Chapter 10
The Structure and Function
of DNA
Laura Coronado
Bio 10
Chapter 10
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.
– Vaccines against the flu are the
best way to protect public
health.
– Because flu viruses mutate
quickly, new vaccines must be
created every year.
Laura Coronado
Bio 10
Chapter 10
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
– DNA and RNA are nucleic acids.
• They consist of chemical units called nucleotides.
• The nucleotides are joined by a sugar-phosphate
backbone.
Laura Coronado
Bio 10
Chapter 10
Phosphate group
Nitrogenous base
Nitrogenous base
(can be A, G, C, or T)
Sugar
Nucleotide
Thymine (T)
DNA
double helix
Phosphate
group
Sugar
(deoxyribose)
DNA nucleotide
Polynucleotide
Sugar-phosphate
backbone
Laura Coronado
Bio 10
Chapter 10
Figure 10.1
Discovery of the Double Helix
• James Watson and Francis
Crick determined that DNA
is a double helix.
• Rosalind Franklin collected
the X-ray crystallography
data.
• Watson and Crick used Xray crystallography data to
reveal the basic shape of
DNA.
Laura Coronado
Bio 10
Chapter 10
X-ray image of DNA
Rosalind Franklin
Laura Coronado
Bio 10
Chapter 10
Figure 10.3b
DNA Structure
– 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.
– 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.
– DNA bases pair in a complementary fashion:
• Adenine (A) pairs with thymine (T)
• Cytosine (C) pairs with guanine (G)
Laura Coronado
Bio 10
Chapter 10
Twist
Laura Coronado
Bio 10
Chapter 10
Figure 10.4
Hydrogen bond
(a) Ribbon model
(c) Computer
model
(b) Atomic model
Laura Coronado
Bio 10
Chapter 10
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.
– DNA replication in eukaryotes:
• Begins at specific sites on a double helix
• Proceeds in both directions
Laura Coronado
Bio 10
Chapter 10
Parental (old)
DNA molecule
Daughter
(new) strand
Daughter
DNA molecules
(double helices)
Laura Coronado
Bio 10
Chapter 10
Figure 10.6
DNA Polymerases
– Are enzymes
– Make the covalent bonds between the
nucleotides of a new DNA strand
– Are involved in repairing damaged DNA
• DNA can be damaged by ultraviolet light.
Laura Coronado
Bio 10
Chapter 10
Origin of
replication
Origin of
replication
Parental strands
Origin of
replication
Parental strand
Daughter strand
Bubble
Two daughter DNA molecules
Laura Coronado
Bio 10
Chapter 10
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.
– 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.
– 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
– Transcription and translation are how genes control: The
structures & the activities of cells
Laura Coronado
Bio 10
Chapter 10
Nucleus
DNA
TRANSCRIPTION
RNA
TRANSLATION
Protein
Cytoplasm
Laura Coronado
Bio 10
Chapter 10
Figure 10.8-3
Overview - The Big Picture
– The function of a gene is to dictate the production
of a polypeptide.
• 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.
– When DNA is transcribed, the result is an RNA
molecule.
– RNA is then translated into a sequence of amino
acids in a polypeptide.
– A protein may consist of two or more different
polypeptides.
Laura Coronado
Bio 10
Chapter 10
– The RNA message is translated into a polypeptide by
using a codon is a triplet of bases, which codes for
one amino acid.
– The genetic code is:
• The set of rules relating nucleotide sequence to amino
acid sequence
• Shared by all organisms
– There are 64 codons:
• 61 code for amino acids
• 3 are stop codons, indicating the end of a polypeptide
Laura Coronado
Bio 10
Chapter 10
Gene 1
DNA molecule
Gene 2
Gene 3
DNA strand
TRANSCRIPTION
RNA
TRANSLATION
Codon
Polypeptide
Amino acid
Laura Coronado
Bio 10
Chapter 10
Figure 10.10
Second base of RNA codon
First base of RNA codon
Leucine
(Leu)
Leucine
(Leu)
Isoleucine
(Ile)
Tyrosine
(Tyr)
Serine
(Ser)
Stop
Stop
Histidine
(His)
Proline
(Pro)
Glutamine
(Gln)
Threonine
(Thr)
Met or start
Valine
(Val)
Laura Coronado
Tryptophan (Trp)
Arginine
(Arg)
Serine
(Ser)
Lysine
(Lys)
Arginine
(Arg)
Glutamic
acid (Glu)
Bio 10
Stop
Asparagine
(Asn)
Aspartic
acid (Asp)
Alanine
(Ala)
Cysteine
(Cys)
Chapter 10
Glycine
(Gly)
Figure 10.11
Third base of RNA codon
Phenylalanine
(Phe)
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.
– 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
Laura Coronado
Bio 10
Chapter 10
RNA Elongation
– During the second phase of transcription, called
elongation:
• The RNA grows longer
• The RNA strand peels away from the DNA template
– 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
Laura Coronado
Bio 10
Chapter 10
RNA polymerase
DNA of gene
Promoter
DNA
Initiation
RNA
Elongation
Terminator DNA
Area shown in
part (a) at left
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
Laura Coronado
RNA
polymerase
(b) Transcription of a gene
Bio 10
Chapter 10
Figure 10.13
Laura Coronado
Bio 10
Chapter 10
Figure 10.12
The Processing of Eukaryotic RNA
– After transcription:
• Eukaryotic cells process RNA
• Prokaryotic cells do not
– RNA processing includes:
• Adding a cap and tail
• Removing introns
• Splicing exons together to form messenger RNA
(mRNA)
Laura Coronado
Bio 10
Chapter 10
DNA
RNA
transcript
with cap
and tail
Transcription
Addition of cap and tail
Cap
Introns removed
Tail
Exons spliced together
mRNA
Coding sequence
Nucleus
Cytoplasm
Laura Coronado
Bio 10
Chapter 10
Figure 10.14
Translation: The Players
– Translation is the conversion from the nucleic acid
language to the protein language.
– Translation requires:
•
•
•
•
•
Messenger RNA (mRNA)
ATP
Enzymes
Ribosomes
Transfer RNA (tRNA)
Laura Coronado
Bio 10
Chapter 10
Transfer RNA (tRNA)
– Transfer RNA (tRNA):
• Acts as a molecular interpreter
• Carries amino acids
• Matches amino acids with codons in mRNA
using anticodons
Laura Coronado
Bio 10
Chapter 10
Amino acid attachment site
Hydrogen bond
RNA polynucleotide
chain
Anticodon
tRNA polynucleotide
(ribbon model)
tRNA
(simplified
representation)
Laura Coronado
Bio 10
Chapter 10
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)
• A fully assembled ribosome holds tRNA and
mRNA for use in translation.
Laura Coronado
Bio 10
Chapter 10
Next amino acid
to be added to
polypeptide
tRNA binding sites
P site
mRNA
binding
site
Growing
polypeptide
A site
Large
subunit
mRNA
Small
subunit
Codons
(b) The “players” of translation
(a) A simplified diagram
of a ribosome
Laura Coronado
tRNA
Ribosome
Bio 10
Chapter 10
Figure 10.16
Translation: The Process
– Translation is divided into three phases:
• Initiation
• Elongation
• Termination
Laura Coronado
Bio 10
Chapter 10
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.
– 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.
Laura Coronado
Bio 10
Chapter 10
Cap
Start of genetic
message
End
Tail
Laura Coronado
Bio 10
Chapter 10
Figure 10.17
Met
Large
ribosomal
subunit
Initiator
tRNA
P site
A site
mRNA
Start
codon
Small ribosomal
subunit
Laura Coronado
Bio 10
Chapter 10
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.
• 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
• Step 3, translocation:
– The P site tRNA leaves the ribosome
– The tRNA carrying the polypeptide moves from the A to
the P site
Laura Coronado
Bio 10
Chapter 10
Termination
– Elongation continues until:
• The ribosome reaches a stop codon
• The completed polypeptide is freed
• The ribosome splits into its subunits
Laura Coronado
Bio 10
Chapter 10
Amino acid
Polypeptide
P site
mRNA
Anticodon
A
site
Codons
Codon recognition
ELONGATION
Stop
codon
New
peptide
bond
Peptide bond formation
mRNA
movement
Translocation
Bio 10 Chapter 10
Laura Coronado
Figure 10.19-4
Review: DNA RNA Protein
– In a cell, genetic information flows from DNA to
RNA in the nucleus and RNA to protein in the
cytoplasm.
Laura Coronado
Bio 10
Chapter 10
Transcription
RNA polymerase
Polypeptide
Nucleus
DNA
mRNA
Stop
codon
Intron
RNA processing
Cap
Tail
Intron
Termination
mRNA
Amino acid
Ribosomal
subunits
tRNA
ATP
Enzyme
Amino acid
attachment
Initiation
of translation
Laura Coronado
Bio 10
Chapter 10
Anticodon
Codon
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.
Laura Coronado
Bio 10
Chapter 10
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
– 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
Laura Coronado
Bio 10
Chapter 10
– Insertions and deletions can:
• Change the reading frame of the genetic
message
• Lead to disastrous effects
• Have beneficial effects
Laura Coronado
Bio 10
Chapter 10
Normal hemoglobin DNA
Mutant hemoglobin DNA
mRNA
mRNA
Normal hemoglobin
Sickle-cell hemoglobin
Laura Coronado
Bio 10
Chapter 10
Figure 10.21
Mutagens
– Mutations may result from:
• Errors in DNA replication
• Physical or chemical agents called mutagens
– Although mutations are often harmful, they
are the source of genetic diversity, which is
necessary for evolution by natural selection.
Laura Coronado
Bio 10
Chapter 10
mRNA and protein from a normal gene
(a) Base substitution
Deleted
(b) Nucleotide deletion
Inserted
(c) Nucleotide
insertion
Laura Coronado Bio 10
Chapter 10
Figure 10.22
VIRUSES AND OTHER NONCELLULAR
INFECTIOUS AGENTS
– Viruses exhibit some, but
not all, characteristics of
living organisms. Viruses:
DNA
• Possess genetic material in
the form of nucleic acids
• Are not cellular and
cannot reproduce on their
own.
– Bacteriophages, or phages,
are viruses that attack
bacteria.
Laura Coronado
Bio 10
Protein coat
Chapter 10
Head
Bacteriophage
(200 nm tall)
Tail
Tail
fiber
DNA
of
virus
Bacterial cell
Colorized TEM
Laura Coronado
Bio 10
Chapter 10
Figure 10.25
Bacteriophages Reproduction
– 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)
(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
Laura Coronado
Bio 10
Chapter 10
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
Phage DNA is inserted into
the bacterial chromosome.
New phage DNA and
proteins are synthesized.
Laura Coronado
Bio 10
Chapter 10
Figure 10.26-2
Plant Viruses
– Viruses that infect plants can:
• Stunt growth
• Diminish plant yields
• Spread throughout the entire plant
– Viral plant diseases:
• Have no cure
• Are prevented by producing plants that resist
viral infection, controversial
Laura Coronado
Bio 10
Chapter 10
RNA
Protein
Tobacco mosaic virus
Laura Coronado
Bio 10
Chapter 10
Figure 10.27
Animal Viruses
– Viruses that infect
animals are:
Protein spike
• Common causes of
disease
• May have RNA or DNA
genomes
RNA
– Some animal viruses
steal a bit of host cell
membrane as a
protective envelope.
Laura Coronado
Membranous
envelope
Protein coat
Bio 10
Chapter 10
Viral Reproductive Cycle
– The reproductive cycle of an enveloped RNA
virus can be broken into seven steps.
1.
2.
3.
4.
5.
6.
7.
Entry
Uncoating
RNA Synthesis
Protein Synthesis
RNA Synthesis (other strand)
Assembly
Exit
Laura Coronado
Bio 10
Chapter 10
Viral RNA (genome)
Virus
Plasma membrane
of host cell
Entry
Uncoating
Viral RNA
(genome)
mRNA
Protein spike
Protein coat
Envelope
RNA synthesis
by viral enzyme
RNA synthesis
(other strand)
Protein
synthesis
Assembly
New viral proteins
Template
New viral
genome
Exit
Laura Coronado
Bio 10
Chapter 10
Figure 10.29
Mumps virus
Protein spike
Colorized TEM
Envelope
Laura Coronado
Bio 10
Chapter 10
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.
Laura Coronado
Bio 10
Chapter 10
The Process of Science:
Do Flu Vaccines Protect the Elderly?
– 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
Laura Coronado
Bio 10
Chapter 10
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.
– The behavior of HIV nucleic acid in an infected cell
can be broken into six steps.
– AIDS (acquired immune deficiency syndrome) is:
• Caused by HIV infection and
• Treated with drugs that interfere with the reproduction
of the virus
Laura Coronado Bio 10 Chapter 10
Envelope
Surface protein
Protein
coat
RNA
(two identical
strands)
Reverse
transcriptase
Laura Coronado
Bio 10
Chapter 10
Figure 10.31
Viral RNA
Reverse
transcriptase
Cytoplasm
Nucleus
Double
stranded
DNA
Viral
RNA
and
proteins
Chromosomal
DNA
Provirus
RNA
SEM
DNA
strand
HIV (red dots) infecting
a white blood cell
Laura Coronado
Bio 10
Chapter 10
Figure 10.32
Thymine
(T)
Part of a T nucleotide
AZT
Laura Coronado
Bio 10
Chapter 10
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
– 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
Laura Coronado
Bio 10
Chapter 10
Evolution Connection: Emerging Viruses
• Emerging viruses are
viruses that have:
– Appeared suddenly or
– Have only recently
come to the attention
of science
Laura Coronado
Bio 10
Chapter 10
Figure 10.35
Evolution Connection: Emerging Viruses
– Avian flu:
• Infects birds
• Infected 18 people in 1997
• Since has spread to Europe and Africa infecting 300 people
and killing 200 of them
• If avian flu mutates to a form that can easily spread between
people, the potential for a major human outbreak is
significant.
– New viruses can arise by:
• Mutation of existing viruses
• Spread to new host species
Laura Coronado
Bio 10
Chapter 10
Laura Coronado
Bio 10
Chapter 10
Figure 10.UN7