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© 2010 Pearson Education, Inc.
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
© 2010 Pearson Education, Inc.
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.
Animation: Hershey-Chase Experiment
Animation: DNA and RNA Structure
Sugar-phosphatebackbone
Phosphate group
Nitrogenous base
DNA nucleotide
Nucleotide Thymine (T)
Sugar
Polynucleotide
DNAdouble helix
Sugar(deoxyribose)
Phosphategroup
Nitrogenous base(can be A, G, C, or T)
Figure 10.1
Sugar-phosphatebackbone
Phosphate group
Nitrogenous base
DNA nucleotide
Nucleotide Thymine (T)
Sugar
Polynucleotide
Sugar(deoxyribose)
Phosphategroup
Nitrogenous base(can be A, G, C, or T)
Figure 10.1a
DNA nucleotide
Thymine (T)
Sugar(deoxyribose)
Phosphategroup
Nitrogenous base(can be A, G, C, or T)
Figure 10.1b
© 2010 Pearson Education, Inc.
• 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.
© 2010 Pearson Education, Inc.
Watson and Crick’s Discovery of the Double Helix
• James Watson and Francis Crick determined that DNA is a double helix.
James Watson (left) and Francis CrickFigure 10.3a
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• Watson and Crick used X-ray crystallography data to reveal the basic shape of DNA.
– Rosalind Franklin collected the X-ray crystallography data.
X-ray image of DNA
Rosalind Franklin
Figure 10.3b
© 2010 Pearson Education, Inc.
• 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.
TwistFigure 10.4
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• DNA bases pair in a complementary fashion:
– Adenine (A) pairs with thymine (T)
– Cytosine (C) pairs with guanine (G)
Animation: DNA Double Helix
Blast Animation: Hydrogen Bonds in DNA
Blast Animation: Structure of DNA Double Helix
(c) Computermodel
(b) Atomic model(a) Ribbon model
Hydrogen bond
Figure 10.5
© 2010 Pearson Education, Inc.
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.
Animation: DNA Replication Review
Animation: DNA Replication Overview
© 2010 Pearson Education, Inc.
• 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
© 2010 Pearson Education, Inc.
• DNA replication in eukaryotes:
– Begins at specific sites on a double helix
– Proceeds in both directions
Animation: Origins of Replication
Animation: Leading Strand
Animation: Lagging Strand
Origin ofreplication
Origin ofreplication
Origin ofreplication
Parental strands
Parental strand
Daughter strand
Two daughter DNA molecules
Bubble
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.
© 2010 Pearson Education, Inc.
© 2010 Pearson Education, Inc.
• 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
DNA
Cytoplasm
Nucleus
Figure 10.8-1
RNA
TRANSCRIPTION
DNA
Cytoplasm
Nucleus
Figure 10.8-2
TRANSLATION
Protein
RNA
TRANSCRIPTION
DNA
Cytoplasm
Nucleus
Figure 10.8-3
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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 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
TRANSLATION
Amino acid
RNA
TRANSCRIPTION
DNA strand
Polypeptide
Codon
Gene 1
Gene 3
Gene 2DNA molecule
Figure 10.10
TRANSLATION
Amino acid
RNA
TRANSCRIPTION
DNA strand
Polypeptide
Codon
Figure 10.10
© 2010 Pearson Education, Inc.
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
Blast Animation: Roles of RNA
Blast Animation: Transcription
© 2010 Pearson Education, Inc.
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)
Animation: Transcription
(b) Transcription of a gene
RNA polymerase
Completed RNA
Growing RNATermination
Elongation
Initiation Terminator DNA
Area shown in part (a) at leftRNA
Promoter DNA
RNA polymerase
DNA of gene
Figure 10.13b
© 2010 Pearson Education, Inc.
Translation: The Players
• Translation is the conversion from the nucleic acid language to the protein language.
© 2010 Pearson Education, Inc.
Messenger RNA (mRNA)
• Translation requires:
– mRNA
– ATP
– Enzymes
– Ribosomes
– Transfer RNA (tRNA)
TranscriptionAddition of cap and tail
Coding sequence
mRNA
DNA
Cytoplasm
Nucleus
Exons spliced together
Introns removed Tail
CapRNAtranscriptwith capand tail
Exon Exon ExonIntronIntron
Figure 10.14
© 2010 Pearson Education, Inc.
Transfer RNA (tRNA)
• Transfer RNA (tRNA):
– Acts as a molecular interpreter
– Carries amino acids
– Matches amino acids with codons in mRNA using anticodons
tRNA polynucleotide(ribbon model)
RNA polynucleotidechain
Anticodon
Hydrogen bond
Amino acid attachment site
tRNA (simplifiedrepresentation)
Figure 10.15
© 2010 Pearson Education, Inc.
Ribosomes
• Ribosomes are organelles that:
– Coordinate the functions of mRNA and tRNA
– Are made of two protein subunits
– Contain ribosomal RNA (rRNA)
© 2010 Pearson Education, Inc.
• A fully assembled ribosome holds tRNA and mRNA for use in translation.
Next amino acidto be added to polypeptide
Growingpolypeptide
tRNA
mRNA
tRNA binding sites
Codons
Ribosome
(b) The “players” of translation(a) A simplified diagramof a ribosome
Largesubunit
Smallsubunit
P site
mRNA binding
site
A site
Figure 10.16
tRNA binding sites
Ribosome
(a) A simplified diagramof a ribosome
Largesubunit
Smallsubunit
P site
mRNA binding
site
A site
Figure 10.16a
Next amino acidto be added to polypeptide
Growingpolypeptide
tRNA
mRNA
Codons
(b) The “players” of translationFigure 10.16b
© 2010 Pearson Education, Inc.
Translation: The Process
• Translation is divided into three phases:
– Initiation
– Elongation
– Termination
© 2010 Pearson Education, Inc.
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.
Blast Animation: Translation
Start of geneticmessage
Tail
End
Cap
Figure 10.17
© 2010 Pearson Education, Inc.
• 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.
InitiatortRNA
mRNA
Startcodon
Met
P site
Small ribosomalsubunit
A site
Large ribosomalsubunit
Figure 10.18
© 2010 Pearson Education, Inc.
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.
© 2010 Pearson Education, Inc.
– 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
© 2010 Pearson Education, Inc.
– Step 3, translocation:
– The P site tRNA leaves the ribosome
– The tRNA carrying the polypeptide moves from the A to the P site
Animation: Translation
Figure 10.19-1
mRNA
P site
Polypeptide
ELONGATION
Codon recognition
Asite
Codons
Anticodon
Amino acid
mRNA
P site
Peptide bond formation
Polypeptide
ELONGATION
Codon recognition
Asite
Codons
Anticodon
Amino acid
Figure 10.19-2
Newpeptidebond
mRNAmovement
mRNA
P site
Translocation
Peptide bond formation
Polypeptide
ELONGATION
Codon recognition
Asite
Codons
Anticodon
Amino acid
Figure 10.19-3
Newpeptidebond
Stopcodon
mRNAmovement
mRNA
P site
Translocation
Peptide bond formation
Polypeptide
ELONGATION
Codon recognition
Asite
Codons
Anticodon
Amino acid
Figure 10.19-4
© 2010 Pearson Education, Inc.
Termination
• Elongation continues until:
– The ribosome reaches a stop codon
– The completed polypeptide is freed
– The ribosome splits into its subunits
© 2010 Pearson Education, Inc.
Review: DNA RNA Protein• In a cell, genetic information flows from DNA to RNA in the
nucleus and RNA to protein in the cytoplasm.
Transcription RNA polymerase
mRNA DNA
Intron
Nucleus
Figure 10.20-1
Transcription RNA polymerase
mRNA DNA
Intron
Nucleus
mRNAIntron
TailCap
RNA processing
Figure 10.20-2
Transcription RNA polymerase
mRNA DNA
Intron
Nucleus
mRNAIntron
TailCap
RNA processing
tRNA
Amino acid
Amino acidattachment
EnzymeATP
Figure 10.20-3
Transcription RNA polymerase
mRNA DNA
Intron
Nucleus
mRNAIntron
TailCap
RNA processing
tRNA
Amino acid
Amino acidattachment
EnzymeATP Initiation
of translation
Ribosomalsubunits
Figure 10.20-4
Transcription RNA polymerase
mRNA DNA
Intron
Nucleus
mRNAIntron
TailCap
RNA processing
tRNA
Amino acid
Amino acidattachment
EnzymeATP Initiation
of translation
Ribosomalsubunits
Elongation
AnticodonCodon
Figure 10.20-5
Transcription RNA polymerase
mRNA DNA
Intron
Nucleus
mRNAIntron
TailCap
RNA processing
tRNA
Amino acid
Amino acidattachment
EnzymeATP Initiation
of translation
Ribosomalsubunits
Elongation
AnticodonCodon
Termination
Polypeptide
Stopcodon
Figure 10.20-6
© 2010 Pearson Education, Inc.
• 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.
© 2010 Pearson Education, Inc.
• Transcription and translation are how genes control:
– The structures
– The activities of cells
© 2010 Pearson Education, Inc.
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
© 2010 Pearson Education, Inc.
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
Normal hemoglobin DNA
mRNA
Normal hemoglobin
Mutant hemoglobin DNA
mRNA
Sickle-cell hemoglobin
Figure 10.21
© 2010 Pearson Education, Inc.
• Insertions and deletions can:
– Change the reading frame of the genetic message
– Lead to disastrous effects
© 2010 Pearson Education, Inc.
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.
mRNA and protein from a normal gene
(a) Base substitution
Figure 10.22a
Deleted
(b) Nucleotide deletion
mRNA and protein from a normal gene
Figure 10.22b
mRNA and protein from a normal gene
Inserted
(c) Nucleotide insertionFigure 10.22c
© 2010 Pearson Education, Inc.
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.
© 2010 Pearson Education, Inc.
Bacteriophages
• Bacteriophages, or phages, are viruses that attack bacteria.
Animation: Phage T2 Reproductive Cycle
Protein coat DNA
Figure 10.24
Figure 10.24
© 2010 Pearson Education, Inc.
• 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)
© 2010 Pearson Education, Inc.
(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
Bacterial cell
Head
Tail
Tailfiber
DNAofvirus
Bacteriophage(200 nm tall)
Colorized TEM
Figure 10.25
© 2010 Pearson Education, Inc.
Plant Viruses
• Viruses that infect plants can:
– Stunt growth
– Diminish plant yields
– Spread throughout the entire plant
Animation: Phage T4 Lytic Cycle
Animation: Phage Lambda Lysogenic and Lytic Cycles
Phage
Phage DNA
Phage injects DNA
Phageattachesto cell.
Bacterialchromosome (DNA)
Phage DNAcircularizes.
Phages assemble
OR
New phage DNA andproteins are synthesized.
LYTIC CYCLE
Cell lyses,releasingphages.
Figure 10.26-1
Phage
Phage DNA
Phage injects DNA
Phageattachesto cell.
Bacterialchromosome (DNA)
Phage DNAcircularizes.
Phages assemble
OR
New phage DNA andproteins are synthesized.
LYTIC CYCLE
Cell lyses,releasingphages.
LYSOGENIC CYCLE
Phage DNA is inserted intothe bacterial chromosome.
Many cell divisions
Prophage
Occasionally aprophagemay leave the bacterialchromosome.
Lysogenic bacteriumreproduces normally,replicating the prophageat each cell division.
Figure 10.26-2
Phage
Phage DNA
Phage injects DNA
Phage attachesto cell.
Bacterialchromosome (DNA)
Phage DNA circularizes.Phages assemble
New phage DNA andproteins are synthesized.
LYTIC CYCLE
Cell lyses,releasingphages.
Figure 10.26a
Phage DNA
Phage injects DNA
Phage attachesto cell.
Bacterialchromosome (DNA)
Phage DNAcircularizes.
LYSOGENIC CYCLE
Many cell divisions
Prophage
Occasionally a prophagemay leave the bacterialchromosome.
Lysogenic bacteriumreproduces normally,replicating the prophageat each cell division.
Phage DNA is inserted into thebacterial chromosome.
Phage
Figure 10.26b
Phage lambda
E. coli
Figure 10.26c
© 2010 Pearson Education, Inc.
• Viral plant diseases:
– Have no cure
– Are best prevented by producing plants that resist viral infection
Tobacco mosaic virus
RNA
Protein
Figure 10.27
Figure 10.27b
Tobacco mosaic virus
RNA
Protein
Figure 10.27a
© 2010 Pearson Education, Inc.
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.
Protein coat
RNA
Protein spike
Membranousenvelope
Figure 10.28
© 2010 Pearson Education, Inc.
• The reproductive cycle of an enveloped RNA virus can be broken into seven steps.
Animation: Simplified Viral Reproductive Cycle
Protein coat
mRNA
Protein spike
Plasma membraneof host cell
Envelope
Template
New viralgenomeNew viral proteins
Exit
VirusViral RNA (genome)
Viral RNA(genome)
Entry
Uncoating
Assembly
RNA synthesisby viral enzyme
RNA synthesis(other strand)
Protein synthesis
Figure 10.29
Protein coat
Protein spike
Plasma membraneof host cell
Envelope
Virus
Viral RNA (genome)
Viral RNA(genome)
Entry
Uncoating
RNA synthesisby viral enzyme
Figure 10.29a
mRNA Template
New viralgenome
Newviral proteins
Exit
Assembly
RNA synthesis(other strand)
Protein synthesis
Figure 10.29b
© 2010 Pearson Education, Inc.
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.
Envelope
Reverse
transcriptase
Surface protein
Protein
coat
RNA
(two identical
strands)
Figure 10.31
© 2010 Pearson Education, Inc.
• The behavior of HIV nucleic acid in an infected cell can be broken into six steps.
Animation: HIV Reproductive Cycle
Blast Animation: AIDS Treatment Strategies
Reversetranscriptase
HIV (red dots) infecting a white blood cell
DNAstrand Chromosomal
DNA
Cytoplasm
Nucleus
Provirus
RNA
Viral RNA
DoublestrandedDNA
ViralRNAand proteins
SE
M
Figure 10.32
Reversetranscriptase
DNAstrand Chromosomal
DNA
Cytoplasm
Nucleus
Provirus
RNA
Viral RNA
DoublestrandedDNA
ViralRNAand proteins
Figure 10.32a
© 2010 Pearson Education, Inc.
• AIDS (acquired immune deficiency syndrome) is:
– Caused by HIV infection and
– Treated with drugs that interfere with the reproduction of the virus
HIV (red dots) infecting a white blood cell
SE
M
Figure 10.32b
Part of a T nucleotide
Thymine(T)
AZT
Figure 10.33
© 2010 Pearson Education, Inc.
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
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
© 2010 Pearson Education, Inc.
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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
© 2010 Pearson Education, Inc.
• If avian flu mutates to a form that can easily spread between people, the potential for a major human outbreak is significant.
Figure 10.35
© 2010 Pearson Education, Inc.
• New viruses can arise by:
– Mutation of existing viruses
– Spread to new host species
Figure 10.UN1
Figure 10.UN2
Polynucleotide
Phosphategroup
Nucleotide
Sugar
DNA
Nitrogenousbase
Nitrogenousbase
Number ofstrands
Sugar
DNA RNA
RiboseDeoxy-ribose
CGAT
CGAU
12
Figure 10.UN3
Polynucleotide
Phosphategroup
Nucleotide
Sugar
DNA
Nitrogenousbase
Figure 10.UN3a
Nitrogenous base
Number of strands
Sugar
DNA RNA
RiboseDeoxyribose
CGAT
CGAU
12
Figure 10.UN3b
New daughterstrand
ParentalDNA molecule
IdenticaldaughterDNA molecules
Figure 10.UN4
Polypeptide
TRANSLATIONTRANSCRIPTION
mRNA
DNA
Gene
Figure 10.UN5
Growingpolypeptide
mRNA
Codons
Large ribosomalsubunit
tRNA
Small ribosomalsubunit
Anticodon
Amino acid
Figure 10.UN6
Figure 10.UN7