Answers to Review Questions – chapter 20
1.List four consequences of cleavage. In what way do cleavage divisions differ from conventional mitotic divisions? (pp. 448–450)
As consequences of cleavage the embryonic cells decrease in size, restoring the surface to area volume much closer to normal somatic cells. Different blastomeres throughout the embryo receive different types of molecules and thus these cells ultimately perform different functions. A multicellular embryo arises and the nucleus to cytoplasm ratio in blastomeres is closer to the ‘norm’ than in a gamete.
Cleavage differs from a conventional mitotic division in that immediately after cytokinesis the cells enter the S phase and then move directly into the M phase—the G1 and G2 phases are eliminated. Because the growth phases are missed, the cell does not grow in size and individual cells actually get much smaller, though the overall size of the embryo stays the same.
2.Explain why the blastomeres in the animal hemisphere of the frog blastula are smaller than those in the vegetal hemisphere. (p. 448)
In amphibians, cytokinesis takes place throughout the egg but division proceeds more slowly in the vegetal region than in the animal region due to the higher concentration of yolk. Thus over a given time period more divisions take place in the animal region than in the vegetal region and the cells as a result are smaller.
3.‘Gastrulation usually establishes the basic body plan of an animal.’ Illustrate this statement with reference to sea urchin and frog embryos. (pp. 452–453)
In both the sea urchin and the frog, the final body plan can be seen by the end of gastrulation. Thus in the sea urchin, essentially a round animal, the gastrula is round with ectoderm on the outside and a central gut cavity. In contrast, in the frog, which is essentially a ‘head to tail’ animal, by the end of gastrulation the body, though still round in external appearance, has a definite head-to-tail orientation. In all cases by the end of gastrulation, three discrete tissue layers, the germ layers, can be identified. This is best illustrated by reference to Figure 20.7 in the text.
4.(a) How are microfilaments involved in the folding of epithelial sheets? (p. 456)
When bundles of microfilaments in adhesion belts in certain epithelial cells contract, they cause a decrease in the diameter of the cells at that point. If these cells retain adhesive contact with their neighbours (through what are called adherent junctions), the epithelial cell sheet as a whole will bend around the contracting epithelial cells.
(b) How do changes in cell shape and cell adhesion underlie the morphogenetic movements of neural crest cells? (p. 457)
Refer to Figure 20.13.
5.Explain the term ‘induction’ and give two examples of this phenomenon in amphibian development. (pp. 462–463)
Embryonic induction occurs when cell fate during development is regulated by interactions between cells or tissues. For example, if animal pole blastomeres and vegetal pole blastomeres are separated in an early blastula, the animal pole blastomeres develop features of ectoderm and vegetal pole blastomeres develop features of endoderm. No cells with mesoderm characteristics are formed. However, if cells from the two poles are placed adjacent to each other, some animal pole cells develop mesoderm. Cell interaction has caused a developmental switch.
In the amphibian embryo specification of dorsal tissues, such as the central nervous system, involves interaction with a neighbouring tissue called the organiser. The organiser in turn results from a preceding inductive interaction between animal and vegetal hemisphere cells.
6.What are maternal-effect genes? Give a molecular explanation for the action of maternal-effect genes. (p. 464)
Maternal-effect genes are genes located in the maternal genome, whose mRNA suffuses the oocyte and, after translation, whose gene products regulate the activities of zygotic genes. An example is the bicoid gene, whose mRNA is transcribed from the genes of the mother during oogenesis and becomes localised in the anterior region of the egg. After fertilisation, the Bicoid protein is formed from translation of the bicoid mRNA and diffuses away from the anterior pole, establishing a concentration gradient. The Bicoid protein is a homeodomain transcription factor, and directly regulates the transcription of the zygotic segmentation gap genes, which establish the anterior/posterior spatial organisation of the egg. Note that the mRNA gradient approach works only in single cells. (The Drosophila embryo remains a syncytium, a multinucleate single cell, in the early stages of its development).
7.Give an example of a developmental gene that controls cell fate. Draw a diagram to show where such a gene operates in a hierarchy of genetic interactions. (p. 463)
The protein Sonic hedgehog (SHH) induces cells to differentiate as motor neurones and floor plate cells in the vertebrate neural tube. SHH protein is the product of the sonic hedgehog gene, identified in vertebrates from the homology cloning of the hedgehog gene in Drosophila.Cell fate genes sit high in the hierarchy and regulate the activity of numerous ‘downstream’ genes lower in the hierarchy.
8.The genetic control of the pattern of a morphological structure can be independent of the control of the differentiation of the cells in that structure. What line of evidence has led developmental geneticists to that conclusion? Give an example of a patterning gene. (Box 20.3)
Patterning of the vertebrate neural tubeappears to be under the control of a set of Hox genes, which are homeodomain transcription factors, while cell fates seem to be under the control of cell signalling proteins such as SHH which bind to specific receptors on the cell membrane. An understanding of the molecular genetics of Drosophila development has highlighted the role of homeotic genes in controlling the spatial patterns of segmentation (see Figure 20.33).
9.Give an example of a developmental gene that is involved in cell–cell signalling. What sort of protein does this gene encode and how does it act at a molecular level? (p. 463)
The Sonic hedgehog (Shh) gene is involved in cell-to-cell signalling. The Shh gene is expressed in notochordal cells at a time when the neural tube is undergoing patterning. The gene product, Sonic hedgehog protein (SHH), induces neural tube cells to differentiate as motor neurones and floor plate cells. SHH is an extracellular signalling molecule. Binding SHH to a receptor on its target cell triggers an intracellular signalling pathway, leading to the transcriptional activation of a specific set of ‘downstream’ genes.
10.Draw a diagram to show regulatory interactions between genes involved in segmentation in Drosophila. (pp. 467–468)
Students should be directed to Figure 20.32, which schematically summarises the interactions between bicoid, gap and pair-rule genes.