Exam 21: Development of Multicellular Organisms

Mutations in certain components of the cell-cycle machinery in Drosophila melanogaster can be used to slow down the rate of progression through the cell cycle in the wing imaginal discs of the fly larvae, without a major effect on the rate of cell growth. As a result, compared to wild-type flies, the wing in the mutant flies would …

You have grown neurons in a culture dish containing laminin and have induced axon outgrowth. You now incubate the cells in the presence of various concentrations of two secreted proteins (1 and 2). You then fix the cells, stain them with rhodamine-phalloidin (a fluorescent dye that selectively labels actin filaments), and finally use a fluorescence microscope to examine the axons. Some of the axon tips have lost their spread morphology (with lamellipodia shrunk and filopodia retracted) and lack significant fluorescence. You call these "collapsed cones." In the following graph, the percentage of cones that have collapsed is plotted as a function of concentration of proteins 1 and 2. Do you expect these proteins to be chemoattractants or chemorepellents in guiding axonal growth cones? Which one is more potent?

Indicate true (T) and false (F) statements below regarding Hox genes. Your answer would be a four-letter string composed of letters T and F only, e.g. TTTF. ( ) The order of expression of Hox genes along the body corresponds to their order in the Hox complex. ( ) Generally, the more anterior of the Hox genes dominate (or suppress) posterior Hox genes. ( ) When a posterior Hox gene is artificially expressed in an anterior region of the embryo, the tissue maintains its anterior character. ( ) Hox genes control the A-P axis in both vertebrates and invertebrates.

The Steel factor/Kit signaling pathway plays an important role in the migration of many types of cells during development as well as in the adult animal. Not surprisingly, loss-of-function mutations in Steel factor result in cell migration defects. Selective inactivation in these cells of which of the following proteins might be expected to rescue (i.e. partially restore to normal) the defective phenotype of Steel loss-of-function mutants?

Which of the following is true regarding maternal-effect genes?

You grow neurons in culture in the presence or absence of nerve growth factor (NGF). You perform this experiment using either wild-type neurons or neurons lacking caspase-3, an effector protein required for programmed cell death by apoptosis in these cells. After a day, you examine cell survival in the culture using a microscope. The survival scores (percentage of cells surviving the treatment) are presented in the following table. Which column (a or b) corresponds to the experiment in the presence of NGF? Which row (1 or 2) corresponds to the experiment using wild-type cells?

In classical experiments done half a century ago, the cells of early frog embryos were disaggregated and later reaggregated in desired combinations. The cells managed to rearrange and sort themselves out into an overall arrangement similar to that of a normal embryo. This effect is mainly due to …

You have obtained leaf samples from three strawberry varieties A, B, and A) Variety A B) Variety B C) Variety C C) You isolate the cells, extract their nuclei, and stain them with propidium iodide, a fluorescent dye that binds quantitatively to DNA. You then use a fluorescence-activated cell sorter (FACS) machine to sort the nuclei based on their fluorescence. Guessing from the results, presented in the following histograms, which variety would you expect to yield larger strawberries? D) They should all be the same

Indicate whether each of the following organs or tissues arises from ectoderm (C), mesoderm (M), or endoderm (N). Your answer would be a four-letter string composed of letters C, M, and N only, e.g. MMCC. ( ) Blood ( ) Liver and pancreas ( ) Brain ( ) Bone and cartilage

If explants that can produce axons from either the anterior or posterior retina are placed on a culture substratum that is coated by either anterior or posterior tectal membranes, …

The following graph qualitatively represents the gradients of Nodal and Lefty gene products in an early frog embryo. The position along which primary axis is defined by these gradients? Which curves correspond to these two proteins?

In a developing Drosophila melanogaster embryo, a hierarchy of gene regulatory interactions subdivides the embryo to regulate progressively finer details of patterning. For each of the following proteins, indicate which expression pattern (1 to 4) in the schematic drawing below is more appropriate. Your answer would be a four-digit number composed of digits 1 to 4 only, with each digit used once, e.g. 3412. ( ) Hunchback (product of a gap gene) ( ) Engrailed (product of a segment-polarity gene) ( ) Bicoid (product of an egg-polarity gene) ( ) Even-skipped (product of a pair-rule gene)

An axonal growth cone has reached a muscle fiber and is forming a neuromuscular junction. Which of the following events does NOT normally occur in this process?

Indicate whether each of the following conditions favors a larger (L) or smaller (S) tissue or body size. Your answer would be a four-letter string composed of letters L and S only, e.g. SSSS. ( ) Hippo overexpression ( ) Insulin-like growth factor overexpression ( ) Myostatin deletion ( ) Growth hormone deficiency

Assuming that the only function of Short gastrulation (Sog) in fruit flies is to regulate Decapentaplegic (Dpp), which of the following genetic interactions would you NOT expect to observe?

Consider Agrin signaling in the formation of neuromuscular junctions in vertebrates. Shp2 is a protein tyrosine phosphatase that counteracts the tyrosine kinase activity of MuSK. Rapsyn interacts with the acetylcholine receptors and induces their agrin-dependent clustering. Indicate whether each of the following is expected to enhance (E) or suppress (S) acetylcholine receptor clustering in this process. Your answer would be a four-letter string composed of letters E and S only, e.g. EEEE. ( ) Specific inhibition of Shp2 ( ) Loss of Rapsyn ( ) Loss of LRP4 ( ) Constitutive activation of MuSK

In the following graph, which line (1 to 3) better represents the changes in the level of EphA receptors on retinal axons along the anterior-posterior axis of the mouse retina? Which line better represents the changes in the abundance of EphrinA along the anterior-posterior axis of the mouse optical tectum? Write down your answer as a two-digit number, e.g. 22.

A cross section of a developing spinal cord in a vertebrate embryo is shown in the schematic drawing below. Indicate which feature in the drawing (labeled A to E) better matches each of the following descriptions. Your answer would be a four-letter string composed of letters A to E only, e.g. AAEB. ( ) It contains motor neurons. ( ) It is the floor plate. ( ) It secretes BMP and Wnt signals. ( ) It contains sensory neurons.

TSH is a pituitary hormone that stimulates the production of thyroid hormone by the thyroid gland. This production can be blocked by sodium perchlorate (SP), which inhibits the cellular import of iodine necessary for thyroid hormone synthesis. What would be the effect of TSH or SP exposure, respectively, on the timing of metamorphosis in frog larvae?

CFSE (carboxyfluorescein succinimidyl ester) is a cell-tracing molecule that, once inside a cell, is modified to yield a highly fluorescent molecule that is retained within the cell. Due to its stability, it can be used to trace cells and follow their divisions: each division dilutes the fluorescent dye twofold. You label T cells from a healthy mouse with CFSE and inject them into either a wild-type strain of mouse or a strain with T cell deficiency, both of which are immunologically compatible with the donor mouse. You later collect and analyze peripheral T cells from the recipient mice and draw the following histograms according to the fluorescent intensity per cell. Based on these results, do you think the total number of T cells in the mouse body is (1) controlled by intracellular programs (as in the thymus, for example) or is (2) regulated as a whole (as in the spleen)? Write down 1 or 2 as your answer.