IP 28: Organisms Development EK 2E1: Timing and coordination of specific events are necessary for the normal development of an organism, and these events are regulated by a variety of mechanisms

Organism Development Development: going from a single celled zygote to a complex multicellular organism For normal development to take place it requires the timing and coordination of specific events. – Example: seed germination Requires specific temperate and water

Three main mechanisms of development: Mitosis Cell Differentiation Morphogenesis

Mitosis Division of diploid somatic cells; produces identical copies (therefore all the cells have the same exact genes)

Cell Differentiation Initially all the cells that result from mitosis are stem cells (undifferentiated cells that can became any type of cell in the body) Although every cell has the same DNA what makes one cell different from another cell is the types of genes that are expressed Type of genes expressed are determined by translation factors (review gene expression)

Cell Differentiation Continued Cell differentiation is the process by which cells become specialized. Once specialized a cell can not become a different type of cell Results from the expression of genes for tissue-specific proteins

How does a cell know what genes to express? Two sources of information are used to instruct cells in an embryo which genes to express at appropriate times: – Cytoplasmic determinants – Embryonic Induction

Cytoplasmic Determinants An egg’s cytoplasm contains RNA, proteins, and other substances that are distributed unevenly in the egg Cytoplasmic determinants: maternal substances in the egg that influence early development As the zygote divides by mitosis, cells contain different cytoplasmic determinants, which lead to different gene expression (a) Cytoplasmic determinants in the egg Two different cytoplasmic determinants Unfertilized egg cell Sperm Fertilization Zygote Mitotic cell division Two-celled embryo Nucleus

Embryonic Induction The other important source of developmental information is the environment around the cell, especially signals from nearby embryonic cells Embryotic induction: process by which one group of embryotic cells influences the development of another; usually by causing changes in gene expression (b) Induction by nearby cells Signal molecule (inducer) Signal transduction pathway Early embryo (32 cells) NUCLEUS Signal receptor

Role of microRNAs (miRNAs) in gene regulation miRNAs – small (18-25 nucleotides) single stranded RNA molecule associated with one or more proteins in a complex that can degrade or prevent translation of an mRNA strand Relatively new discovery; 1 st one documented in 1993 but it was thought the discovery was unique to the C. elegans (soil worm); we now know miRNA are common Acts as a repressor; turns off genes New research on treatment for disease

Morphogenesis Development of the body shape and organization Apoptosis plays a significant role in morphogenesis – Ex: development of fingers and toes in humans Homeotic genes (aka HOX genes):genes that regulate the placement of anatomical structures in animals, plants, and fungi

Homeotic gene mutants

Apoptosis Programmed cell death Cell is chopped and packaged into vesicles that are digested by scavenger cells Prevents enzymes from leaking out of a dying cell and damaging neighboring cells Important in shaping an organism during embryonic development The role of apoptosis in embryonic development was first studied in C. elegans – In C. elegans, apoptosis results when specific proteins override proteins that “put the brakes” on apoptosis Apoptosis evolved early in animal evolution and is essential for the development and maintenance of all animals

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Fig Ced-9 protein (active) inhibits Ced-4 activity Mitochondrion Receptor for death- signaling molecule Ced-4 Ced-3 Inactive proteins (a) No death signal Ced-9 (inactive) Cell forms blebs Death- signaling molecule Other proteases Active Ced-4 Active Ced-3 Nucleases Activation cascade (b) Death signal

Fig a Ced-9 protein (active) inhibits Ced-4 activity Mitochondrion Ced-4Ced-3 Receptor for death- signaling molecule Inactive proteins (a) No death signal

Fig Interdigital tissue 1 mm