Passage Reading and English Comprehension

Nearly a century ago, biologists found that if they separated an invertebrate animal embryo into two parts at an early stage of its life, it would survive and develop as two normal embryos. This led them to believe that the cells in the early embryo are undetermined in the sense that each cell has the potential to develop in a variety of different ways. Later biologists found that the situation was not so simple. It matters in which plane the embryo is cut. If it is cut in a plane different from the one used by the early investigators, it will not form two whole embryos.

A debate arose over what exactly was happening. Which embryo cells are determined, just when do they become irreversibly committed to their fates, and what are the “morphogenetic determinants” that tell a cell what to become? But the debate could not be resolved because no one was able to ask the crucial questions in a form in which they could be pursued productively. Recent discoveries in molecular biology, however, have opened up prospects for a resolution of the debate. Now investigators think they know at least some of the molecules that act as morphogenetic determinants in early development. They have been able to show that, in a sense, cell determination begins even before an egg is fertilized.

Studying sea urchins, biologist Paul Gross found that an unfertilized egg contains substances that function as morphogenetic determinants. They are located in the cytoplasm of the egg cell; i.e., in that part of the cell’s protoplasm that lies outside of the nucleus. In the unfertilized egg, the substances are inactive and are not distributed homogeneously. When the egg is fertilized, the substances become active and, presumably, govern the behavior of the genes they interact with. Since the substances are unevenly distributed in the egg, when the fertilized egg divides, the resulting cells are different from the start and so can be qualitatively different in their own gene activity.

The substances that Gross studied are maternal messenger RNA’s—products of certain of the maternal genes. He and other biologists studying a wide variety of organisms have found that these particular RNA’s direct, in large part, the synthesis of histones, a class of proteins that bind to DNA. Once synthesized, the histones move into the cell nucleus, where section of DNA wrap around them to form a structure that resembles beads, or knots, on a string. The beads are DNA segments wrapped around the histones; the string is the intervening DNA. And it is the structure of these beaded DNA strings that guide the fate of the cells in which they are located.

782. The passage is most probably directed at which kind of audience?

	(a) State legislators deciding about funding levels for a state-funded biological laboratory
	(b) Scientists specializing in molecular genetics
	(c) Readers of an alumni newsletter published by the college that Paul Gross attended
	(d) Marine biologists studying the processes that give rise to new species

783. It can be inferred from the passage that the morphogenetic determinants present in the early embryo are

	(a) located in the nucleus of the embryo cells
	(b) evenly distributed unless the embryo is not developing normally
	(c) inactive until the embryo cells become irreversibly committed to their final function
	(d) identical to those that were already present in the unfertilized egg

784. The main topic of the passage is

	(a) the early development of embryos of lower marine organisms
	(b) the main contribution of modern embryology to molecular biology
	(c) the role of molecular biology in disproving older theories of embryonic development
	(d) cell determination as an issue in the study of embryonic development

785. According to the passage, when biologists believed that the cells in the early embryo were undetermined, they made which of the following mistakes?

	(a) They did not attempt to replicate the original experiment of separating an embryo into two parts
	(b) They did not realize that there was a connection between the issue of cell determination and the outcome of the separation experiment
	(c) They assumed that the results of experiments on embryos did not depend on the particular animal species used for such experiments
	(d) They assumed that different ways of separating an embryo into two parts would be equivalent as far as the fate of the two parts was concerned

786. It can be inferred from the passage that the initial production of histones after an egg is fertilized takes place

	(a) in the cytoplasm
	(b) in the maternal genes
	(c) throughout the protoplasm
	(d) in the beaded portions of the DNA strings

787. It can be inferred from the passage that which of the following is dependent on the fertilization of an egg?

	(a) Copying of maternal genes to produce maternal messenger RNA’s
	(b) Synthesis of proteins called histones
	(c) Division of a cell into its nucleus and the cytoplasm
	(d) Determination of the egg cell’s potential for division

788. According to the passage, the morphogenetic determinants present in the unfertilized egg cell are which of the following?

	(a) Proteins bound to the nucleus
	(b) Histones
	(c) Maternal messenger RNA’s
	(d) Cytoplasm (E) Nonbeaded intervening DNA

789. The passage suggests that which of the following plays a role in determining whether an embryo separated into two parts will develop as two normal embryos?

I. The stage in the embryo’s life at which the separation occurs
II. The instrument with which the separations is accomplished
III. The plane in which the cut is made that separates the embryo

	(a) I only
	(b) II only
	(c) I and II only
	(d) I and III only

TOTAL

Detailed Solution

1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20
21	22	23	24	25	26	27	28	29	30	31	32	33	34	35	36	37	38	39	40
41	42	43	44	45	46	47	48	49	50	51	52	53	54	55	56	57	58	59	60
61	62	63	64	65	66	67	68	69	70	71	72	73	74	75	76	77	78	79	80
81	82	83	84	85	86	87	88	89	90	91	92	93	94	95	96	97	98	99	100
101	102	103	104	105	106	107	108	109	110	111	112	113	114	115	116	117	118	119	120
121	122	123	124	125	126	127	128	129	130	131	132	133	134	135	136	137	138	139	140
141	142	143	144	145	146	147	148	149	150	151	152	153	154	155	156	157	158	159	160
161	162	163	164	165	166	167	168	169	170	171	172	173	174	175	176	177	178	179	180
181	182	183	184	185	186	187	188	189	190	191	192	193	194	195	196	197	198	199	200
201	202	203	204	205	206	207	208	209	210	211	212	213	214	215	216	217	218	219	220
221	222	223	224	225	226	227	228	229	230	231	232	233	234	235	236	237	238	239	240
241	242	243	244	245	246	247	248	249	250	251	252	253	254	255	256	257	258	259	260
261	262	263	264	265	266	267	268	269	270	271	272	273	274	275	276	277	278	279	280
281	282	283	284	285	286	287	288	289	290	291	292	293	294	295	296	297	298	299	300
301	302	303	304	305	306	307	308	309	310	311	312	313	314	315	316	317	318	319	320
321	322	323	324	325	326	327	328	329	330	331	332	333	334	335	336	337	338	339	340
341	342	343	344	345	346	347	348	349	350	351	352	353	354	355	356	357	358	359	360
361	362	363	364	365	366	367	368	369	370	371	372	373	374	375	376	377	378	379	380
381	382	383	384	385	386	387	388	389	390	391	392	393	394	395	396	397	398	399	400
401	402	403	404	405	406	407	408	409	410	411	412	413	414	415	416	417	418	419	420
421	422	423	424	425	426	427	428	429	430	431	432	433	434	435	436	437	438	439	440
441	442	443	444	445	446	447	448	449	450	451	452	453	454	455	456	457	458	459	460
461	462	463	464	465	466	467	468	469	470	471	472	473	474	475	476	477	478	479	480
481	482	483	484	485	486	487	488	489	490	491	492	433	494	495	496	497	498	499	500

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