• DNA is the molecule that
carries all of the inherited
information in the cell.
• DNA was discovered as
“nucleic acid”—an acidic
material in the nucleus in
the later 1800’s.
• Its importance was
discovered until later. For
a long time, DNA was
considered too simple to
carry genetic information.
An Experiment
How it was learned that DNA was the
hereditary material: experiments by Griffith
and then by Avery, Macleod, and McCarthy
in the 1920’s through 1940’s.
They used bacteria called Streptococcus
pneumoniae, one cause of pneumonia.
They had 2 strains: R (formed “rough”
colonies) and S (formed “smooth” colonies
due to a polysaccharide coat).
When injected into mice, S bacteria caused
pneumonia and killed them. R bacteria
didn’t hurt the mice.
When he killed the S bacteria by heating
them, they no longer caused pneumonia.
Same for R bacteria.
Here’s the important result: if he injected live
R along with the heat-killed S bacteria, the
mice developed pneumonia and died. And,
they contained live S bacteria.
What happened: the hereditary material
from the S bacteria survived the death of the
bacteria themselves, and it “transformed”
the live R bacteria into S bacteria..
Demonstrated that the hereditary material is
separate from the property of being alive.
More Experiment
Later work showed that the “hereditary
material’ was DNA.
Crude extracts were made from the S
cells: breaking them apart. The
extracts transformed live R cells into S
cells and killed the mice, just like heatkilled S cells.
The extracts were treated with various
enzymes known to digest different
cellular components: protein, RNA,
DNA, etc.
DNAase, the enzyme that digested
DNA, stopped the transformation
effect, but none of the other enzymes
did. This demonstrated that DNA was
the active material in transformation,
the hereditary material.
Numerous other experiments, using
different organisms and procedures,
continued to show that DNA, and not
protein or some other type of
molecule, was responsible for
Structure of DNA
• DNA is a macromolecule, a
large molecule composed of
many subunits. The subunits
of DNA are nucleotides.
• Each nucleotide is composed
of 3 parts: a nitrogenous base,
a sugar (called deoxyribose),
and a phosphate group (which
is a phosphorus atom bonded
to 4 oxygen atoms).
• There are 4 kinds of
nitrogenous bases in DNA:
adenine (A), guanine (G),
cytosine (C), and thymine (T).
More DNA Structure
The nucleotides are joined into long
chains that connect the phosphate of
one nucleotide to the sugar of the next
The nitrogenous bases of 2 different
chains pair with each other, giving a
DNA molecule that has 2 sugarphosphate chains on the outside, with
bases paired in the center.
Base pairing occurs by hydrogen
bonds: partial positive and negative
charges attract each other. Hydrogen
bonds are weak, but there are lots of
them in a DNA molecule.
The 2 chains are “anti-parallel”—they
run in opposite directions. They are
twisted together into a corkscrew
shape: a double helix.
Base pairing is very specific: A pairs
with T, and G pairs with C.
DNA Replication
How DNA makes copies of itself.
Occurs during the S phase of the cell
cycle, when each chromosome starts
with 1 chromatid and ends with 2
identical chromatids. Each chromatid
is 1 molecule of DNA.
Involves an enzyme: DNA polymerase.
The DNA double helix unwinds into 2
separate strands, and a new strand is
build on each old one. Thus, each
new DNA molecule consists of 1 old
strand plus 1 new strand. This is
called “semi-conservative” replication.
DNA polymerase makes the new
strands, using the old strands as a
template, with normal base pairing: A
with T, and G with C.
The energy for this comes from the
nucleotide precursors. They all have 3
phosphates on them, like ATP, and 2 of
the phosphates are removed to make
the DNA.
Gene Expression
Each gene is a short section of a
chromosome’s DNA that codes for a
Recall that polypeptides are linear chains of
amino acids, and that proteins are
composed of one or more polypeptides,
sometimes with additional small molecules
attached. The proteins then act as enzymes
or structures to do the work of the cell.
All cells have the same genes. What makes
one type of cell different from another is
which genes are expressed or not
expressed in the cell. For example, the
genes for hemoglobin are on in red blood
cells, but off in muscle and nerve cells.
“Expressed” = making the protein product.
Genes are expressed by first making an
RNA copy of the gene (transcription) and
then using the information on the the RNA
copy to make a protein (translation).
This process: DNA transcribed into RNA,
then RNA translated into protein, is called
the “Central Dogma of Molecular Biology”.
• RNA is a nucleic acid, like DNA, with a few small differences:
RNA is single stranded, not double stranded like DNA
RNA is short, only 1 gene long, where DNA is very long and
contains many genes
RNA uses the sugar ribose instead of deoxyribose in DNA
RNA uses the base uracil (U) instead of thymine (T) in DNA.
• There are 3 main types of RNA in the cell:
1. messenger RNA: copies of the individual genes
2. ribosomal RNA: part of the ribosome, the machine that
translates messenger RNA into protein.
3. transfer RNA, which is an adapter between the messenger
RNA and the amino acids it codes for.
Transcription is the process of making an
RNA copy of a single DNA gene.
The copying is done by an enzyme: RNA
polymerase. Recall that in replication, a
DNA copy of DNA is made by the enzyme
DNA polymerase.
The bases of RNA pair with the bases of
DNA: A with T (or U in RNA), and G with C.
The RNA copy of a gene is just a
complementary copy of the DNA strand.
RNA polymerase attaches to a signal at the
beginning of the gene, the promoter. Then
RNA polymerase moves down the gene,
adding new bases to the RNA copy, until it
reaches a termination signal at the end of
the gene.
More Transcription
RNA processing
Oddly, most genes in eukaryotes are not
continuous. They are interrupted by long regions of
DNA that don’t code for protein, called “introns”.
Introns have no known function. The useful parts
of the gene, the parts that code for proteins, are
called “exons”. Some genes are more than 99%
introns, with only 1% of the gene useful: the cystic
fibrosis gene is like this.
The entire gene, introns and exons, is transcribed
into an RNA copy, but the introns need to be
removed before it can be converted to protein.
After transcription, snips out the introns, leaving
only the protein coding portion of the gene in the
Also, the cell adds a protective cap to one end, and
a tail of A’s to the other end. These both function to
protect the RNA from enzymes that would degrade
it starting on an end and moving inward.
Thus, an RNA copy of a gene is converted into
messenger RNA by doing 2 things: 1. cut out the
introns. 2. add protective bases to the ends.
Transcription of RNA processing occur in the
nucleus. After this, the messenger RNA moves to
the cytoplasm for translation.
Genetic Code
There are only 4 bases in DNA and
RNA, but there are 20 different amino
acids that go into proteins. How can
DNA code for the amino acid
sequence of a protein?
Each amino acid is coded for by a
group of 3 bases, a codon. 3 bases of
DNA or RNA = 1 codon.
Since there are 4 bases and 3
positions in each codon, there are 4 x
4 x 4 = 64 possible codons.
This is far more than is necessary, so
most amino acids use more than 1
3 of the 64 codons are used as STOP
signals; they are found at the end of
every gene and mark the end of the
One codon is used as a START signal:
it is at the start of every protein.
Transfer RNA
Transfer RNA molecules act as adapters
between the codons on messenger RNA and
the amino acids. Transfer RNA is the physical
manifestation of the genetic code.
Each transfer RNA molecule is twisted into a
knot that has 2 ends.
At one end is the “anticodon”, 3 RNA bases that
matches the 3 bases of the codon. This is the
end that attaches to messenger RNA.
At the other end is an attachment site for the
proper amino acid.
A special group of enzymes pairs up the proper
transfer RNA molecules with their
corresponding amino acids.
Transfer RNA brings the amino acids to the
ribosomes, which are RNA/protein hybrids that
move along the messenger RNA, translating the
codons into the amino acid sequence of the
• Three main players here:
messenger RNA, the
ribosome, and the transfer
RNAs with attached amino
• First step: initiation. The
messenger RNA binds to a
ribosome, and the transfer
RNA corresponding to the
START codon binds to this
complex. Ribosomes are
composed of 2 subunits (large
and small), which come
together when the messenger
RNA attaches during the
initiation process.
More Translation
Step 2 is elongation: the ribosome
moves down the messenger RNA,
adding new amino acids to the growing
polypeptide chain.
The ribosome has 2 sites for binding
transfer RNA. The first RNA with its
attached amino acid binds to the first
site, and then the transfer RNA
corresponding to the second codon
bind to the second site.
The ribosome then removes the amino
acid from the first transfer RNA and
attaches it to the second amino acid.
At this point, the first transfer RNA is
empty: no attached amino acid, and
the second transfer RNA has a chain
of 2 amino acids attached to it.
Translation, part 3
• The ribosome then
slides down the
messenger RNA 1
codon (3 bases).
• The first transfer RNA
is pushed off, and the
second transfer RNA,
with 2 attached amino
acids, moves to the
first position on the
Translation, part 4
• The elongation cycle
repeats as the
ribosome moves
down the messenger
RNA, translating it
one codon and one
amino acid at a time.
• Repeat until a STOP
codon is reached.
Translation, end
• The final step in translation is
termination. When the
ribosome reaches a STOP
codon, there is no
corresponding transfer RNA.
• Instead, a small protein called
a “release factor” attaches to
the stop codon.
• The release factor causes the
whole complex to fall apart:
messenger RNA, the two
ribosome subunits, the new
• The messenger RNA can be
translated many times, to
produce many protein copies.
Summary of translation
• The new polypeptide is now floating loose in the
cytoplasm. It might also be inserted into a
membrane, if the ribosome it was translated on
was attached to the rough endoplasmic
• Polypeptides fold spontaneously into their active
configuration, and they spontaneously join with
other polypeptides to form the final proteins.
• Sometimes other molecules are also attached to
the polypeptides: sugars, lipids, phosphates, etc.
All of these have special purposes for protein
Any change in the base sequence of a DNA molecule is a mutation. Mutation is a completely
random process: any DNA base can be mutated, whether it is in a gene or not.
Basic types:
1. base substitutions: convert one base into another, such as changing an A into a G.
2. Insertions or deletions of large pieces of DNA.
3. Combining parts of 2 different genes together.
Mutations are very common: every cell contains multiple mutations. Also, everyone is genetically
different from every other person due to the accumulation of mutations.
Genetic load: on average, each person has 3 recessive lethal mutations in all cells. We survive
because the dominant normal alleles cover up the recessive lethals. Inbreeding—mating with
close blood relatives—often causes defective children because the recessive lethals inherited
from the common ancestor become homozygous.
Many mutations occur in regions where they have no effect: between the genes, or in the introns
that are spliced out of the messenger RNA. Only mutations within genes can affect the organism.
Base substitution mutations within a gene can alter or destroy the gene’s protein product. The
protein may not function at all, or it might be less efficient, or it might have an altered pH optimum
or temperature optimum. Many of these changes have little or no effect on the organism: these
are called “neutral” mutations, because they are neither good nor bad for the organism.
The larger changes that occur with insertions, deletions, and rearrangements are usually harmful,
because they usually destroy at least one gene. However, new useful genes sometimes arise
from these rearrangements. One event in particular: attaching the control regions of one gene
onto the protein-coding part of another gene. This causes the protein to be synthesized in a new
time and place within the organism.
Mutation Causes and Rate
• Rate: for typical genes, base substitutions occur about once in every
10,000 to 1,000,000 cells. Since we have about 6 billion bases of
DNA in each cell, this implies that virtually every cell in your body
contains several mutations. Clearly, most mutations are neutral:
have no effect.
• Only mutations in the germ line cells: cells that become sperm or
eggs—are passed on to future generations. Mutations in other body
cells only cause trouble when they cause cancer or related
• Causes: The natural replication of DNA produces occasional errors.
DNA polymerase has an editing mechanism that decreases the rate,
but it still exists.
• Radiation and certain chemical compounds also cause mutations.
Chemicals that cause cancer—carcinogens—almost all work by
causing mutations.
Gene Control
• Most regulation of genes works by controlling
transcription, the process of making an RNA
copy of a single gene. Thus, a gene is “on” when
it is being transcribed, and “off” when it is not
being transcribed.
• There are many ways to regulate genes—a lot of
contemporary biology research is devoted to
discovering these mechanisms.
• I will describe a few simple mechanisms.
Lac Operon
• The common gut bacterium
Escherichia coli (E. coli) has
been studied by scientists for a
long time and much is known
about it.
• E. coli, like most organisms,
used glucose as its primary
food. However, in the absence
of glucose, it can used lactose
(milk sugar).
• Lactose is a disaccharide. E.
coli cells produce an enzyme
that breaks it down into
glucose. This enzyme is called
beta-galactosidase, and it is
made by a gene called the lac
• The lac operon also makes
other proteins that help in the
Lac Operon Basics
The operon itself consists of 3
regions that code for protein,
called Z, Y, and A. The Z gene
codes for the important enzyme,
A single messenger RNA is made
from the entire lac operon.
Transcription of the RNA starts at
the promoter at the left end of the
The operator is a region of DNA
between the promoter and the Z,
Y, and Z genes. It is an important
part of the regulatory system.
Another gene makes the “lac
repressor”, the protein involved in
gene regulation.
Lac Regulation
To conserve its resources, the E. coli
need to have the lac operon ON when
lactose is present and OFF when
lactose is absent.
To accomplish this, the repressor
protein can be in two different states:
the repressor can either bind to
lactose, or it can bind to the operator
DNA sequence.
When lactose is present, the repressor
binds to lactose and not to the
operator. This allows RNA polymerase
to transcribe the gene: it is ON.
When lactose is absent, the repressor
binds to the operator. This blocks RNA
polymerase from transcription, and the
gene is OFF.
If lactose is added, the repressor falls
off the operator and binds the lactose,
which allows transcription to start.
Positive Control
The lac operon is an example of negative
regulation: the regulatory protein (the lac repressor)
causes transcription to stop.
Positive control, where the regulatory protein
causes transcription to start, is more common.
The lac operon of E. coli also has an example of
positive control. When the cell’s glucose level is
high, it doesn’t need to use lactose at all. So, only
when the glucose level drops is it necessary to try
using lactose.
The positive control mechanism turns the lac
operon ON when the glucose level drops.
This mechanism uses a different protein, called
CAP, and the signaling molecule cyclic AMP
cAMP is generated when the glucose level is low.
cAMP then binds to CAP, and the cAMP-CAP
complex attaches itself to the promoter. This
complex attracts RNA polymerase and allows
transcription to occur at a high rate.
The negative regulation system acts on top of the
positive system: transcription is allowed whenever
glucose is low, but only occurs is lactose is present.
X Chromosome Inactivation
Many genes in multicellular organisms are
shut down permanently in different tissues.
They are never needed because they make
products used only in other types of cells.
A model for this permanent inactivation is
the X chromosome in females. Males have
only 1 X and females have 2 X’s. For any
other chromosome, this imbalance would be
lethal, but it is the normal condition of the X.
How can this occur?
In every female body cell, only 1 X is active.
The other X gets converted into a
condensed, inactive blob on the nuclear
membrane called a Barr body. This is a
simple way to telling male cells from female
cells: female cells have Barr bodies and
male cells don’t.
Only one X is active in every cell: all others
are converted to Barr bodies. People with
XXY (Klinefelter’s syndrome) are male in
appearance, but their cells have Barr
bodies. People with Turner’s syndrome
(XO, only 1 X) are female but have no Barr
bodies. People with 3 X’s (XXX) have 2
Barr bodies in each cell.
More X Inactivation
When the embryo has about 200 cells, each cell
independently inactivates all but 1 X chromosome. The
inactive X stays inactive in all cells descended from the
original cell, throughout the individual’s life.
The inactivation can lead to interesting effects. Tortoiseshell
cats are a mixture of black and orange. They are always
female (occasionally Klinefelter’s –XXY – males), because
they need 2 X’s. The gene for coat color is on the X and it
has a black allele and an orange allele. A heterozygote has
one black and one orange allele. But: only 1 X is active in
each cell, which means that either the black allele is active or
the orange allele is active, but not both. So, as the cells of
the embryo develop into patches of skin, the active allele is
expressed and you get a black and orange pattern.
Calico cats also have white spots. Their black and orange
pattern is the result of X inactivation, but the white spots are
due to another, autosomal gene. Calico cats are also always
There is a human condition called anhidrotic ectodermal
dysplasia that cause a loss of sweat glands in the skin. In
females it leads to patches with and without sweat glands. It
is lethal in males.
Hormone Signaling
Hormones are small molecules that
circulate in the blood and alter the
expression of genes in many tissues.
Hormones are either steroids (lipids
with 4 rings of atoms in a characteristic
shape) or peptides (small proteins).
Steroid hormones can enter the cell
directly through the membranes.
Once in the cell, they bind to receptor
proteins, then move to the nucleus
where they stimulate transcription.
Peptide hormones bind to receptor
proteins on the surface of the cell.
The receptor proteins then activate
other proteins within the cell, in a
cascade that ends up activating
“transcription factors” in the nucleus.
Transcription factors stimulate RNA
transcription of particular genes.
Plant Response to Light
Many plants will only flower when days are
short, while other plants require a long day.
Plants are able to determine the length of
the dark period. Even a very short –10
second– pulse of light in the middle of the
dark period prevents short day/long night
plants from flowering.
The mechanism for determining day length
uses a blue-green pigment called
“phytochrome”. Phytochrome is activated by
red light, which dominates during the day.
The active phytochrome slowly converts
back to the inactive form during the night.
The amount of active phytochrome left at the
end of the night is proportional to the length
of the night.
When present at the appropriate level,
phytochrome stimulates enzymes in the
plant cell to start or stop transcription.
Different plants have different critical levels
of active phytochrome.
Genetic Engineering
• We have been modifying living things for a long
time: domestication of various plants and
animals, hybridization of different species (such
as horse x donkey = mule), selective breeding
for useful traits.
• Recently it has become possible to directly
modify the DNA of living organisms, in the hope
of producing a more useful plant or animal.
Also, we can artificially produce natural body
proteins that can be used as medicinal drugs.
Molecular Cloning
Molecular cloning means taking a
gene, a piece of DNA, out of the
genome and growing it in bacteria.
The bacteria (usually E. coli) produce
large amounts of this particular gene.
The cloned gene can then be used for
further research, or to produce large
amounts of protein, or (sometimes) to
be inserted into cells that lack the
gene (people with genetic disease, for
The basic tools:
1. plasmid vector: small circle of
DNA that grows inside the bacteria. It
carries the gene being cloned
2. Restriction enzymes: cut the
DNA at specific spots, allowing the
isolation of specific genes.
3. DNA ligase, an enzyme that
attached pieces of DNA together.
Plasmid Vectors
Bacterial chromosomes are large DNA
circles. Bacteria have a single
Plasmids are small circles of DNA inside
bacteria that replicate independently of the
bacterial chromosome. They exist naturally,
and usually confer some useful but not
necessary trait on the bacteria: drug
resistance, for example. Each plasmid only
has a small number of genes on it.
Some plasmids can produce hundreds of
copies of themselves inside each bacterial
It is possible to insert foreign DNA into the
plasmid. This DNA becomes part of the
enlarged circle of the plasmid. It replicates
along with the plasmid.
The plasmid DNA can be manipulated in
vitro, outside the cell, then put back into the
cell through the process of transformation.
Because bacteria can be grown quickly and
easily, you can produce large quantities of
any DNA inserted into a plasmid
Restriction Enzymes
Restriction enzymes are part of the
defense system of bacteria: they
digest foreign DNA that enters the
bacterial cell.
Each species of bacteria has its own
set of restriction enzymes. Each
enzyme cuts DNA at a specific short
base sequence. For instance, EcoR1
cuts the DNA at the sequence
GAATTC, and BamH1 cuts at
GGATCC. There are hundreds of
restriction enzymes known.
Using properly chosen enzymes, the
gene you want can be cut out of the
chromosome intact, with very little
extra DNA.
Many restriction enzymes give a
staggered cut across the DNA double
helix. This produces short single
stranded regions, called “sticky ends”.
The ends are sticky because they
spontaneously pair with similar ends.
DNA Ligase
• “ligate” means to tie together.
DNA ligase is an enzyme that
attaches 2 pieces of DNA
together. It forms covalent
bonds between the sugars and
phosphates of the DNA
• Especially useful: if 2 different
DNAs were cut with the same
restriction enzyme, they will
have matching sticky ends.
DNA ligase can then combine
these 2 different DNA
molecules into a single DNA.
This hybrid DNA molecule is
called “recombinant DNA”.
The Cloning Process
• 1. Cut genomic DNA with
a restriction enzyme.
• 2. Cut plasmid vector with
the same restriction
• 3. Mix the two DNAs
together and join them
with DNA ligase.
• 4. Put the recombinant
DNA back into E. coli by
• 5. Grow lots of the E. coli
containing your gene.
Using Cloned DNA
One major use of cloned genes is to produce large amounts of their protein products
to be used as medicinal drugs.
As an example, human growth hormone is made in the pituitary gland, a very small
organ at the base of the brain. Some people do not produce enough of it, resulting in
very short stature and various health-related issues. HGH injections during childhood
help. However, pituitary glands must be harvested from human cadavers, and are
often contaminated with viruses.
Another example: insulin is needed by people with diabetes. In former times it was
isolated from pigs or sheep. However, the animal forms had a few amino acid
differences from human insulin, and sometimes caused bad immune responses.
The solution to these problems is to isolate the human genes using the techniques of
recombinant DNA, then cause these genes to express themselves, to produce their
protein products. The proteins can then be isolated from the bacteria.
The proteins are the normal human forms, despite having been made in bacteria.
They are not contaminated with human viruses, and they won’t cause an immune
Getting the genes to express is simple (in principle): RNA copies of genes are made if
they have a promoter sequence in front of them. Similarly, proteins are made if the
messenger RNA has the appropriate signals on it. So, it is merely a matter of
including the proper signal sequences on the plasmid.
But of course: complications arise.
Gene Therapy
One way to cure genetic diseases is to
insert good (non-mutant) copies of the
defective gene into the cells of the
affected person.
Big problem: how to get the gene into
all of the cells.
In mice, inject the gene directly into
the zygote. The gene incorporates
into the chromosomes at some
random location, and (usually)
Humans usually don’t know about the
presence of the disease until the baby
is born.
In some cases, blood cells can be
used. Blood cell precursors are in the
bone marrow, which can be extracted,
have recombinant genes inserted,
then put back into the body.
Gene Therapy Problems
Two big problems:
1. The recombinant genes insert
into random locations. Sometimes
they insert into oncogenes, which
causes cancer. Leukemia usually,
cancer of the blood cells.
2. There are very few blood stem
cells in the body. Stem cells divide
repeatedly and never differentiate into
the final blood cells. Genes inserted
into stem cells are permanently in the
body. Genes inserted into cells that
have already started differentiating into
blood cells will stay in the body for a
few weeks, then be lost as the blood
cells wear out and die.
The bottom line (so far): gene therapy
has not been very successful except in
a small handful of cases.
Genetically Modified Plants
It is fairly easy to insert genes into
plants, using a special plasmid vector
derived from crown gall tumors. This
vector grows in bacteria, but transfers
its DNA into the plant genome after
One use of this techniques is to insert
nutrient genes into plants. For
instance, rice is the staple food for a
large part of the world’s population. It
contains no carotene, the orange
pigment that is the precursor for the
main visual pigment retinol. People
who live on rice alone often develop
blindness because they don’t eat
enough carotene.
Golden rice was developed to solve
this problem: genes fro producing
carotene were put into rice. This
pigment gives the rice its color.
Problems: will people eat this oddlycolored food? Will it work well in
cooking? Will they accept
More Plant Genetic Engineering
In the US, the main uses of genetically modified
crop plants are herbicide resistance and insect
resistance. If your crop plants are resistant to a
herbicide that kills the weeds, you can spray more
effectively. Similarly, plants that are given genes
for insect resistance don’t need to be sprayed with
Very effective—farmers find these plants cheaper
to grow.
However, there is a lot of resistance to eating these
plants in Europe. Maybe the genes will somehow
leak out and affect people—we digest DNA down to
nucleotide subunits before taking it into our bodies,
so this shouldn’t happen. Maybe it will affect other
plants—probably does happen naturally at a slow
Another idea: “pharming” putting genes for useful
proteins like insulin into plants, and letting the plant
synthesize them in large amounts. Can also be
done with animals: proteins secreted into milk.
Nuclear Cloning
• Why not just take the nucleus
from any cell and put it into an
egg, producing a new person
genetically identical to the
original? This is the idea
behind nuclear cloning, and it
does work on occasion.
• Dolly the Sheep was the first
example: a nucleus from her
parent’s mammary gland was
extracted and put into an egg
whose own nucleus was
removed. The egg was them
implanted into the uterus of
another sheep, and Dolly was
born. She is genetically
identical to the donor sheep.
More Nuclear Cloning
• Although a clone’s nuclear DNA is identical to the donor parent, the
parent and offspring are not likely to be exactly identical. Similarly,
identical twins are not exactly identical. Events occur after the
identical zygotes form: random environmental influences, patterns of
development, etc.
• It is very difficult to get a nucleus of a cell to regress to the
“totipotent” state of an embryonic cell. A totipotent cell can turn into
any cell type. After a few cell divisions of the embryo, the cells are
restricted: they can no longer become any type. Many genes are
permanently inactivated by modifying the DNA molecule. Removing
these modifications is, at the moment, more of an art than a science.
• The result: the failure rate in cloning is very high: less than 1%
success rate. And, of the clones that are born alive, many suffer
defects that only become apparent later in life. Dolly, for instance,
died at a young age.
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