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CHAPTER 19

DEVELOPMENTAL

GENETICS

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Development

Refers to a series of changes in the state of the cell, tissue, organ, or organism

Underlying process that gives rise to the structure and function of living organisms

Developmental genetics aimed at understanding how gene expression controls this process

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Sperm and egg unite to produce a zygote That diploid cell divides and develops into the

embryo Cells divide and begin to arrange themselves Each cell becomes determined – destined to

become a particular cell type Commitment occurs before differentiation –

cell’s function and morphology have permanently changed into a highly specialized cell type

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Genome is a set of genes that constitute the program of development

Unicellular organisms – controls structure and function of the single cell

Multicellular – controls cellular features and the arrangement of cells

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Model organisms Fruit fly Drosophila melanogaster

Advanced techniques for generating and analyzing mutants

Large enough for easy study but small enough to determine where genes are expressed

Nematode worm Caenorhabditis elegans Simplicity – only about a thousand somatic cells Pattern of cell division and fate of each cell known

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House mouse Mus musculus Zebrafish Brachydanio rerio Thale cress Arabidopsis thaliana

Wild mustard family Short generation time, small genome

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Pattern formation

Coordination of events leading to the formation of a body with a particular pattern

Formation of an adult body with 3 axesDorsoventral, anteroposterior, and right-leftMay also be segmentedPlant bodies are formed along a root-shoot

axis in a radial pattern

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Positional information Each cell in the body must become the

appropriate cell type based on its relative position

Each cell receives positional information that tells it where to go and what to become

Cell may respond byCell division, cell migration, cell differentiation or

cell death (apoptosis)

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2 main mechanisms used to communicate positional informationMorphogens Cell adhesion

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Morphogens

Give positional information and promote cellular changes

Act in a concentration dependent manner with a critical threshold concentration

Distributed asymmetrically In the oocyte or egg precursor In the embryo by secretion and transport

Induction – cells govern fate of neighboring cells

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Cell adhesion

Each cell makes its own cell adhesion molecules (CAMs)

Positioning of a cell within a multicellular organism is strongly influenced by the combination of contacts it makes with other cells and with the extracellular matrix

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Hierarchy of transcription factors

Four general phases for body formation Organize body along major axes Organize into smaller regions (organs, legs) Cells organize to produce body parts Cells themselves change morphologies and become

differentiated Differential gene regulation – certain genes expressed

at specific phase of development in a particular cell type

Parallel between phases and expression of specific transcription factors

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Animal development

Drosophila model Oocyte establishes pattern for adult

Elongated cell with positional information After fertilization, zygote develops into blastoderm

Series of nuclear divisions without cytoplasmic division (produces many free nuclei) synctial blastoderm

Individual cells are created after nuclei line up along cell membrane (cellular blastoderm)

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Gastrulation involves cells migrating to the interior3 cell layers formed- ectoderm, mesoderm and

endoderm Segmented body plan develops

Head, thorax and abdomen Larva – free living Pupa – undergoes metamorphosis Adult Egg to adult in 10 days

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Phase 1 Pattern development

First phase is establishment of body axes Morphogens are distributed prior to fertilization Bicoid, example morphogen

Mutation results in larva with 2 posterior ends Nurse cells are located near anterior end of oocyte Bicoid gene transcribed in nurse cells and mRNA transported

into anterior end of oocyte Maternal effect Transcription factor that activates particular genes at specific

times Asymmetrical distribution means activated only in certain regions

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Phase 2 Segments Normal Drosophila embryo divided into 15

segments3 head, 3 thoracic and 9 abdominalEach will give rise to unique morphological

features in adult

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Bithorax gene complexNormal – wings on 2nd thoracic segment and 2

halteres on 3rd thoracic segmentMutant – 3rd segment has wings so 2 sets of

wings and no halteres

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3 classes of segmentation genes

Gap genesMutation – several adjacent segments are missing

Pair-rule genesMutation – alternating segments or parts of

segments deleted Segment-polarity genes

Mutation – portions of segments to be missing either anterior or posterior region and adjacent regions to mirror each other

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To create a segment in phase 2, a group of genes acts sequentially to govern the fate of a given body region

Maternal effect genes, which promote phase 1 pattern development, activate gap genes Seen as broad bands of gap gene expression in the embryo

Gap genes and maternal effect genes then activate the pair-rule genes in alternating stripes in the embryo

Once the pair-rule genes are activated, their gene products then regulate the segment-polarity genes

Expression of a segment-polarity gene corresponds to portions of segments in the adult fly

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Phase 3 Segment characteristics Each segment begins to develop its own unique

characteristics Cell fate – ultimate morphological features that

a cell or group of cells will adopt Mutations in homeotic genes alter cell fate

Bithorax is an example of a homeotic mutation Order of homeotic genes along chromosome

corresponds to their expression along the anteroposterior axis of bodyColinearity rule

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Role of homeotic genes to determine identity of particular segments

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Homeotic genes encode homeotic proteins that functionas transcription factors Activate transcription of

specific genes that promote developmental changes

Homeobox – coding sequence of homeotic genes contains 180-bp sequence Encodes homeodomain for

DNA binding

A Homologous Group of Homeotic Genes Is Found in All Animals

Identify vertebrate genes that are homologous to those that control development in simpler organisms such as DrosophilaHybridization techniques

Homologous genes are evolutionarily derived from the same ancestral gene and have similar DNA sequences

Hox genes in miceFollow colinearity ruleKey role in patterning anteroposterior axis

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Phase 4 Cell differentiation

Emphasis shifts to cell differentiation Studied in mammalian cell culture lines Differential gene expression underlies cell

differentiation Stem cell characteristics

Capacity to divideDaughter cells can differentiate into 1 or more cell

types

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Stem cell categories

TotipotentUltimate stem cell is fertilized eggCan produce all adult cell types

PluripotentEmbryonic stem cells (ES cells)Embryonic germ cells (EG cells)Can differentiate into almost any cell but a

single cell has lost the ability to produce an entire individual

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MultipotentCan differentiate far fewer types of cellsHematopoietic stem cells (HScs)

UnipotentDaughter cells become only one cell typeStem cells in testis produce only sperm

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Robert Davis, Harold Weintraub, and AndrewLasser Identified Genes Encoding TranscriptionFactors That Promote Muscle Cell Differentiation

What causes stem cells to differentiate into a particular cell type?

Certain proteins function as “master transcription factors”

Initial experimental strategy to identify genes expressed only in differentiating muscle cells

Narrowed down to 3 genes Would any of these 3 genes cause nonmuscle cells to

differentiate into muscle cells?

MyoD was the only one to cause fibroblasts to differentiate into muscle cells

Belongs to myogenic bHLH genes Found in all vertebrates and activated during skeletal muscle

development Features promoting muscle cell differentiation

Basic domain binds specifically to an enhancer DNA sequence that is adjacent to genes that are expressed only in muscle cells

Protein contains an activation domain that stimulates the ability of RNA polymerase to initiate transcription

Interacts with other cellular proteins Id – prevents muscle differentiation too soon E – activates gene expression

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Plant development 2 key features of complex plant morphology

Root-shoot axisRadial pattern

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Differs from animal developmentNo cell migrationNo morphogensAn entirely new individual can be regenerated

from somatic cells (totipotent) Similarities to animal development

Use differential gene expressionUse of transcription factors

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Arabidopsis model for development After fertilization, the first cellular division is

asymmetrical and produces a smaller apical cell and a larger basal cell Apical cell – gives rise to most of embryo and shoot Basal cell – root and suspensor cell for seed

formation Heart stage – about 100 cells

Basic organization established Root meristem – gives rise only to root Shoot meristem – all aerial parts of plant – stem,

leaves, flowers 2 cotyledons to store nutrients

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Seedling3 main regions

Apical region – leaves and flowers

Central region – stem Basal region – roots

Shoot meristem organizing center

Central zone – maintains undifferentiated stem cells

Peripheral zone – dividing cells that will differentiate

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Apical, central and basal regions express different sets of genes

Apical-basal patterning genes

Defects can cause dramatic effects

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Plant homeotic genes

First known homeotic genes discovered in plants

Normal Arabidopsis flower composed of 4 concentric whorls

1. Sepals – outer whorl, protects bud2. Petals3. Stamens – make pollen (male gametophyte)4. Carpel – produces female gametophyte

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ABC model for flower development3 gene classes A, B, C and EWhorl 1 – gene A product = sepalsWhorl 2 – gene A, B and E products = petalsWhorl 3 – gene B, C and E products =

stamensWhorl 4 – gene C and E products = carpel

Leaf structure is default pathway If A,B,C and E are defective, leaves result

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