
Cytoplasm comprises the majority of the cell and
contains large amounts of the cell’s molecular components, with the exception of DNA. All cellular metabolic
functions occur in the cytoplasm. The major functions
of the cytoplasm are listed in Box 6.7.
Cytoplasmic Organelles
The cytoplasm contains all the miniature cellular components that enable the cell to function in a highly
organized manner. These small organs of the cell are
collectively referred to as cytoplasmic organelles,
which consist of the following:
• Tubules (small tubes)
• Vesicles (small cavities or sacs containing liquid)
• Granules (small insoluble, nonmembranous particles found in cytoplasm)
• Fibrils (minute fibers or strands that are frequently
part of a compound fiber)
Together these structures perform the major func-
tions of the cell in a systematized manner. DNA, which
is located in the cell nucleus, separated from the cytoplasm, determines the function of each cytoplasmic
organelle; mRNA carries the DNA code from the
nucleus into the cytoplasm.
Endoplasmic Reticulum—The “Highway” of the Cell.
The endoplasmic reticulum (ER) is a vast, irregular
network of tubules and vesicles spreading and interconnecting in all directions throughout the cytoplasm. The ER enables the cell to communicate with
the extracellular environment and transfer food and
molecules from one part of the cell to another. Thus,
it functions as the highway system of the cell. For
example, mRNA travels from the nucleus to different
locations in the cytoplasm through the ER, and lipids
and proteins are also routed into and out of the
nucleus through the ER tubular network.
Cells have two types of endoplasmic reticulum:
• Rough surfaced (granular)
• Smooth (agranular)
When numerous ribosomes (the sites where mRNA
and tRNA assemble amino acids into proteins) are
present on the surface of the ER, the surface is rough
or granular. If they are not present, the surface is
smooth or agranular. The “smooth” or “rough” distinction refers to the endoplasmic reticulum’s appearance
when viewed with an electron microscope. The cell
type determines the type of ER. For example, cells that
actively manufacture proteins for export, such as
the pancreatic cells, which produce insulin, need more
ribosomes and therefore have an extensive rough or
granular endoplasmic reticulum. A lesser amount of
rough or granular ER is found in cells that synthesize
proteins mainly for their own use.
Golgi Apparatus or Complex—Hauls “Freight” Within and Out of the Cell. The Golgi apparatus con-
tains minute vesicles that extend from the nucleus to
the cell membrane. The vesicles consist of tubes and
a tiny sac located near the nucleus. This structure
unites large carbohydrate molecules (i.e., various types
of sugars) and then combines them with proteins,
which are typically found floating in or around the
membrane of cells, to form glycoproteins. Glycoproteins are involved in nearly every process in cells.
Glycoproteins have diverse functions throughout the
body within the immune system, in communication
between cells, and in the reproductive systems. In
addition, when the cell manufactures glycoproteins
that function as enzymes and hormones, the Golgi
apparatus concentrates, packages, and transports them
through the cell membrane so they can exit the cell,
enter the bloodstream, and be carried to the areas of
the body where they are required.
Mitochondria—The “Power-Generating Station” of the Cell. The large, double-membranous, oval or bean-shaped
CHAPTER 6 Overview of Cell Biology
101
structures called mitochondria function as the “powerhouses” of the cell because they supply the energy for
all cellular function. They contain highly organized
enzymes in their inner membranes that produce this
energy for cellular activity by breaking down nutrients
such as:
• Carbohydrates
• Fats
• Proteins
This breakdown of nutrients occurs through the
process of oxidative metabolism. Oxidation is any
chemical reaction in which atoms lose electrons. The
substance that loses electrons is said to have been
oxidized, and chemical energy is released in the process. The oxidation of iron, for example, which occurs
in a moist environment in the presence of oxygen,
produces iron oxide (Fe2O3) commonly known as
rust. In the case of rust, the iron atoms give up electrons to oxygen atoms, creating a bond between iron
and oxygen.
Destructive metabolism (also known as catabolism)
is the breaking down of large molecules (e.g., polysaccharides, lipids, proteins) into smaller molecules. Oxidative metabolism is the oxidation of these smaller
molecules to release energy. Some of this energy is
lost as heat, and the remainder is primarily used with
the assistance of the enzymes contained within the
mitochondria to produce the compound adenosine tri-
phosphate (ATP).* ATP is the prime energy-containing
molecule in the cell. ATP is essential for sustaining life
and performs a significant role in active transport
within the cell. As mentioned previously, in active transport molecules are moved or pumped through cell
membranes. This happens regardless of the relative
concentrations of particles on either side of the membrane. This process will therefore often require energy.
The needed energy is supplied by ATP. ATP functions
by losing its endmost phosphate group (Fig. 6.11) when
*The ATP molecule is composed of three molecular subgroups. At the center is a sugar molecule, ribose (the same
sugar that forms the basis of RNA). Attached to one side of
this is a base (a group consisting of linked rings of carbon and
nitrogen atoms); in this case the base is adenine. The other
side of the sugar is attached to a string or chain of phosphate
groups. These phosphates are the key to the energy activity of
ATP (see Fig. 6.11).
ADENINE (BASE)
NH
2
C
C
C
N
2
O
RIBOSE
N
CH
N
PHOSPHATE CHAIN
O O
−
O
Fig. 6.11 Molecular structure of adenosine triphosphate
(ATP).
O
P P POOCH
−
O
O
−
O
O
N
HC
−
H H
OH OH
instructed to do so by enzymes. This reaction releases a
large amount of energy, which the organism can then
also use to build proteins, contract muscles, etc. When
the organism is resting and energy is not immediately
needed, the reverse reaction takes place and the phosphate group is reattached to the molecule, using energy
obtained from food or sunlight. Thus, the ATP molecule
acts as a chemical “battery,” storing energy when it is
not needed, but is able to release it instantly when
the organism requires it. The number of mitochondria
in cells varies from a few hundred to several thousand.
The greatest number of mitochondria is found in cells
exhibiting the greatest activity.
Lysosomes—”Garbage Bags” With “Poison Pills.”
Lysosomes are small, pea-like sacs or single-membrane
spherical bodies that are of great importance for digestion within the cytoplasm. Lysosomes contain a group
of different digestive enzymes that target proteins, and
their primary function appears to be the breaking down
of unwanted large molecules that either penetrate into
the cell through microscopic channels or are drawn in
by the cell membrane itself. If lysosomes fail in their
cellular “garbage disposal” tasks, the resulting accumulation of large molecules can ultimately obstruct normal
functions in organs. Lysosomes are sometimes referred
to as suicide bags, because their enzymes break down
and digest not only proteins and certain carbohydrates,
but also will do the same to the cell itself should the
lysosome’s surrounding membrane rupture. Exposure
to radiation may induce such a rupture. When this
occurs, the cell is likely to die.
102
CHAPTER 6 Overview of Cell Biology
Mother centriole
Microtubule
triplets
Top view
Daughter centriole
Fig. 6.12 Centriole configuration. The centrioles are cylindrical-
shaped cellular organelles that occur in pairs. Each centriole is
made up of groups of microtubules that are arranged in a pattern, forming a ring of nine trio microtubules known as triplets.
As shown, the centrioles are arranged at right angles to one
another. In human cells, the centrioles facilitate the organizing
and assembly of microtubules during the process of cell division.
Ribosomes—”Manufacturing Facilities” of the Cell.
Ribosomes are very small, spherical organelles that
attach to the endoplasmic reticulum. They consist of:
• Two-thirds RNA
• One-third protein
Ribosomes are commonly referred to as the cell’s
protein factories because their role is to manufacture
(synthesize) the various proteins that cells require by
using the blueprints provided by mRNA. Ribosomes’
role in the assembly of amino acids into proteins was
detailed earlier in this chapter.
Chromatin
Nucleolus
Centrosome
Microtubules
Centrioles
Fig. 6.13 Centrosome structure.
Nuclear envelope
Nuclear pore
Endoplasmic
reticulum
Ribosomes
Centrosomes—”Weavers of the Spindle.” Centro-
somes are located in the center of the cell near the nucleus. Centrosomes contain centrioles, which are a pair
of small, hollow, cylindrical structures within each centrosome (Fig. 6.12) oriented at right angles to each
other and embedded in a material mass of more than
100 proteins.
When a cell divides and produces two new identical
cells, first, the cell must create two copies of its DNA.
Each copy of DNA will subsequently travel to a new cell
so that the new cells have the identical DNA as the
original cell.
The centrosome and centrioles have crucial roles in
this process: during cell division, two centrioles combine together with some other special proteins and
form the centrosome. Fig. 6.13 is an image of a centrosome, comprised of two centrioles and microtubules.
The centrosome serves as the main microtubuleorganizing center of the cell, as well as a regulator
of cell-cycle progression.
Fig. 6.14 Anatomy of the cell nucleus.
The centrioles’ pair duplicates within a cell, and the
resultant two pairs migrate to the opposite ends of the
cell to form and organize the mitotic spindle.* The
stages of cell division are discussed in detail later in this
chapter.
Nucleus—Information-Processing and Administrative Center
Separated from the other parts of the cell by a doublewalled membrane with pores, called the nuclear envelope,
the nucleus is a highly specialized cellular component
that is the information-processing and administrative
center of the living cell (Fig. 6.14). The nucleus consists
*The mitotic spindle is essentially a protein machine that segregates chromosomes into two daughter cells during the cell
division process.
CHAPTER 6 Overview of Cell Biology
TABLE 6.1 Summary of Cell Components
Component Site Activity
Cell
membrane
Endoplasmic
reticulum
Golgi
apparatus
Mitochondria Cytoplasm Power-generating stations – Produce energy for cellular activity by breaking down nutri-
Lysosomes Cytoplasm Garbage bags with poison pills – Dispose of large particles such as bacteria and food,
Ribosomes Cytoplasm Manufacturing facilities – Manufacture the various proteins that cells require.
Centrosomes Cytoplasm Spindle weaver – Plays an important role in organizing the formation of the mitotic
Nucleus Nucleus Information-processing and administrative center of the cell – Contains the genetic, or
DNA Nucleus The blueprints – Contains the genetic material; controls cell division and multiplication
Nucleolus Nucleus RNA copy center – Holds a large amount of RNA and synthesizes ribosomes.
Cytoplasm Plastic storage bag – Functions as a barricade to protect cellular contents from their
environment and controls the passage of water and other materials into and out of
the cell; performs many additional functions, such as elimination of wastes and refining of material for energy through breakdown of the materials.
Cytoplasm The highway – Enables the cell to communicate with the extracellular environment and
transfers food from one part of the cell to another.
Cytoplasm Freight hauling – Unites large carbohydrate molecules and combines them with
proteins to form glycoproteins; transports enzymes and hormones through the cell
membrane so that they can exit the cell, enter the bloodstream, and be carried to
areas of the body in which they are required.
ents through a process of oxidation.
as well as smaller particles; also contain hydrolytic enzymes that can break down and
digest proteins, certain carbohydrates, and the cell itself if the lysosome’s surrounding membrane breaks.
spindle during cell division.
hereditary, material, DNA, and proteins. Also contains the nucleolus. The nucleus controls cell division and multiplication and the biochemical reactions that occur within
the cell. Also directs protein synthesis.
and biochemical reactions that occur within the living cell.
103
of a spherical mass of semifluid protoplasm, known as
nucleoplasm, which contains the genetic or hereditary
material, DNA (the blueprints, or instructions, for building proteins in the cell), and proteins. The pores in the
nuclear envelope allow molecules of specific types and
sizes to pass back and forth between the nucleus and the
cytoplasm.
Proteins and DNA within the nucleoplasm are
arranged in long threads called chromatin. Chromatin
is essentially a less condensed or less tightly packed form
of the cell’s DNA that, together with various proteins,
during the division of a cell contracts into the tiny rodshaped bodies that are called chromosomes. The genetic
history of the cell is contained within the chromosomes
in the segments of DNA called genes.
The cell nucleus also contains at least one very small,
rounded body called the nucleolus. The nucleolus is the
RNA copy center. This nuclear organelle manufactures
and contains a large amount of RNA and protein. The
nucleolus synthesizes ribosomes, which are proteinproducing machines.
In summary, the nucleus controls cell division, multiplication, and the biochemical reactions that occur within
the cell. By directing protein synthesis, the nucleus plays
an essential role in the following:
• Active transport
• Metabolism
• Growth
• Heredity
A summary of cell components is presented in Table 6.1.
CELL DIVISION
Cell division is the multiplication process whereby one
cell divides to form two or more cells (Fig. 6.15). The
two types of cell divisions that occur in the body are:
• Mitosis
• Meiosis
104
CHAPTER 6 Overview of Cell Biology
Cell Division
2 2
4
8 8 8 8
Fig. 6.15 Cell division is the multiplication process whereby
one cell divides to form two or more cells. (From Radiobiology
and radiation protection: Mosby’s radiographic instructional
series, St. Louis, 1999, Elsevier.)
4 4 4
32
new cells
64
128
256
512
1024
A never-ending process
……
When somatic cells (all cells in the human body
other than the germ cells) divide, they undergo mitosis,
a process in which the nucleus first divides, followed
by the division of the cytoplasm. Genetic cells (the
oogonium, or female germ cell, and the spermatogonium, or male germ cell), however, undergo meiosis,
a process of reduction division.
Mitosis
When mitosis (M) (Fig. 6.16) occurs, a parent cell divides to form two daughter cells identical to the parent
cell. Mitosis results in an approximately equal distribution of all cellular material between the two daughter
cells. The entire cellular life cycle may be depicted as
shown in Fig. 6.17. Differing degrees of cell growth,
maturation, and division occur in each phase. Four distinct phases of the cellular life cycle are identifiable:
• G1 (pre-DNA synthesis)
• S (synthesis)
• G2 (post-DNA synthesis)
• M (mitosis)
In addition, the M phase, itself, can be divided into
four subphases:
• Prophase
• Metaphase
• Anaphase
• Telophase
Mitosis should be thought of as the division phase of the
cellular life cycle and therefore is actually the last phase
of the cycle. After mitosis has commenced, it takes about
1 hour to complete division in all cells. Just prior to
mitosis, however, there is a relatively brief time of cell
growth. This interval is called Interphase and is itself
composed of three phases:
1. G
1
2. S
3. G
2
G1 is the earliest period among reproductive events.
G1 is the gap in the growth of the cell that occurs
between mitosis and DNA synthesis. Depending on
the types of cells involved, this phase may take a few
minutes, or it may take several hours. G1 is designated as
the pre-DNA synthesis period. During G1, a form of
RNA is manufactured in the cells that are to reproduce.
This RNA is needed before actual DNA creation can
efficiently begin. S is the actual DNA synthesis period.
While in S phase, each DNA molecule contained within
the chromosome (Fig. 6.18) is first copied (replicated)
and then is divided into two individual sister components called chromatids,* each containing DNA molecules. By the end of the S phase these chromatids will
join together to form a new chromosome that has an
X-shaped structure (see Fig. 6.18). Thus, each of the
identical genetic pieces has now become one half of a
new chromosome. The region of this chromosome
where the two chromatids join together is the centro-
mere (see Fig. 6.18). Note, that during the anaphase
portion of Mitosis (described later), the paired sister
chromatids separate from one another to form individual daughter chromosomes.
When compared with G1 and G2, the S portion of
Interphase is relatively long, lasting up to 15 hours.
G2 is the post-DNA manufacturing interval in the
cellular life cycle. G2 is of comparatively short duration, lasting approximately 1 to 5 hours. During G2,
cells manufacture certain proteins and RNA molecules,
which are needed for initiating and completing the
subsequent Mitosis process. Directly after G2, cells enter
the first phase of Mitosis, and the process of division
commences.
*A chromatid is a highly coiled strand; one of the two duplicated portions of DNA in a replicated chromosome that
appear during cell division.
centrosome
centrioles
parent cell
(whole four-armed structure)
Anaphase
Interphase
duplicated
chromosome
CHAPTER 6 Overview of Cell Biology
mitotic
spindle
Prophase
centromere
chromosome
(two-armed structure)
Metaphase
105
Telophase
Fig. 6.16 Diagram of mitosis. An animal cell with four chromosomes first multiplies (duplicates its DNA) and
then divides, forming two new daughter cells, each of which contains exactly the same genetic material as
the parent cell.
The Four Phases of Mitosis. In the discussion
which follows, it will be helpful to the reader to refer
to Fig. 6.16.
Prophase. During prophase, the first phase of cell
division, the nucleus enlarges, the DNA complex (the
chromatid network of threads) coils up tightly, and the
chromatids become visible on stained microscopic
slides. Chromosomes enlarge, and the DNA begins to
assume structural form. Next, the nuclear membrane
disappears, and the centrioles (small hollow, cylindrical
daughter cells
structures) migrate to opposite sides of the cell and begin to regulate the formation of the mitotic spindle, the
delicate fibers that are attached to the centrioles and
extend from one side of the cell to the other across the
equator of the cell.
Metaphase. As metaphase begins, the mitotic spin-
dle forms between the centrioles. Each chromosome
which now consists of two chromatids, lines up in the
center, or equator, of the cell attached by its centromere
to the mitotic spindle. This configuration establishes the
106
Chromosome
Chromosome
Fig. 6.17 The entire cellular life cycle may be depicted as four
distinct, identifiable phases: G1, S, G2, and M. M may be divided into four subphases: prophase, metaphase, anaphase,
and telophase. (From Bushong SC: Radiologic science for tech-
nologists: physics, biology and protection, ed 11, St. Louis,
2017, Elsevier.)
Centromere
Fig. 6.18 A single-strand chromosome gets duplicated during
S phase, and the duplicates are joined together in an X-shape
configuration to form a new chromosome. Each crossed arm of
this new chromosome is called a sister chromatid.
CHAPTER 6 Overview of Cell Biology
Metaphase
Prophase
G
2
(before S phase)
Anaphase
M
S
I
n
t
e
e
s
r
p
a
h
(after S phase)
Telophase
G
1
Sister
chromatid
Short arm
Centromere
Long arm
Sister
chromatid
equatorial plate (see Fig. 6.16). During metaphase, cell
division can be stopped, and visible chromosomes can
be examined under a microscope. Chromosome damage caused by radiation can then be evaluated.
Anaphase. Anaphase begins with the breakdown of
a protein called securing, which maintains chromosome
stability by inhibiting the action of a protein called
separase, whose primary function is to break down the
protein complex cohesin. Cohesin proteins hold sister
chromatids together after DNA replication by maintaining the integrity of the centromeres attached to the
microtubules forming the mitotic spindle. In anaphase,
dissolution of cohesin by active separase proteins leads
to the separation of sister chromatids. With the removal
of active cohesion, the centromeres are severed and the
sister chromatids move apart and are subsequently
pulled toward opposite poles of the spindle. During this
progression, the chromatids acquire a shape that is
similar to a V placed on its side (see Fig. 6.16). This
process causes the cell to stretch or elongate into an oval
shape. The cell is now ready to begin the last phase of its
division process.
Telophase. During telophase, the chromatids un-
dergo changes in appearance by uncoiling and becoming long, loosely spiraled threads. Simultaneously, the
nuclear membrane forms anew, and two nuclei (one for
each new daughter cell) appear. The cytoplasm of the
parent cell then divides into two daughter cells (cytokinesis) near its equator to separately surround each new
nucleus. After this cell division is complete, each daughter cell has a whole cell membrane and contains exactly
the same amount of genetic material (46 chromosomes)
as the parent cell.
Meiosis
Meiosis is a special type of cell division that reduces
the number of chromosomes in each daughter cell
to half the number of chromosomes in the parent cell
(Fig. 6.19). Box 6.8 provides terms associated with
the female reproductive cell. Male and female germ
cells, or sperm and ova, of sexually mature individuals
each begin meiosis with 46 chromosomes. However,
before the male and female germ cells unite to produce
a new organism, the number of chromosomes in each
must be reduced by one half to ensure that the daughter
cells (called zygotes) formed when they unite will contain only the standard number of 46 chromosomes.
Hence meiosis is actually a process of reduction division
(Fig. 6.20).
Meiosis begins with a doubling of the amount of
genetic material. This doubling of DNA is called replica-
tion and occurs during interphase. As a result of DNA
replication, each one-chromatid chromosome duplicates, thus forming a two-chromatid chromosome. This