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Chapter 21. The Genetic Basis of Development. Introduction. The development of a multicellular organism from a single cell is one of the most fascinating & complex processes to study in biology.
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Chapter 21 The Genetic Basis of Development
Introduction The development of a multicellular organism from a single cell is one of the most fascinating & complex processes to study in biology. How & when do cells become specialized? How & when are certain genes silenced or expressed? How are cells “put” in the right place – i.e. how do we become spatially 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 that biologists now understand have come from studying the development of fruit flies, mice, fish, worms and plants – read about some of these “model organisms” on pages 412 & 413.
Embryonic Development A.) Embryonic development relies on cell division, differentiation & morphogenesis. 1.) Cell differentiation: the process by which cells become specialized in structure & function. 2.) Morphogenesis: the physical processes that give an organism its shape
a.) Lays out the basic body plan – which ends are head & tail, dorsal & ventral, where do appendages go. B.) Differences between plant & animal development: 1.) In animals, cell movement is necessary to transform the embryo to its characteristic form. 2.) Plants have perpetually embryonic regions called apical meristems that allow for morphogenesis & differentiation throughout their lives.
Cell Differentiation A.) Different cell types result from differential gene expression. 1.) Experimental evidence supports the idea that all cells of an organism have the same genes. a.) Cells taken from mature plants can divide & produce an entire organism. b.) Any cell with the ability to give rise to all the specialized cell types in a mature organism is said to be totipotent.
c.) Using somatic cells from a mature individual to produce another identical individual is called cloning. 2.) Somatic cells from animals have also been used to produce clones.
a.) Many animal clones suffer from various ailments. b.) This could be due to the fact that the chromatin of somatic cells becomes altered as organisms age. These changes may be reversed before the cloning procedure but many times this “reprogramming” could be incomplete. Will this ever happen???
B.) Stem Cells in Animals 1.) Stem cell: an unspecialized cell that can reproduce itself indefinitely & differentiate into one or more types of cells (under appropriate conditions). a.) Embryonic stem cells are totipotent – can become any cell type. b.) Adult stem cells in bone marrow, the intestines & the brain are pluripotent: can give rise to multiple (but not all) cell types.
2.) Goals of stem cell research: a.) Produce & supply cells for the repair of damaged or diseased tissues. 3.) Conflicts of embryonic stem cell research: a.) Cell are obtained from early stage embryos – usually donated by adults undergoing infertility treatments or from long-dividing lines of previously donated cells. Dilemma: when does “life” begin?
Transcriptional Regulation of Gen A.) Determination: the events that lead to the observable differentiation of a cell. 1.) At the end of the determination process, a cell is committed to its final fate – you can move it somewhere else & it will still become the cell that is its normal fate.
B.) The outcome of determination is the production of tissue specific proteins. 1.) These are proteins that are only found in the specific cell type & give the cells its characteristic structure & function. a.) Examples: liver cells produce albumin, lens cells in eye produce crystallins (proteins that enable lens to transmit/focus light), etc. b.) Read page 420 about the regulatory gene myoD in muscle cells. This gene produces a transcription factor that turns on many more genes that are specific to muscle cells.
C.) But what triggers expression of FIRST specific gene in a cell? (For example – what triggered myoD to be transcribed & turn that cell into a muscle cell?) For that matter, what generates the first differences that arise among cells in an early embryo???
1.) One source of information in development comes for the molecules in the egg cell’s cytoplasm (proteins, RNA). 2.) Maternal substances that influence the course of development are called cytoplasmic determinants. 3.) These molecules are distributed unevenly in the egg & this impacts development.
a.) After fertilization, mitosis divides up the egg’s cytoplasm unevenly into the separate embryonic cells. b.) The differences in cytoplasmic determinants present at this stage helps determine each cell’s fate.
A simplified view of the unequal distribution of cytoplasmic determinants.
4.) The surrounding environment also plays a role in the differentiation of embryonic cells. a.) Contact with other embryonic cells surfaces & reception of growth factors, etc released by neighboring cells b.) Induction: the process by which signal molecules cause changes in nearby target cells during development.
Pattern Formation A.) Pattern formation: the development of a spatial organization in which tissues & organs of an organism are all in their characteristics places. B.) Best studied in fruit flies (Drosophila melanogaster)
1.) Earliest work involved breeding flies carrying mutations and examining phenotypes of offspring to provide insight into the affects of different genes involved in pattern formation. A fruit fly with legs instead of antennae.
2.) Cytoplasmic determinants establish the body’s axes. a.) These determinants are encoded by genes of the mother – call these genes maternal effect genes. b.) If these are mutant in the mother, they result in a mutant phenotype in the offspring because they will be present in a mutated form 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.
The gradient is essential for correct pattern formation. 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.
3.) Proteins encoded by maternal effect genes (also called egg polarity genes) can regulate expression of some of the embryo’s own genes. a.) These include segmentation genes which are responsible for directing the formation of body segments.
4.) Homeotic genes: specify the types of appendages/structures that will form on each segment. a.) Mutations in these put structures in the wrong places.
b.) Homeotic genes encode transcription factors that turn on genes responsible for the development of specific appendages. Read about the development of the worm C. elegans starting on page 425.
Important Lessons learned from C. elegans A.) Sequential inductions drive organ formation – one cell signals another & then that cells signals another, etc. B.) The effect of an inducer can depend on its concentration (just like with cytoplasmic determinants)
C.) Inducers produce their effects via signal transduction pathways. D.) Induced cell’s response is usually the activation of genes specific to a certain type of cell.
E.) Programmed cell suicide, apoptosis, is crucial in normal development. 1.) Normal development of nervous system, immune system & formation of hands & feet rely on apoptosis during development.
Evolutionary Developmental Biology A.) Study the evolution of developmental processes & how changes in these processes can modify existing phenotypes or lead to new ones. B.) Homeotic genes of fruit flies all have a specific nucleotide sequence called the homeobox. 1.) This same sequence has been discovered in the homeotic genes of many other animals.
2.) Homeotic genes in animals are usually called Hox genes. 3.) The presence of the homeobox sequence among diverse animals suggests it evolved very early & its conservation suggests its importance.
Even the placement of homeotic genes on chromosomes is the same among some very different organisms.
C.) Small changes in homeotic (Hox) genes can lead to big changes in body structures & organization.
