BIOL 114 Test 4
December 11, 2006

Name __________________________________

Total Points: 50

Note: These responses are a "skeleton key;" the bare minimum of what's needed for an answer. Typical student answers would take a few more sentences. Excessive length, however (more than the box provided) does not help.

1. (8 pts) Explain how the reproductive cycle of Caulobacter crescentus serves as a model system for study of differentiation. What molecular mechanism controls the cycle; and how is this related to a molecular process in human development?

See Caulobacter.

The reproductive cycle of Caulobacter serves as a model because one type of cell (a stalked cell) divides into two cells of very different form (stalked and swarmer). This reproductive event offers a simple model of differentiation, which can be generated in quantity and studied at the molecular level.

The key molecular event of determination [discussed in class; not yet on the website] is that as the DNA replicates, each daughter helix has a different set of genes methylated previously. The new DNA is not methylated right away. So as the cell is dividing, there is a period when each daughter cell expresses different developmental genes, for becoming a stalk vs. a swarmer. This mechanism is related to the imprinting mechanism of human development, in which germ cells contain different patterns of methylated DNA depending on maturation through the ovaries or the testes. The human imprinting mechanism however is more complicated and involves control by multicellular tissues.

2. (8 pts) Explain three fundamental differences between development in plants and animals.

See Plant development.

* Plant cells do not migrate, as do bacteria or animal cells. Plant shape forms based on the rate of cell division and the direction of elongation. Although plants develop three basic tissue systems (dermal, ground, and vascular), they don't rely on gastrulation to establish this layered system of tissues.

* Plant development is continuous. New plant organs are formed throughout their life by clusters of embryonic cells called meristem.

* Plants have tremendous developmental plasticity. Lost plant parts can be regenerated by meristems, and even entire plants can be regenerated from single cells. In addition, environmental factors such as light and temperature can greatly influence overall plant form.

Other answers may involve the fertilization system.

3.  (8 pts) In Drosophila, you find two genes encoding transcription factors (genes A and B) expressed early in development.

Explain an experiment to determine whether gene product A regulates expression of gene B, or vice versa.

4. (8 pts) Compare and contrast the processes of gastrulation in the sea urchin; the frog; and the chick. Brief diagrams will be helpful, as well as explanation.

For images see Gastrulation.

In the sea urchin, the blastoderm invaginates to form endoderm, whose fate includes the future digestive system; and some cells ingress to form mesenchyme, the future internal organs.

In the frog, vegetal cells at the blastopore involute beneath the ectoderm. Some ectoderm cells ingress to form the mesoderm.

In the chick, the blastula is flattened into a pocket. Outer cells (called epiblast) involute through the primitive streak, forming the endoderm. Some epiblast cells ingress to form mesoderm.

5. (8 pts) Explain a developmental process that occurs in the human and the frog, but not in Drosophila. Explain why not.

In neurulation, the notochord (formed out of mesoderm) induced epidermal cells above it to pucker in, forming first the neural plate, then closing over to form the neural tube.

Neurulation generates the spinal column and central nervous system, which are found only in human and frog, because they are vertebrates. Drosophila is an invertebrate (lacking a backbone).

6. (10 pts) In Figure 2, explain the technique that was used to label the cells.

The epiblast tissue was electroporated with a plasmid expressing GFP attached to one of the Hoxb promoters (Hoxb1, Hoxb4, Hoxb7, Hoxb9). The control has a non-Hox promoter. A slice of this labeled tissue was grafted into an unlabeled embryo.

What stage embryos were labeled, and what stage is observed?

Early epiblast embryos were labeled, at the primitive streak stage before neurulation. Embryos were observed after neurulation (a-e), and at a later stage when the body cavity is forming (f-j).

Explain what can be learned about expression of the Hoxb genes.

From this figure we can see that the higher-numbered Hoxb genes (such as Hoxb9) are expressed in more posterior tissues than the lower-numbered genes (Hoxb1). The order of gene expression in the body plan very roughly approximates the order of genes on the chromosome. This is particuarly evident looking at the most posterior extent of expression of each GFP in panels g, h, i, j.

What about the role of hoxb genes in development is NOT learned from this figure, but can be learned form other experiments in the paper?

From other experiments we learn that Hoxb gene expression actually determines when and where the epiblast cells ingress into mesoderm. The second part of Figure 2 shows this with double reporter plasmids, in which a Hoxb gene is overexpressed while GFP is also expressed.

Iimura & Pourquie (2006), Collinear activation of Hoxb, Figure 2.