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DNA, Genes, and
Biotechnology
Chapter 22
Structure of the
Hereditary Material
• Experiments in the 1950s
showed that DNA is the
hereditary material
• Scientists raced to
determine the structure of
DNA
• 1953 - Watson and Crick
proposed that DNA is a
double helix
Structure of Nucleotides
in DNA
• Each nucleotide consists of
– Deoxyribose (5-carbon sugar)
– Phosphate group
– A nitrogen-containing base
• Four bases
– Adenine, Guanine, Thymine, Cytosine
(A)
(G)
(T)
(C)
Nucleotide Bases
ADENINE
(A)
phosphate
group
GUANINE
(G)
deoxyribose
THYMINE
(T)
CYTOSINE
(C)
Watson-Crick Model
• DNA consists of two nucleotide strands
• Strands run in opposite directions
• Strands are held together by hydrogen
bonds between bases
• A binds with T, C with G
• Molecule is a double helix
Structure of
DNA
A gene is a
sequence of
nucleotides in a
DNA molecule
DNA Structure Helps
Explain How It Duplicates
• DNA is two nucleotide strands held
together by hydrogen bonds
• Hydrogen bonds between two strands
are easily broken
• Each single strand then serves as
template for new strand
DNA
Replication
• Each parent
strand remains
intact
• Every DNA
molecule is half
“old” and half
“new”
new
old
old
new
Base-Pairing
during
Replication
Each old strand
serves as the
template for
complementary
new strand
Errors in DNA Replication
• Mistakes can occur during replication
• Enzymes may detect and correct the
problem, restoring the proper DNA
sequence
• When the error is not corrected, the
result is a mutation
Gene Mutations
Base-Pair Substitutions
Insertions
Deletions
Effect of Base-Pair
Substitution
original
base triplet
in a DNA
strand
a base
substitution
within the
triplet (red)
As DNA is replicated, proofreading
enzymes detect the mistake and
make a substitution for it:
POSSIBLE OUTCOMES:
OR
One DNA molecule
carries the original,
unmutated sequence
The other DNA
molecule carries
a gene mutation
Frameshift Mutations
• Insertion
– Extra base added into gene region
• Deletion
– Base removed from gene region
• Both shift the reading frame
• Result in many wrong amino acids
Frameshift Mutation
mRNA
PARENTAL DNA
amino acid sequence
ARGININE
GLYCINE
TYROSINE
TRYPTOPHAN
ASPARAGINE
ARGININE
GLYCINE
LEUCINE
LEUCINE
GLUTAMATE
altered mRNA
BASE INSERTION
altered amino-acid sequence
Transposable Elements
• DNA segments that move
spontaneously about the genome
• When they insert into a gene
region, they usually inactivate
that gene
• Neurofibromatosis
DNA to RNA to Proteins
DNA
transcription
RNA
translation
protein
Three Classes of RNAs
• Messenger RNA
– Carries protein-building instruction
• Ribosomal RNA
– Major component of ribosomes
• Transfer RNA
– Delivers amino acids to ribosomes
Base-Pairing during
Transcription
• A new RNA strand can be put together
on a DNA region
DNA
G
C
A
T
RNA
G
C
A U
DNA
C
G
T
A
DNA
C
G
T A
base-pairing in DNA replication
base-pairing in transcription
Promoter
• A base sequence in the DNA that
signals the start of a gene
• For transcription to occur, RNA
polymerase must first bind to a
promoter
Gene Transcription
transcribed DNA
winds up again
DNA to be
transcribed unwinds
mRNA
transcript
RNA polymerase
Transcript Modification
unit of transcription in a DNA strand
3’
exon
intron
exon
transcription
intron
5’
exon
into pre-mRNA
poly-A
tail
3’
cap
5’
snipped
out
snipped
out
5’
3’
mature mRNA transcript
Gene Regulation
• Most cells of your body carry the same
genes
• Each uses only a tiny subset at any
time, and some are never turned on
• Regulatory proteins can speed up or
halt transcription
Genetic Code
• Set of 64 base triplets
• Codons
– Nucleotide bases read in
blocks of three
• 61 specify amino acids
• 3 stop translation
Code Is Redundant
• Twenty kinds of amino acids are specified by
61 codons
• Most amino acids can be specified by more
than one codon
• Six codons specify leucine
– UUA, UUG, CUU, CUC, CUA, CUG
tRNA Structure
codon in mRNA
anticodon
in tRNA
amino
acid
tRNA molecule’s
attachment site
for amino acid
OH
Ribosomes
tunnel
small ribosomal subunit large ribosomal subunit
intact ribosome
Three Stages of Translation
Initiation
Elongation
Termination
Initiation
• Initiator tRNA binds to
small ribosomal subunit
• Small subunit/tRNA
complex attaches to
mRNA and moves along
it to an AUG “start”
codon
• Large ribosomal subunit
joins complex
Binding Sites on
Large Subunit
binding site for mRNA
P (first
binding site
for tRNA)
A (second
binding site
for tRNA)
Elongation
Termination
• A stop codon in the mRNA moves onto
the ribosomal binding site
• No tRNA has a corresponding anticodon
• Proteins called release factors bind to
the ribosome
• mRNA and polypeptide are released
Making Recombinant DNA
Restriction enzymes cut DNA into fragments
5’
G
3’
C T T A A
A A T T C
G
one DNA fragment
another DNA fragment
5’
G
A A T T C
3’
3’
C
T T A A
5’
G
Fragments base-pair at sticky ends
Using Plasmids
• Plasmid is small circle of bacterial DNA
• Foreign DNA can be inserted into
plasmid
– Forms recombinant plasmids
– Plasmid is a cloning vector
– Can be used to deliver DNA into
another cell
Using Plasmids
DNA
fragments
+
enzymes
recombinant
plasmids
host cells containing
recombinant plasmids
Polymerase Chain Reaction
• Sequence to be copied is heated
• Primers are added and bind to ends of
single strands
• DNA polymerase uses free nucleotides
to create complementary strands
• Doubles number of copies of DNA
double-stranded
DNA to copy
PCR (1)
DNA heated to
90°– 94°C
primers added to
base-pair with
ends
mixture cooled;
base-pairing of
primers and ends
of DNA strands
DNA polymerases
assemble new
DNA strands
PCR (2)
mixture heated
again; makes all DNA
fragments unwind
mixture cooled; basepairing between
primers and ends of
single DNA strands
DNA polymerase
action again
doubles number of
identical DNA
fragments
DNA Sequencing:
Reaction Mixture
• Copies of DNA to be sequenced
• Primer
• DNA polymerase
• Standard nucleotides
• Modified nucleotides
Reactions Proceed
• Nucleotides are assembled to create
complementary strands
• When a modified nucleotide is included,
synthesis stops
• Result is millions of tagged copies of
varying length
TCCATGGACC
TCCATGGAC
TCCATGGA
Recording
the
Sequence
TCCATGG
TCCATG
TCCAT
TCCA
TCC
•DNA is placed on gel
•Fragments move off
TC
T
electrophoresis
gel
one of the many
fragments of
DNA migrating
through the gel
gel in size order; pass
through laser beam
•Color each fragment
fluoresces is recorded
one of the DNA fragments
passing through a laser beam
after moving through the gel
on printout
T C C A T G G A C C A
The Human Genome Project
Goal - Map the entire human genome
• Initially thought by many to be a waste of
resources
• Process accelerated when Craig Ventner
used bits of cDNAs as hooks to find genes
• Sequencing was completed ahead of
schedule in early 2001
Surprising Discoveries
• Coding regions (exons) make up only
1.5% of our DNA
• About half of remaining DNA is repeated
segments
• Around 1.4 million SNPs in the human
genome (single nucleotide
polymorphisms)
Using Human Genes
• Even with gene in hand it is difficult to
manipulate it to advantage
• Viruses usually used to insert genes
into cultured human cells but procedure
has problems
• Very difficult to get modified genes to
work where they should
DNA Fingerprints
• Unique array of DNA fragments
• Inherited from parents in Mendelian
fashion
• Even full siblings can be distinguished
from one another by this technique
Tandem Repeats
• Short regions of DNA that differ
substantially among people
• Many sites in genome where tandem
repeats occur
• Each person carries a unique
combination of repeat numbers
RFLPs
• Restriction fragment-length polymorphisms
• DNA from areas with tandem repeats is cut
with restriction enzymes
• Because of the variation in the amount of
repeated DNA, the restriction fragments
vary in size
• Variation is detected by gel electrophoresis
Biotechnology Concerns
• Mutation of transgenic bacteria or
viruses could yield new pathogens
• Bioengineered plants and animals could
alter ecological balance
• Genetic screening could lead to
discrimination
Engineered Bacteria
• Transgenic bacteria can be used to
grow medically valuable proteins
– Insulin, interferon, blood-clotting factors
– Vaccines
• Human gene is inserted into bacteria,
which are then grown in huge vats
Engineered Plants
and Animals
• Transgenic goats produce human
tissue plasminogen activator (tPA)
• Chinese hamster ovary cells produce
blood-clotting factor VIII
• Aspen plants produce less lignin and
more cellulose
Jefferson’s Genes
• It was suspected that Jefferson had at
least one child with his slave, Sally
Hemings
• DNA evidence showed that a modern
male descendant of Sally Hemings has
a Y chromosome that had been passed
down to him through the generations
from Thomas Jefferson