Chapter 17: Developmental Genetics

Cell Differentiation and Nuclear Equivalence

• An organism contains many types of cells that are specialized to carry out specific functions.
  • These cells are the product of a process of gradual commitment, called cell determination, which ultimately leads to the final step in cell specialization, called cell differentiation.
• Nuclear equivalence is the concept that, with a few exceptions, all the nuclei of the differentiated somatic cells of an organism are identical to one another and to the nudes of the single cell from which they descended.
  • Totipotency is the capability of cells to direct the development of an entire organism.
• Steward induced mature carrot root cells to become undifferentiated and express their totipotency, forming an entire plant.
• Gurdon injected nuclei from tadpole intestinal cells into enucleated eggs; a small fraction of these eggs developed into tadpoles, demonstrating the totipotency of the injected nucleus.
• Wilmut fused a differentiated cell from an adult sheep cell with an enucleated egg and implanted it into the uterus of a host mother, where it developed into a normal lamb.
• Stem cells can divide to produce differentiated descendants yet retain the ability to divide to maintain the stem cell population.
  • Totipotent stem cells give rise to all cell types of the body and placenta, whereas pluripotent stem cells give rise to many, but not all, types of cells in an organism.
• Embryonic stem (ES) cells, formed after a zygote has undergone several rounds of cell division to become a five- or six-day-old blastocyst, are pluripotent and have the potential to develop into any type of cell in the body; ES cells are not totipotent because they cannot form cells of the placenta.
  • Stem cells show promise in treating diseases, such as Parkinson's disease and diabetes mellitus.

The Genetic Control of Development

• Developmental mutations have been identified in the fruit fly, Drosophila, many of which affect the organism's segmented body plan.
  • Many developmental genes discovered in the fruit fly are now known to be important in the growth and development of all animals.
• Caenorhabditis elegans is a roundworm with mosaic development, a rigid developmental pattern in which the fates of cells are restricted early in development.
  • The lineage of every somatic cell in the adult is known, and each can be traced to a single founder cell in the early embryo.
• The laboratory mouse, M. musculus, is extensively used in studies of mammalian development.
  • The mouse shows regulative development; the very early embryo is a self-regulating whole and can develop normally even if it has extra or missing cells.
  • Transgenic mice, in which foreign genes have been incorporated, have helped researchers determine how genes are activated and regulated during development
• Genes affecting development have also been identified in certain plants, such as A.thaliana.
  • The ABC model of interactions among three kinds of genes hypothesizes how floral organs develop in Arabidopsis.
  • Mutations in these homeotic genes cause one flower part to be substituted for another.
• The earliest developmental events to operate in the egg are established by maternal effect genes in the surrounding maternal tissues, which are active prior to fertilization.
  • Some produce gradients of morphogens, chemical agents that affect the differentiation and development of form.
  • Maternal effect genes establish polarity in the embryo.
• Segmentation genes generate a repeating pattern of body segments within the embryo.
  • Gap genes are segmentation genes that begin organizing the body into anterior, middle, and posterior regions; pair-rule genes and segment polarity genes affect all segments.
• The later-acting homeotic genes are responsible for specifying the identity of each segment.
• Transcription factors are DNA-binding proteins that regulate transcription in eukaryotes.
  • Some genes that code for transcription factors contain a DNA sequence called a homeobox, which codes for a protein with a DNA-binding region called a homeodomain.
• Some homeobox genes are organized into complexes that appear to be systems of master genes specifying an organism's body plan.
  • Parallels exist between the homeobox complexes of Drosophila and those of other animals.
• Induction refers to developmental interactions with neighboring cells.
  • During development in C. elegans, the anchor cell induces cells on the surface to organize to form the vulva, the structure through which the eggs are laid.
• Apoptosis is programmed cell death.
  • During human development, the human hand forms as a webbed structure, but the fingers become individualized when the cells between them die

Cancer and Cell Development

• The traits of cancer cells are due to accumulations of mutations and epigenetic changes in cancer driver genes that typically include oncogenes and tumor suppressor genes.
  • Oncogenes are growth-activating genes that arise from changes in the expression of normal genes called proto-oncogenes, which exist in all cells and are involved in the control of growth and development.
  • Tumor suppressor genes are normal genes that prevent cells from undergoing inappropriate growth and cell division.
  • These are typically inactivated in cancer cells.