The Genetic Basis of Development How do cells with the same genes grow up to be so different?

Three Procceses of Development The transformation from a zygote into an organism The transformation from a zygote into an organism Results from three interrelated processes: cell division, cell differentiation, and morphogenesis Results from three interrelated processes: cell division, cell differentiation, and morphogenesis Figure 21.3a, b (a) Fertilized eggs of a frog (b) Tadpole hatching from egg

Through a succession of mitotic cell divisions Through a succession of mitotic cell divisions The zygote gives rise to a large number of cells The zygote gives rise to a large number of cells In cell differentiation In cell differentiation Cells become specialized in structure and function Cells become specialized in structure and function Morphogenesis encompasses the processes Morphogenesis encompasses the processes That give shape to the organism and its various parts That give shape to the organism and its various parts

Some key stages of development in animals and plants Animal development. Most animals go through some variation of the blastula and gastrula stages. The blastula is a sphere of cells surrounding a fluid-filled cavity. The gastrula forms when a region of the blastula folds inward, creating a tube—a rudimentary gut. Once the animal is mature, differentiation occurs in only a limited way—for the replacement of damaged or lost cells. Plant development. In plants with seeds, a complete embryo develops within the seed. Morphogenesis, which involves cell division and cell wall expansion rather than cell or tissue movement, occurs throughout the plant’s lifetime. Apical meristems (purple) continuously arise and develop into the various plant organs as the plant grows to an indeterminate size. Zygote (fertilized egg) Eight cells Blastula (cross section) Gastrula (cross section) Adult animal (sea star) Cell movement Gut Cell division Morphogenesis Observable cell differentiation Seed leaves Shoot apical meristem Root apical meristem Plant Embryo inside seed Two cells Zygote (fertilized egg) (a) (b)

Differential gene expression Nearly all the cells of an organism have genomic equivalence, that is, they have the same genes Differences between cells in a multicellular organism Differences between cells in a multicellular organism differences in gene expression differences in gene expression not from differences in the cells’ genomes not from differences in the cells’ genomes

yields a variety of cell types yields a variety of cell types each expressing a different combination of genes each expressing a different combination of genes multicellular eukaryotes multicellular eukaryotes cells become specialized as a zygote develops into a mature organism cells become specialized as a zygote develops into a mature organism Cell Differentiation

Different types of cells Different types of cells Make different proteins because different combinations of genes are active in each type Make different proteins because different combinations of genes are active in each type Muscle cellPancreas cellsBlood cells Cell Diferentiation

may retain all of their genetic potential may retain all of their genetic potential Most retain a complete set of genes Most retain a complete set of genes May be totipotent May be totipotent Differentiated cells

Totipotency in Plants EXPERIMENT Transverse section of carrot root 2-mg fragments Fragments cultured in nutrient medium; stirring causes single cells to shear off into liquid. Single cells free in suspension begin to divide. Embryonic plant develops from a cultured single cell. Plantlet is cultured on agar medium. Later it is planted in soil. RESULTS A single Somatic (nonreproductive) carrot cell developed into a mature carrot plant. The new plant was a genetic duplicate(clone) of the parent plant. Adult plant CONCLUSION At least some differentiated (somatic) cells in plants are toipotent, able to reverse their differentiation and then give rise to all the cell types in a mature plant.

DNA packing in a eukaryotic chromosome Wound around clusters of histone proteins, forming a string of beadlike nucleosomes Wound around clusters of histone proteins, forming a string of beadlike nucleosomes This beaded fiber is further wound and folded This beaded fiber is further wound and folded DNA packing tends to block gene expression DNA packing tends to block gene expression Presumably by preventing access of transcription proteins to the DNA Presumably by preventing access of transcription proteins to the DNA DNA double helix (2-nm diamet er) Histones Linker “Beads on a string” Nucleosome (10-nm diameter) Tight helical fiber (30-nm diameter) Supercoil (300-nm diameter) Metaphase chromosome 700 nm TEM

X chromosome inactivation in the cells of female mammals X chromosome inactivation in the cells of female mammals Early embryo X chromosomes Allele for orange fur Allele for black fur Cell division and random X chromosome inactivation Two cell populations in adult Active X Inactive X Active X Orange fur Black fur An Extreme Example of DNA Packing

Nuclear Transplantation The nucleus of an unfertilized egg cell or zygote is replaced with the nucleus of a differentiated cell The nucleus of an unfertilized egg cell or zygote is replaced with the nucleus of a differentiated cell

How to Clone a Sheep How to Clone a Sheep Bill Ritchie (Produced for the 1997 Royal Agricultural Show) The Egg The Egg The unfertilized eggs are flushed out of a sheep which has been induced to produce a larger than normal number of eggs. The unfertilized eggs are flushed out of a sheep which has been induced to produce a larger than normal number of eggs. The Cell The Cell Previously a sample of tissue was from the udder of a six year old ewe was taken and cultured in a dish (Dolly 1). Previously a sample of tissue was from the udder of a six year old ewe was taken and cultured in a dish (Dolly 1). The cultured cells are starved to send them into a resting or quiescent state. The cultured cells are starved to send them into a resting or quiescent state. The fusion The fusion A cell is placed beside the egg and an electric current used to fuse the couplet. A cell is placed beside the egg and an electric current used to fuse the couplet.

How to Clone a Sheep Culture Culture The reconstructed embryo is put into culture and grows for seven days. The reconstructed embryo is put into culture and grows for seven days. Development Development Embryos which grow successfully are taken and transferred to a sheep which is at the the same stage of the oestrus cycle as the egg. Embryos which grow successfully are taken and transferred to a sheep which is at the the same stage of the oestrus cycle as the egg. The sheep becomes pregnant and produces a lamb after 21 weeks (Dolly). The sheep becomes pregnant and produces a lamb after 21 weeks (Dolly).

“Copy Cat” Was the first cat ever cloned Was the first cat ever cloned Figure 21.8

The Stem Cells of Animals A stem cell A stem cell Is a relatively unspecialized cell Is a relatively unspecialized cell Can reproduce itself indefinitely Can reproduce itself indefinitely Can differentiate into specialized cells of one or more types, given appropriate conditions Can differentiate into specialized cells of one or more types, given appropriate conditions

Totipotent cells Liver cells Nerve cells Early human embryo at blastocyst stage (mammalian equiva- lent of blastula) From bone marrow in this example Pluripotent cells Cultured stem cells Different culture conditions Different types of differentiated cells Blood cells Embryonic stem cells Adult stem cells Embryonic and Adult Stem Cells Stem cells can be isolated Stem cells can be isolated From early embryos at the blastocyst stage From early embryos at the blastocyst stage Adult stem cells pluripotent, able to give rise to multiple but not all cell types

Complex assemblies of proteins control eukaryotic transcription Complex assemblies of proteins control eukaryotic transcription A variety of regulatory proteins interact with DNA and with each other A variety of regulatory proteins interact with DNA and with each other To turn the transcription of eukaryotic genes on or off To turn the transcription of eukaryotic genes on or off Transcriptional Regulation of Gene Expression During Development

Assist in initiating eukaryotic transcription Enhancers Promoter Gene DNA Activator proteins Other proteins Transcription factors RNA polymerase Bending of DNA Transcription Transcription Factors

DNA OFF Master control gene myoDOther muscle-specific genes Nucleus Embryonic precursor cell Determination and differentiation of muscle cells Determination and differentiation of muscle cells

DNA OFF mRNA MyoD protein (transcription factor) Master control gene myoD Determination. Signals from other cells lead to activation of a master regulatory gene called myoD, and the cell makes MyoD protein, a transcription factor. The cell, now called a myoblast, is irreversibly committed to becoming a skeletal muscle cell. 1 Other muscle-specific genes Nucleus Myoblast (determined) Embryonic precursor cell Determination and differentiation of muscle cells Determination and differentiation of muscle cells

DNA OFF mRNA Another transcription factor MyoD Muscle cell (fully differentiated) MyoD protein (transcription factor) Myoblast (determined) Embryonic precursor cell Myosin, other muscle proteins, and cell-cycle blocking proteins Other muscle-specific genesMaster control gene myoD Nucleus Determination. Signals from other cells lead to activation of a master regulatory gene called myoD, and the cell makes MyoD protein, a transcription factor. The cell, now called a myoblast, is irreversibly committed to becoming a skeletal muscle cell. 1 Differentiation. MyoD protein stimulates the myoD gene further, and activates genes encoding other muscle-specific transcription factors, which in turn activate genes for muscle proteins. MyoD also turns on genes that block the cell cycle, thus stopping cell division. The nondividing myoblasts fuse to become mature multinucleate muscle cells, also called muscle fibers. 2 Determination and differentiation of muscle cells Determination and differentiation of muscle cells

Cytoplasmic Determinants and Cell-Cell Signals in Cell Differentiation Cytoplasmic determinants in the cytoplasm of the unfertilized egg Cytoplasmic determinants in the cytoplasm of the unfertilized egg Regulate the expression of genes in the zygote that affect the developmental fate of embryonic cells Regulate the expression of genes in the zygote that affect the developmental fate of embryonic cells Sperm Molecules of another cyto- plasmic deter- minant Unfertilized egg cell Molecules of a a cytoplasmic determinant Fertilization Zygote (fertilized egg) Mitotic cell division Two-celled embryo Nucleus Sperm

Induction Signal molecules from embryonic cells cause transcriptional changes in nearby target cells Signal molecules from embryonic cells cause transcriptional changes in nearby target cells Early embryo (32 cells) NUCLEUS Signal transduction pathway Signal receptor Signal molecule (inducer) Induction by nearby cells. The cells at the bottom of the early embryo depicted here are releasing chemicals that signal nearby cells to change their gene expression. (b)

Pattern Formation Pattern formation in animals and plants results from similar genetic and cellular mechanisms Pattern formation in animals and plants results from similar genetic and cellular mechanisms Pattern formation Pattern formation Is the development of a spatial organization of tissues and organs Is the development of a spatial organization of tissues and organs Occurs continually in plants Occurs continually in plants Is mostly limited to embryos and juveniles in animals Is mostly limited to embryos and juveniles in animals

Cell Positioning Positional information Positional information Consists of molecular cues that control pattern formation Consists of molecular cues that control pattern formation Tells a cell its location relative to the body’s axes and to other cells Tells a cell its location relative to the body’s axes and to other cells

Cascades of gene expression and cell-to-cell signaling direct the development of an animal Early understanding of the relationship between gene expression and embryonic development Early understanding of the relationship between gene expression and embryonic development Came from studies of mutants of the fruit fly Drosophila melanogaster Came from studies of mutants of the fruit fly Drosophila melanogaster THE GENETIC CONTROL OF EMBRYONIC DEVELOPMENT Eye Antenna Leg SEM 50  Head of a normal fruit fly Head of a developmental mutant

Key Developmental Genes are Very Ancient Homeotic genes contain nucleotide sequences, called homeoboxes Homeotic genes contain nucleotide sequences, called homeoboxes That are very similar in many kinds of organisms That are very similar in many kinds of organisms Fly chromosome Mouse chromosomes Fruit fly embryo (10 hours) Mouse embryo (12 days) Adult fruit fly Adult mouse

Key developmental events in the life cycle of Drosophila Key developmental events in the life cycle of Drosophila Figure Follicle cell Nucleus Egg cell Fertilization Nurse cell Egg cell developing within ovarian follicle Laying of egg Egg shell Nucleus Fertilized egg Embryo Multinucleate single cell Early blastoderm Plasma membrane formation Late blastoderm Cells of embryo Yolk Segmented embryo Body segments 0.1 mm Hatching Larval stages (3) Pupa Metamorphosis Head Thorax Abdomen 0.5 mm Adult fly Dorsal Anterior Posterior Ventral BODY AXES

Bicoid Mutation Head Wild-type larva Tail Mutant larva (bicoid) Drosophila larvae with wild-type and bicoid mutant phenotypes. A mutation in the mother’s bicoid gene leads to tail structures at both ends (bottom larva). The numbers refer to the thoracic and abdominal segments that are present. (a) T1 T2 T3 A1 A2 A3 A4 A5 A6 A7 A8 A7A6A7 A8 Tail

Summary of Gene Activity During Drosophila Development Hierarchy of Gene Activity in Early Drosophila Development Maternal effect genes (egg-polarity genes) Gap genes Pair-rule genes Segment polarity genes Homeotic genes of the embryo Other genes of the embryo Segmentation genes of the embryo

C. elegans: The Role of Cell Signaling The complete cell lineage The complete cell lineage Of each cell in the nematode roundworm C. elegans is known Of each cell in the nematode roundworm C. elegans is known Figure Zygote Nervous system, outer skin, mus- culature Musculature, gonads Outer skin, nervous system Germ line (future gametes) Musculature First cell division Time after fertilization (hours) 0 10 Hatching Intestine Eggs Vulva ANTERIOR POSTERIOR 1.2 mm

Induction As early as the four-cell stage in C. elegans As early as the four-cell stage in C. elegans Cell signaling helps direct daughter cells down the appropriate pathways, a process called induction Cell signaling helps direct daughter cells down the appropriate pathways, a process called induction Figure 21.16a 4 Anterior EMBRYO Posterior Receptor Signal protein Signal Anterior daughter cell of 3 Posterior daughter cell of 3 Will go on to form muscle and gonads Will go on to form adult intestine (a)

Induction also critical later in nematode development also critical later in nematode development As the embryo passes through three larval stages prior to becoming an adult As the embryo passes through three larval stages prior to becoming an adult Figure 21.16b Epidermis GonadAnchor cell Signal protein Vulval precursor cells Inner vulva Outer vulva Epidermis ADULT

Programmed Cell Death (Apoptosis) In apoptosis In apoptosis Cell signaling is involved in programmed cell death Cell signaling is involved in programmed cell death 2 µm Figure 21.17

In vertebrates In vertebrates Apoptosis is essential for normal morphogenesis of hands and feet in humans and paws in other animals Apoptosis is essential for normal morphogenesis of hands and feet in humans and paws in other animals Figure Interdigital tissue 1 mm

Plant Development: Cell Signaling and Transcriptional Regulation Thanks to DNA technology and clues from animal research Thanks to DNA technology and clues from animal research Plant research is now progressing rapidly Plant research is now progressing rapidly

Mechanisms of Plant Development In general, cell lineage In general, cell lineage Is much less important for pattern formation in plants than in animals Is much less important for pattern formation in plants than in animals The embryonic development of most plants The embryonic development of most plants Occurs inside the seed Occurs inside the seed

Pattern Formation in Flowers Floral meristems Floral meristems Contain three cell types that affect flower development Contain three cell types that affect flower development Figure Carpel Petal Stamen Sepal Anatomy of a flower Floral meristem Tomato flower Cell layers L1 L2 L3

Organ Identity Genes Organ identity genes Organ identity genes Determine the type of structure that will grow from a meristem Determine the type of structure that will grow from a meristem Are analogous to homeotic genes in animals Are analogous to homeotic genes in animals Figure Wild typeMutant