Deck 21: Development of Multicellular Organisms

Question 1

Indicate whether each of the following groups of genes typically creates a transient pattern in the developing embryo (T) or a long-lived pattern that is preserved (L). Your answer would be a five-letter string composed of letters T and L only, e.g. TTTLT.
( ) Egg-polarity genes
( ) Pair-rule genes
( ) Hox genes
( ) Gap genes
( ) Segment-polarity genes

Answer 1

T
T
L
T
L

Question 2

Indicate true (T) and false (F) statements below regarding animal development. Your answer would be a four-letter string composed of letters T and F only, e.g. FFFF.
( ) A fertilized egg is totipotent.
( ) Differences in their regulatory DNA can largely explain the differences between animal species.
( ) Inductive signaling is mostly mediated through G-protein-coupled receptors.
( ) A cell's response to a signal depends on its exposure to other signals at that present time as well as in the past.

Answer 2

T
T
F
T

Question 3

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)

Answer 3

2
4
1
3

Question 4

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.

Question 5

Imagine a morphogen gradient established from left to right in a field of cells in a developing tissue, as shown in the following schematic diagram. Below a first threshold of morphogen concentration, cells do not respond to the morphogen and express gene "red" by default. Cells exposed to morphogen concentrations above this threshold respond by expressing gene "white" instead, while those exposed to even higher concentrations, above a second threshold, express gene "blue." As shown, the initial pattern resembles a French flag with equally wide blue, white, and red expression domains. With no other change, if the diffusion rate of the morphogen is increased (by a modification that decreases its affinity for heparan sulfate proteoglycans, for example), the gradient profile changes from the gray curve to the black curve, as indicated. Under this new condition, indicate whether each of the following would be expected to increase (I), decrease (D), or remain unchanged (U) in its range. Your answer would be a three-letter string composed of letters I, D, and U only, e.g. UUI.

( ) Blue expression domain
( ) White expression domain
( ) Red expression domain

Question 6

Sort the following organisms from the least to the most degree of asymmetry in the unfertilized egg (i.e. pre-defined axes of polarization). Your answer would be a three-letter string composed of letters A to C only, e.g. BCA.
(A) M. musculus
(B) D. melanogaster
(C) X. laevis

Question 7

Which of the following is correct regarding the regulation and maintenance of Hox gene expression in Drosophila?

A) Proteins of the Polycomb and Trithorax groups maintain inactive and active states of Hox gene expression, respectively.
B) If Polycomb or Trithorax group genes are defective, Hox gene expression patterns are still initiated, but cannot be correctly maintained.
C) The remodeled chromatin at the Hox complex is heritable through cell generations.
D) If all the Hox genes in an embryo are deleted, segmentation still occurs but distinct segment identities are lost.
E) All of the above.

Question 8

The gene clusters known as the Bithorax complex and the Antennapedia complex contain…

A) a subset of segment-polarity genes.
B) segmentation genes.
C) homeobox-containing genes.
D) genes encoding chromatin repressors.
E) genes encoding chromatin remodelers.

Question 9

In Drosophila melanogaster, the expression of genes Ultrabithorax and Antennapedia can normally be observed in the third thoracic segment which bears a pair of legs as well as a pair of halteres. Homeotic mutations associated with these two genes can give rise to remarkable disturbances in the organization of the adult fly: two pairs of wings in the case of Ultrabithorax, and legs in the place of antennae in the case of Antennapedia. Would you expect these to be gain-of-function or loss-of-function mutations?

A) Loss-of-function for both genes
B) Loss-of-function for Ultrabithorax and gain-of-function for Antennapedia
C) Gain-of-function for Ultrabithorax and loss-of-function for Antennapedia
D) Gain-of-function for both genes

Question 10

Cortical rotation following fertilization in X. laevis places the …(1) pole at the point of sperm entry, while Wnt11 mRNA is transported to the …(2) pole.

A) 1: anterior; 2: posterior
B) 1: posterior; 2: anterior
C) 1: animal; 2: vegetal
D) 1: dorsal; 2: ventral
E) 1: ventral; 2: dorsal

Question 11

Indicate whether each of the following proteins is most concentrated near the dorsal (D) or ventral (V) side of the Drosophila embryo. Your answer would be a four-letter string composed of letters D and V only, e.g. DDDV.
( ) Decapentaplegic
( ) Dorsal (nuclear fraction)
( ) Toll (active form)
( ) Twist

Question 12

The simplified drawing below depicts early stages of animal development. Indicate which letter (A to E) in the drawing corresponds to each of the following terms. Your answer would be a five-letter string composed of letters A to E only, e.g. ECDBA.

( ) Blastula
( ) Ectoderm
( ) Endoderm
( ) Mesoderm
( ) Gastrula

Question 13

Indicate whether each of the following descriptions better applies to patterning by lateral inhibition (L), reaction-diffusion systems (R), or sequential induction (S). Your answer would be a four-letter string composed of letters L, R, and S only, e.g. SSRR.
( ) It does NOT generate asymmetrical patterns from an initial noisy field.
( ) It is based on short-range activation and long-range inhibition.
( ) It can readily generate complex patterns resembling the spots of a leopard or stripes of a zebra.
( ) It is commonly mediated by Notch signaling.

Question 14

Which of the following schematic drawings better depicts the regulatory network that maintains Engrailed (en), Hedgehog (hh), and Wingless (wg) expression following cellularization in a developing Drosophila embryo?

Question 15

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

Question 16

Indicate whether each of the following descriptions better applies to these poles established in an early embryo: anterior (A), posterior (P), dorsal (D), ventral (V), animal (N), or vegetal (G). Your answer would be a six-letter string composed of letters A, D, G, N, P, and V only, e.g. ADGPVN.
( ) It defines the part to become internalized in gastrulation.
( ) It defines the parts to remain external.
( ) It defines the location of the future head.
( ) It defines the location of the future tail.
( ) It defines the location of the future belly.
( ) It defines the location of the future back.

Question 17

Indicate whether each of the following descriptions better applies to egg-polarity genes (E), gap genes (G), pair-rule genes (P), or segment-polarity genes (S). Your answer would be a four-letter string composed of letters E, G, P, and S only, e.g. SSGG.
( ) Mutations in these genes show a maternal effect.
( ) Mutations in these genes leave the embryo with only half the number of normal segments.
( ) Mutations in these genes produce a normal number of segments but with part of each segment replaced by a mirror-image duplicate of other parts of the segment.
( ) Mutations in these genes can eliminate one or two groups of adjacent segments altogether.

Question 18

Plants and animals use different developmental strategies and have very different ways of life. In which of the following fundamental cellular processes during development are plants most different to animals?

A) Cell proliferation
B) Cell-cell interactions
C) Cell specialization
D) Cell movement

Question 19

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

A) Bicoid and Nanos are maternal-effect genes.
B) A female homozygous for a loss-of-function maternal-effect gene mutation can be fully normal, but her offspring will show the phenotype.
C) The offspring of a female homozygous for a loss-of-function maternal-effect gene mutation will show the phenotype regardless of the paternal genotype.
D) The second-generation offspring of a male homozygous for a loss-of-function maternal-effect gene mutation can show the phenotype.
E) All of the above.

Question 20

Sort the following primary axes in the order that they are established during the development of Xenopus laevis. Your answer would be a three-letter string composed of letters A to C only, e.g. ACB.
(A) A-P
(B) D-V
(C) A-V

Question 21

Does the nuclear-to-cytoplasmic ratio increase (I) or decrease (D) during cleavage in early vertebrate development? Would you expect tetraploid embryos to undergo the maternal-zygotic transition earlier (E) or later (L) compared to diploids? If the availability of a transcription regulatory protein determines the timing of this transition, should the protein be a transcriptional activator (A) or repressor (R)? Write down your answer as a three-letter string using the letters in the parentheses above, e.g. IEA.

Question 22

Stromal cell-derived factor 1 (SDF1) is a secreted protein that plays a major role in guiding the migration of various cells during development. It binds to a G-protein-coupled receptor on the surface of the migrating cell, which in turn activates a trimeric G protein containing a Gᵢ subunit. Pertussis toxin (PTX) modifies and inactivates the Gᵢ subunit. What would you expect to observe if migrating cells are treated with PTX?

A) They migrate toward the source of SDF1.
B) They migrate away from the source of SDF1.
C) They migrate in a certain direction chosen randomly.
D) They move in small steps in random directions.
E) They lose their motility.

Question 23

During branching morphogenesis in lung development, …

A) FGF10 is secreted by epithelial cells at the tip of the growing epithelial tubes.
B) FGF10 inhibits Shh production by the epithelial cells at the tip of the growing epithelial tubes.
C) Shh inhibits FGF10 production by the epithelial cells at the tip of the growing epithelial tubes.
D) Shh is produced by epithelial cells at the tip of the growing epithelial tubes.
E) None of the above.

Question 24

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?

A) Kit, the receptor for Steel
B) Bax, an essential apoptotic protein
C) Endothelin-3, a survival factor for neural crest cells
D) Fibronectin, a multi-adhesive protein of the extracellular matrix
E) FGF4, a growth factor

Question 25

The qualitative graph below shows the molecular changes accompanying vernalization in the flowering plant Arabidopsis thaliana. Indicate which curve (A to D) in the graph better represents the temporal changes in each of the following variables. Your answer would be a four-letter string composed of letters A to D only, with each letter used once, e.g. BACD.

( ) Level of Coolair noncoding RNA
( ) Level of Flowering locus C (FlC) gene product (transcriptional repressor)
( ) Level of Flowering locus T (Ft) gene product (transcriptional activator)
( ) Level of repressive chromatin marks at the FlC locus

Question 26

Which of the following evolutionary changes better explains morphological differences between different animals despite many common molecular mechanisms governing their development?

A) Changes in the types of genes responsible for key developmental processes.
B) Changes in regulatory DNA controlling expression of key developmental genes.
C) Changes in DNA-binding domains of key transcription regulators involved in development.
D) Changes in coding sequences of key developmental genes.
E) Changes in copy number of key developmental genes.

Question 27

The gene encoding Hes7 in mouse contains three introns. Any or all of these introns can be deleted to alter the delay associated with transcription and splicing of Hes7 mRNA. As a result of such deletions, either the oscillation frequency of this gene's expression can change (result 1) or the oscillatory behavior can disappear altogether (result 2). One of these two results is obtained when only one intron is deleted from the gene, while the other result is obtained when all three introns are removed. Would you expect the oscillation frequency to increase or decrease if result 1 is obtained? Which result (1 or 2) would you expect to observe when all three introns are deleted?

A) Increase; result 1
B) Increase; result 2
C) Decrease; result 1
D) Decrease; result 2

Question 28

Which of the following is NOT true regarding cell-type specification in developing animals?

A) Members of the MyoD/myogenin family of transcription regulators drive cells to differentiate into muscle cells.
B) Members of the Achaete/Scute family of transcription regulators drive the cells to form endothelial layers.
C) Many proteins that induce the differentiation of particular cell types belong to the bHLH (basic helix-loop-helix) class of transcription regulators.
D) Eyeless is both necessary and sufficient to generate eye structures composed of various specialized cell types in Drosophila.
E) In vertebrates, loss of the Eyeless homolog Pax6 leads to an animal without eyes.

Question 29

The role of Chordin and Noggin in patterning in developing vertebrates is equivalent to that of …(1) in Drosophila. They are secreted from the …(2) pole and antagonize the activity of bone morphogenetic factors that are secreted throughout the embryo.

A) 1: Decapentaplegic; 2: dorsal
B) 1: Decapentaplegic; 2: ventral
C) 1: Short gastrulation; 2: dorsal
D) 1: Short gastrulation; 2: ventral

Question 30

Fill in the blank in the following paragraph describing collective cell rearrangements. Do not use abbreviations.
"Cells form lamellipodia and attempt to crawl over one another, essentially pulling their neighbors inward into a narrow zone. This is accompanied by elongation along the long axis of the narrow zone. This process of … depends on Wnt signaling and is observed multiple times during the development of a vertebrate."

Question 31

Which of the following is NOT true regarding vertebrate segmentation?

A) The presomitic mesoderm retreats tailward as new somites are generated.
B) The period of gene-expression oscillations in the presomitic mesoderm determines the size of each somite.
C) If feedback delays in the gene-expression oscillations are increased, somite size will consequently increase as well.
D) Notch-mediated lateral inhibition ensures that the segmentation clock is different in neighboring cells in the presomitic mesoderm.

Question 32

Cactus is a maternal-effect gene coding for an inhibitory protein in the Drosophila Toll signaling pathway that binds to Dorsal in the cytosol and keeps it from nuclear entry. Would you expect maternal loss-of-function mutations in Cactus to give rise to dorsalized or ventralized embryos? What about loss-of-function mutations in Decapentaplegic (Dpp)?

A) Dorsalized; dorsalized
B) Dorsalized; ventralized
C) Ventralized; dorsalized
D) Ventralized; ventralized

Question 33

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?

A) D-V axis; curves 1 and 2
B) D-V axis; curves 1 and 3
C) A-V axis; curves 1 and 2
D) A-V axis; curves 1 and 3

Question 34

Indicate true (T) and false (F) statements below regarding developmental timing in animals. Your answer would be a four-letter string composed of letters T and F only, e.g. TTTT.
( ) Cell-division cycles generally serve as an intracellular timer that controls the timing of cellular differentiation.
( ) Developmental transitions are often regulated by microRNAs.
( ) Intracellular developmental programs are often followed even if the cells are taken from the developing embryo and maintained in culture.
( ) Timing of developmental transitions can be coordinated by cell-cell interactions as well as globally by hormones.

Question 35

The following schematic drawings show an epithelial sheet. Which example shows a mutant with planar-cell-polarity defects but no other defects?

Question 36

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?

A) Premature metamorphosis; premature metamorphosis
B) Premature metamorphosis; delayed metamorphosis
C) Delayed metamorphosis; premature metamorphosis
D) Delayed metamorphosis; delayed metamorphosis

Question 37

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 …

A) chemotactic cell motility.
B) asymmetric cell division.
C) cell adhesion.
D) convergent extension.
E) lateral inhibition.

Question 38

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?

A) Mutant embryos lacking both Sog and Dpp phenotypically resemble those lacking only Dpp.
B) Partial loss of Sog suppresses the phenotype of partial loss of Dpp.
C) Gain of Dpp suppresses the phenotype of partial loss of Sog.
D) Gain of Sog suppresses the phenotype of gain of Dpp.

Question 39

The result of Notch-mediated competitive lateral inhibition in a patch of wild-type cells is depicted on the left in the following schematic drawing, in which the black cells have become specialized. Consider a genetic mutation that interferes with intracellular Notch signaling in such a way that Notch can no longer regulate Delta effectively. Which drawing (1 or 2) better represents the outcome with cells bearing this mutation? Do the specialized cells (black hexagons) have active or inactive Notch at their surface?

A) Drawing 1; active Notch
B) Drawing 1; inactive Notch
C) Drawing 2; active Notch
D) Drawing 2; inactive Notch

Question 40

In the following fate map for the Xenopus blastula, indicate which zone in the map (A to D) better corresponds to each of the following fates. Your answer would be a four-letter string composed of letters A to D only, e.g. DCAB.

( ) Lung
( ) Blood
( ) Brain
( ) Skin

Question 41

Indicate true (T) and false (F) statements below regarding the formation of orderly neural maps in the mammalian brain. Your answer would be a four-letter string composed of letters T and F only, e.g. FFFF.
( ) As a general rule, neurons that fire together avoid each other.
( ) In the tonotopic map of the auditory system, brain neurons are arranged in the auditory cortex according to the pitch of the sound they respond to, like the keys of a piano.
( ) In the optic tectum in the midbrain, brain neurons are arranged along the anterior-posterior axis according to the frequency of light they respond to, like the colors of a rainbow.
( ) If the optic nerve of a frog is cut and the eye is rotated 180° in its socket, the retinal ganglion cells reconnect to their original targets in the optic tectum, therefore creating an inverted map.

Question 42

After reaching sexual maturity, the nematode Caenorhabditis elegans normally doubles in size within about two weeks. This doubling is mainly due to …

A) cell division.
B) DNA replication without cell division.
C) reduced cell death.
D) deposition of extracellular matrix.

Question 43

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?

A) a; 1
B) a; 2
C) b; 1
D) b; 2

Question 44

Robo3.1 is an alternative splice form of Robo3 that blocks the reception of Slit signals by the Robo1 and Robo2 receptors. Considering the guidance of commissural neurons crossing the ventral midline in the embryonic spinal cord of a vertebrate, would you expect Robo3.1 activity to increase (I) or decrease (D) as the growth cone crosses the midline? Write down I or D as your answer.

Question 45

Consider the embryonic cerebral cortex of a mammal. Near which side of the cortex are the cell bodies of radial glial cells located? To which side of the cortex do the first-born neurons eventually migrate?

A) Inner surface; inner surface
B) Inner surface; outer surface
C) Outer surface; inner surface
D) Outer surface; outer surface

Question 46

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, …

A) anterior axons grow only on the anterior tectal membranes, whereas posterior axons grow only on the posterior tectal membranes.
B) anterior axons grow only on the posterior tectal membranes, whereas posterior axons grow only on the anterior tectal membranes.
C) anterior axons grow only on the anterior tectal membranes, whereas posterior axons grow on both anterior and posterior tectal membranes.
D) anterior axons grow only on the posterior tectal membranes, whereas posterior axons grow on both anterior and posterior tectal membranes.
E) anterior axons grow on both the anterior and posterior tectal membranes, whereas posterior axons grow only on the anterior tectal membranes.

Question 47

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.

Question 48

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?

A) Chemoattractants; protein 1 is more potent.
B) Chemoattractants; protein 2 is more potent.
C) Chemorepellents; protein 1 is more potent.
D) Chemorepellents; protein 2 is more potent.
E) Protein 1 is a chemoattractant, and protein 2 is a chemorepellent; protein 2 is more potent.

Question 49

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?

A) Agrin secreted from the growth cone serves as a signaling molecule affecting the muscle cell.
B) LRP4 on the surface of the growth cone serves as a signaling molecule affecting the muscle cell.
C) Synaptic vesicles containing the neurotransmitter acetylcholine appear in the growth cone.
D) Acetylcholine receptors cluster on the muscle cell plasma membrane at the junction.
E) A thin layer of basal lamina forms at the cleft between the two cell membranes at the junction.

Question 50

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

Question 51

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

Question 52

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

Question 53

Which of the following is correct regarding neuronal self-avoidance in both invertebrates and vertebrates?

A) They both rely on alternative splicing to generate the diversity required to uniquely identify each neuron.
B) They both result in tiling, in which not more than one neuron occupies a certain territory.
C) They both rely on protocadherins.
D) They both use a strategy in which homophilic recognition results in avoidance.
E) All of the above.

Question 54

Which cell behavior depicted below is involved in the tube formation that underlies the development of lungs and trachea?

Question 55

The axons and dendrites of neurons that lack DSCAM1 altogether …

A) avoid other neurons as well as themselves.
B) avoid other neurons but not themselves.
C) avoid themselves but not other neurons.
D) avoid neither themselves nor other neurons.

Question 56

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.

Question 57

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.

Question 58

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 …

A) have a larger size (area) with a higher number of relatively normal cells.
B) have the same size, but with a higher number of relatively smaller cells.
C) have the same size, but with a smaller number of relatively larger cells.
D) have a smaller size with the same number of relatively smaller cells.
E) have a smaller size with a smaller number of relatively normal cells.

Question 59

Sort the following phases in the order that they take place in neural development in vertebrates. Your answer would be a four-letter string composed of letters A to D only, e.g. DACB.
(A) Neurons are assigned specific characters according to the place and time of their birth.
(B) Neurons form synapses with other neurons or with muscle cells.
(C) Synaptic connections are refined and adjusted.
(D) Neurons extend axons and dendrites toward their target cells.

Question 60

In the following schematic drawing, a growing axon (in red) is extended to a target neuron to form three synapses. In its journey, it is guided by a variety of mechanisms. Sort the following mechanisms to reflect the order in which they guide the navigation of the growth cone toward the target neuron in this example. Ignore the final chemoattraction step toward the target neuron itself. Your answer would be a six-letter string composed of letters A to F only, e.g. FACBDE.

(A) Chemoattraction
(B) Chemorepulsion
(C) Cell surface adhesion
(D) Contact inhibition
(E) Extracellular matrix adhesion
(F) Guidance by pioneer neuron

Question 61

A cat's left eye was covered during the critical period of development after eye opening. At the age of 6 months, a radioactive tracer molecule was injected into the right eye and allowed to reach the primary visual cortex. The cortex was then sectioned and subjected to autoradiography to reveal the ocular dominance columns. Which stripes would you expect to be wider: the dark stripes due to radioactivity (D) or light stripes (L)? Write down D or L as your answer.