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PowerPoint to accompany
Genetics: From Genes to Genomes
Fourth Edition
Hartwell ● Hood ● Goldberg ● Reynolds ● Silver
Reference
B
Prepared by Malcolm Schug
University of North Carolina Greensboro
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B-1
Arabadopsis thaliana:
Genetic Portrait of a Model Plant
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B-2
A. thaliana plant
Hybrid camellia vs. wild-type
Fig. B.1
Fig. B.2
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B-3
Outline of Reference B
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Structure and organization of the genome
Plant’s anatomy and life cycle
Techniques of mutational analysis
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Genetic analysis applied to various aspects of
development
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Chemical and radiation procedures
Insertional mutagenesis
Analysis of mutations to identify gene function
Embryogenesis
Hormonal control systems
Responses to environmental signals
Genetic analysis of flowering: a comprehensive
example
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B-4
Genome Structure and Organization
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Genome size: 125 Mb
Smallest genomes known in plant kingdom
Five pairs of small chromosomes
Well-defined banding patterns on
chromosomes
Complete sequence of genome in 2000
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B-5
Comparison of Genetic and Physical
Maps
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Ecotypes – plant varieties analogous to animal
strains with common origin, shape and phenotypes
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Columbia (Col)
Landsberg erecta (Ler)
Differ by DNA polymorphisms and phenotypes
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Segregation patterns of self-pollinating plants from crosses
between ecotypes
Linkage between DNA markers themselves
Linkage between DNA markers and morphological loci
Create maps of DNA markers and phenotypes and integrate
them
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B-6
Characteristics of A.thaliana Genome
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Genome has little repetitive DNA and tight
arrangement of genes
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Protein coding genes 40% of genome
One gene every 4 kb
65% of proteins encoded by > 1 gene
Derived from ancient tetraploid
Multiple copies of transposable elements
DNA that originated in mitochondria and chloroplasts
Centromeres are heterochromatic regions
Genetic and physical distances vary widely
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B-7
Functional Analysis of Arabadobsis
Genes
Figure B.4
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B-8
The Basic Body Plan
Fig. B.5
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B-9
Life Cycle from Fertilization to
Flowering to Senescence
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Each ovule has six mononucleate cells
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One nucleus and two nuclei
Pollen triggers fertilization by landing on stigma
Fig. B.6 a,b
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B-10
Fertilization
Fig. B.6 c
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B-11
Insert figure B.7 from Hartwell 3e as shown
on slide pages B12 and B13.
Figure B.7
Stages of
Embryonic
Development
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B-12
From heart stage
Final Stages
of
Embryonic
Development
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B-13
Favorable Environmental Conditions Trigger
Seed Germination and Vegetative Growth
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Gravity sensed by
root and shoot
Light hypocotyl
develops rapidly in
dark and apical
meristem and
cotyledon growth
suppressed

Fig. B.8
photomorphogenesis
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B-14
Reproductive Development Begins when
Leaf-producing Apical Meristem Switches to
Flower-producing Apical Meristem
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Environmental signals
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Fig. B.9
Photoperiod – day length
Vernalization – exposing to
cold
Scenescence – vegetative
plant ages and dies
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B-15
Mutagenesis by Chemical and Irradiation Produces
Different Ratios of Various Mutations
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Seed contains dormant embryo from which
1-3 cells from apical meristem are destined
to form the germ line
Mutational segregation depends on how
many gamete cells are present during
mutagenesis
One cell – ¼ progeny have mutation
 Two cells – 1/8 progeny
 Three cells – 1/12 progeny
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B-16
Transformation by T-DNA
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Bacterium Agrobacterium
tumefaciens is agent of
transformation
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Fig. B.10 a
Transfer of plasmid DNA
called T-DNA into genome
of wounded plant
Antibiotic resistance
engineered into plasmids
provide selectable markers
GUS reporter gene with no
promoter stain blue when
inserted in gene of plant
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B-17
Transformation by Transposon
Tagging
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Transposon tagging using transposable
elements from Corn and Arabidobsis
Insertional mutagenesis not only allows
generation of mutations but also facilitates
molecular characterization
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Transposon or T-DNA can be used as a probe to
identify and clone gene insert disrupts
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B-18
Silencing Specific Genes through Antisense RNAs or RNAs
Carrying Inverted Duplications of Gene Fragments
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Activation of defense mechanism evolved to
protect plants from invading viruses or
transposons
Double-stranded RNAs are targeted by
ribonuclease (Dicer) and cleaved into 21 bp
fragments
Fragments of dsRNA recognize homologous
molecules in cell and promote degradation
Gene knockout
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Induction of recombinant gene that carries inverted
repeat of gene of interest (endogene)
RNA forms hairpin loop and is degraded by Dicer
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B-19
Fig. B.11
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B-20
Genetic Analysis of Embryogenesis
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Researcher have found very few maternal-effect
mutations in Arabadopsis
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Occur in maternal genes whose products are deposited
in egg and alter embryogenesis
Major difference between plants and certain
animals such as Drosophila
Lack of maternal-effect mutants is mostly due to
cytoplasm that is produced by zygote’s own
genome
Few zygotic mutations affect embryogenesis
before globular stage
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Zygotic genes encode proteins that are functionally
redundant with those of maternal origin
Many genes are members of multigene families
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B-21
Screens for Mutations that Arrest Development at
Specific Stages of Embryogenesis Help Identify
Regulatory Processes
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(a) LEC genes help
regulate seed
maturation
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Figure B.12 a,b
LEC2 encodes
transcription factor
controlling
differentiating cells into
embryo
(b) TWIN represses
embryo development
program in suspensor
cells
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B-22
Mutations that Disrupt Hormone Activity Clarify
Biological Significance and Biosynthetic Pathway
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Fig. B.13
gal-1 is an x-ray
induced deletion
that helped
identify it as gene
in gibberellin
biosynthetic
pathway
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B-23
Mutations that Render Arabadopsis Insensitive to a
Hormone Reveal how Plants Perceive and Transduce
Hormone Signals
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Ethylene exposure inhibits shoot and root
elongation and accentuation of apical hook
Mutant seedlings grow tall in presence of ethylene
Figure 14
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B-24
Ethylene Biosynthetic Pathway
Fig. B. 15
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B-25
Mutational Analysis has Helped Characterize the
Photoreceptor Molecules by Which Plants Receive
Light Signals
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hy (hypocotyle) mutants show reduced sensitivity to light
hy3 and hy8 coincide with phyA and phyB on linkage maps
– members of phytochrome gene family
 hy3 altered response to red wavelengths
 hy8 altered response to far-red wavelengths
 Same phytochrome family
hy4 decreased sensitivity to blue light
hy5 encodes transcription factor controls expression of
genes contributing to morphogenesis
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B-26
Mutations that Affect Response to Light
Fig. B.16 a
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B-27
Mutational Studies of How Plants
Process Light Signals
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cop (constitutive photomorphogenesis)
det (de-etiolated – released from development without
chlorophyll)
Grow in dark as if they received light
Recessive
COPI is negative regulator, mediating degradation of
transcription factors needed for photomorphogenesis
COPI also interacts directly with cytochrome blue-light
photoreceptors
det2 mutant shows degradation of light-regulating genes,
but does not develop chloroplasts – dwarf adult
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B-28
Genetic Analysis of Flowering:
A Comprehensive Example
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Switch to reproductive growth (flowering)
involves reprogramming apical meristem
Becomes inflorescence meristem (IM)
 Produces smaller leaves, elongated stem, and
many side shoots
 IM is indeterminate because it produces floral
meristems (FMs) indefinitely
 FMs are determinant because they produce a
fixed number of floral organs
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B-29
Fig. B.17
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B-30
Analysis of Homeotic Mutations Reveal Three
Types of Single-gene Products Influencing
Floral Pattern Formation
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Homeotic gene – plays a role in determining a
tissue’s identity during development
Homeotic mutation – cells misinterpret their
position and become normal organs in
inappropriate tissues
Three classes of mutations
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Class A – carpels instead of sepals in first whorl and
stamens instead of petals in second whorl
Class B – sepals in first and second whorl, and carpels
in third and fourth whorls
Class C – abnormal radial pattern of sepals, petals,
petals, sepals
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B-31
Insert
B.18a
again –
this
image
blurry
Fig. B.18 a
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B-32
How Three Classes of Genes Could
Determine Identity of Floral Organs
Fig. B.18
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B-33
How Three Classes of Genes Could Determine
Identity of Floral Organs
c) ap2 / ap3, ap2 / ag, and ap3 /ag double mutants
ap2/ap3/ag triple mutant
Fig. B.18
AP3 activity
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B-34
Early Acting Genes Specify Identity
of Floral Meristem
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FM produces flower primordium that differentiates
into four whorls of organs, two of which contain
gametes
Loss of function single and double mutant analysis
Gain of function transgenic plants
Fig. B.19
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B-35
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B-36
Some Genes Control Timing of FM
Formation and Flowering
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B-37
Hundreds of Mutational Snalyses have Generated
Preliminary Model-to-Guide Future Research on
Flowering-a Preliminary Genetic Model of Process
Fig. B.20
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B-38