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 maternaleffect 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.
Shh expressed in notochordal cell
SHH produced and diffuses out
SHH binds to ligand on neural tube cell
Intracellular signalling pathway
Transcriptional activation of downstream genes
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 tube appears 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.