Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 18.4 A Program of Differential Gene Expression Leads to the Different Cell Types in a Multicellular Organism
Multicellular organisms develop from a single fertilized egg through a highly orchestrated process of differential gene expression. Cells become specialized, forming distinct tissues and organs through regulated gene expression during embryonic development.
Genetic Program for Embryonic Development
- Cell division: increases cell number.
- Cell differentiation: cells specialize in structure and function.
- Morphogenesis: arrangement and organization of cells into specific structures.
Cytoplasmic Determinants and Inductive Signals
- Early embryonic cells receive developmental instructions from:
- Cytoplasmic determinants:
- Substances (RNA, proteins) unevenly distributed in the egg cytoplasm by the mother.
- After fertilization and division, these determinants lead cells down different developmental pathways by regulating gene expression.
- Inductive signals:
- External signals from neighboring embryonic cells.
- Cells influence one another's developmental paths via signaling molecules and surface receptors, activating gene expression pathways.
- Cytoplasmic determinants:
Sequential Regulation of Gene Expression in Differentiation
- Determination: when embryonic cells irreversibly commit to a specific cell fate. Initially subtle, detectable through gene expression.
- Differentiation: actual physical and functional specialization of cells, marked by production of tissue-specific proteins.
- Muscle cell differentiation example:
- Early embryonic cells become myoblasts, committed precursors to muscle cells.
- MyoD gene encodes MyoD protein, a master regulator that triggers other muscle-specific genes and halts cell division.
- Result: formation of mature skeletal muscle fibers from fused myoblasts.
Pattern Formation and Axis Establishment
- Pattern formation: the spatial arrangement of tissues and organs.
- Controlled by positional information provided by cytoplasmic determinants and inductive signals.
- Body axes (anterior-posterior, dorsal-ventral, left-right) are established early.
- Drosophila studies:
- Homeotic genes direct pattern formation and body plan development.
- Maternal effect genes (egg-polarity genes) deposit determinants in the egg, setting body axes.
- bicoid gene: specifies anterior (head) structures; bicoid mRNA and protein form gradients that guide anterior-posterior axis development.
In a Nutshell
Differential gene expression orchestrates the development of multicellular organisms from a single fertilized egg. Cytoplasmic determinants, inductive signals, and sequential gene activation guide embryonic cells through determination, differentiation, and morphogenesis, creating specialized cell types and organized body structures.