PowerLecture:
Chapter 22
DNA, Genes, and
Biotechnology
Learning Objectives




Understand how the instructions for
producing heritable traits are encoded in
DNA.
Know the parts of a nucleotide, and know
how nucleotides are linked together to make
DNA.
Understand how DNA is replicated and what
materials are needed for replication.
Know how the structure and behavior of
DNA determine the structure and behavior
of the forms of RNA during transcription.
Learning Objectives (cont’d)



Know how the structure and behavior of the
three forms of RNA determine the primary
structure of polypeptide chains during
translation.
Know the various ways that gene activity
(replication and transcription) are turned on
(activated) and off (inactivated).
Understand what plasmids are and how
they may be used to insert new genes into
recombinant DNA molecules.
Learning Objectives (cont’d)


Know how DNA can be cleaved, spliced,
cloned, and sequenced.
Be aware of several limits and possibilities
for future research in genetic engineering.
Impacts/Issues
Ricin and
Your Ribosomes
Ricin and Your Ribosomes

Ricin could be used as a biochemical weapon.



Ricin was identified as a biochemical weapon as long
ago as the 1880s and was considered for use during
WWII by both the US and England.
It is a poisonous byproduct formed during
production of castor oil from the castor bean.
Ricin inactivates ribosomes by damaging the part
of the ribosome where amino acids are joined
together; protein synthesis stops and the person
dies because there is no antidote.
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 Given
the threat of biochemical warfare,
would you be willing to be vaccinated – or
does the threat seem too remote?


a. Yes, serious bioterror threats can be lethal in
small doses and easy to manufacture.
b. No, we must be selective in what diseases to
vaccinate against.
Section 1
DNA: A Double Helix
DNA: A Double Helix
DNA is built of four kinds of nucleotides.


Each nucleotide consists of a five-carbon
sugar (deoxyribose), a phosphate group, and
one of four bases—adenine (A), guanine (G),
thymine (T), cytosine (C).
Figure 22.1
adenine
A
base with a
double-ring
structure
thymine
T
base with a
single-ring
structure
sugar
(deoxyribose)
guanine
G
base with a
double-ring
structure
cytosine
C
base with a
single-ring
structure
Fig 22.1b, p.406
DNA: A Double Helix

Watson and Crick were the first to discover the
structure of DNA.
•
•
DNA consists of two strands of nucleotides twisted into
a double helix.
Nucleotides are joined along the molecule’s length by
covalent bonds and run in opposite directions; the two
strands are held together with weaker hydrogen
bonds.
Figure 22.2
The pattern of base
pairing (A only with T
and G only with C) is
consistent with the
known composition
of DNA (A=T and
G=C
Fig 22.2, p.407
DNA: A Double Helix
Chemical “rules” determine which
nucleotide bases in DNA can pair up.




Base pairs are formed by the hydrogen
bonding of A with T, and G with C.
In a DNA molecule, the amount of adenine
always equals thymine, and G = C.
The base pairs can occur in any sequence
along the length of the DNA molecule.
DNA: A Double Helix
A gene is a sequence of nucleotides.



A gene is a sequence of nucleotides in a DNA
molecule.
The nucleotide sequence of each gene codes
for a given polypeptide chain.
Figure 22.3
Section 2
Passing on
Genetic Instructions
Passing on Genetic Instructions
How is a DNA molecule duplicated?


DNA replication is the process that duplicates
DNA before a cell divides.
•
•

First, the two strands of DNA unwind and expose
their bases to serve as a template.
Then, unattached nucleotides are linked by
hydrogen bonds to exposed bases according to
base pairing rules.
Replication results in DNA molecules each
consisting of one “old” strand and one “new”
strand—semiconservative replication.
A. Parent DNA
molecule, two
complimentary strands
of base-paired
nucleotides.
B. Replication begin;
the two strands unwind
and separate from
each other at specific
sites along the length
of the DNA molecule.
C. Each “old” strand
serves as a structural
pattern (a template) for
the addition of bases
according to the base
paring rule.
D. Bases positioned on
each old strand are
joined together into a
“new” strand. Each
half-old, half new DNA
molecule is just like the
parent molecule
Fig 22.4, p.408
Passing on Genetic Instructions
Mistakes and damage in DNA can be
repaired.


DNA polymerases and other enzymes are
involved in DNA repair mechanisms.
•
•
These enzymes detect and correct the sequence of
bases if it becomes altered; they do so by reading
the complementary sequence on the other strand
and restoring it.
If an error is not fixed, a mutation results.
Passing on Genetic Instructions


DNA is vulnerable to damage from chemicals
and UV light, which can form thymine dimers
that increase replication errors.
Thymine dimers can lead to the genetic
disorder xeroderma pigmentosum, which
further increases an individual’s chance of
developing lethal skin cancers.
Figure 22.5
Passing on Genetic Instructions
A mutation is a change in the sequence of
a gene’s nucleotides.


Gene mutations are small-scale changes in
the nucleotide sequence of genes.
•
Base-pair
substitutions can
result in the
substitution of one
amino acid for
another in a
protein, as in
sickle-cell anemia.
Figure 22.6a-b
Passing on Genetic Instructions
•
A deletion occurs when a base has been lost.
•
In an expansion mutation, a nucleotide sequence
is repeated many times, as in Huntington disease
and fragile X syndrome.
Figures 22.6a, c, and 22.7a
Passing on Genetic Instructions


Neurofibromatosis is caused by segments of
DNA called transposable elements (in this
case, a specific element called Alu); such
elements can move from location to location in
the chromosomes.
Mutations are only inherited if they occur in the
germ cells that form the gametes.
Figure 22.7b
Section 3
DNA into RNA—The First
Step in Making Proteins
DNA into RNA—
The First Step in Making Proteins
Genes become proteins through the
processes of transcription and translation.


RNA is involved in both processes; RNA is
single-stranded, contains the sugar ribose, and
substitutes the base uracil for the thymine of
DNA.
•
•
In transcription, molecules of RNA are produced on
the DNA templates in the nucleus.
In translation, RNA molecules are shipped from the
nucleus to the cytoplasm to be used in polypeptide
assembly.
DNA into RNA—
The First Step in Making Proteins

Genes are transcribed into three kinds of RNA:
•
•
•
Ribosomal RNA (rRNA) combines with proteins to
form ribosomes upon which polypeptides are
assembled.
Messenger RNA (mRNA) carries the protein “code”
to the ribosome.
Transfer RNA (tRNA) brings the correct amino acid
to the ribosome and pairs up with an mRNA
nucleotide code for that amino acid.
DNA into RNA—
The First Step in Making Proteins
In transcription, DNA is decoded into RNA.


Transcription differs from replication in three
ways:
•
•
•
Only one region of one DNA strand is used as a
template.
RNA polymerase is used instead of DNA
polymerase.
The result of transcription is a single-stranded RNA.
DNA
RNA
DNA
DNA
© 2007 Thomson Higher Education
base-pairing in DNA replication
base-pairing in transcription
In-text Fig, p.410
DNA into RNA—
The First Step in Making Proteins

Transcription begins when RNA polymerase
binds to a promoter region (a base sequence
at the start of a gene) and then moves along to
the end of a gene.
•
•
The result is a RNA transcript, which will have a 5
cap and a 3 tail.
The RNA is also modified: introns (noncoding
portions of the RNA) are removed, and exons (those
portions that will be translated) are stitched together
before the finished transcript leaves the nucleus.
Gene Transcription
[Step art]
Figure 22.8
DNA into RNA—
The First Step in Making Proteins
Gene transcription can be turned on or off.



Most of the cells of the human body carry the
same genes, but only certain genes are
expressed in any given cell at any given time.
Genes are turned on and off by regulatory
proteins that speed up or halt transcription.
Section 4
Reading the
Genetic Code
Reading the Genetic Code
Codons are mRNA “words” for building
proteins.




Three base triplets, a codon, specify each
amino acid to be included into a growing
polypeptide chain.
The genetic code consists of a total of 64
triplet codons: most specify amino acids, one is
a start codon (AUG) and three are stop
codons (UAA, UAG, UGA).
Most amino acids can be specified by more
than one codon.
DNA
mRNA
a
mRNA
codons
amino
acids
b
threonine
proline
glutamate
glutamate
lysine
Fig 22.9, p.412
The Genetic
Code
Figure 22.10
Reading the Genetic Code
tRNA translates the genetic code.


Each kind of tRNA has an anticodon that is
complementary to an mRNA codon; each tRNA
also carries one specific amino acid.
Figure 22.11
Reading the Genetic Code


After the mRNA arrives in the cytoplasm, a
specific anticodon on a tRNA bonds to the
codon on the mRNA by complementary basepairing, and so a correct amino acid is brought
into place.
There are fewer tRNAs than the number of
possible codons because the third position in
the codon-anticodon pairing is loose; the
wobble effect allows some tRNAs to match
multiple amino acids to the right codon.
Reading the Genetic Code
rRNAs are ribosome building blocks.



Translation occurs on the surface of ribosomes
where the tRNAs and mRNA interact.
Ribosomal subunits are synthesized from rRNA
and proteins in the nucleus, then shipped to the
cytoplasm where they are combined into
ribosomes during translation.
Figure 22.12
Section 5
Translating the Genetic
Code into Protein
Translating the Genetic Code into Protein
Translation has three stages.



In initiation, a complex forms in this sequence:
initiator tRNA + small ribosomal subunit +
mRNA (specifically, the AUG start codon) +
large ribosomal subunit.
In elongation, the mRNA passes through the
ribosome attracting a series of tRNAs that
deliver amino acids in sequence by codonanticodon matching; a peptide bond joins each
amino acid to the next in the sequence.
Elongation
intact ribosome
INITIATION
mRNA transcript
© 2007 Thomson Higher Education
Fig. 22.13a-c, p.414
binding site for mRNA
(first binding
site for tRNA)
© 2007 Thomson Higher Education
(second binding
site for tRNA)
Fig. 22.13d-f, p.414
© 2007 Thomson Higher Education
Fig. 22.13f-i, p.415
Translating the Genetic Code into Protein

With termination, a stop codon is reached that
has no corresponding tRNA; release factors
cause the polypeptide chain and the mRNA to
be released.
© 2007 Thomson Higher Education
Fig 22.13j-l, p.415
Translating the Genetic Code into Protein
Cells use newly formed proteins in various
ways.



To increase the efficiency of the translation
process, several ribosomes can be aligned on
one mRNA (polysome), allowing synthesis of
more than one polypeptide at a time.
After new polypeptide chains are complete,
they may join the pool of proteins in the
cytoplasm or may enter the ER for modification.
Transcription Different gene regions of DNA:
Transcript
processing:
mRNA
mature
mRNA
Translation
At ribosome,
a polypeptide
chain is
synthesized at
the binding sites
for mRNA and
tRNAs
rRNA
protein
subunits
ribosomal
subunits
RNAs
converge
tRNA
mature
tRNA
amino
acids,
ribosome
subunits,
and tRNAs
in the
cytoplasm
FINAL PROTEIN
For use in cell or for export
© 2007 Thomson Higher Education
Fig 22.26, p.424
Section 6
Tools for
“Engineering” Genes
Tools for “Engineering” Genes
Recombinant DNA technology
encompasses a range of techniques that
allow for the specific creation of genetic
changes in DNA.



DNA from different species can be cut, spliced
together, and inserted into bacteria, which then
multiply the recombinant DNA molecules.
Genetic engineering involves the isolation,
modification, and reinsertion of DNA back into
an organism.
Tools for “Engineering” Genes
Enzymes and plasmids from bacteria are
basic tools.



Many bacteria possess plasmids, circular DNA
molecules that carry only a few genes and
which can replicate independently of the single
“main” chromosome.
Restriction enzymes are used by bacteria to
cut apart DNA; this capability makes them
useful to researchers as tools for doing genetic
recombination in the laboratory.
Tools for “Engineering” Genes
•
•

Restriction enzymes produce DNA fragments with
staggered cuts resulting in sticky ends; some
fragments may be thousands of bases long, allowing
the study of the genome (all of the DNA in a haploid
set of chromosomes).
The sticky ends of the fragments can be spliced
together by other enzymes to create a recombinant
DNA molecule.
Using these tools, it is possible to insert foreign
DNA into bacterial plasmids, creating DNA
clones; DNA clones are sometimes called
cloning vectors.
A. A selected restriction
enzyme cuts wherever a
specific base sequence
occurs in a molecule
of chromosomal DNA
or cDNA.
C. DNA or cDNA
fragments with
sticky ends.
E. The foreign DNA, the plasmid
DNA, and modification
enzymes are mixed together.
F. A
collection of
recombinant
plasmids
containing
foreign DNA.
B. The same enzyme
cuts the same
sequence in plasmid
DNA.
D. Plasmid
DNA with
sticky ends
G. Host cells able to divide
rapidly take up recombinant
plasmids.
© 2007 Thomson Higher Education
Fig 22.14, p.416
Tools for “Engineering” Genes
The polymerase chain reaction (PCR) is a
faster way to copy DNA.


The reactions are done in test tubes, starting
with primers.
•
•
Primers are man-made, short nucleotide sequences
that will base pair with sequences of DNA that are to
be amplified.
A heat stable DNA polymerase is also needed.
Figure 22.15
Tools for
“Engineering” Genes

The steps are relatively simple:
•
•
•
Researchers mix primers, polymerase, DNA of
choice, and nucleotides.
The mixture is exposed to precise temperature cycles
in a dedicated machine.
Starting with tiny quantities of DNA, the procedure
doubles the DNA molecules in each round.
a. PCR starts with a
fragment of doublestranded DNA
b. The DNA is heated to 90º94ºC to unwind it. The single
strands will be templates.
c. Primers designed to basepair with ends of the DNA
strands will be mixed with
the DNA.
d. The mixture is cooled. The
lower temperature promotes
Base pairing between the
primers and the ends of the
DNA strands.
e. DNA polymerases recognize
the primers as start tags. They
assemble complimentary
sequences on the strands.
This doubles the number of
identical DNA fragments.
© 2007 Thomson Higher Education
Fig. 22.15, p.417
f. The mixture is heated
again. The higher
temperature make all of
the double-stranded
DNA fragments unwind.
g. The mixture is
cooled. The lower
temperature promotes
Base pairing between
more primers added to
the mixture and the
single strands
h. DNA polymerase
action again doubles
the number of identical
DNA fragments.
© 2007 Thomson Higher Education
Fig. 22.15, p.417
Section 7
“Sequencing” DNA
“Sequencing” DNA
Automated DNA sequencing can reveal
the sequence of nucleotides in DNA in a few
hours.



The machines are loaded with four standard
nucleotides (A, T, G, and C) and four modified
versions of the nucleotides, which fluoresce a
different color.
The DNA molecule to be sequenced, primer,
and polymerase are also added.
“Sequencing” DNA

A series of segments tagged with
fluorescing molecules are separated into
sets of fragments, which are analyzed by
the machine to reveal the original DNA’s
nucleotide sequence.
printout
of DNA
sequence:
T
C
C
A
T
G
G
A
C
C
A
Figure 22.16
“Sequencing” DNA

To identify a particular gene among many in
a gene library (say, inside bacteria),
researchers use a radioactive probe that will
match up with the DNA nucleotide
sequence of interest.
Section 8
Mapping the
Human Genome
Mapping the Human Genome

Results of the Human Genome Project
indicate that the human genome is
composed of roughly 2.9 billion nucleotide
bases subdivided into about 21,500 genes.
Figure 22.17
Mapping the Human Genome
Genome mapping provides basic biological
information.



Exons comprise only 1.5% of our DNA; the
remainder is non-coding DNA but should not be
labeled “junk.”
Our DNA is sprinkled with SNPs (single
nucleotide polymorphisms), each of which has
a change in one nucleotide in sequence; these
account for the slightly different versions of the
genes that make us all different.
Mapping the Human Genome
DNA chips help identify
mutations and diagnose diseases.


Each chip is a microarray of thousands of DNA
sequences stamped onto a small glass plate.
•
•

When a sample of body tissue is placed on the plate,
the reactions can pinpoint which genes are silent and
which are being expressed.
Some chips are being used to design better drug
therapies for disease.
As new genes are identified, it may be possible
to derive a complete genetic profile from a
small sample of a person’s blood.
Figure 22.18a
Mapping the Human Genome
Chromosome mapping shows where genes
are located.


Sequencing of the genome can identify where
specific genes are located on chromosomes.
•
•
We know that chromosome 21 carries genes for
early-onset Alzheimer’s, epilepsy, and amyotrophic
lateral sclerosis (ALS).
More than 60 disorders have been mapped to
chromosome 14.
P
1
12
11
Amyloidosis, cerebroarterial, Dutch-type
Alzheimer’s disease, one form
Schizophrenia, chronic, one form
Amyotrophic lateral sclerosis, one form
q
2
21
22
Down syndrome (critical region)
Epilepsy, progressive myoclonus
Hemolytic anemia due to
phosphofructokinase deficiency
Homocystinuria, B6 responsive and
B6 unresponsive
Leukemia, acute myeloid
Leukocyte adhesion deficiency
Fig 22.19, p.419
Mapping the Human Genome

But there may be a down side to all of this
progress if genetic profiling leads to
discrimination in employment or insurance
coverage.
Figure 22.18b
Section 9
Some Applications
of Biotechnology
Some Applications of Biotechnology
Researchers are exploring gene therapy.



There are 15,500 known genetic disorders in
humans.
Gene therapy attempts to replace mutated
genes with normal ones, or to insert genes that
restore normal controls over gene activity.
Genes can be inserted two ways.


Transformation involves exposing cells
cultured in the laboratory to DNA that contains
the gene of interest; some small portion of the
DNA will be taken up by the cells and integrated
into the host’s genome.
Some Applications of Biotechnology

In transfection, DNA segments are inserted
into viruses (often retroviruses) and then the
modified viruses are allowed to infect the target
host cell; infection generally leads to insertion of
the DNA into the host genome.
Normal gene
Clone normal
gene into
retrovirus vector
Infect patient’s
white blood cells
with virus
In some cells viral DNA
Inserts into chromosome
Inject cells
into patient
© 2007 Thomson Higher Education
Fig 22.20, p.420
Some Applications of Biotechnology
Results of gene therapy have been mixed.



Severe combined immune
deficiency (SCID-X1) was one of the
first successfully treated diseases
using gene therapy; however, several
of the initial children treated for the
disease went on to develop cancer.
Cystic fibrosis therapy trials have attempted to
deliver the corrective gene into the body using
a viral vector in a nasal spray; results have
been disappointing.
Figure 22.21
Some Applications of Biotechnology

One of the most successful gene therapy efforts
is the treatment of some cancers such as
malignant melanoma, leukemia, and lung
cancer.
•
•
Viruses have been used to introduce interleukin
encoding genes to tumor cells; the interleukins serve
as a “suicide tag,” encouraging destruction of the
tumor by T cells.
“Lipoplexes” composed of plasmid wrapped in lipid
have also been used to deliver markers to tumors to
stimulate T cell destruction.
Some Applications of Biotechnology
Genetic analysis also is used to read DNA
fingerprints.


Each of us has a unique set of DNA fragments
inherited from our parents in a Mendelian
pattern—a DNA fingerprint.
•
•
Fingerprints form from short repeated segments
called tandem repeats.
Tandem repeats can be separated and visualized by
gel electrophoresis.
Some Applications of Biotechnology

Variation can also be detected using
restriction fragment length polymorphisms
(RFLPs); in RFLP analysis, DNA is cut into
fragments using restriction enzymes and then
separated by electrophoresis.
Figure 22.28
Section 10
Issues for a
Biotechnological Society
Issues for a Biotechnological Society
Some important concerns brought up for
discussion in recent years include:




The possibility that transgenic bacteria or
viruses could mutate, possibly becoming new
pathogens.
Bioengineered plants could escape from test
plots and become “superweeds” resistant to
herbicidal control measures.
Crop plants with added insect resistance could
bring forth new, even more formidable, insect
pests.
Issues for a Biotechnological Society

Transgenic species, such as fish, could feed
voraciously and displace natural species.
Genetically modified plants, especially
those used for food, are particularly
controversial.


Critics allege that these
“Frankenfoods” may be
toxic, less nutritious, and
could promote antibiotic
resistance.
Figure 22.23
Issues for a Biotechnological Society

On the other hand, advocates for such foods
envision a Green Revolution where these
plants may help feed the world’s hungry people
or be used to clean up pollution in a process
called bioremediation.
Figures 22.23 and 22.24
Section 11
Engineering Bacteria,
Animals, and Plants
Engineering Bacteria, Animals, and Plants


Bacteria were the first bioengineered
organisms; today they help produce many
important human medicines such as human
growth hormone, insulin, and interferons.
Animals have been used in bioengineering
experiments, and transgenic barnyard
animals may become the sources for
pharmaceuticals: the blood clot dissolver
tPA, CFTR protein for cystic fibrosis, and
blood-clotting factor VIII.
Engineering Bacteria, Animals, and Plants

Animals have been used in bioengineering
experiments, and transgenic barnyard
animals may become the sources for
pharmaceuticals: the blood clot dissolver
tPA, CFTR protein for cystic fibrosis, and
blood-clotting factor VIII.
Figures 22.23a and 22.27
Engineering Bacteria, Animals, and Plants

Plants can be conferred with desirable traits
in the laboratory, such as resistance to
pathogens or herbicides, and then grown in
the field.
Figure 22.25b-c