Chapter 21 GENOMES AND THEIR EVOLUTION
Overview: From Single Cell to Multicellular Organism • How does a complex multicellular organism develop from a single cell? • Mutations—one way to deduce • Example: in 1995, found that a particular gene functions as a master switch that triggers development of the eye in Drosophila. It was expressed in an abnormal body location, causing extra eyes.
Model Organisms used for Genetic Studies • Drosophila—fruit fly • Nematode (worm) • Mouse • Zebrafish • Common wall cress (plants)
Drosophila • Generation time—two weeks • Produces many offspring • Embryos develop outside the mother’s body
Mouse (Mus Musculus) • Manipulation of mouse genes to make transgenic mice, and mice in which particular genes are “knocked out” by mutation. • Generation time—9 weeks. • Embryos develop within the mother’s uterus, hidden from view (disadvantage) • Similar to the human genome
Concept 21.1: Embryonic development involves cell division, cell differentiation, and morphogenesis • In embryonic development of most organisms, a single-celled zygote gives rise to cells of many different types, different structure and function • Development involves three processes: • cell division • cell differentiation • morphogenesis (“creation of form”)
Development • Cell differentiation —cells become specialized in structure and function, well-organized • Morphogenesis —meaning the physical process that gives an organism its shape, and its body plan
LE 21-3 Fertilized egg of a frog Tadpole hatching from egg
LE 21-4 Animal development Gut Cell movement Zygote (fertilized egg) Eight cells Blastula (cross section) Gastrula (cross section) Adult animal (sea star) Cell division Morphogenesis Observable cell differentiation Seed leaves Plant development Shoot apical meristem Root apical meristem Two cells Zygote (fertilized egg) Embryo inside seed Plant
Development of plant and animal differentiation • Animal and plants develop primarily in 2 different ways: • In animals, but not in plants: • movements of cells and tissues are necessary to transform the early embryo into the characteristic three-dimensional form of the organism.
In plants, but not in animals: • Morphogenesis and growth are not limited to embryonic and juvenile periods • occur throughout the life of the plant
Concept 21.2: Different cell types result from differential gene expression in cells with the same DNA • Differences between cells in a multicellular organism come almost entirely from gene expression, not differences in the cells’ genomes • Regulatory mechanisms turn genes off and on
Evidence for Genomic Equivalence—are our genes similar in structure and number? • Research: nearly all cells of an organism have genomic equivalence (the same genes). • A key question that is asked is whether genes are irreversibly inactivated during differentiation.
Totipotency in Plants • One experimental approach for testing genomic equivalence is to see whether a differentiated cell can generate a whole organism??? • A totipotent cell (considered a stem cell) is one that can generate a complete new organism.
Differentiated cells taken from the root of a carrot and placed in culture medium • Grows into normal adult plants, each genetically identical to the “parent”. • This showed that differentiation does not necessarily involve irreversible changes in the DNA.
Nuclear Transplantation in Animals • This same type of growth in plants does not happen in animals….can’t put them in petri dish with a seed and create a new human! • In nuclear transplantation-- the nucleus of an unfertilized egg cell or zygote is replaced with the nucleus of a differentiated cell.
LE 21-6 Frog embryo Frog egg cell Frog tadpole UV Fully differ- entiated (intestinal) cell Less differ- entiated cell Donor nucleus trans- planted Donor nucleus trans- planted Enucleated egg cell Most develop into tadpoles <2% develop into tadpoles
Something in the nucleus does change as animal cells differentiate. • Various cell types in the body of an animal differ in structure and function not because they contain different genes…. • Because they express different sets of genes from a common genome.
Stem Cell Research • http://www.youtube.com/watch?v=LETYf0Ybp-4 • http://www.youtube.com/watch?v=cMtFkfcQqec&feature=related
Reproductive Cloning of Mammals • In 1997, Scottish researchers announced the birth of Dolly • Lamb cloned from an adult sheep by nuclear transplantation from a differentiated mammary cell. • Dolly’s premature death in 2003 with complications of arthritis. • Felt cells were “older” than those of a normal sheep • reflecting incomplete reprogramming of the original transplanted nucleus.
Since 1997, cloning has been demonstrated in many mammals, including mice, cats, cows, horses, and pigs. • “Copy Cat” was the first cat cloned. • However, cloned animals of the same species do not always look or behave identically.
The Stem Cells of Animals • Stem cell -- is a relatively unspecialized cell that can reproduce itself indefinitely and differentiate into specialized cells of one or more types • Embryonic stem cells – cells isolated from early embryos at the blastocyst stage • The adult body also has stem cells, which replace nonreproducing specialized cells • Embryonic stem cells are totipotent, able to differentiate into all cell types • Adult stem cells are pluripotent, able to give rise to multiple but not all cell types
Example: stem cells in the bone marrow give rise to all the different kinds of blood cells. • Example: in intestinal wall, stem cells regenerate forming the lining of the intestine. • Example: new information tells us that the adult brain has stem cells which continue to produce certain kinds of nerve cells.
LE 21-9 Embryonic stem cells Adult stem cells Totipotent cells Pluripotent cells Cultured stem cells Different culture conditions Different types of differentiated cells Liver cells Nerve cells Blood cells
Transcriptional Regulation of Gene Expression During Development • As the tissues and organs of an embryo take shape, the cells become visibly different in structure and function. • Cell determination comes before differentiation and involves expression of genes for tissue-specific proteins. • At the end of this process, an embryonic cell is committed to its final fate, and is said to be determined. • Tissue-specific proteins-- enable differentiated cells to carry out their specific tasks
LE 21-10_3 Nucleus Master control gene myoD Other muscle-specific genes DNA Embryonic precursor cell OFF OFF Determination mRNA OFF MyoD protein (transcription factor) Myoblast (determined) Differentiation mRNA mRNA mRNA mRNA Myosin, other muscle proteins, and cell-cycle blocking proteins MyoD Another transcription factor Muscle cell (fully differentiated)
LE 21-11b Early embryo (32 cells) Signal transduction pathway NUCLEUS Signal receptor Signal molecule (inducer) Induction by nearby cells
Genetic Analysis of Early Development: Scientific Inquiry • Study of developmental mutants laid the groundwork for understanding the mechanisms of development • Mutations that affect segmentation are likely to be embryonic lethals, leading to death at the embryonic or larval stage
Segmentation Pattern—in animalsInquiry: the Fly • Segmentation genes --produce proteins that direct formation of segments after the embryo’s major body axes are formed. • Positional information is provided by activation of three sets of segmentation genes: • gap genes • pair-rule genes • segment-polarity genes
Identity of Body Parts • The anatomical identity of , for example, Drosophila segments is set by master regulatory genes called homeotic genes. • Mutations to homeotic genes produce flies with strange traits, such as legs growing from the head in place of antennae.
Programmed Cell Death (Apoptosis) • Cell signaling is involved in apoptosis, which is defined as programmed cell death. • During apoptosis, a cell shrinks and becomes lobed (called “blebbing”); the nucleus condenses; and the DNA is fragmented (cell suicide!). • Neighboring cells quickly engulf and ingest the remains, leaving no trace.
LE 21-17 Apoptosis of human white blood cell normal abnormal 2 mm
In vertebrates, apoptosis is part of: • normal development of the nervous system • operation of the immune system • morphogenesis of hands and feet in humans and paws in other mammals
LE 21-19 The cells that appear yellow, in between the digits, die on purpose, to create the space for the paws, or “fingers” Interdigital tissue 1 mm
Mechanisms of Plant Development • In general, cell lineage is much less important for pattern formation in plants than in animals. • The embryonic development of most plants occurs inside the seed, and thus is relatively inaccessible to study. • However, other important aspects of plant development are observable in plant meristems, particularly apical meristems at the tips of shoots.
Meristems • A meristem is the tissue in all plants consisting of undifferentiated cells, found in zones of the plant where growth can take place. • In meristems, cell division, morphogenesis, and differentiation give rise to new organs.
Pattern Formation in Flowers • A flower, however, develops from 3 layers in a floral meristem: • Cell division • Enlargement • Differentiation • Four types of “organs” in flowers: • carpels • stamens • petals • sepals
LE 21-20 Carpel Petal Stamen L1 Cell layers L2 L3 Sepal Floral meristem Anatomy of a flower Tomato flower
Researchers chose to experiment on plants and grafted stems from a mutant tomato plant onto a wild-type plant and then grew new plants from the shoots at the graft sites. • The new plants were called chimeras -- organisms with a mixture of genetically different cells.
In contrast to genes controlling organ number in flowers, genes controlling organ identity (organ identity genes) determine the types of structures that will grow from a meristem. • Organ identity genes --are analogous to homeotic genes in animals. • Mutations cause plant structures to grow in unusual places, such as carpels in place of sepals. • “Homeotic”—meaning this mutation causes a specific organ to be missing or repeated.
LE 21-22 Wild type Mutant
Widespread Conservation of Developmental Genes Among Animals • Analysis of the homeotic genes in Drosophila has shown that they all include a sequence called a homeobox. • Homeobox—a nucleotide sequence which specifies a 60-amino-acid sequence.