Chapter 21 The Genetic Basis of Development “Embryology is to me by far the strongest single class of facts in favor of change of forms, and not one, I think, of my reviewers has alluded to this.” -Charles Darwin, 1860 “Embryology is to me by far the strongest single class of facts in favor of change of forms, and not one, I think, of my reviewers has alluded to this.” -Charles Darwin, 1860

Zygote and Cell Division F When the zygote divides, it undergoes 3 major changes: F 1. Cell division F 2. Cell differentiation F 3. Morphogenesis F When the zygote divides, it undergoes 3 major changes: F 1. Cell division F 2. Cell differentiation F 3. Morphogenesis

Cell Signaling F Cell signaling is largely responsible for the developmental processes. Movie

1. Cell Division F Cell division gives rise to numerous cells.

2. Cell Differentiation F Cell differentiation is the process by which cells become specialized in form and function. These cells undergo changes that organize them into tissues and organs.

3. Morphogenesis F As the dividing cells begin to take form, they are undergoing morphogenesis which means the “creation of form.” F Morphogenetic events lay out the development very early on in development as cell division, cell differentiation and morphogenesis overlap. F As the dividing cells begin to take form, they are undergoing morphogenesis which means the “creation of form.” F Morphogenetic events lay out the development very early on in development as cell division, cell differentiation and morphogenesis overlap.

3. Morphogenesis F These morphogenetic events “tell” the organism where the head and tail are, which is the front and back, and what is left and right. F As time progresses, later morphogenetic events will give instructions as to where certain appendages will be located. F These morphogenetic events “tell” the organism where the head and tail are, which is the front and back, and what is left and right. F As time progresses, later morphogenetic events will give instructions as to where certain appendages will be located.

Morphogenetic Events F Morphogenetic events, as well as cell division and differentiation, take place in all multicellular organisms. F Morphogenesis differs in 2 major ways in plants and animals: F 1. In animals, movements of cells and tissues are required for the transformation of the early embryo into the characteristic 3D form of the organism. F 2. In plants, morphogenesis and growth in overall size are not limited to embryonic and juvenile periods, they occur throughout the life of the plant. F Morphogenetic events, as well as cell division and differentiation, take place in all multicellular organisms. F Morphogenesis differs in 2 major ways in plants and animals: F 1. In animals, movements of cells and tissues are required for the transformation of the early embryo into the characteristic 3D form of the organism. F 2. In plants, morphogenesis and growth in overall size are not limited to embryonic and juvenile periods, they occur throughout the life of the plant.

Apical Meristems F For example, apical meristems of plants are responsible for a plant’s continued growth and development and the formation of new organs throughout the plant’s life. These are perpetually embryonic regions in the tips of shoots and roots.

The Experiments of F.C. Steward F In the 1950’s, Steward was working with carrot plants. F He showed that cells taken from the root of the plant would grow into an adult carrot when cultured in growth medium. These plants were clones of the original. F It demonstrated that differentiation doesn’t involve irreversible changes in DNA; that cells can dedifferentiate; some cells are totipotent while other cells are pluripotent. F In the 1950’s, Steward was working with carrot plants. F He showed that cells taken from the root of the plant would grow into an adult carrot when cultured in growth medium. These plants were clones of the original. F It demonstrated that differentiation doesn’t involve irreversible changes in DNA; that cells can dedifferentiate; some cells are totipotent while other cells are pluripotent.

Totipotent Cells F Totipotency is the ability of a single cell to divide and produce all the differentiated cells in an organism, including extraembryonic tissues. F Totipotent cells are formed as a result of sexual reproduction. F Totipotency is the ability of a single cell to divide and produce all the differentiated cells in an organism, including extraembryonic tissues. F Totipotent cells are formed as a result of sexual reproduction.

Pluripotent  In cell biology, the definition of pluripotency has come to refer to a stem cell that has the potential to differentiate into any of the three germ layers: endoderm, mesoderm, or ectoderm.  Pluripotent stem cells can give rise to any fetal or adult cell type. However, alone they cannot develop into a fetal or adult animal because they lack the potential to contribute to extraembryonic tissue, such as the placent.  In cell biology, the definition of pluripotency has come to refer to a stem cell that has the potential to differentiate into any of the three germ layers: endoderm, mesoderm, or ectoderm.  Pluripotent stem cells can give rise to any fetal or adult cell type. However, alone they cannot develop into a fetal or adult animal because they lack the potential to contribute to extraembryonic tissue, such as the placent.

Animals F The ongoing development in adult animals is normally restricted to the generation of cells that need to be continually replenished: blood cells, skin cells and the cells lining the intestine for example.

Multicellular Organisms F The cells of multicellular organisms come almost entirely from differences in gene expression. Regulatory mechanisms turn certain genes on and off during development. F These regulatory mechanisms are what makes cells different because nearly all cells have the same genetic complement. F The cells of multicellular organisms come almost entirely from differences in gene expression. Regulatory mechanisms turn certain genes on and off during development. F These regulatory mechanisms are what makes cells different because nearly all cells have the same genetic complement.

Cloning F Using the somatic cells of a multicellular organism to generate a new organism is called cloning. Each clone is genetically identical to the parent plant. F Differentiated cells don’t usually divide in culture, so researchers had to take a different approach to decide if animal cells were totipotent. F Using the somatic cells of a multicellular organism to generate a new organism is called cloning. Each clone is genetically identical to the parent plant. F Differentiated cells don’t usually divide in culture, so researchers had to take a different approach to decide if animal cells were totipotent.

What Researchers Did… F They removed the nucleus of an unfertilized egg and replaced it with one from a differentiated cell. F The process is called nuclear transplantation. F If the transplanted cell retains all of its genetic information, the recipient cell should develop with all of the necessary tissues and organs. F They removed the nucleus of an unfertilized egg and replaced it with one from a differentiated cell. F The process is called nuclear transplantation. F If the transplanted cell retains all of its genetic information, the recipient cell should develop with all of the necessary tissues and organs.

Nuclear Transplantation F As these experiments were conducted on frogs, it was determined that something in the DNA does change. F In tadpoles, normal development proceeded, but as the age of the donor nucleus increased, the percentage of organisms that developed correctly decreased. F As these experiments were conducted on frogs, it was determined that something in the DNA does change. F In tadpoles, normal development proceeded, but as the age of the donor nucleus increased, the percentage of organisms that developed correctly decreased.

Nuclear Transplantation F Continued research showed that the DNA remains the same for the most part, but the chromatin changes in a way that problems arise.

Nuclear Transplantation F Often times, the histones get modified or DNA is methylated and these changes in the chromatin prevent dedifferentiation. F Sometimes the process is reversible, but usually it isn’t. One thing is certain, most scientists agree that all cells contain the necessary genetic information to make an entire organism. However, the different cell types exist because of the variations in gene expression. F Often times, the histones get modified or DNA is methylated and these changes in the chromatin prevent dedifferentiation. F Sometimes the process is reversible, but usually it isn’t. One thing is certain, most scientists agree that all cells contain the necessary genetic information to make an entire organism. However, the different cell types exist because of the variations in gene expression.

Nuclear Transplanting and Cloning F In 1997, Scottish researchers cloned a sheep named Dolly. F They used cells from mammary tissue in an adult sheep, implanted the nucleus from the cell into egg cells from which the nucleus had been removed and implanted into the uterus of a lamb. F In 1997, Scottish researchers cloned a sheep named Dolly. F They used cells from mammary tissue in an adult sheep, implanted the nucleus from the cell into egg cells from which the nucleus had been removed and implanted into the uterus of a lamb.

Nuclear Transplanting and Cloning F Analysis of the DNA from Dolly showed it was identical to that of the original sheep, and its mitochondria matched that of the mother lamb. F However, Dolly’s cells appeared older than her age would indicate. F Analysis of the DNA from Dolly showed it was identical to that of the original sheep, and its mitochondria matched that of the mother lamb. F However, Dolly’s cells appeared older than her age would indicate.

Dolly’s Problems F She suffered from a lung disease seen in older sheep. F She had arthritis. F These results indicate that not all of the DNA had been reprogrammed. F She suffered from a lung disease seen in older sheep. F She had arthritis. F These results indicate that not all of the DNA had been reprogrammed.

Problems With Animal Cloning In General: F Many of the animals exhibit a variety of defects such as obesity and premature death. F Only a small percentage of the embryos created develop correctly resulting in live birth. F Possible reasons for these results include: F Epigenetic changes in chromatin (acetylation of histones and/or methylation of DNA) result in only a small number of genes being turned on while the others remain suppressed. F Many of the animals exhibit a variety of defects such as obesity and premature death. F Only a small percentage of the embryos created develop correctly resulting in live birth. F Possible reasons for these results include: F Epigenetic changes in chromatin (acetylation of histones and/or methylation of DNA) result in only a small number of genes being turned on while the others remain suppressed.

Stem Cells F The use of stem cells, especially embryonic stem cells, has many obvious medical applications. F There are obvious ethical dilemmas that arise from the research. F There are moral issues on both sides: F One is that it is immoral to tamper with human embryos for medical purposes. F The other is that it is immoral not to because the benefits outweigh the cost of doing nothing. F The use of stem cells, especially embryonic stem cells, has many obvious medical applications. F There are obvious ethical dilemmas that arise from the research. F There are moral issues on both sides: F One is that it is immoral to tamper with human embryos for medical purposes. F The other is that it is immoral not to because the benefits outweigh the cost of doing nothing.

Cell Differentiation F There are 2 major things telling a cell when and how to differentiate: F 1. The “stuff” found within the egg at the time of conception. F 2. The environment in which the embryo develops. F There are 2 major things telling a cell when and how to differentiate: F 1. The “stuff” found within the egg at the time of conception. F 2. The environment in which the embryo develops.

1. The “Stuff” in the Egg F The egg cell’s cytoplasm contains RNA and protein molecules encoded by the mother’s DNA. F mRNA, proteins, organelles, and other substances are scattered unevenly throughout the cytoplasm of an unfertilized egg. F These maternal substances influence the course of early development called cytoplasmic determinants. F The egg cell’s cytoplasm contains RNA and protein molecules encoded by the mother’s DNA. F mRNA, proteins, organelles, and other substances are scattered unevenly throughout the cytoplasm of an unfertilized egg. F These maternal substances influence the course of early development called cytoplasmic determinants.

Cytoplasmic Determinants F Following fertilization, mitotic divisions distribute the zygote’s cytoplasm into separate cells. F The nuclei of these cells are subjected to many different cytoplasmic determinants. F What has been received will determine the developmental fate of each of the cells. F Following fertilization, mitotic divisions distribute the zygote’s cytoplasm into separate cells. F The nuclei of these cells are subjected to many different cytoplasmic determinants. F What has been received will determine the developmental fate of each of the cells.

Cytoplasmic Determinants F Cytoplasmic determinants help to create an animal’s 3D arrangement before morphogenesis can shape the animal.

2. The Environment F The environment in which the embryo develops plays an important factor in outcome of the developing organism. F The surface contact of cell-to-cell interaction helps to signal development. F By the process of induction, an embryo’s genes signal the expression of proteins that cause changes in nearby target cells. F These signals send a cell down a specific developmental pathway--inducing further differentiation of the many specialized cells within the new organism. F The environment in which the embryo develops plays an important factor in outcome of the developing organism. F The surface contact of cell-to-cell interaction helps to signal development. F By the process of induction, an embryo’s genes signal the expression of proteins that cause changes in nearby target cells. F These signals send a cell down a specific developmental pathway--inducing further differentiation of the many specialized cells within the new organism.

Pattern Formation F Pattern formation is the development of spatial organization in which the tissues and organs of an organism are all in their characteristic places. F In plants, pattern formation occurs through the life of the plant. F In animals, it is restricted to the embryonic or juvenile stage. F Pattern formation is the development of spatial organization in which the tissues and organs of an organism are all in their characteristic places. F In plants, pattern formation occurs through the life of the plant. F In animals, it is restricted to the embryonic or juvenile stage.

Pattern Formation F Pattern formation in animals begins in the embryo when the major axes are determined. F Before tissues and organs within an animal can be formed, the 3D arrangement must be established. Recall that this occurs as a result of cytoplasmic determinants. F This process has been extensively studied in many animals such as the fruit fly, sea urchin, frog, nematode, and chicken. F Pattern formation in animals begins in the embryo when the major axes are determined. F Before tissues and organs within an animal can be formed, the 3D arrangement must be established. Recall that this occurs as a result of cytoplasmic determinants. F This process has been extensively studied in many animals such as the fruit fly, sea urchin, frog, nematode, and chicken.

Pattern Formation, An Example F Here is an example of pattern formation and cell signaling as seen in the fruit fly. Movie

Apoptosis F Apoptosis is the programmed cell death that occurs through the normal course of development. F It is usually triggered by signals that activate a cascade of signal proteins in cells that are to die. F During the process, the cell shrinks, the nucleus breaks down and the nearby cells quickly engulf and break down the contents of the cell. F Apoptosis is the programmed cell death that occurs through the normal course of development. F It is usually triggered by signals that activate a cascade of signal proteins in cells that are to die. F During the process, the cell shrinks, the nucleus breaks down and the nearby cells quickly engulf and break down the contents of the cell.

Apoptosis F Apoptosis is essential to the development of all cells. The process helps in the growth and development of the major structures and systems of an organism. F It controls cell division helping to slow or stop division in certain cells. F Apoptosis is essential to the development of all cells. The process helps in the growth and development of the major structures and systems of an organism. F It controls cell division helping to slow or stop division in certain cells.

Homeotic Genes F Looking across species, there are many similarities in the genes controlling development.

Homeotic Genes F A homeobox is a DNA sequence found within genes that are involved in the regulation of patterns of development (morphogenesis). F Genes that have a homeobox are called homeobox genes and form the homeobox gene family. F A homeobox is a DNA sequence found within genes that are involved in the regulation of patterns of development (morphogenesis). F Genes that have a homeobox are called homeobox genes and form the homeobox gene family.

Homeotic Genes  The most studied and the most conserved group of homeodomain proteins are the Hox genes, which control segmental patterning during development.  Not all homeodomain proteins are Hox proteins.  The most studied and the most conserved group of homeodomain proteins are the Hox genes, which control segmental patterning during development.  Not all homeodomain proteins are Hox proteins.

Homeotic Genes F Many distantly related eukaryotes such as plants and yeasts also have these Hox genes (regulatory sequences). F Such similarities indicate that the homeobox sequence is very useful in development and arose very early on in evolution and has been conserved for hundreds of millions of years. F Many distantly related eukaryotes such as plants and yeasts also have these Hox genes (regulatory sequences). F Such similarities indicate that the homeobox sequence is very useful in development and arose very early on in evolution and has been conserved for hundreds of millions of years.

Homeotic Genes F Not all Hox genes are homeotic genes-- not all of them control body parts. However, most are involved in development.

Homeotic Genes F Research has revealed that the homeobox- encoded region is part of the protein that functions as a transcription regulator. F The shape of the encoded region allows it to bind to any DNA segment, but by itself, it cannot select a specific sequence. The variable regions within the whole protein allow it to interact with other transcription factors and enhancers within the DNA. F In this way, the homeobox genes work to switch certain developmental genes on and off. F Research has revealed that the homeobox- encoded region is part of the protein that functions as a transcription regulator. F The shape of the encoded region allows it to bind to any DNA segment, but by itself, it cannot select a specific sequence. The variable regions within the whole protein allow it to interact with other transcription factors and enhancers within the DNA. F In this way, the homeobox genes work to switch certain developmental genes on and off.

Homeotic Genes F There are many other regions of DNA that are highly conserved among species. F The common question is how can the same genes code for different body forms? F It is likely that the small changes in the regulatory sequences lead to major changes in body form--the basis of the next unit. F There are many other regions of DNA that are highly conserved among species. F The common question is how can the same genes code for different body forms? F It is likely that the small changes in the regulatory sequences lead to major changes in body form--the basis of the next unit.