Chapter 21Chapter 21

The Genetic Basis of The Genetic Basis of DevelopmentDevelopment

IntroductionIntroduction

The development of a multicellular organism The development of a multicellular organism from a single cell is one of the most from a single cell is one of the most fascinating & complex processes to study in fascinating & complex processes to study in biology.biology.

How & when do cells become specialized? How & when do cells become specialized? How & when are certain genes silenced or How & when are certain genes silenced or expressed? How are cells “put” in the right expressed? How are cells “put” in the right place – i.e. how do we become spatially place – i.e. how do we become spatially organized in our recognizable patterns?organized in our recognizable patterns?

Complex gene interactions control the developmental process – it doesn’t quit at 9 months either! Think about the changes that occur after birth!

Studying the development of other organisms has provided insight into our own development.

►Many of the developmental principles Many of the developmental principles that biologists now understand have that biologists now understand have come from studying the development come from studying the development of fruit flies, mice, fish, worms and of fruit flies, mice, fish, worms and plants – read about some of these plants – read about some of these “model organisms” on pages 412 & “model organisms” on pages 412 & 413.413.

Embryonic DevelopmentEmbryonic Development

A.) Embryonic development relies on cell A.) Embryonic development relies on cell division, differentiation & morphogenesis.division, differentiation & morphogenesis.

1.) Cell differentiation: the process by 1.) Cell differentiation: the process by which cells become specialized in which cells become specialized in structure & function.structure & function.

2.) Morphogenesis: the physical 2.) Morphogenesis: the physical processes that give an organism its shapeprocesses that give an organism its shape

a.) Lays out the basic body plan – a.) Lays out the basic body plan – which ends are head & tail, dorsal & which ends are head & tail, dorsal & ventral, where do appendages go.ventral, where do appendages go.

B.) Differences between plant & animal B.) Differences between plant & animal development:development:

1.) In animals, cell movement is 1.) In animals, cell movement is necessary to transform the embryo to its necessary to transform the embryo to its characteristic form.characteristic form.

2.) Plants have perpetually embryonic 2.) Plants have perpetually embryonic regions called apical meristems that allow regions called apical meristems that allow for morphogenesis & differentiation for morphogenesis & differentiation throughout their lives. throughout their lives.

Cell DifferentiationCell DifferentiationA.) Different cell types result from A.) Different cell types result from

differential gene expression.differential gene expression.

1.) Experimental evidence supports the 1.) Experimental evidence supports the idea that all cells of an organism have the idea that all cells of an organism have the same genes.same genes.

a.) Cells taken from mature plants a.) Cells taken from mature plants can divide & produce an entire organism.can divide & produce an entire organism.

b.) Any cell with the ability to give b.) Any cell with the ability to give rise to all the specialized cell types in a rise to all the specialized cell types in a mature organism is said to be totipotent.mature organism is said to be totipotent.

c.) Using somatic cells from a c.) Using somatic cells from a mature individual to produce another mature individual to produce another identical individual is called cloning.identical individual is called cloning.

2.) Somatic cells from animals have 2.) Somatic cells from animals have also been used to produce clones. also been used to produce clones.

a.) Many animal clones suffer from various a.) Many animal clones suffer from various ailments.ailments.

b.) This could be due to the fact that the b.) This could be due to the fact that the chromatin of somatic cells becomes altered as chromatin of somatic cells becomes altered as organisms age. These changes may be reversed organisms age. These changes may be reversed before the cloning procedure but many times this before the cloning procedure but many times this “reprogramming” could be incomplete.“reprogramming” could be incomplete.

Will this ever happen???

B.) Stem Cells in AnimalsB.) Stem Cells in Animals

1.) Stem cell: an unspecialized cell that can 1.) Stem cell: an unspecialized cell that can reproduce itself indefinitely & differentiate reproduce itself indefinitely & differentiate into one or more types of cells (under into one or more types of cells (under appropriate conditions). appropriate conditions).

a.) Embryonic stem cells are a.) Embryonic stem cells are totipotent – can become any cell type.totipotent – can become any cell type.

b.) Adult stem cells in bone marrow, b.) Adult stem cells in bone marrow, the intestines & the brain are pluripotent: the intestines & the brain are pluripotent: can give rise to multiple (but not all) cell can give rise to multiple (but not all) cell types.types.

2.) Goals of stem cell research:2.) Goals of stem cell research:

a.) Produce & supply cells for the a.) Produce & supply cells for the repair of damaged or diseased tissues.repair of damaged or diseased tissues.

3.) Conflicts of embryonic stem cell 3.) Conflicts of embryonic stem cell research:research:

a.) Cell are obtained from early stage a.) Cell are obtained from early stage embryosembryos – usually donated by adults – usually donated by adults undergoing infertility treatments or from undergoing infertility treatments or from long-dividing lines of previously donated long-dividing lines of previously donated cells. Dilemma: when does “life” begin?cells. Dilemma: when does “life” begin?

One side of the debate…

And another side of the debate…

Transcriptional Regulation of Transcriptional Regulation of GenGen

A.) Determination: the events that lead A.) Determination: the events that lead to the observable differentiation of a to the observable differentiation of a cell. cell.

1.) At the end of the determination 1.) At the end of the determination process, a cell is committed to its final process, a cell is committed to its final fate – you can move it somewhere else fate – you can move it somewhere else & it will still become the cell that is its & it will still become the cell that is its normal fate.normal fate.

B.) The outcome of determination is the B.) The outcome of determination is the production of tissue specific proteins.production of tissue specific proteins.

1.) These are proteins that are only found in 1.) These are proteins that are only found in the specific cell type & give the cells its the specific cell type & give the cells its characteristic structure & function.characteristic structure & function.

a.) Examples: liver cells produce a.) Examples: liver cells produce albumin, lens cells in eye produce crystallins albumin, lens cells in eye produce crystallins (proteins that enable lens to transmit/focus (proteins that enable lens to transmit/focus light), etc.light), etc.

b.) Read page 420 about the regulatory b.) Read page 420 about the regulatory gene myoD in muscle cells. This gene gene myoD in muscle cells. This gene produces a transcription factor that turns on produces a transcription factor that turns on many more genes that are specific to muscle many more genes that are specific to muscle cells.cells.

C.) But what triggers expression of C.) But what triggers expression of FIRST specific gene in a cell? (For FIRST specific gene in a cell? (For example – what triggered myoD to example – what triggered myoD to be transcribed & turn that cell into a be transcribed & turn that cell into a muscle cell?)muscle cell?)

For that matter, what generates the For that matter, what generates the first differences that arise among first differences that arise among cells in an early embryo???cells in an early embryo???

1.) One source of information in 1.) One source of information in development comes for the development comes for the molecules in the egg cell’s cytoplasm molecules in the egg cell’s cytoplasm (proteins, RNA).(proteins, RNA).

2.) Maternal substances that 2.) Maternal substances that influence the course of development influence the course of development are called cytoplasmic determinants.are called cytoplasmic determinants.

3.) These molecules are distributed 3.) These molecules are distributed unevenly in the egg & this impacts unevenly in the egg & this impacts development.development.

a.) After fertilization, mitosis a.) After fertilization, mitosis divides up the egg’s cytoplasm divides up the egg’s cytoplasm unevenly into the separate unevenly into the separate embryonic cells.embryonic cells.

b.) The differences in b.) The differences in cytoplasmic determinants present at cytoplasmic determinants present at this stage helps determine each this stage helps determine each cell’s fate.cell’s fate.

A simplified view of the unequal distribution of cytoplasmic determinants.

4.) The surrounding environment also 4.) The surrounding environment also plays a role in the differentiation of plays a role in the differentiation of embryonic cells.embryonic cells.

a.) Contact with other embryonic a.) Contact with other embryonic cells surfaces & reception of growth cells surfaces & reception of growth factors, etc released by neighboring factors, etc released by neighboring cellscells

b.) Induction: the process by which b.) Induction: the process by which signal molecules cause changes in signal molecules cause changes in nearby target cells during development.nearby target cells during development.

Pattern FormationPattern Formation

A.) Pattern formation: the development A.) Pattern formation: the development of a spatial organization in which of a spatial organization in which tissues & organs of an organism are all tissues & organs of an organism are all in their characteristics places. in their characteristics places.

B.) Best studied in fruit flies (B.) Best studied in fruit flies (Drosophila Drosophila melanogastermelanogaster))

1.) Earliest work 1.) Earliest work involved breeding involved breeding flies carrying flies carrying mutations and mutations and examining examining phenotypes of phenotypes of offspring to provide offspring to provide insight into the insight into the affects of different affects of different genes involved in genes involved in pattern formation.pattern formation.

A fruit fly with legs instead of antennae.

2.) Cytoplasmic determinants 2.) Cytoplasmic determinants establish the body’s axes.establish the body’s axes.

a.) These determinants are a.) These determinants are encoded by genes of the mother – call encoded by genes of the mother – call these genes maternal effect genes.these genes maternal effect genes.

b.) If these are mutant in the b.) If these are mutant in the mother, they result in a mutant mother, they result in a mutant phenotype in the offspring because phenotype in the offspring because they will be present in a mutated form they will be present in a mutated form in the egg.in the egg.

This shows a bicoid mutant fruit fly – one with 2 posterior ends & no anterior end. If the bicoid gene is mutated in the mother, the protein doesn’t function as it should in the embryo and a fruit fly larva is produced with 2 posterior ends – and it dies.

This shows where the bicoid protein is concentrated – wherever it is more concentrated is where an anterior end forms. Read about this on page 424! If the gene is mutated, protein product is mutated – no gradient is set up and no anterior end forms in embryo.

The gradient is essential for correct pattern formation.

3.) Proteins encoded by maternal 3.) Proteins encoded by maternal effect genes (also called egg polarity effect genes (also called egg polarity genes) can regulate expression of genes) can regulate expression of some of the embryo’s own genes.some of the embryo’s own genes.

a.) These include segmentation a.) These include segmentation genes which are responsible for genes which are responsible for directing the formation of body directing the formation of body segments.segments.

4.) Homeotic genes: specify the types of 4.) Homeotic genes: specify the types of appendages/structures that will form on appendages/structures that will form on each segment.each segment.

a.) Mutations in these put structures a.) Mutations in these put structures in the wrong places.in the wrong places.

b.) Homeotic genes encode b.) Homeotic genes encode transcription factors that turn on transcription factors that turn on genes responsible for the development genes responsible for the development of specific appendages. of specific appendages.

Read about the development of the Read about the development of the worm worm C. elegans C. elegans starting on page 425.starting on page 425.

Important Lessons learned from Important Lessons learned from C. elegansC. elegans

A.) Sequential inductions drive organ A.) Sequential inductions drive organ formation – one cell signals another & formation – one cell signals another & then that cells signals another, etc.then that cells signals another, etc.

B.) The effect of an inducer can depend B.) The effect of an inducer can depend on its concentration (just like with on its concentration (just like with cytoplasmic determinants)cytoplasmic determinants)

C.) Inducers produce their effects via C.) Inducers produce their effects via signal transduction pathways.signal transduction pathways.

D.) Induced cell’s response is usually D.) Induced cell’s response is usually the activation of genes specific to a the activation of genes specific to a certain type of cell.certain type of cell.

E.) Programmed cell suicide, apoptosis, is E.) Programmed cell suicide, apoptosis, is crucial in normal development.crucial in normal development.

1.) Normal development of nervous system, 1.) Normal development of nervous system, immune system & formation of hands & feet immune system & formation of hands & feet rely on apoptosis during development.rely on apoptosis during development.

Read about plant development – pages Read about plant development – pages 429 & 430429 & 430

Evolutionary Developmental Evolutionary Developmental BiologyBiology

A.) Study the evolution of developmental A.) Study the evolution of developmental processes & how changes in these processes can processes & how changes in these processes can modify existing phenotypes or lead to new ones.modify existing phenotypes or lead to new ones.

B.) Homeotic genes of fruit flies all have a specific B.) Homeotic genes of fruit flies all have a specific nucleotide sequence called the homeobox.nucleotide sequence called the homeobox.

1.) This same sequence has been 1.) This same sequence has been discovered in the homeotic genes of many other discovered in the homeotic genes of many other animals.animals.

2.) Homeotic genes in animals 2.) Homeotic genes in animals are usually called Hox genes.are usually called Hox genes.

3.) The presence of the 3.) The presence of the homeobox sequence among diverse homeobox sequence among diverse animals suggests it evolved very early animals suggests it evolved very early & its conservation suggests its & its conservation suggests its importance. importance.

Even the placement of homeotic genes on chromosomes is the same among some very different organisms.

C.) Small changes in homeotic (Hox) C.) Small changes in homeotic (Hox) genes can lead to big changes in body genes can lead to big changes in body structures & organization.structures & organization.