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MCAT Sample Questions : Biological Sciences

Passage I

Approximately 100 years ago, two biologists performed separate experiments to study the process by which a fertilized egg differentiates into the many cell types found in a complete organism.

Biologist 1: The Mosaic Hypothesis

Biologist 1 worked with two-celled frog embryos, killing one cell of each embryo with a hot needle, but leaving the dead cell attached. The surviving cell formed only half of an embryo, and the biologist concluded that cells of the developing embryo were independent; that is, they acted as individual pieces of a mosaic. The biologist assumed that "determinants" (i.e., genes) were portioned out qualitatively as the egg divided, until each cell contained only the substances needed for its own development. Biologist 1 concluded that the fate of developing cells is determined by the cells' unequal content of determinants, and that cell lineage is unaffected by external conditions or by the position of a cell in the embryo.

Biologist 2: The Regulative Hypothesis

Biologist 2 worked with sea urchin embryos. When a tube of seawater containing embryos was shaken, the two cells of each embryo separated, and each cell later developed into a complete but slightly smaller embryo. This suggested that each cell retained a complete set of determinants. Biologist 2 viewed the embryo not as a mosaic, but as a harmonious equipotential system; that is, each cell is capable of developing into a complete organism, and the cells interact to regulate development. Thus, Biologist 2 concluded that the fate of developing cells depends mainly on environmental factors and on their position in the embryo.

Following are some sample questions on this passage:

  1. Which of the following pieces of experimental evidence best supports the Mosaic Hypothesis?

    1. Identical twins or triplets are derived from a single fertilized egg.
    2. In some developmental accidents, embryos with two normal-sized heads are produced.
    3. Separated cells of two-celled embryos continue to divide, producing partial embryos.
    4. Nuclei of dividing eggs in a single organism all contain the same genetic information.

    Answer: C

    Explanation: The Regulative Hypothesis proposes that each cell contains complete information for construction of the multicellular organism. Choice D is just a restating of this hypothesis. It can not be a correct answer, because the Regulative Hypothesis contradicts the Mosaic Hypothesis. If a single egg produces identical twins or triplets, identical plans for each individual must have been transferred from the egg in accordance with the Regulative Hypothesis, so A is also a wrong answer. For an embryo to have two normal-sized heads, identical construction information must have been introduced into both parts as stated in the Regulative Hypothesis, so B is also wrong. The Mosaic Hypothesis, states that the parts of the plan are handed out to the cells that need them at the time of division. The production of partial embryos is consistent with the mosaic theory because it implies that each embryo received just part of the total construction plan. Answer choice C is the only one of the four statements that supports the Mosaic Hypothesis.

  2. The nucleus of a frog egg is destroyed by radiation and replaced by a nucleus from a differentiated gut cell of a tadpole. The resulting egg is activated and develops into an adult frog. Are the results of this experiment more consistent with the Regulative Hypothesis or the Mosaic Hypothesis?

    1. The Regulative Hypothesis, because an environmental factor (radiation) activated the fertilized egg to develop into a frog
    2. The Regulative Hypothesis, because the activated egg resulting from the experiment developed into a complete organism
    3. The Mosaic Hypothesis, because some genes were retained in the nucleus of the frog egg cell after the radiation treatment
    4. The Mosaic Hypothesis, because the frog egg was unable to develop into an adult frog until genes from another cell were added

    Answer: B

    Explanation: Construction information for an organism must be coded in the form of the DNA in the nucleus of the egg. If the nucleus of an egg cell can be replaced by the nucleus of a gut cell and still direct the production of a complete individual, then the gut cell must contain all the construction information an egg nucleus contains. This is an example of the Regulative Hypothesis. This narrows the choice of answers to either A or B. Answer choice A suggests that we should decide that the experiment supports the Regulative Hypothesis because an environmental factor induced the development of the egg, but, in fact, the way the egg was activated to begin development is irrelevant to the question of the source of the information guiding development. Only answer choice B points out a relevant fact: that the egg with a gut nucleus developed into a complete organism.

  3. The validity of the Regulative Hypothesis could best be demonstrated in an organism by showing that:

    1. incomplete embryos develop from separated cells.
    2. cell fate is dependent on factors within the cell.
    3. transplanted embryonic cells show position-dependent development.
    4. genes determining cell development are distributed asymmetrically.

    Answer: C

    Explanation: The development of incomplete embryos, described in choice A, falsifies the Regulative Hypothesis. Answer choice B states that the fate of a cell depends on its own internal factors, but this would be true whether the Mosaic Hypothesis or the Regulative Hypothesis was true. Cells must have genes to guide their fate. The question is whether they have a full complement of the organism's genes or just a part. Answer choice D restates the Mosaic Hypothesis, that genes are distributed unevenly to daughter cells during development. Answer choice C, the correct answer, however, supports the Regulative Hypothesis. If all cells contain identical information for construction of the organism, then some external factor must tell different parts what they should become. If specific courses of development were shown to be dependent on the position of the cell in the embryo, it would explain this aspect of the Regulative Hypothesis. By supplying an answer to the question of how the parts of an animal develop differently when they have the same genes, that information buttresses the Regulative Hypothesis. Thus, answer choice C is the best answer.

  4. In a developmental study of a nematode worm, the positions of two embryonic cells (designated A and B) are switched. Cell A typically develops into the worm's pharynx; however, after the switch, descendants of Cell B form the pharynx. Which of the following statements regarding the nematode's development best explains these results?

    1. The nematode undergoes position-dependent cellular interactions.
    2. Different cells of the nematode receive different sets of genes.
    3. Embryogenesis in the nematode is mainly determined by individual cells.
    4. Development in the nematode is invariant.

    Answer: A

    Explanation: When the position of the cells was switched, the cells took on a fate consistent with their new positions. Answer choice A is the correct answer. It would be hard to deny that there must have been "position-dependent cellular interactions" involved. Answer choice B can not be true because the switched A cells did not develop the way A cells normally do, but instead developed the way B cells developed. They took on B cell characteristics, which would be impossible if they had a different set of genes. Answer choice C is not relevant to the experiment and is true of development in general. It is a statement of the obvious: that embryogenesis is a process dependent on the organism's individual cells. Choice D is contradicted by the experiment. Development did vary when the position of the cells was switched. There fore, answer choice A is the best answer.

  5. Which of the two hypotheses in the passage most closely fits our present-day understanding of human differentiation?

    1. The Mosaic Hypothesis, because each germ cell loses half of its genetic material during meiosis
    2. The Mosaic Hypothesis, because only specific genes are activated during the differentiation of each cell type
    3. The Regulative Hypothesis, because each embryonic cell receives a complete set of genes, and cell position is unrelated to differentiation
    4. The Regulative Hypothesis, because each embryonic cell receives a complete set of genes, and cell position helps to determine differentiation

    Answer: D

    Explanation: After meiosis, human germ cells contain half the genetic material of other cells in the body, but they still retain one copy of each gene. The normal complement of genes is re-established at fertilization, before development of the embryo begins. The facts stated in answer choice A are therefore irrelevant. Answer choice B is not good evidence for the Mosaic Hypothesis. It just suggests that all genes are not active at all times. The Regulative Hypothesis fits our understanding of human development best and because cell position does have an effect on development in humans. The correct answer must be answer choice D.

  6. The cell nucleus below contains the chromosomes of a sea urchin embryo at the two-cell stage.

    Which of the diagrams below best represents the nucleus of an embryo at the 64-cell stage grown from this cell?

    Answer: D

    Explanation: Answer choice D is the only choice that shows that all cells of the body have the same genetic makeup, because it is the only one in which the chromosome content at the 64 cell stage matches the chromosome complement at the two cell stage. Answer choices A and B show a reduction in the number of chromosomes and answer choice D shows an increase. Thus, answer choice D is the best answer.

  7. The validity of the Mosaic Hypothesis could best be demonstrated in an organism by showing that:

    1. all embryonic cells have the same developmental potential.
    2. interactions occur among all embryonic cells.
    3. the fate of all cells depends on external conditions.
    4. the fate of transplanted embryonic cells is independent of their new position in the embryo.

    Answer: D

    Explanation: Data that showed that all embryonic cells have the same developmental potential (answer A), that interactions occur among cells (answer B) or that the fate of cells depends on external conditions (answer C), would constitute evidence for the Regulative Hypothesis not the Mosaic Hypothesis. The only choice that is consistent with the Mosaic Hypothesis is answer choice D. Evidence that the fate of cells depends on their position disproves the theory that their fate depends on restrictions in the number of genes the cell contains. Cells can not acquire features based on their position unless they already have genes for those features.

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