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Chapter 10
The Structure and Function
of DNA
PowerPoint® Lectures for
Campbell Essential Biology, Fourth Edition
– Eric Simon, Jane Reece, and Jean Dickey
Campbell Essential Biology with Physiology, Third Edition
– Eric Simon, Jane Reece, and Jean Dickey
Lectures by Chris C. Romero, updated by Edward J. Zalisko
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Biology and Society:
Tracking a Killer
• The influenza virus is one of the deadliest pathogens in the world.
• Each year in the United States, over 20,000 people die from
influenza infection.
• In the flu of 1918–1919, about 40 million people died worldwide.
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Figure 10.00a
Figure 10.00b
• Vaccines against the flu are the best way to protect public health.
• Because flu viruses mutate quickly, new vaccines must be created
every year.
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DNA: STRUCTURE AND REPLICATION
• DNA:
– Was known to be a chemical in cells by the end of the nineteenth century
– Has the capacity to store genetic information
– Can be copied and passed from generation to generation
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DNA and RNA Structure
• DNA and RNA are nucleic acids.
– They consist of chemical units called nucleotides.
– The nucleotides are joined by a sugar-phosphate backbone.
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Phosphate group
Nitrogenous base
Sugar
Nucleotide
DNA
double helix
Nitrogenous base
(can be A, G, C, or T)
Thymine (T)
Phosphate
group
Sugar
(deoxyribose)
DNA nucleotide
Polynucleotide
Sugar-phosphate
backbone
Figure 10.1
• The four nucleotides found in DNA differ in their nitrogenous
bases. These bases are:
– Thymine (T)
– Cytosine (C)
– Adenine (A)
– Guanine (G)
• RNA has uracil (U) in place of thymine.
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Watson and Crick’s Discovery of the Double
Helix
• James Watson and Francis Crick determined that DNA is a
double helix.
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James Watson (left) and Francis Crick
Figure 10.3a
• Watson and Crick used X-ray crystallography data to reveal the
basic shape of DNA.
– Rosalind Franklin collected the X-ray crystallography data.
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X-ray image of DNA
Rosalind Franklin
Figure 10.3b
• The model of DNA is like a rope ladder twisted into a spiral.
– The ropes at the sides represent the sugar-phosphate backbones.
– Each wooden rung represents a pair of bases connected by hydrogen
bonds.
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Twist
Figure 10.4
• DNA bases pair in a complementary fashion:
– Adenine (A) pairs with thymine (T)
– Cytosine (C) pairs with guanine (G)
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Hydrogen bond
(a) Ribbon model
(b) Atomic model
(c) Computer
model
Figure 10.5
DNA Replication
• When a cell reproduces, a complete copy of the DNA must pass
from one generation to the next.
• Watson and Crick’s model for DNA suggested that DNA
replicates by a template mechanism.
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Parental (old)
DNA molecule
Daughter
(new) strand
Daughter
DNA molecules
(double helices)
Figure 10.6
• DNA can be damaged by ultraviolet light.
• DNA polymerases:
– Are enzymes
– Make the covalent bonds between the nucleotides of a new DNA strand
– Are involved in repairing damaged DNA
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• DNA replication in eukaryotes:
– Begins at specific sites on a double helix
– Proceeds in both directions
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Origin of
replication
Origin of
replication
Parental strands
Origin of
replication
Parental strand
Daughter strand
Bubble
Two daughter DNA molecules
Figure 10.7
THE FLOW OF GENETIC INFORMATION
FROM DNA TO RNA TO PROTEIN
• DNA functions as the inherited directions for a cell or organism.
• How are these directions carried out?
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How an Organism’s Genotype Determines Its
Phenotype
• An organism’s genotype is its genetic makeup, the sequence of
nucleotide bases in DNA.
• The phenotype is the organism’s physical traits, which arise from
the actions of a wide variety of proteins.
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• DNA specifies the synthesis of proteins in two stages:
– Transcription, the transfer of genetic information from DNA into an
RNA molecule
– Translation, the transfer of information from RNA into a protein
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Nucleus
DNA
TRANSCRIPTION
RNA
TRANSLATION
Protein
Cytoplasm
Figure 10.8-3
• The function of a gene is to dictate the production of a
polypeptide.
• A protein may consist of two or more different polypeptides.
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From Nucleotides to Amino Acids: An Overview
• Genetic information in DNA is:
– Transcribed into RNA, then
– Translated into polypeptides
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• What is the language of nucleic acids?
– In DNA, it is the linear sequence of nucleotide bases.
– A typical gene consists of thousands of nucleotides.
– A single DNA molecule may contain thousands of genes.
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• When DNA is transcribed, the result is an RNA molecule.
• RNA is then translated into a sequence of amino acids in a
polypeptide.
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Figure 10.9
• What are the rules for translating the RNA message into a
polypeptide?
• A codon is a triplet of bases, which codes for one amino acid.
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The Genetic Code
• The genetic code is:
– The set of rules relating nucleotide sequence to amino acid sequence
– Shared by all organisms
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• Of the 64 triplets:
– 61 code for amino acids
– 3 are stop codons, indicating the end of a polypeptide
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Gene 1
DNA molecule
Gene 2
Gene 3
DNA strand
TRANSCRIPTION
RNA
TRANSLATION
Codon
Polypeptide
Amino acid
Figure 10.10
Second base of RNA codon
First base of RNA codon
Leucine
(Leu)
Leucine
(Leu)
Isoleucine
(Ile)
Serine
(Ser)
Stop
Stop
Proline
(Pro)
Threonine
(Thr)
Met or start
Valine
(Val)
Tyrosine
(Tyr)
Alanine
(Ala)
Histidine
(His)
Glutamine
(Gln)
Cysteine
(Cys)
Stop
Tryptophan (Trp)
Arginine
(Arg)
Asparagine
(Asn)
Serine
(Ser)
Lysine
(Lys)
Arginine
(Arg)
Aspartic
acid (Asp)
Glutamic
acid (Glu)
Third base of RNA codon
Phenylalanine
(Phe)
Glycine
(Gly)
Figure 10.11
Transcription: From DNA to RNA
• Transcription:
– Makes RNA from a DNA template
– Uses a process that resembles DNA replication
– Substitutes uracil (U) for thymine (T)
• RNA nucleotides are linked by RNA polymerase.
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Initiation of Transcription
• The “start transcribing” signal is a nucleotide sequence called a
promoter.
• The first phase of transcription is initiation, in which:
– RNA polymerase attaches to the promoter
– RNA synthesis begins
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RNA Elongation
• During the second phase of transcription, called elongation:
– The RNA grows longer
– The RNA strand peels away from the DNA template
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Termination of Transcription
• During the third phase of transcription, called termination:
– RNA polymerase reaches a sequence of DNA bases called a terminator
– Polymerase detaches from the RNA
– The DNA strands rejoin
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The Processing of Eukaryotic RNA
• After transcription:
– Eukaryotic cells process RNA
– Prokaryotic cells do not
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• RNA processing includes:
– Adding a cap and tail
– Removing introns
– Splicing exons together to form messenger RNA (mRNA)
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RNA polymerase
DNA of gene
Promoter
DNA
Initiation
Terminator DNA
Elongation
Area shown in
part (a) at left
RNA
RNA nucleotides
RNA polymerase
Termination
Growing RNA
Newly
made
RNA
Completed RNA
Direction of
transcription
Template
strand of DNA
(a) A close-up view of transcription
RNA
polymerase
(b) Transcription of a gene
Figure 10.13
Translation: The Players
• Translation is the conversion from the nucleic acid language to
the protein language.
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Messenger RNA (mRNA)
• Translation requires:
– mRNA
– ATP
– Enzymes
– Ribosomes
– Transfer RNA (tRNA)
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DNA
Cap
RNA
transcript
with cap
and tail
Transcription
Addition of cap and tail
Introns removed Tail
Exons spliced together
mRNA
Coding sequence
Nucleus
Cytoplasm
Figure 10.14
Transfer RNA (tRNA)
• Transfer RNA (tRNA):
– Acts as a molecular interpreter
– Carries amino acids
– Matches amino acids with codons in mRNA using anticodons
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Amino acid attachment site
Hydrogen bond
RNA polynucleotide
chain
Anticodon
tRNA polynucleotide
(ribbon model)
tRNA
(simplified
representation)
Figure 10.15
Ribosomes
• Ribosomes are organelles that:
– Coordinate the functions of mRNA and tRNA
– Are made of two protein subunits
– Contain ribosomal RNA (rRNA)
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• A fully assembled ribosome holds tRNA and mRNA for use in
translation.
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Next amino acid
to be added to
polypeptide
tRNA binding sites
P site
Growing
polypeptide
A site
mRNA
binding
site
(a) A simplified diagram
of a ribosome
Large
subunit
Small
subunit
Ribosome
tRNA
mRNA
Codons
(b) The “players” of translation
Figure 10.16
Translation: The Process
• Translation is divided into three phases:
– Initiation
– Elongation
– Termination
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Initiation
• Initiation brings together:
– mRNA
– The first amino acid, Met, with its attached tRNA
– Two subunits of the ribosome
• The mRNA molecule has a cap and tail that help it bind to the
ribosome.
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Cap
Start of genetic
message
End
Tail
Figure 10.17
• Initiation occurs in two steps:
– First, an mRNA molecule binds to a small ribosomal subunit, then an
initiator tRNA binds to the start codon.
– Second, a large ribosomal subunit binds, creating a functional ribosome.
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Met
Large
ribosomal
subunit
Initiator
tRNA
P site
A site
mRNA
Start
codon
Small ribosomal
subunit
Figure 10.18
Elongation
• Elongation occurs in three steps.
– Step 1, codon recognition:
–
the anticodon of an incoming tRNA pairs with the mRNA codon at
the A site of the ribosome.
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– Step 2, peptide bond formation:
–
The polypeptide leaves the tRNA in the P site and attaches to the
amino acid on the tRNA in the A site
–
The ribosome catalyzes the bond formation between the two amino
acids
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– Step 3, translocation:
–
The P site tRNA leaves the ribosome
–
The tRNA carrying the polypeptide moves from the A to the P site
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Amino acid
Polypeptide
P site
mRNA
Anticodon
A
site
Codons
Codon recognition
ELONGATION
Stop
codon
New
peptide
bond
Peptide bond formation
mRNA
movement
Translocation
Figure 10.19-4
Termination
• Elongation continues until:
– The ribosome reaches a stop codon
– The completed polypeptide is freed
– The ribosome splits into its subunits
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Review: DNA RNA Protein
• In a cell, genetic information flows from DNA to RNA in the
nucleus and RNA to protein in the cytoplasm.
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Transcription
RNA polymerase
Polypeptide
Nucleus
DNA
mRNA
Stop
codon
Intron
RNA processing
Cap
Tail
Termination
mRNA
Intron
Anticodon
Ribosomal Codon
subunits
Amino acid
tRNA
ATP
Enzyme
Amino acid
attachment
Initiation
of translation
Elongation
Figure 10.20-6
• As it is made, a polypeptide:
–
Coils and folds
– Assumes a three-dimensional shape, its tertiary structure
• Several polypeptides may come together, forming a protein with
quaternary structure.
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• Transcription and translation are how genes control:
– The structures
– The activities of cells
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Mutations
• A mutation is any change in the nucleotide sequence of DNA.
• Mutations can change the amino acids in a protein.
• Mutations can involve:
– Large regions of a chromosome
– Just a single nucleotide pair, as occurs in sickle cell anemia
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Types of Mutations
• Mutations within a gene can occur as a result of:
– Base substitution, the replacement of one base by another
– Nucleotide deletion, the loss of a nucleotide
– Nucleotide insertion, the addition of a nucleotide
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Normal hemoglobin DNA
Mutant hemoglobin DNA
mRNA
mRNA
Normal hemoglobin
Sickle-cell hemoglobin
Figure 10.21
• Insertions and deletions can:
– Change the reading frame of the genetic message
– Lead to disastrous effects
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Mutagens
• Mutations may result from:
– Errors in DNA replication
– Physical or chemical agents called mutagens
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• Although mutations are often harmful, they are the source of
genetic diversity, which is necessary for evolution by natural
selection.
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mRNA and protein from a normal gene
(a) Base substitution
Deleted
(b) Nucleotide deletion
Inserted
(c) Nucleotide insertion
Figure 10.22
VIRUSES AND OTHER NONCELLULAR
INFECTIOUS AGENTS
• Viruses exhibit some, but not all, characteristics of living
organisms. Viruses:
– Possess genetic material in the form of nucleic acids
– Are not cellular and cannot reproduce on their own.
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Bacteriophages
• Bacteriophages, or phages, are viruses that attack bacteria.
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Protein coat
DNA
Figure 10.24
Figure 10.24
• Phages have two reproductive cycles.
(1) In the lytic cycle:
–
Many copies of the phage are made within the bacterial cell, and
then
–
The bacterium lyses (breaks open)
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(2) In the lysogenic cycle:
–
The phage DNA inserts into the bacterial chromosome and
–
The bacterium reproduces normally, copying the phage at each cell
division
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Head
Bacteriophage
(200 nm tall)
Tail
Tail
fiber
DNA
of
virus
Bacterial cell
Colorized TEM
Figure 10.25
Plant Viruses
• Viruses that infect plants can:
– Stunt growth
– Diminish plant yields
– Spread throughout the entire plant
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Phage
Phage
attaches
to cell.
Cell lyses,
releasing
phages.
Phage DNA
Bacterial
chromosome (DNA)
Phage injects DNA
Many cell divisions
Occasionally a
prophage
may leave the bacterial
chromosome.
LYTIC CYCLE
Phages assemble
Phage DNA
circularizes.
LYSOGENIC
CYCLE
Prophage
Lysogenic bacterium
reproduces normally,
replicating the prophage
at each cell division.
OR
New phage DNA and
proteins are synthesized.
Phage DNA is inserted into
the bacterial chromosome.
Figure 10.26-2
Phage lambda
E. coli
Figure 10.26c
• Viral plant diseases:
– Have no cure
– Are best prevented by producing plants that resist viral infection
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RNA
Protein
Tobacco mosaic virus
Figure 10.27
Figure 10.27b
RNA
Protein
Tobacco mosaic virus
Figure 10.27a
Animal Viruses
• Viruses that infect animals are:
– Common causes of disease
– May have RNA or DNA genomes
• Some animal viruses steal a bit of host cell membrane as a
protective envelope.
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Membranous
envelope
Protein spike
RNA
Protein coat
Figure 10.28
• The reproductive cycle of an enveloped RNA virus can be broken
into seven steps.
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Viral RNA (genome)
Virus
Plasma membrane
of host cell
Entry
Viral RNA
(genome)
mRNA
Protein spike
Protein coat
Envelope
Uncoating
RNA synthesis
by viral enzyme
RNA synthesis
(other strand)
Protein
synthesis
Assembly
New viral proteins
Template
New viral
genome
Exit
Figure 10.29
Mumps virus
Protein spike
Colorized TEM
Envelope
Figure 10.29c
The Process of Science:
Do Flu Vaccines Protect the Elderly?
• Observation: Vaccination rates among the elderly rose from 15%
in 1980 to 65% in 1996.
• Question: Do flu vaccines decrease the mortality rate among
those elderly people who receive them?
• Hypothesis: Elderly people who were immunized would have
fewer hospital stays and deaths during the winter after
vaccination.
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• Experiment: Tens of thousands of people over the age of 65 were
followed during the ten flu seasons of the 1990s.
• Results: People who were vaccinated had a:
– 27% less chance of being hospitalized during the next flu season and
– 48% less chance of dying
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Percent reduction in severe illness
and death in vaccinated group
50
48
40
30
27
20
16
10
0
0
Winter months
(flu season)
Hospitalizations
Summer months
(non-flu season)
Deaths
Figure 10.30a
Figure 10.30b
HIV, the AIDS Virus
• HIV is a retrovirus, an RNA virus that reproduces by means of a
DNA molecule.
• Retroviruses use the enzyme reverse transcriptase to synthesize
DNA on an RNA template.
• HIV steals a bit of host cell membrane as a protective envelope.
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Envelope
Surface protein
Protein
coat
RNA
(two identical
strands)
Reverse
transcriptase
Figure 10.31
• The behavior of HIV nucleic acid in an infected cell can be
broken into six steps.
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Viral RNA
Reverse
Cytoplasm
transcriptase
Nucleus
Double
stranded
DNA
Viral
RNA
and
proteins
Chromosomal
DNA
Provirus
RNA
SEM
DNA
strand
HIV (red dots) infecting
a white blood cell
Figure 10.32
• AIDS (acquired immune deficiency syndrome) is:
– Caused by HIV infection and
– Treated with drugs that interfere with the reproduction of the virus
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SEM
HIV (red dots) infecting
a white blood cell
Figure 10.32b
Thymine
(T)
Part of a T nucleotide
AZT
Figure 10.33
Viroids and Prions
• Two classes of pathogens are smaller than viruses:
– Viroids are small circular RNA molecules that do not encode proteins
– Prions are misfolded proteins that somehow convert normal proteins to
the misfolded prion version
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• Prions are responsible for neurodegenerative diseases including:
– Mad cow disease
– Scrapie in sheep and goats
– Chronic wasting disease in deer and elk
– Creutzfeldt-Jakob disease in humans
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Figure 10.34
Evolution Connection:
Emerging Viruses
• Emerging viruses are viruses that have:
– Appeared suddenly or
– Have only recently come to the attention of science
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• Avian flu:
– Infects birds
– Infected 18 people in 1997
– Since has spread to Europe and Africa infecting 300 people and killing
200 of them
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• If avian flu mutates to a form that can easily spread between
people, the potential for a major human outbreak is significant.
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Figure 10.35
• New viruses can arise by:
– Mutation of existing viruses
– Spread to new host species
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