Biology | Answers to Review Questions

Author Dr. George B. Johnson, Washington University
Author Dr. Peter H. Raven, Missouri Botanical Gardens & Washington University
Contributor Dr. Susan Singer, Carleton College
Contributor Dr. Jonathan Losos, Washington University

Answers to Review Questions

Chapter 17 (p. 360)

1. Cleavage is the initial division of a fertilized egg. It proceeds so rapidly that the zygote does not increase in size. The fate of cells immediately post-cleavage can often be determined by where they are located (e.g., animal or vegetal pole).

2. A blastula is a hollow ball of cells formed from the rapid cleavage divisions of the zygote. As cell division progresses, the blastula invaginates to form a gastrula.

3. A gastrula is an invaginated blastula that continues in rapid cell division. The wall of cells that invaginates and becomes an internal fold is the endoderm, destined to become the gut. The external surface of cells is the ectoderm, which will ultimately form skin and nervous system. The region of cells between the endoderm and the ectoderm is the mesoderm.

4. A thickened band of ectoderm overlying the notochord develops during neurulation, immediately following gastrulation. The thickening, triggered by the notochord beneath the ectoderm, continues until a long tube overlying the dorsal side of the body pinches off, forming the neural tube.

5. Cadherins attach to actin or intermediate fibers of the migrating cell's cytoskeleton and extend through the plasma membrane. The cadherins then attach to an adjacent cell with Ca⁺⁺ binding sites. Contracting fibers move the cell along. Integrins attached to the cytoskeleton protrude from the cell surface and grasp components of the intercellular matrix. Changes in the cytoskeleton move the cell along.

6. In mosaic development, cells have specific developmental destinies at the outset. In regulative development, all cells have equal capacity to develop into different tissues.

7. Organizers produce diffusible signal molecules (morphogens) that convey positional information to other cells.

8. Determination is the commitment to a specialized developmental path, whereas differentiation is the specialization at the end of that path.

9. Maternal mRNA is concentrated in the cephalic end of a Drosophila embryo. When fertilization stimulates production of a protein from one of the maternal genes (bicoid), the result is the creation of a morphogen gradient, which regulates development.

10. Homeotic genes determine what each body segment during development will ultimately turn into (i.e., wings, legs, etc.).

11. Differences in development begin early. The mammalian zygote divides into equal, totipotent cells. They are totipotent through the eight-cell stage. Cells migrate to the site of specialization. Chemical gradients in the insect egg create a polarity that directs embryonic development. Segmentation genes begin setting up segments before many of the blastomere nuclei are fully separated. Seventeen segments form and develop without migration. Tissue patterns in plants are established when the embryo is first formed. Embryonic tissues celled meristems committed to specific development patterns are established early. Thereafter, development proceeds in much the same way as animals.

12. Telomeres of chromosomes in normal tissues are depleted through successive cell divisions. The loss of protection of the telomeres results in chromosome damage and aging. Since cancer cells are not subject to this aging, one could conclude that their chromosomes do not suffer a loss of telomeres.