At a microscopic level, we are all composed of cells. Look
at yourself in a mirror -- what you see is about 10 trillion
cells divided into about 200 different types. Our muscles are
made of muscle cells, our livers of liver cells, and there are
even very specialized types of cells that make the enamel for
our teeth or the clear lenses in our eyes!
If you want to understand how your body works, you need to
understand cells. Everything from reproduction
to infections to repairing a broken bone happens down at the
cellular level. In addition, if you want to understand new
frontiers like biotechnology and genetic engineering, you need
to understand cells as well.
Anyone who reads the paper or any of the scientific
magazines (Scientific American, Discover, Popular Science) is
aware that genes are BIG news these days. Here are some of the
terms you commonly see:
and genetics are rapidly changing the face of medicine,
agriculture and even the legal system!
- Gene splicing
- Human genome
- Genetic engineering
- Recombinant DNA
- Genetic diseases
- Gene therapy
- DNA mutations
- DNA fingerprinting or DNA profiling
In this edition of HowStuffWorks,
we will go right down to the molecular level and completely
understand how cells work. We'll look at the simplest cells
possible: bacteria cells. By understanding how bacteria
work, you can understand the basic mechanisms of all of the
cells in your body. This is a fascinating topic both because
of its very personal nature and the fact that it makes these
news stories so much clearer and easier to understand. Also,
once you understand how cells work, you will be able to answer
other related questions like these:
All of these questions have obvious answers
once you understand how cells work -- so let's get started!
- What is a virus and how does it work at the molecular
- What is an antibiotic and how do antibiotics work? Why
don't antibiotics kill normal cells?
- What is a vitamin, and why do we need to take them every
- How do poisons work?
- What does it mean to be alive, at least at the cellular
Your body is made of about 10
trillion cells. The largest human cells are about the
diameter of a human hair, but most human cells are smaller --
perhaps one-tenth of the diameter of a human hair.
Run your fingers through your hair now and look at a single
strand. It is not very thick -- maybe 100 microns in diameter
(a micron is a millionth of a meter, so 100 microns is a tenth
of a millimeter). A typical human cell might be one-tenth of
the diameter of your hair (10 microns). Look down at your
little toe -- it might be 2 or 3 billion cells or so,
depending on how big you are. Imagine a whole house filled
with baby peas. If the house is your little toe, the peas are
the cells. That's a lot of cells!
Bacteria are about the simplest cells that exist today. A
bacteria is a single, self-contained, living cell. An
Escherichia coli bacteria (or E. coli
bacteria) is typical -- it is about one-hundredth the size of
a human cell (maybe a micron long and one-tenth of a micron
wide), so it is invisible without a microscope.
When you get an infection, the bacteria are swimming around
your big cells like little rowboats next to a large ship.
Bacteria are a lot simpler than human cells. A bacterium
consists of an outer wrapper called the cell membrane,
and inside the membrane is a watery fluid called the
cytoplasm. Cytoplasm might be 70-percent water. The
other 30 percent is filled with proteins called enzymes
that the cell has manufactured, along with smaller molecules
like amino acids, glucose molecules and ATP. At the center of
the cell is a ball of DNA (similar to a wadded-up ball of
string). If you were to stretch out this DNA into a single
long strand, it would be incredibly long compared to the
bacteria -- about 1000 times longer!
An E. coli bacterium has a distinctive, capsule shape. The
outer portion of the cell is the cell membrane, shown here in
orange. In E. coli, there are actually two closely-spaced
membranes protecting the cell. Inside the membrane is the
cytoplasm, made up of millions of enzymes, sugars, ATP and
other molecules floating freely in water. At the center of the
cell is its DNA. The DNA is like a wadded-up ball of string.
There is no protection for the DNA in a bacterium -- the
wadded-up ball floats in the cytoplasm roughly in the center
of the cell. Attached to the outside of the cell are long
strands called flagella, which propel the cell. Not all
bacterium have flagella, and no human cells have them besides
Human cells are much more complex than bacteria. They
contain a special nuclear membrane to protect the DNA,
additional membranes and structures like mitochondria and
Golgi bodies, and a variety of other advanced features.
However, the fundamental processes are the same in bacteria
and human cells, so we will start with bacteria.
At any given moment, all of the work
being done inside any cell is being done by enzymes. If
you understand enzymes, you understand cells. A bacterium like
E. coli has about 1,000 different types of enzymes floating
around in the cytoplasm at any given time.
ProteinsA protein is any chain
of amino acids. An amino acid is a small molecule that
acts as the building block of any protein. If you ignore
the fat, your body is about 20-percent protein by
weight. It is about 60-percent water. Most of the rest
of your body is composed of minerals (for example,
calcium in your bones).
Amino acids are called "amino acids" because they
contain an amino group (NH2) and a carboxyl group (COOH) that
is acidic. In the figure above, you can see the chemical
structure of two of the amino acids. You can see that
the top part of each one is the same. That is true of
all amino acids -- the little chain at the bottom (the H
or the CH3 in these two
amino acids) is the only thing varying from one amino
acid to the next. In some amino acids, the variable part
can be quite large. The human body is constructed of 20
different amino acids (there are perhaps 100 different
amino acids available in nature).
As far as your body is concerned there are two
different types of amino acids: essential and
non-essential. Non-essential amino acids are amino acids
that your body can create out of other chemicals found
in your body. Essential amino acids cannot be created,
and therefore the only way to get them is through food.
Here are the different amino acids:
Protein in our diets comes
from both animal and vegetable sources. Most animal
sources (meat, milk, eggs) provide what's called
"complete protein", meaning that they contain all of the
essential amino acids. Vegetable sources usually are low
on or missing certain essential amino acids. For
example, rice is low in isoleucine and lysine. However,
different vegetable sources are deficient in different
amino acids, and so by combining different foods you can
get all of the essential amino acids throughout the
course of the day. Some vegetable sources contain quite
a bit of protein. Nuts, beans and soybeans are all high
in protein. By combining them, you can get complete
coverage of all essential amino acids.
- Alanine (synthesized from pyruvic acid)
- Arginine (synthesized from glutamic acid)
- Asparagine (synthesized from aspartic acid)
- Aspartic acid (synthesized from oxaloacetic
- Cysteine (synthesized from homocysteine, which
comes from methionine)
- Glutamic acid (synthesized from oxoglutaric
- Glutamine (synthesized from glutamic acid)
- Glycine (synthesized from serine and threonine)
- Proline (synthesized from glutamic acid)
- Serine (synthesized from glucose)
- Tryosine (synthesized from phenylalanine)
The digestive system breaks all proteins down into
their amino acids so that they can enter the bloodstream.
Cells then use the amino acids as building blocks to
build enzymes and structural proteins.
See How Food
Works for additional information.
Enzymes have extremely interesting properties that make
them little chemical-reaction machines. The purpose of an
enzyme in a cell is to allow the cell to carry out chemical
reactions very quickly. These reactions allow the cell to
build things or take things apart as needed. This is how a
cell grows and reproduces. At the most basic level, a cell is
really a little bag full of chemical reactions that are made
possible by enzymes!
Enzymes are made from amino acids, and they are
proteins. When an enzyme is formed, it is made by stringing
together between 100 and 1,000 amino acids in a very specific
and unique order. The chain of amino acids then folds into a
unique shape. That shape allows the enzyme to carry out
specific chemical reactions -- an enzyme acts as a very
efficient catalyst for a specific chemical reaction. The
enzyme speeds that reaction up tremendously.
For example, the sugar maltose is made from two glucose
molecules bonded together. The enzyme maltase is shaped
in such a way that it can break the bond and free the two
glucose pieces. The only thing maltase can do is break maltose
molecules, but it can do that very rapidly and efficiently.
Other types of enzymes can put atoms and molecules together.
Breaking molecules apart and putting molecules together is
what enzymes do, and there is a specific enzyme for each
chemical reaction needed to make the cell work properly.
The chemical structure of
Maltose is made of two glucose molecules
bonded together (1). The maltase enzyme is a protein
that is perfectly shaped to accept a maltose molecule
and break the bond (2). The two glucose molecules are
released (3). A single maltase enzyme can break in
excess of 1,000 maltose bonds per second, and will only
You can see in the diagram above the basic action of an
enzyme. A maltose molecule floats near and is captured at a
specific site on the maltase enzyme. The active site on
the enzyme breaks the bond, and then the two glucose molecules
You may have heard of people who are lactose
intolerant, or you may suffer from this problem yourself.
The problem arises because the sugar in milk -- lactose --
does not get broken into its glucose components. Therefore, it
cannot be digested. The intestinal cells of lactose-intolerant
people do not produce lactase, the enzyme needed to
break down lactose. This problem shows how the lack of just
one enzyme in the human body can lead to problems. A person
who is lactose intolerant can swallow a drop of lactase prior
to drinking milk and the problem is solved. Many enzyme
deficiencies are not nearly so easy to fix.
Inside a bacterium there are about 1,000 types of enzymes
(lactase being one of them). All of the enzymes float freely
in the cytoplasm waiting for the chemical they recognize to
float by. There are hundreds or millions of copies of each
different type of enzyme, depending on how important a
reaction is to a cell and how often the reaction is needed.
These enzymes do everything from breaking glucose down for
energy to building cell walls, constructing new enzymes and
allowing the cell to reproduce. Enzymes do all of the work
Enzymes at Work
There are all sorts of
enzymes at work inside of bacteria and human cells, and many
of them are incredibly interesting! Cells use enzymes
internally to grow, reproduce and create energy, and they
often excrete enzymes outside their cell walls as well. For
example, E. coli bacteria excrete enzymes to help break down
food molecules so they can pass through the cell wall into the
cell. Some of the enzymes you may have heard of include:
Bacteria excrete these enzymes outside their cell
walls. Molecules in the environment are broken down into
pieces (proteins into amino acids, starches into simple
sugars, etc.) so they are small enough to pass through the
cell's wall into the cytoplasm. This is how an E. coli eats!
- Proteases and peptidases - A protease is
any enzyme that can break down a long protein into smaller
chains called peptides (a peptide is simply a short amino
acid chain). Peptidases break peptides down into individual
amino acids. Proteases and peptidases are often found in
laundry detergents -- they help remove things like blood
stains from cloth by breaking down the proteins. Some
proteases are extremely specialized, while others break down
just about any chain of amino acids. (You may have heard of
protease inhibitors used in drugs that fight the AIDS
virus. The AIDS virus uses very specialized proteases during
part of its reproductive cycle, and protease inhibitors try
to block them to shut down the reproduction of the virus.)
- Amylases - Amylases break down starch chains into
smaller sugar molecules. Your saliva contains amylase and so
does your small intestine. Maltase, lactase, sucrase
(described in the previous section) finish breaking the
simple sugars down into individual glucose molecules.
- Lipases - Lipases break down fats.
- Cellulases - Cellulases break cellulose molecules
down into simpler sugars. Bacteria in the guts of cows and
termites excrete cellulases, and this is how cows and
termites are able to eat things like grass and wood.
Inside a cell, hundreds of highly specialized enzymes carry
out extremely specific tasks that the cell needs to live its
life. Some of the more amazing enzymes found inside cells
A cell really is
nothing but a set of chemical reactions, and enzymes make
those reactions happen properly!
- Energy enzymes - A set of 10 enzymes allows a
cell to perform glycosis. Another eight enzymes
control the citric-acid cycle (also known as the
Krebs cycle). These two processes together allow a cell to
turn glucose and oxygen into adenosine triphosphate, or ATP.
In an oxygen-consuming cell like E. coli or a human cell,
one glucose molecule forms 36 ATP molecules (in something
like a yeast cell, which lives its life without oxygen, only
glycosis occurs and it produces only two ATP molecules per
glucose molecule). ATP is a fuel molecule that is able to
power enzymes by performing "uphill" chemical reactions.
- Restriction enzymes - Many bacteria are able to
produce restriction enzymes, which recognize very specific
patterns in DNA chains and break the DNA at those patterns.
When a virus injects its DNA into a bacterium, the
restriction enzyme recognizes the viral DNA and cuts it,
effectively destroying the virus before it can reproduce.
- DNA-manipulation enzymes - There are specialized
enzymes that move along DNA strands and repair them. There
are other enzymes that can untwist DNA strands to reproduce
them (DNA polymerase). Still others can find small patterns
on DNA and attach to them, blocking access to that section
of DNA (DNA-binding proteins).
- Enzyme-production enzymes - All of these enzymes
have to come from somewhere, so there are enzymes that
produce the cell's enzymes! Ribonucleic acid (RNA), in three
different forms (messenger RNA, transfer RNA and ribosomal
RNA), is a big part of the process.
Making Enzymes: DNA
As long as a cell's
membrane is intact and it is making all of the enzymes it
needs to function properly, the cell is alive. The
enzymes it needs to function properly allow the cell to create
energy from glucose, construct the pieces that make up its
cell wall, reproduce and, of course, produce new enzymes.
So where do all of these enzymes come from? And how does
the cell produce them when it needs them? If a cell is just a
collection of enzymes causing chemical reactions that make the
cell do what it does, then how can a set of chemical reactions
create the enzymes it needs, and how can the cell reproduce?
Where does the miracle of life come from?
The answer to these questions lies in the DNA, or
deoxyribonucleic acid. You have certainly heard of DNA,
chromosomes and genes. DNA guides the cell in
its production of new enzymes.
The DNA in a cell is really just a pattern made up of four
different parts, called nucleotides or bases.
Imagine a set of blocks that has only four different shapes,
or an alphabet that has only four different letters. DNA is a
long string of blocks or letters. In an E. coli cell, the DNA
pattern is about 4 million blocks long. If you were to stretch
out this single stand of DNA, it would be 1.36 mm long --
pretty long considering the bacteria itself is 1,000 times
smaller. In bacteria, the DNA strand is like a wadded-up ball
of string. Imagine taking 1,000 feet (300 meters) of
incredibly thin thread and wadding it up -- you could easily
hold it in your hand. [A human's DNA is about 3 billion blocks
long, or almost 1,000 times longer than an E. coli's. Human
DNA is so long that the wadded-up approach does not work.
Instead, human DNA is tightly wrapped into 23 structures
called chromosomes to pack it more tightly and fit it
inside a cell.]
The amazing thing about DNA is this: DNA is nothing more
than a pattern that tells the cell how to make its proteins!
That is all that DNA does. The 4 million bases in an E. coli
cell's DNA tell the cell how to make the 1,000 or so enzymes
that an E. coli cell needs to live its life. A gene is
simply a section of DNA that acts as a template to form an
Let's look at the entire process of how DNA is turned into
an enzyme so you can understand how it works.
You have probably heard of the DNA molecule referred to as
the "double-helix". DNA is like two strings twisted together
in a long spiral. DNA is found in all cells as base
pairs made of four different nucleotides. Each base
pair is formed from two complementary nucleotides bonded
together. The four bases in DNA's alphabet are:
Adenine and thymine always bond
together as a pair, and cytosine and guanine bond together as
a pair. The pairs link together like rungs in a ladder:
Base pairs in DNA bond together to form a
ladder-like structure. Because bonding occurs at angles
between the bases, the whole structure twists into a
In an E. coli bacterium, this ladder is about 4 million
base pairs long. The two ends link together to form a ring,
and then the ring gets wadded up to fit inside the cell. The
entire ring is known as the genome, and scientists have
completely decoded it. That is, scientists know all 4 million
of the base pairs needed to form an E. coli bacterium's DNA
exactly. The human genome project is in the process of
finding all 3 billion or so of the base pairs in a typical
You may remember from a previous section that enzymes are
formed from 20 different amino acids strung together in a
specific order. Therefore the question is this: How do you get
from DNA, made up of only four nucleotides, to an enzyme
containing 20 different amino acids? There are two answers to
What this means is that every three base
pairs in the DNA chain encodes for one amino acid in an
enzyme. Three nucleotides in a row on a DNA strand is
therefore referred to as a codon. Because DNA consists
of four different bases, and because there are three bases in
a codon, and because 4 * 4 * 4 = 64, there are 64 possible
patterns for a codon. Since there are only 20 possible amino
acids, this means that there is some redundancy -- several
different codons can encode for the same amino acid. In
addition, there is a stop codon that marks the end of a
gene. So in a DNA strand, there is a set of 100 to 1,000
codons (300 to 3,000 bases) that specify the amino acids to
form a specific enzyme, and then a stop codon to mark the end
of the chain. At the beginning of the chain is a section of
bases that is called a promoter. A gene, therefore,
consists of a promoter, a set of codons for the amino acids in
a specific enzyme, and a stop codon. That is all that a gene
- An extremely complex and amazing enzyme called a
ribosome reads messenger RNA, produced from the DNA,
and converts it into amino-acid chains.
- To pick the right amino acids, a ribosome takes the
nucleotides in sets of three to encode for the 20 amino
A gene consists of a promoter, the codons for
an enzyme and a stop codon. Two genes are shown above.
The long strand of DNA in an E. coli bacterium encodes
about 4,000 genes, and at any time those genes specify
about 1,000 enzymes in the cytoplasm of an E. coli cell.
Many of the genes are
To create an enzyme, the cell must first transcribe
the gene in the DNA into messenger RNA. The
transcription is performed by an enzyme called RNA
polymerase. RNA polymerase binds to the DNA strand at the
promoter, unlinks the two strands of DNA and then makes a
complementary copy of one of the DNA strands into an RNA
strand. RNA, or ribonucleic acid, is very similar to
DNA except that it is happy to live in a single-stranded state
(as opposed to DNA's desire to form complementary
double-stranded helixes). So the job of RNA polymerase is to
make a copy of the gene in DNA into a single strand of
messenger RNA (mRNA).
The strand of messenger RNA then floats over to a
ribosome, possibly the most amazing enzyme in nature. A
ribosome looks at the first codon in a messenger RNA strand,
finds the right amino acid for that codon, holds it, then
looks at the next codon, finds its correct amino acid,
stitches it to the first amino acid, then finds the third
codon, and so on. The ribosome, in other words, reads the
codons, converts them to amino acids and stitches the amino
acids together to form a long chain. When it gets to the last
codon -- the stop codon -- the ribosome releases the chain.
The long chain of amino acids is, of course, an enzyme. It
folds into its characteristic shape, floats free and begins
performing whatever reaction that enzyme performs.
Obviously, this is not a simple process. A ribosome is an
extremely complex structure of enzymes and ribosomal RNA
(rRNA) bonded together into a large molecular machine. A
ribosome is helped by ATP, which powers it as it walks along
the messenger RNA and as it stitches the amino acids together.
It is also helped by transfer RNA (tRNA), a collection
of 20 special molecules that act as carriers for the 20
different individual amino acids. As the ribosome moves down
to the next codon, the correct tRNA molecule, complete with
the correct amino acid, moves into place. The ribosome breaks
the amino acid off the tRNA and stitches it to the growing
chain of the enzyme. The ribosome then ejects the "empty" tRNA
molecule so it can go get another amino acid of the correct
- An RNA polymerase enzyme (yellow) attaches to a
DNA strand at a gene's promoter. It then walks down
the DNA and creates a copy of it into a strand of
messenger RNA (mRNA).
- The mRNA strand floats free and finds a ribosome.
- A ribosome (green) attaches to and walks down the
mRNA strand to form a chain of amino acids for the
enzyme that the gene represents.
- The amino-acid chain folds into the enzyme's
characteristic shape and starts doing its thing.
As you can see, inside every cell there are a variety of
processes keeping the cell alive:
The cytoplasm of
any cell is swimming with ribosomes, RNA polymerases, tRNA and
mRNA molecules and enzymes, all carrying out their reactions
independently of each other.
- There is an extremely long and very precise DNA molecule
that defines all of the enzymes the cell needs.
- There are RNA polymerase enzymes attaching to the DNA
strand at the starting points of different genes and copying
the DNA for the gene into an mRNA molecule.
- The mRNA molecule floats over to a ribosome, which reads
the molecule and stitches together the string of amino acids
that it encodes.
- The string of amino acids floats away from the ribosome
and folds into its characteristic shape so it can start
catalyzing its specific reaction.
As long as the enzymes in a cell are active and all of the
necessary enzymes are available, the cell is alive. An
interesting side note: If you take a bunch of yeast cells and
mistreat them (for example, place them in a blender) to
release the enzymes, the resulting soup will still do the
sorts of things that living yeast cells do (for example,
produce carbon dioxide and alcohol from sugar) for some period
of time. However, since the cells are no longer intact and
therefore are not alive, no new enzymes are produced.
Eventually, as the existing enzymes wear out, the soup stops
reacting. At this point, the cells and the soup have "died."
The hallmark of all living
things is the ability to reproduce.
A bacterium reproduction is simply another enzymatic behavior.
An enzyme called DNA polymerase, along with several
other enzymes that work alongside it, walks down the DNA
strand and replicates it. In other words, DNA polymerase
splits the double helix and creates a new double helix along
each of the two strands. Once it reaches the end of the DNA
loop, there are two separate copies of the loop floating in
the E. coli cell. The cell then pinches its cell wall in the
middle, divides the two DNA loops between the two sides and
splits itself in half.
Under the proper conditions, an E. coli cell can split like
this every 20 or 30 minutes! The enzymatic process of growing
the cell, replicating the DNA loop and splitting happens very
For more information, see How
Human Reproduction Works.
Poisons and Antibiotics
You can now see that
the life of a cell is dependent on a rich soup of enzymes that
float in the cell's cytoplasm. Many different poisons work by
disrupting the balance of the soup in one way or another.
For example, diphtheria toxin works by gumming up the
action of a cell's ribosomes, making it impossible for the
ribosome to walk along the mRNA strand. The toxin in a
death-cap mushroom, on the other hand, gums up the action of
RNA polymerase and halts the transcription of DNA. In both
cases, the production of new enzymes shuts down and the cells
affected by the toxin can no longer grow or reproduce.
An antibiotic is a poison that works to destroy
bacterial cells while leaving human cells unharmed. All
antibiotics take advantage of the fact that there are many
differences between the enzymes inside a human cell and the
enzymes inside a bacterium. If a toxin is found, for example,
that affects an E. coli ribosome but leaves human ribosomes
unharmed, then it may be an effective antibiotic. Streptomycin
is an example of an antibiotic that works in this way.
Penicillin was one of the first antibiotics. It gums up a
bacterium's ability to build cell walls. Since bacterial cell
walls and human cell walls are very different, penicillin has
a big effect on certain species of bacteria but no effect on
human cells. The sulfa drugs work by disabling an enzyme that
manages the creation of nucleotides in bacteria but not in
humans. Without nucleotides, the bacteria cannot reproduce.
You can see that the search for new antibiotics occurs down
at the enzyme level, hunting for differences between the
enzymes in human and bacterial cells that can be exploited to
kill bacteria without affecting human cells.
The unfortunate problem with any antibiotic is that it
becomes ineffective over time. Bacteria reproduce so quickly
that the probability for mutations is high. In your body,
there may be millions of bacteria that the antibiotic kills.
But if just one of them has a mutation that makes it immune to
the antibiotic, that one cell can reproduce quickly and then
spread to other people. Most bacterial diseases have become
immune to some or all of the antibiotics used against them
through this process.
Viruses are absolutely amazing.
Although they are not themselves alive, a virus can reproduce
by hijacking the machinery of a living cell. The article How
Viruses Work describes viruses in detail -- below is a
A virus particle consists of a viral jacket wrapped around
a strand of DNA or RNA. The jacket and its short strand of DNA
can be extremely small -- a thousand times smaller than a
bacterium. The jacket normally is studded with chemical
"feelers" that can bond to the outside of a cell. Once docked,
the viral DNA (or RNA, depending on the virus) is injected
into the cell, leaving the jacket on the outside of the cell.
In the simplest virus, the DNA or RNA strand is now
floating freely inside a cell. RNA polymerase transcribes the
DNA strand, and ribosomes create the enzymes that the viral
DNA specifies. The enzymes that the viral DNA creates are able
to create new viral jackets and other components of the virus.
In simple viruses, the jackets then self-assemble around
replicated DNA strands. Eventually the cell is so full of new
viral particles that the cell bursts, freeing the particles to
attack new cells. Using this system, the speed at which a
virus can reproduce and infect other cells is amazing.
In most cases, the immune
system produces antibodies, which are proteins that bind
to the viral particles and prevent them from attaching to new
cells. The immune system can also detect infected cells by
discovering cells decorated with viral jackets, and can kill
Antibiotics have no effect on a virus because a virus is
not alive. There is nothing to kill! Immunizations work by
pre-infecting the body so it knows how to produce the right
antibodies as soon as the virus starts reproducing.
See How the
Immune System Works for further details.
Many genetic diseases occur
because a person is missing the gene for a single enzyme. Here
are some of the more common problems caused by missing genes:
Other genetic diseases include
Tay-Sachs disease (damage to the gene for the enzyme
hexosaminidase A leads to an accumulation of a chemical in the
brain that destroys it), sickle cell anemia (improper coding
of the gene that produces hemoglobin), hemophilia (lack of a
gene for a blood-clotting factor) and muscular dystrophy
(caused by a defective gene on the X chromosome). There are
something like 60,000 genes in the human genome, and over
5,000 of them, if damaged or missing, are known to lead to
genetic diseases. It is amazing that damage to just one enzyme
can lead, in many cases, to life-threatening or disfiguring
- Lactose intolerance - The inability to digest
lactose (the sugar in milk) is caused by a missing lactase
gene. Without this gene, no lactase is produced by
- Albinism - In albinos, the gene for the enzyme
tyrosinase is missing. This enzyme is necessary for the
production of melanin, the pigment that leads to sun
tans, hair color and eye color. Without tyrosinase,
there is no melanin.
- Cystic fibrosis - In cystic fibrosis, the gene
that manufactures the protein called cystic fibrosis
transmembrane conductance regulator is damaged. According to
The defect (or mutation) found in the gene on
chromosome 7 of persons with cystic fibrosis causes the
production of a protein that lacks the amino acid
phenylalanine. This flawed protein somehow distorts the
movement of salt and water across the membranes that line
the lungs and gut, resulting in dehydration of the mucus
that normally coats these surfaces. The thick, sticky
mucus accumulates in the lungs, plugging the bronchi and
making breathing difficult. This results in chronic
respiratory infections, often with Staphylococcus aureus
or Pseudomonas aeruginosa. Chronic cough, recurrent
pneumonia, and the progressive loss of lung function are
the major manifestations of lung disease, which is the
most common cause of death of persons with cystic
So what is
biotechnology and genetic engineering? There are
three major developments that act as the signature of biotech,
with many more surprises coming down the road:
some of the techniques used in biotechnology, lets look at how
bacteria have been modified to produce human insulin.
- Bacterial production of substances like human
interferon, human insulin and human growth hormone. That is,
simple bacteria like E. coli are manipulated to produce
these chemicals so that they are easily harvested in vast
quantities for use in medicine. Bacteria have also been
modified to produce all sorts of other chemicals and
- Modification of plants to change their response to the
environment, disease or pesticides. For example, tomatoes
can gain fungal resistance by adding chitinases to their
genome. A chitinase breaks down chitin, which forms the cell
wall of a fungus cell. The pesticide Roundup kills all
plants, but crop plants can be modified by adding genes that
leave the plants immune to Roundup.
of people by their DNA. An individual's DNA is unique,
and various, fairly simple tests let DNA samples found at
the scene of a crime be matched with the person who left it.
This process has been greatly aided by the invention of the
polymerase chain reaction (PCR) technique for taking
a small sample of DNA and magnifying it millions of times
over in a very short period of time.
Insulin is a simple protein normally produced by the
pancreas. In people with diabetes,
the pancreas is damaged and cannot produce insulin. Since
insulin is vital to the body's processing of glucose, this is
a serious problem. Many diabetics, therefore, must inject
insulin into their bodies daily. Prior to the 1980s, insulin
for diabetics came from pigs and was very expensive.
To create insulin inexpensively, the gene that produces
human insulin was added to the genes in a normal E. coli
bacteria. Once the gene was in place, the normal cellular
machinery produced it just like any other enzyme. By culturing
large quantities of the modified bacteria and then killing and
opening them, the insulin could be extracted, purified and
used very inexpensively.
The trick, then, is in getting the new gene into the
bacteria. The easiest way is to splice the gene into a
plasmid -- a small ring of DNA that bacteria often pass
to one another in a primitive form of sex. Scientists have
developed very precise tools for cutting standard plasmids and
splicing new genes into them. A sample of bacteria is then
"infected" with the plasmid, and some of them take up the
plasmid and incorporate the new gene into their DNA. To
separate the infected from the uninfected, the plasmid also
contains a gene giving the bacteria immunity to a certain
antibiotic. By treating the sample with the antibiotic, all of
the cells that did not take up the plasmid are killed. Now a
new strain of insulin-producing E. coli bacteria can be
cultured in bulk to create insulin.
For more information on cells, bacteria, enzymes and
related topics, check out the links on the next page.
Lots More Information!
Other Great Links
E. coli and Disease
Thread of Life: The Story of Genes and Genetic
Engineering," by Susan Aldridge
Way Life Works," by Mahlon Hoagland, Bert Dodson
Coloring Book," by Robert D. Griffin, Lawrence M. Elson
Machinery of Life," by David S. Goodsell
Molecular Nature: The Body's Motors, Machines and
Messages," by David S. Goodsell
Biology of the Cell," by Bruce Alberts, Dennis Bray,
Julian Lewis, Martin Raff, James D. Watson, Keith Roberts