chapter 16- end review. fig. 16-7 (c) space-filling model hydrogen bond 3 end 5 end 3.4 nm 0.34 nm 3...
TRANSCRIPT
Chapter 16- endREVIEW
Fig. 16-7
(c) Space-filling model
Hydrogen bond 3 end
5 end
3.4 nm
0.34 nm
3 end
5 end
(b) Partial chemical structure
(a) Key features of DNA structure
1 nm
Fig. 16-9-3
A T
GC
T A
TA
G C
(a) Parent molecule
A T
GC
T A
TA
G C
(c) “Daughter” DNA molecules, each consisting of one parental strand and one new strand
(b) Separation of strands
A T
GC
T A
TA
G C
A T
GC
T A
TA
G C
Fig. 16-12Origin of replication
Parental (template) strand Daughter (new) strand
Replication forkReplication bubble
Two daughter DNA molecules(a) Origins of replication in E. coliOrigin of replicationDouble-stranded DNA
moleculeParental (template) strandDaughter (new) strand
Bubble Replication fork
Two daughter DNA molecules(b) Origins of replication
in eukaryotes
0.5 µm
0.25 µm
Double-strandedDNA molecule
Fig. 16-17
OverviewOrigin of replicationLeading strand
Leading strand
Lagging strand
Lagging strandOverall
directions of
replicationLeading strand
Lagging strand
Helicase
Parental DNA
DNA pol III
PrimerPrimase
DNA ligase
DNA pol IIIDNA pol I
Single-strand
binding protein
53
5
55
5
3
3
33
13 2
4
Fig. 16-19Ends of parental DNA strands
Leading strand
Lagging strand
Lagging strand
Last fragment Previous fragment
Parental strand
RNA primer
Removal of primers and replacement with DNA where a 3 end is available
Second round of replication
New leading strand
New lagging strand
Further rounds of replication
Shorter and shorter daughter molecules
5
3
3
3
3
3
5
5
5
5
Fig. 17-3
TRANSCRIPTION
TRANSLATION
DNA
mRNARibosome
Polypeptide
(a) Bacterial cell
Nuclearenvelope
TRANSCRIPTION
RNA PROCESSINGPre-mRNA
DNA
mRNA
TRANSLATION Ribosome
Polypeptide
(b) Eukaryotic cell
Fig. 17-4
DNAmolecule
Gene 1
Gene 2
Gene 3
DNAtemplatestrand
TRANSCRIPTION
TRANSLATION
mRNA
Protein
Codon
Amino acid
Fig. 17-7
Promoter Transcription unit
Start pointDNA
RNA polymerase
5533
Initiation1
2
3
5533
UnwoundDNA
RNAtranscript
Template strandof DNA
Elongation
RewoundDNA
5
55
5
5
333
3
RNAtranscript
Termination
5533
35Completed RNA transcript
Newly madeRNA
Templatestrand of DNA
Direction oftranscription(“downstream”)
3 end
RNApolymerase
RNA nucleotides
Nontemplatestrand of DNA
Elongation
Fig. 17-8A eukaryotic promoterincludes a TATA box
3
1
2
3
Promoter
TATA box Start point
Template
TemplateDNA strand
535
Transcriptionfactors
Several transcription factors mustbind to the DNA before RNApolymerase II can do so.
5533
Additional transcription factors bind tothe DNA along with RNA polymerase II,forming the transcription initiation complex.
RNA polymerase IITranscription factors
55 53
3
RNA transcript
Transcription initiation complex
Fig. 17-10
Pre-mRNA
mRNA
Codingsegment
Introns cut out andexons spliced together
5 Cap
Exon Intron5
1 30 31 104
Exon Intron
105
Exon
146
3Poly-A tail
Poly-A tail5 Cap
5 UTR 3 UTR1 146
Fig. 17-17
3355U
UA
ACGMet
GTP GDPInitiator
tRNA
mRNA5 3
Start codon
mRNA binding siteSmallribosomalsubunit
5
P site
Translation initiation complex
3
E A
Met
Largeribosomalsubunit
Fig. 17-18-4
Amino endof polypeptide
mRNA
5
3E
Psite
Asite
GTP
GDP
E
P A
E
P A
GDPGTP
Ribosome ready fornext aminoacyl tRNA
E
P A
Fig. 17-19-3
Releasefactor
3
5Stop codon(UAG, UAA, or UGA)
5
32
Freepolypeptide
2 GDP
GTP
5
3
Fig. 17-21
Ribosome
mRNA
Signalpeptide
Signal-recognitionparticle (SRP)
CYTOSOL Translocationcomplex
SRPreceptorprotein
ER LUMEN
Signalpeptideremoved
ERmembrane
Protein
Fig. 17-23Wild-type
3DNA template strand5
5
53
3
Stop
Carboxyl endAmino end
Protein
mRNA
33
3
55
5
A instead of G
U instead of C
Silent (no effect on amino acid sequence)
Stop
T instead of C
33
3
55
5
A instead of G
Stop
Missense
A instead of T
U instead of A
33
3
5
5
5
Stop
Nonsense No frameshift, but one amino acid missing (3 base-pair deletion)
Frameshift causing extensive missense (1 base-pair deletion)
Frameshift causing immediate nonsense (1 base-pair insertion)
5
5
533
3
Stop
missing
missing
3
3
3
5
55
missing
missing
Stop
5
5533
3
Extra U
Extra A
(a) Base-pair substitution (b) Base-pair insertion or deletion
Fig. 17-24RNA polymerase
DNA
Polyribosome
mRNA
0.25 µmDirection oftranscription
DNA
RNApolymerase
Polyribosome
Polypeptide(amino end)
Ribosome
mRNA (5 end)
Fig. 17-25
TRANSCRIPTION
RNA PROCESSING
DNA
RNAtranscript
3
5RNApolymerase
Poly-A
Poly-A
RNA transcript(pre-mRNA)
Intron
Exon
NUCLEUS
Aminoacyl-tRNAsynthetase
AMINO ACID ACTIVATIONAminoacid
tRNACYTOPLASM
Poly-A
Growingpolypeptide
3
Activatedamino acid
mRNA
TRANSLATION
Cap
Ribosomalsubunits
Cap
5
E
PA
AAnticodon
Ribosome
Codon
E
Fig. 18-3
Polypeptide subunits that make upenzymes for tryptophan synthesis
(b) Tryptophan present, repressor active, operon off
Tryptophan(corepressor)
(a) Tryptophan absent, repressor inactive, operon on
No RNA made
Activerepressor
mRNA
Protein
DNA
DNA
mRNA 5
Protein Inactiverepressor
RNApolymerase
Regulatorygene
Promoter Promoter
trp operon
Genes of operon
OperatorStop codonStart codon
mRNA
trpA
5
3
trpR trpE trpD trpC trpB
ABCDE
Fig. 18-3a
Polypeptide subunits that make upenzymes for tryptophan synthesis
(a) Tryptophan absent, repressor inactive, operon on
DNA
mRNA 5
Protein Inactiverepressor
RNApolymerase
Regulatorygene
Promoter Promoter
trp operon
Genes of operon
OperatorStop codonStart codon
mRNA
trpA
5
3
trpR trpE trpD trpC trpB
ABCDE
Fig. 18-3b-1
(b) Tryptophan present, repressor active, operon off
Tryptophan(corepressor)
No RNA made
Activerepressor
mRNA
Protein
DNA
Fig. 18-3b-2
(b) Tryptophan present, repressor active, operon off
Tryptophan(corepressor)
No RNA made
Activerepressor
mRNA
Protein
DNA
Fig. 18-4
(b) Lactose present, repressor inactive, operon on
(a) Lactose absent, repressor active, operon off
mRNA
Protein
DNA
DNA
mRNA 5
Protein Activerepressor
RNApolymerase
Regulatorygene
Promoter
Operator
mRNA5
3
Inactiverepressor
Allolactose(inducer)
5
3
NoRNAmade
RNApolymerase
Permease Transacetylase
lac operon
-Galactosidase
lacYlacZ lacAlacI
lacI lacZ
Fig. 18-4a
(a) Lactose absent, repressor active, operon off
DNA
ProteinActiverepressor
RNApolymerase
Regulatorygene
Promoter
Operator
mRNA5
3
NoRNAmade
lacI lacZ
Fig. 18-4b
(b) Lactose present, repressor inactive, operon on
mRNA
Protein
DNA
mRNA 5
Inactiverepressor
Allolactose(inducer)
5
3
RNApolymerase
Permease Transacetylase
lac operon
-Galactosidase
lacYlacZ lacAlacI
Repressible and Inducible Operons: Two Types of Negative Gene Regulation
• A repressible operon is one that is usually on; binding of a repressor to the operator shuts off transcription
• The trp operon is a repressible operon• An inducible operon is one that is usually off; a
molecule called an inducer inactivates the repressor and turns on transcription
• The lac operon is an inducible operon and contains genes that code for enzymes used in the hydrolysis and metabolism of lactose
• By itself, the lac repressor is active and switches the lac operon off
• A molecule called an inducer inactivates the repressor to turn the lac operon on
• Inducible enzymes usually function in catabolic pathways; their synthesis is induced by a chemical signal
• Repressible enzymes usually function in anabolic pathways; their synthesis is repressed by high levels of the end product
• Regulation of the trp and lac operons involves negative control of genes because operons are switched off by the active form of the repressor
Positive Gene Regulation
• Some operons are also subject to positive control through a stimulatory protein, such as catabolite activator protein (CAP), an activator of transcription
• When glucose (a preferred food source of E. coli) is scarce, CAP is activated by binding with cyclic AMP
• Activated CAP attaches to the promoter of the lac operon and increases the affinity of RNA polymerase, thus accelerating transcription
• When glucose levels increase, CAP detaches from the lac operon, and transcription returns to a normal rate
• CAP helps regulate other operons that encode enzymes used in catabolic pathways
Fig. 18-5
(b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized
cAMP
DNA
Inactive lacrepressor
Allolactose
InactiveCAP
lacI
CAP-binding site
Promoter
ActiveCAP
Operator
lacZRNApolymerasebinds andtranscribes
Inactive lacrepressor
lacZ
OperatorPromoter
DNA
CAP-binding site
lacI
RNApolymerase lesslikely to bind
InactiveCAP
(a) Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized
Fig. 18-6
DNA
Signal
Gene
NUCLEUS
Chromatin modification
Chromatin
Gene availablefor transcription
Exon
Intron
Tail
RNA
Cap
RNA processing
Primary transcript
mRNA in nucleus
Transport to cytoplasm
mRNA in cytoplasm
Translation
CYTOPLASM
Degradationof mRNA
Protein processing
Polypeptide
Active protein
Cellular function
Transport to cellulardestination
Degradationof protein
Transcription
Fig. 18-6a
DNA
Signal
Gene
NUCLEUS
Chromatin modification
Chromatin
Gene availablefor transcription
Exon
Intron
Tail
RNA
Cap
RNA processing
Primary transcript
mRNA in nucleus
Transport to cytoplasm
CYTOPLASM
Transcription
Fig. 18-6b
mRNA in cytoplasm
Translation
CYTOPLASM
Degradationof mRNA
Protein processing
Polypeptide
Active protein
Cellular function
Transport to cellulardestination
Degradationof protein
Fig. 18-6
DNA
Signal
Gene
NUCLEUS
Chromatin modification
Chromatin
Gene availablefor transcription
Exon
Intron
Tail
RNA
Cap
RNA processing
Primary transcript
mRNA in nucleus
Transport to cytoplasm
mRNA in cytoplasm
Translation
CYTOPLASM
Degradationof mRNA
Protein processing
Polypeptide
Active protein
Cellular function
Transport to cellulardestination
Degradationof protein
Transcription
Fig. 18-6a
DNA
Signal
Gene
NUCLEUS
Chromatin modification
Chromatin
Gene availablefor transcription
Exon
Intron
Tail
RNA
Cap
RNA processing
Primary transcript
mRNA in nucleus
Transport to cytoplasm
CYTOPLASM
Transcription
Fig. 18-6b
mRNA in cytoplasm
Translation
CYTOPLASM
Degradationof mRNA
Protein processing
Polypeptide
Active protein
Cellular function
Transport to cellulardestination
Degradationof protein
• Proximal control elements are located close to the promoter
• Distal control elements, groups of which are called enhancers, may be far away from a gene or even located in an intron
Enhancers and Specific Transcription Factors
• An activator is a protein that binds to an enhancer and stimulates transcription of a gene
• Bound activators cause mediator proteins to interact with proteins at the promoter
Animation: Initiation of Transcription
Fig. 18-9-1
Enhancer TATAbox
PromoterActivators
DNAGene
Distal controlelement
Fig. 18-9-2
Enhancer TATAbox
PromoterActivators
DNAGene
Distal controlelement
Group ofmediator proteins
DNA-bendingprotein
Generaltranscriptionfactors
Fig. 18-9-3
Enhancer TATAbox
PromoterActivators
DNAGene
Distal controlelement
Group ofmediator proteins
DNA-bendingprotein
Generaltranscriptionfactors
RNApolymerase II
RNApolymerase II
Transcriptioninitiation complex RNA synthesis
Fig. 18-10
Controlelements
Enhancer
Availableactivators
Albumin gene
(b) Lens cell
Crystallin geneexpressed
Availableactivators
LENS CELLNUCLEUS
LIVER CELLNUCLEUS
Crystallin gene
Promoter
(a) Liver cell
Crystallin genenot expressed
Albumin geneexpressed
Albumin genenot expressed
Fig. 18-11
or
RNA splicing
mRNA
PrimaryRNAtranscript
Troponin T gene
Exons
DNA
Protein Processing and Degradation
• After translation, various types of protein processing, including cleavage and the addition of chemical groups, are subject to control
• Proteasomes are giant protein complexes that bind protein molecules and degrade them
Animation: Protein Degradation
Animation: Protein Processing
Fig. 18-12
Proteasomeand ubiquitinto be recycledProteasome
Proteinfragments(peptides)Protein entering a
proteasome
Ubiquitinatedprotein
Protein tobe degraded
Ubiquitin
Structure of Viruses
• Viruses are not cells• Viruses are very small infectious particles consisting
of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope
Fig. 19-3
RNA
Capsomere
Capsomereof capsid
DNA
Glycoprotein18 250 nm 70–90 nm (diameter)
Glycoproteins
80–200 nm (diameter) 80 225 nm
Membranousenvelope RNA
Capsid
HeadDNA
Tailsheath
Tailfiber
50 nm50 nm50 nm20 nm(a) Tobacco mosaic virus
(b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4
Concept 19.2: Viruses reproduce only in host cells
• Viruses are obligate intracellular parasites, which means they can reproduce only within a host cell
• Each virus has a host range, a limited number of host cells that it can infect
Transcriptionand manufactureof capsid proteins
Self-assembly of new virus particles and their exit from the cell
Entry anduncoating
Fig. 19-4VIRUS1
2
3
DNA
Capsid
4
Replication
HOST CELL
Viral DNA
mRNA
Capsidproteins
Viral DNA
Fig. 19-6
PhageDNA
Phage
The phage injects its DNA.
Bacterialchromosome
Phage DNAcircularizes.
Daughter cellwith prophage
Occasionally, a prophageexits the bacterialchromosome,initiating a lytic cycle.
Cell divisionsproducepopulation ofbacteria infectedwith the prophage.
The cell lyses, releasing phages.
Lytic cycle
Lytic cycleis induced or Lysogenic cycle
is entered
Lysogenic cycle
Prophage
The bacterium reproduces,copying the prophage andtransmitting it to daughter cells.
Phage DNA integrates intothe bacterial chromosome,becoming a prophage.
New phage DNA and proteinsare synthesized andassembled into phages.
Fig. 19-7
Capsid
RNA
Envelope (withglycoproteins)
Capsid and viral genomeenter the cell
HOST CELL
Viral genome (RNA)
Template
mRNA
ER
Glyco-proteins
Capsidproteins Copy of
genome (RNA)
New virus
Fig. 19-8aGlycoprotein
Reversetranscriptase HIV
RNA (twoidenticalstrands)
Capsid
Viral envelope
HOST CELL
Reversetranscriptase
Viral RNA
RNA-DNAhybrid
DNA
NUCLEUS
Provirus
ChromosomalDNA
RNA genomefor thenext viralgeneration
mRNA
New virus
Evolution of Viruses
• Viruses do not fit our definition of living organisms• Since viruses can reproduce only within cells, they
probably evolved as bits of cellular nucleic acid• Candidates for the source of viral genomes are
plasmids, circular DNA in bacteria and yeasts, and transposons, small mobile DNA segments
• Plasmids, transposons, and viruses are all mobile genetic elements
Overview: The DNA Toolbox• Sequencing of the human genome was completed
by 2007• DNA sequencing has depended on advances in
technology, starting with making recombinant DNA• In recombinant DNA, nucleotide sequences from
two different sources, often two species, are combined in vitro into the same DNA molecule
• Methods for making recombinant DNA are central to genetic engineering, the direct manipulation of genes for practical purposes
• DNA technology has revolutionized biotechnology, the manipulation of organisms or their genetic components to make useful products
• An example of DNA technology is the microarray, a measurement of gene expression of thousands of different genes
• Gene cloning involves using bacteria to make multiple copies of a gene
• Foreign DNA is inserted into a plasmid, and the recombinant plasmid is inserted into a bacterial cell
• Reproduction in the bacterial cell results in cloning of the plasmid including the foreign DNA
• This results in the production of multiple copies of a single gene
Fig. 20-2
DNA of chromosome
Cell containing geneof interest
Gene inserted intoplasmid
Plasmid put intobacterial cell
RecombinantDNA (plasmid)
Recombinantbacterium
Bacterialchromosome
Bacterium
Gene ofinterest
Host cell grown in cultureto form a clone of cellscontaining the “cloned”gene of interest
Plasmid
Gene ofInterest
Protein expressedby gene of interest
Basic research andvarious applications
Copies of gene Protein harvested
Basicresearchon gene
Basicresearchon protein
Gene for pest resistance inserted into plants
Gene used to alter bacteria for cleaning up toxic waste
Protein dissolvesblood clots in heartattack therapy
Human growth hor-mone treats stuntedgrowth
2
4
1
3
Using Restriction Enzymes to Make Recombinant DNA
• Bacterial restriction enzymes cut DNA molecules at specific DNA sequences called restriction sites
• A restriction enzyme usually makes many cuts, yielding restriction fragments
• The most useful restriction enzymes cut DNA in a staggered way, producing fragments with “sticky ends” that bond with complementary sticky ends of other fragments
Animation: Restriction Enzymes
Fig. 20-3-3Restriction site
DNA
Sticky end
Restriction enzymecuts sugar-phosphatebackbones.
53
35
1
One possible combination
Recombinant DNA molecule
DNA ligaseseals strands.
3
DNA fragment addedfrom another moleculecut by same enzyme.Base pairing occurs.
2
Fig. 20-5
Bacterial clones
Recombinantplasmids
Recombinantphage DNA
or
Foreign genomecut up withrestrictionenzyme
(a) Plasmid library (b) Phage library (c) A library of bacterial artificial chromosome (BAC) clones
Phageclones
Large plasmidLarge insertwith many genes
BACclone
Fig. 20-6-5
DNA innucleus
mRNAs in cytoplasm
Reversetranscriptase Poly-A tail
DNAstrand
Primer
mRNA
DegradedmRNA
DNA polymerase
cDNA
• A probe can be synthesized that is complementary to the gene of interest
• For example, if the desired gene is
– Then we would synthesize this probe G5 3… …G GC C CT T TA A A
C3 5C CG G GA A AT T T
Fig. 20-85
Genomic DNA
TECHNIQUE
Cycle 1yields
2molecules
Denaturation
Annealing
Extension
Cycle 2yields
4molecules
Cycle 3yields 8
molecules;2 molecules
(in whiteboxes)
match targetsequence
Targetsequence
Primers
Newnucleo-tides
3
3
3
3
5
5
51
2
3
Fig. 20-9a
Mixture ofDNA mol-ecules ofdifferentsizes
Powersource
Longermolecules
Shortermolecules
Gel
AnodeCathode
TECHNIQUE
1
2
Powersource
– +
+–
Fig. 20-10
Normalallele
Sickle-cellallele
Largefragment
(b) Electrophoresis of restriction fragments from normal and sickle-cell alleles
201 bp175 bp
376 bp
(a) DdeI restriction sites in normal and sickle-cell alleles of -globin gene
Normal -globin allele
Sickle-cell mutant -globin allele
DdeI
Large fragment
Large fragment
376 bp
201 bp175 bp
DdeIDdeI
DdeI DdeI DdeI DdeI
Fig. 20-11TECHNIQUE
Nitrocellulosemembrane (blot)
Restrictionfragments
Alkalinesolution
DNA transfer (blotting)
Sponge
Gel
Heavyweight
Papertowels
Preparation of restriction fragments Gel electrophoresis
I II III
I II IIII II III
Radioactively labeledprobe for -globin gene
DNA + restriction enzyme
III HeterozygoteII Sickle-cellallele
I Normal-globinallele
Film overblot
Probe detectionHybridization with radioactive probe
Fragment fromsickle-cell-globin allele
Fragment fromnormal -globin allele
Probe base-pairswith fragments
Nitrocellulose blot
1
4 5
32
DNA Sequencing• Relatively short DNA fragments can be sequenced
by the dideoxy chain termination method
• Modified nucleotides called dideoxyribonucleotides (ddNTP) attach to synthesized DNA strands of different lengths
• Each type of ddNTP is tagged with a distinct fluorescent label that identifies the nucleotide at the end of each DNA fragment
• The DNA sequence can be read from the resulting spectrogram
Fig. 20-12
DNA(template strand)
TECHNIQUE
RESULTS
DNA (template strand)
DNA polymerase
Primer Deoxyribonucleotides
Shortest
Dideoxyribonucleotides(fluorescently tagged)
Labeled strands
Longest
Shortest labeled strand
Longest labeled strand
Laser
Directionof movementof strands
Detector
Last baseof longest
labeledstrand
Last baseof shortest
labeledstrand
dATP
dCTP
dTTP
dGTP
ddATP
ddCTP
ddTTP
ddGTP
Fig. 20-12a
DNA(template strand)
TECHNIQUE
DNA polymerase
Primer Deoxyribonucleotides Dideoxyribonucleotides(fluorescently tagged)
dATP
dCTP
dTTP
dGTP
ddATP
ddCTP
ddTTP
ddGTP
Fig. 20-12bTECHNIQUE
RESULTS
DNA (template strand)
Shortest
Labeled strands
Longest
Shortest labeled strand
Longest labeled strand
Laser
Directionof movementof strands
Detector
Last baseof longest
labeledstrand
Last baseof shortest
labeledstrand
Studying the Expression of Single Genes
• Changes in the expression of a gene during embryonic development can be tested using– Northern blotting– Reverse transcriptase-polymerase chain reaction
• Both methods are used to compare mRNA from different developmental stages
• Northern blotting combines gel electrophoresis of mRNA followed by hybridization with a probe on a membrane
• Identification of mRNA at a particular developmental stage suggests protein function at that stage
• Reverse transcriptase-polymerase chain reaction (RT-PCR) is quicker and more sensitive
• Reverse transcriptase is added to mRNA to make cDNA, which serves as a template for PCR amplification of the gene of interest
• The products are run on a gel and the mRNA of interest identified
Fig. 20-13
TECHNIQUE
RESULTS
Gel electrophoresis
cDNAs
-globingene
PCR amplification
Embryonic stages
Primers
1 2 3 4 5 6
mRNAscDNA synthesis 1
2
3
Identifying Protein-Coding Genes Within DNA Sequences
• Computer analysis of genome sequences helps identify sequences likely to encode proteins
• Comparison of sequences of “new” genes with those of known genes in other species may help identify new genes
Understanding Genes and Their Products at the Systems Level
• Proteomics is the systematic study of all proteins encoded by a genome
• Proteins, not genes, carry out most of the activities of the cell
Concept 21.5: Duplication, rearrangement, and mutation of DNA contribute to genome evolution
• The basis of change at the genomic level is mutation, which underlies much of genome evolution
• The earliest forms of life likely had a minimal number of genes, including only those necessary for survival and reproduction
• The size of genomes has increased over evolutionary time, with the extra genetic material providing raw material for gene diversification
Alterations of Chromosome Structure
• Humans have 23 pairs of chromosomes, while chimpanzees have 24 pairs
• Following the divergence of humans and chimpanzees from a common ancestor, two ancestral chromosomes fused in the human line
• Duplications and inversions result from mistakes during meiotic recombination
• Comparative analysis between chromosomes of humans and 7 mammalian species paints a hypothetical chromosomal evolutionary history
Rearrangements of Parts of Genes: Exon Duplication and Exon Shuffling
• The duplication or repositioning of exons has contributed to genome evolution
• Errors in meiosis can result in an exon being duplicated on one chromosome and deleted from the homologous chromosome
• In exon shuffling, errors in meiotic recombination lead to some mixing and matching of exons, either within a gene or between two nonallelic genes
Darwin’s Focus on Adaptation
• In reassessing his observations, Darwin perceived adaptation to the environment and the origin of new species as closely related processes
• From studies made years after Darwin’s voyage, biologists have concluded that this is indeed what happened to the Galápagos finches
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 22-6
(a) Cactus-eater (c) Seed-eater
(b) Insect-eater
Descent with Modification
• Darwin never used the word evolution in the first edition of The Origin of Species
• The phrase descent with modification summarized Darwin’s perception of the unity of life
• The phrase refers to the view that all organisms are related through descent from an ancestor that lived in the remote past
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• In the Darwinian view, the history of life is like a tree with branches representing life’s diversity
• Darwin’s theory meshed well with the hierarchy of Linnaeus
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 22-8a
Elephas maximus(Asia)
Stegodon
Mammuthus
Loxodontaafricana(Africa)
Loxodonta cyclotis(Africa)
010425.52434
Millions of years ago Years ago
Platybelodon
Artificial Selection, Natural Selection, and Adaptation
• Darwin noted that humans have modified other species by selecting and breeding individuals with desired traits, a process called artificial selection
• Darwin then described four observations of nature and from these drew two inferences
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 22-9
Kale
Kohlrabi
Brussels sprouts
Leaves
Stem
Wild mustard
Flowersand stems
Broccoli
Cauliflower
Flowerclusters
Cabbage
Terminalbud
Lateralbuds
• Observation #1: Members of a population often vary greatly in their traits
• Observation #2: Traits are inherited from parents to offspring
• Observation #3: All species are capable of producing more offspring than the environment can support
• Observation #4: Owing to lack of food or other resources, many of these offspring do not survive
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Inference #1: Individuals whose inherited traits give them a higher probability of surviving and reproducing in a given environment tend to leave more offspring than other individuals
• Inference #2: This unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in the population over generations
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Natural Selection: A Summary
• Individuals with certain heritable characteristics survive and reproduce at a higher rate than other individuals
• Natural selection increases the adaptation of organisms to their environment over time
• If an environment changes over time, natural selection may result in adaptation to these new conditions and may give rise to new species
Video: Seahorse Camouflage
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Note that individuals do not evolve; populations evolve over time
• Natural selection can only increase or decrease heritable traits in a population
• Adaptations vary with different environments
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The Fossil Record
• The fossil record provides evidence of the extinction of species, the origin of new groups, and changes within groups over time
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 22-15
Bristolia insolens
Bristolia bristolensis
Bristolia harringtoni
Bristolia mohavensis
Latham Shale dig site, SanBernardino County, California
Dep
th (
met
ers
)
0
2
4
6
8
10
12
14
16
18
1
2
3
3
3
1
2
44
Fig. 22-17
Humerus
Radius
Ulna
Carpals
Metacarpals
Phalanges
Human WhaleCat Bat
Fig. 22-18
Human embryoChick embryo (LM)
Pharyngealpouches
Post-analtail
Fig. 22-19
Hawks andother birds
Ostriches
Crocodiles
Lizardsand snakes
Amphibians
Mammals
Lungfishes
Tetrapod limbs
Amnion
Feathers
Homologouscharacteristic
Branch point(common ancestor)
Tetrapo
ds
Am
nio
tes
Bird
s
6
5
4
3
2
1
Convergent Evolution
• Convergent evolution is the evolution of similar, or analogous, features in distantly related groups
• Analogous traits arise when groups independently adapt to similar environments in similar ways
• Convergent evolution does not provide information about ancestry
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 22-20
Sugarglider
Flyingsquirrel
AUSTRALIA
NORTHAMERICA
Overview: The Smallest Unit of Evolution
• One misconception is that organisms evolve, in the Darwinian sense, during their lifetimes
• Natural selection acts on individuals, but only populations evolve
• Genetic variations in populations contribute to evolution
• Microevolution is a change in allele frequencies in a population over generations
• Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals
Concept 23.1: Mutation and sexual reproduction produce the genetic variation that makes evolution possible
• Population geneticists measure polymorphisms in a population by determining the amount of heterozygosity at the gene and molecular levels
• Average heterozygosity measures the average percent of loci that are heterozygous in a population
• Nucleotide variability is measured by comparing the DNA sequences of pairs of individuals
Mutation
• Mutations are changes in the nucleotide sequence of DNA
• Mutations cause new genes and alleles to arise• Only mutations in cells that produce gametes can
be passed to offspring
Animation: Genetic Variation from Sexual Recombination
Gene Pools and Allele Frequencies
• A population is a localized group of individuals capable of interbreeding and producing fertile offspring
• A gene pool consists of all the alleles for all loci in a population
• A locus is fixed if all individuals in a population are homozygous for the same allele
Hardy-Weinberg Equilibrium
• The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation
• In a given population where gametes contribute to the next generation randomly, allele frequencies will not change
• Mendelian inheritance preserves genetic variation in a population
• Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool
• If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then– p2 + 2pq + q2 = 1– where p2 and q2 represent the frequencies of the
homozygous genotypes and 2pq represents the frequency of the heterozygous genotype
• The five conditions for nonevolving populations are rarely met in nature:– No mutations – Random mating – No natural selection – Extremely large population size– No gene flow
• Three major factors alter allele frequencies and bring about most evolutionary change:– Natural selection– Genetic drift– Gene flow
Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population
The Founder Effect
• The founder effect occurs when a few individuals become isolated from a larger population
• Allele frequencies in the small founder population can be different from those in the larger parent population
The Bottleneck Effect
• The bottleneck effect is a sudden reduction in population size due to a change in the environment
• The resulting gene pool may no longer be reflective of the original population’s gene pool
• If the population remains small, it may be further affected by genetic drift
Gene Flow
• Gene flow consists of the movement of alleles among populations
• Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen)
• Gene flow tends to reduce differences between populations over time
• Gene flow is more likely than mutation to alter allele frequencies directly
• Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals
• Selection favors certain genotypes by acting on the phenotypes of certain organisms
Fig. 23-13
Original population
(c) Stabilizing selection(b) Disruptive selection(a) Directional selection
Phenotypes (fur color)Fr
eque
ncy
of in
divi
dual
sOriginalpopulation
Evolvedpopulation
• Because the environment can change, adaptive evolution is a continuous process
• Genetic drift and gene flow do not consistently lead to adaptive evolution as they can increase or decrease the match between an organism and its environment
Sexual Selection
• Sexual selection is natural selection for mating success
• It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics
Fig. 23-15
• Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex
• Intersexual selection, often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates
• Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival
• Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes
• Natural selection will tend to maintain two or more alleles at that locus
• The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance
Heterozygote Advantage
• In frequency-dependent selection, the fitness of a phenotype declines if it becomes too common in the population
• Selection can favor whichever phenotype is less common in a population
Frequency-Dependent Selection
Fig. 23-18
“Right-mouthed”
1981
“Left-mouthed”
Freq
uenc
y of
“left
-mou
thed
” in
divi
dual
s
Sample year
1.0
0.5
0’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90
Why Natural Selection Cannot Fashion Perfect Organisms
1. Selection can act only on existing variations2. Evolution is limited by historical constraints3. Adaptations are often compromises4. Chance, natural selection, and the environment
interact
• CH 24
• Speciation, the origin of new species, is at the focal point of evolutionary theory
• Evolutionary theory must explain how new species originate and how populations evolve
• Microevolution consists of adaptations that evolve within a population, confined to one gene pool
• Macroevolution refers to evolutionary change above the species level
Animation: Macroevolution
• Prezygotic barriers block fertilization from occurring by:– Impeding different species from attempting to mate– Preventing the successful completion of mating– Hindering fertilization if mating is successful
• Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult:– Reduced hybrid viability– Reduced hybrid fertility– Hybrid breakdown
Fig. 24-4a
Habitat Isolation Temporal Isolation
Prezygotic barriers
Behavioral Isolation
Matingattempt
Mechanical Isolation
(f)(e)(c)(a)
(b)
(d)
Individualsof
differentspecies
Fig. 24-4i
Prezygotic barriers
Gametic Isolation
Fertilization
Reduced Hybrid Viability
Postzygotic barriers
Reduced Hybrid Fertility Hybrid Breakdown
Viable,fertile
offspring
(g) (h) (i)
(j)
(l)
(k)
Concept 24.2: Speciation can take place with or without geographic separation
• Speciation can occur in two ways:– Allopatric speciation– Sympatric speciation
Fig. 24-5
(a) Allopatric speciation (b) Sympatric speciation
Polyploidy• Polyploidy is the presence of extra sets of
chromosomes due to accidents during cell division• An autopolyploid is an individual with more than
two chromosome sets, derived from one species
Fig. 24-10-1
2n = 6 4n = 12
Failure of celldivision afterchromosomeduplication givesrise to tetraploidtissue.
Fig. 24-10-2
2n = 6 4n = 12
Failure of celldivision afterchromosomeduplication givesrise to tetraploidtissue.
2n
Gametesproducedare diploid..
Fig. 24-10-3
2n = 6 4n = 12
Failure of celldivision afterchromosomeduplication givesrise to tetraploidtissue.
2n
Gametesproducedare diploid..
4n
Offspring withtetraploidkaryotypes maybe viable andfertile.
• An allopolyploid is a species with multiple sets of chromosomes derived from different species
Fig. 24-11-1
Species A2n = 6
Normalgameten = 3
Meioticerror
Species B2n = 4
Unreducedgametewith 4chromosomes
Fig. 24-11-2
Species A2n = 6
Normalgameten = 3
Meioticerror
Species B2n = 4
Unreducedgametewith 4chromosomes
Hybridwith 7chromosomes
Fig. 24-11-3
Species A2n = 6
Normalgameten = 3
Meioticerror
Species B2n = 4
Unreducedgametewith 4chromosomes
Hybridwith 7chromosomes
Unreducedgametewith 7chromosomes
Normalgameten = 3
Fig. 24-11-4
Species A2n = 6
Normalgameten = 3
Meioticerror
Species B2n = 4
Unreducedgametewith 4chromosomes
Hybridwith 7chromosomes
Unreducedgametewith 7chromosomes
Normalgameten = 3
Viable fertilehybrid(allopolyploid)2n = 10
Patterns in the Fossil Record
• The fossil record includes examples of species that appear suddenly, persist essentially unchanged for some time, and then apparently disappear
• Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibrium to describe periods of apparent stasis punctuated by sudden change
• The punctuated equilibrium model contrasts with a model of gradual change in a species’ existence
Fig. 24-17
(a) Punctuated pattern
(b) Gradual pattern
Time
CH 25 ORIGIN OF LIFE
Protobionts
• Replication and metabolism are key properties of life
• Protobionts are aggregates of abiotically produced molecules surrounded by a membrane or membrane-like structure
• Protobionts exhibit simple reproduction and metabolism and maintain an internal chemical environment
Self-Replicating RNA and the Dawn of Natural Selection
• The first genetic material was probably RNA, not DNA
• RNA molecules called ribozymes have been found to catalyze many different reactions– For example, ribozymes can make complementary
copies of short stretches of their own sequence or other short pieces of RNA
The Fossil Record
• Sedimentary rocks are deposited into layers called strata and are the richest source of fossils
Video: Grand Canyon
Fig. 25-4Present
Dimetrodon
Coccosteus cuspidatus
Fossilizedstromatolite
Stromatolites Tappania, aunicellulareukaryote
Dickinsoniacostata
Hallucigenia
Casts ofammonites
Rhomaleosaurus victor, a plesiosaur
100
mill
ion
year
s ag
o20
017
530
027
040
037
550
052
556
560
03,
500
1,50
0
2.5 cm4.5 cm
1 cm
How Rocks and Fossils Are Dated
• Sedimentary strata reveal the relative ages of fossils• The absolute ages of fossils can be determined by
radiometric dating• A “parent” isotope decays to a “daughter” isotope
at a constant rate• Each isotope has a known half-life, the time
required for half the parent isotope to decay
Fig. 25-5
Time (half-lives)
Accumulating “daughter” isotope
Remaining “parent” isotopeFr
actio
n of
par
e nt
iso t
ope
rem
a in i
ng
1 2 3 4
1/2
1/41/8 1/16
• Radiocarbon dating can be used to date fossils up to 75,000 years old
• For older fossils, some isotopes can be used to date sedimentary rock layers above and below the fossil
• The magnetism of rocks can provide dating information
• Reversals of the magnetic poles leave their record on rocks throughout the world
Photosynthesis and the Oxygen Revolution
• Most atmospheric oxygen (O2) is of biological origin
• O2 produced by oxygenic photosynthesis reacted with dissolved iron and precipitated out to form banded iron formations
• The source of O2 was likely bacteria similar to modern cyanobacteria
The First Eukaryotes
• The oldest fossils of eukaryotic cells date back 2.1 billion years
• The hypothesis of endosymbiosis proposes that mitochondria and plastids (chloroplasts and related organelles) were formerly small prokaryotes living within larger host cells
• An endosymbiont is a cell that lives within a host cell
Fig. 25-9-4
Ancestral photosyntheticeukaryote
Photosyntheticprokaryote
Mitochondrion
Plastid
Nucleus
Cytoplasm
DNAPlasma membrane
Endoplasmic reticulum
Nuclear envelope
Ancestralprokaryote
Aerobicheterotrophicprokaryote
Mitochondrion
Ancestralheterotrophiceukaryote
• Key evidence supporting an endosymbiotic origin of mitochondria and plastids:– Similarities in inner membrane structures and
functions
– Division is similar in these organelles and some prokaryotes
– These organelles transcribe and translate their own DNA
– Their ribosomes are more similar to prokaryotic than eukaryotic ribosomes
The Origin of Multicellularity
• The evolution of eukaryotic cells allowed for a greater range of unicellular forms
• A second wave of diversification occurred when multicellularity evolved and gave rise to algae, plants, fungi, and animals
The Cambrian Explosion
• The Cambrian explosion refers to the sudden appearance of fossils resembling modern phyla in the Cambrian period (535 to 525 million years ago)
• The Cambrian explosion provides the first evidence of predator-prey interactions
The Colonization of Land
• Fungi, plants, and animals began to colonize land about 500 million years ago
• Plants and fungi likely colonized land together by 420 million years ago
• Arthropods and tetrapods are the most widespread and diverse land animals
• Tetrapods evolved from lobe-finned fishes around 365 million years ago
Continental Drift
• At three points in time, the land masses of Earth have formed a supercontinent: 1.1 billion, 600 million, and 250 million years ago
• Earth’s continents move slowly over the underlying hot mantle through the process of continental drift
• Oceanic and continental plates can collide, separate, or slide past each other
• Interactions between plates cause the formation of mountains and islands, and earthquakes
Fig. 25-13
SouthAmerica
Pangaea
Mill
ions
of y
ears
ago
65.5
135
Mes
ozoi
c
251
Pale
ozoi
c
Gondwana
Laurasia
Eurasia
IndiaAfrica
AntarcticaAustralia
North America
Madagascar
Ceno
zoic
Present
The “Big Five” Mass Extinction Events
• In each of the five mass extinction events, more than 50% of Earth’s species became extinct
Fig. 25-14
Tota
l exti
nctio
n ra
te(f
amili
es p
er m
illio
n ye
ars)
:
Time (millions of years ago)
Num
ber o
f fam
ilies
:
CenozoicMesozoicPaleozoicE O S D C P Tr J
542
0
488 444 416 359 299 251 200 145
EraPeriod
5
C P N
65.5
0
0
200
100
300
400
500
600
700
800
15
10
20
• The Permian extinction defines the boundary between the Paleozoic and Mesozoic eras
• This mass extinction occurred in less than 5 million years and caused the extinction of about 96% of marine animal species
• This event might have been caused by volcanism, which lead to global warming, and a decrease in oceanic oxygen
• The Cretaceous mass extinction 65.5 million years ago separates the Mesozoic from the Cenozoic
• Organisms that went extinct include about half of all marine species and many terrestrial plants and animals, including most dinosaurs
Fig. 25-15
NORTHAMERICA
ChicxulubcraterYucatán
Peninsula
• The presence of iridium in sedimentary rocks suggests a meteorite impact about 65 million years ago
• The Chicxulub crater off the coast of Mexico is evidence of a meteorite that dates to the same time
Consequences of Mass Extinctions
• Mass extinction can alter ecological communities and the niches available to organisms
• It can take from 5 to 100 million years for diversity to recover following a mass extinction
• Mass extinction can pave the way for adaptive radiations
Adaptive Radiations
• Adaptive radiation is the evolution of diversely adapted species from a common ancestor upon introduction to new environmental opportunities
Fig. 25-18
Close North American relative,the tarweed Carlquistia muirii
Argyroxiphium sandwicense
Dubautia linearisDubautia scabra
Dubautia waialealae
Dubautia laxa
HAWAII0.4
millionyears
OAHU3.7
millionyears
KAUAI5.1
millionyears
1.3millionyears
MOLOKAIMAUI
LANAI
Changes in Rate and Timing
• Heterochrony is an evolutionary change in the rate or timing of developmental events
• It can have a significant impact on body shape• The contrasting shapes of human and chimpanzee
skulls are the result of small changes in relative growth rates
Animation: Allometric Growth
Fig. 25-19
(a) Differential growth rates in a human
(b) Comparison of chimpanzee and human skull growth
NewbornAge (years)
Adult1552
Chimpanzee fetus Chimpanzee adult
Human fetus Human adult
• Heterochrony can alter the timing of reproductive development relative to the development of nonreproductive organs
• In paedomorphosis, the rate of reproductive development accelerates compared with somatic development
• The sexually mature species may retain body features that were juvenile structures in an ancestral species
Changes in Spatial Pattern
• Substantial evolutionary change can also result from alterations in genes that control the placement and organization of body parts
• Homeotic genes determine such basic features as where wings and legs will develop on a bird or how a flower’s parts are arranged
• Hox genes are a class of homeotic genes that provide positional information during development
• If Hox genes are expressed in the wrong location, body parts can be produced in the wrong location
• For example, in crustaceans, a swimming appendage can be produced instead of a feeding appendage
• Evolution of vertebrates from invertebrate animals was associated with alterations in Hox genes
• Two duplications of Hox genes have occurred in the vertebrate lineage
• These duplications may have been important in the evolution of new vertebrate characteristics
Chapter 27Cell-Surface Structures
• An important feature of nearly all prokaryotic cells is their cell wall, which maintains cell shape, provides physical protection, and prevents the cell from bursting in a hypotonic environment
• Eukaryote cell walls are made of cellulose or chitin
• Bacterial cell walls contain peptidoglycan, a network of sugar polymers cross-linked by polypeptides
Fig. 27-3
Cellwall
Peptidoglycanlayer
Plasma membrane
Protein
Gram-positivebacteria
(a) Gram-positive: peptidoglycan traps crystal violet.
Gram-negativebacteria
(b) Gram-negative: crystal violet is easily rinsed away, revealing red dye.
20 µm
Cellwall
Plasma membrane
Protein
Carbohydrate portionof lipopolysaccharide
Outermembrane
Peptidoglycanlayer
Fig. 27-9
Endospore
0.3 µm
• Prokaryotes have considerable genetic variation• Three factors contribute to this genetic
diversity:– Rapid reproduction– Mutation– Genetic recombination
Concept 27.2: Rapid reproduction, mutation, and genetic recombination promote genetic diversity in prokaryotes
Transformation and Transduction
• A prokaryotic cell can take up and incorporate foreign DNA from the surrounding environment in a process called transformation
• Transduction is the movement of genes between bacteria by bacteriophages (viruses that infect bacteria)
Fig. 27-11-1
Donorcell
A+ B+
A+ B+
Phage DNA
Fig. 27-11-2
A+
Donorcell
A+ B+
A+ B+
Phage DNA
Fig. 27-11-3
Recipientcell
B–
A+
A–
Recombination
A+
Donorcell
A+ B+
A+ B+
Phage DNA
Fig. 27-11-4
Recombinant cell
Recipientcell
A+ B–
B–
A+
A–
Recombination
A+
Donorcell
A+ B+
A+ B+
Phage DNA
Conjugation and Plasmids
• Conjugation is the process where genetic material is transferred between bacterial cells
• Sex pili allow cells to connect and pull together for DNA transfer
• A piece of DNA called the F factor is required for the production of sex pili
• The F factor can exist as a separate plasmid or as DNA within the bacterial chromosome
The F Factor as a Plasmid
• Cells containing the F plasmid function as DNA donors during conjugation
• Cells without the F factor function as DNA recipients during conjugation
• The F factor is transferable during conjugation
Fig. 27-13
F plasmid
F+ cell
F– cell
Matingbridge
Bacterial chromosome
Bacterialchromosome
(a) Conjugation and transfer of an F plasmid
F+ cell
F+ cell
F– cell
(b) Conjugation and transfer of part of an Hfr bacterial chromosome
F factor
Hfr cell A+A+
A+
A+
A+A– A– A–
A– A+
RecombinantF– bacterium
Chapter 39R Plasmids and Antibiotic Resistance
• R plasmids carry genes for antibiotic resistance• Antibiotics select for bacteria with genes that
are resistant to the antibiotics• Antibiotic resistant strains of bacteria are
becoming more common
R Plasmids and Antibiotic Resistance
• R plasmids carry genes for antibiotic resistance• Antibiotics select for bacteria with genes that
are resistant to the antibiotics• Antibiotic resistant strains of bacteria are
becoming more common
Fig. 38-3(a)
Development of a malegametophyte (in pollen grain)
Microsporangium(pollen sac)
Microsporocyte (2n)
4 microspores (n)
Each of 4microspores (n)
Malegametophyte
Generative cell (n)
Ovule
(b) Development of a femalegametophyte (embryo sac)
Megasporangium (2n)
Megasporocyte (2n)
Integuments (2n)
Micropyle
MEIOSIS
Survivingmegaspore (n)
3 antipodal cells (n)
2 polar nuclei (n)
1 egg (n)
2 synergids (n)
Fem
ale gam
etop
hyte
(emb
ryo sa
c)
Ovule
Embryosac
Integuments (2n)
Ragweedpollengrain
Nucleus oftube cell (n)
MITOSIS
100
µm
20 µm
75 µm
Double Fertilization
• After landing on a receptive stigma, a pollen grain produces a pollen tube that extends between the cells of the style toward the ovary
• Double fertilization results from the discharge of two sperm from the pollen tube into the embryo sac
• One sperm fertilizes the egg, and the other combines with the polar nuclei, giving rise to the triploid (3n) food-storing endosperm
Animation: Plant Fertilization
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 38-5Stigma
Pollen tube
2 sperm
Style
Ovary
Ovule
Micropyle
Ovule
Polar nuclei
Egg
Synergid
2 sperm
Endospermnucleus (3n)(2 polar nucleiplus sperm)
Zygote (2n)(egg plus sperm)
Egg
Pollen grain
Polar nuclei
• Fruits are also classified by their development: – Simple, a single or several fused carpels– Aggregate, a single flower with multiple separate
carpels– Multiple, a group of flowers called an inflorescence
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 38-10
FlowerStamenCarpels
Ovary
Stigma
Pea flowerOvule
Seed
Carpel(fruitlet)
Raspberry flower
Stigma
Ovary
Stamen
Stamen
Pineapple inflorescence Apple flower
Stigma
Stamen
Ovule
Each segmentdevelopsfrom thecarpelof oneflower
Pea fruit Raspberry fruit Pineapple fruit Apple fruit
(a) Simple fruit (b) Aggregate fruit (c) Multiple fruit (d) Accessory fruit
Sepal
Petal Style
Ovary(in receptacle)
Sepals
Seed
Receptacle
Remains ofstamens and styles
• Fruits are also classified by their development: – Simple, a single or several fused carpels– Aggregate, a single flower with multiple separate
carpels– Multiple, a group of flowers called an inflorescence
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light
• They observed that a grass seedling could bend toward light only if the tip of the coleoptile was present
• They postulated that a signal was transmitted from the tip to the elongating region
Video: Phototropism
Fig. 39-5RESULTS
Control
Light
Light
Darwin and Darwin: phototropic responseonly when tip is illuminated
Illuminatedside ofcoleoptile
Shadedside of coleoptile
Tipremoved
Light
Tip coveredby opaquecap
Tip coveredby trans-parent cap
Site of curvature covered by opaque shield
Boysen-Jensen: phototropic response when tip separatedby permeable barrier, but not with impermeable barrier
Tip separatedby gelatin(permeable)
Tip separatedby mica(impermeable)
Table 39-1
Concept 39.3: Responses to light are critical for plant success
• Light cues many key events in plant growth and development
• Effects of light on plant morphology are called photomorphogenesis
• Plants detect not only presence of light but also its direction, intensity, and wavelength (color)
• A graph called an action spectrum depicts relative response of a process to different wavelengths
• Action spectra are useful in studying any process that depends on light
Fig. 39-17
Dark (control)
RESULTS
DarkRed
Red Far-red Red Dark Red Far-red Red Far-red
Red Far-red Dark
Fig. 39-19
Synthesis
Pr
Far-redlight
Slow conversionin darkness(some plants)
Enzymaticdestruction
Responses:seed germination,control offlowering, etc.
Pfr
Red light
• Circadian rhythms are cycles that are about 24 hours long and are governed by an internal “clock”
• Circadian rhythms can be entrained to exactly 24 hours by the day/night cycle
• The clock may depend on synthesis of a protein regulated through feedback control and may be common to all eukaryotes
Photoperiodism and Control of Flowering
• Some processes, including flowering in many species, require a certain photoperiod
• Plants that flower when a light period is shorter than a critical length are called short-day plants
• Plants that flower when a light period is longer than a certain number of hours are called long-day plants
• Flowering in day-neutral plants is controlled by plant maturity, not photoperiod
Fig. 39-2124 hours
Light
Criticaldark period
Flashof light
Darkness
(a) Short-day (long-night) plant
Flashof light
(b) Long-day (short-night) plant
Fig. 39-22
24 hours
R
RFR
RFRR
RFRRFR
Critical dark period
Short-day(long-night)
plant
Long-day(short-night)
plant
Defenses Against Herbivores
• Herbivory, animals eating plants, is a stress that plants face in any ecosystem
• Plants counter excessive herbivory with physical defenses such as thorns and chemical defenses such as distasteful or toxic compounds
• Some plants even “recruit” predatory animals that help defend against specific herbivores
Fig. 39-28
Recruitment of parasitoid wasps that lay their eggs within caterpillars
Synthesis and release of volatile attractants
Chemical in saliva
Wounding
Signal transduction pathway
1 1
2
3
4
The Hypersensitive Response
• The hypersensitive response– Causes cell and tissue death near the infection site– Induces production of phytoalexins and PR proteins,
which attack the pathogen– Stimulates changes in the cell wall that confine the
pathogen
Fig. 39-29
Signal
Hypersensitiveresponse
Signal transduction pathway
Avirulent pathogen
Signal transduction
pathway
Acquired resistance
R-Avr recognition andhypersensitive response
Systemic acquiredresistance
Systemic Acquired Resistance
• Systemic acquired resistance causes systemic expression of defense genes and is a long-lasting response
• Salicylic acid is synthesized around the infection site and is likely the signal that triggers systemic acquired resistance
Fig. 39-UN2
Plasma membrane
Reception Response
CELLWALL
CYTOPLASM
Transduction
Receptor
Hormone or environmental stimulus
Relay proteins and
second messengers
Activation of cellular responses
1 2 3