GRE Subject Test: Biology : Understanding Fertilization and Early Development

Study concepts, example questions & explanations for GRE Subject Test: Biology

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Example Questions

Example Question #1 : Animal Development

What is the name of the structure that forms during development when the embryo becomes a fluid-filled ball?

Possible Answers:

Blastocyst

Neurula

Gastrula

Morula

Correct answer:

Blastocyst

Explanation:

The zygote will divide into a morula once eight or more cells are created. The morula is still characterized by cellular cleavage, and does not increase beyond the size of the original zygote cell. Eventually, the morula develops into a blastula, characterized by the introduction of the G2 phase in the cell cycle. This allows for growth and development of the cells before subsequent divisions. The blastula develops a fluid-filled cavity known as the blastoceole, and is given the name blastocyst. The blastocyst will then grow into a gastrula after gastrulation, and later a neurula (after neurulation). 

Example Question #1 : Understanding Fertilization And Early Development

During embryogenesis, the separation of cells into the three individual germ layers first occurs during which of the following?

Possible Answers:

Cleavage

Gastrulation

Fertilization

Organogenesis

Neurulation

Correct answer:

Gastrulation

Explanation:

After fertilization occurs, the zygote undergoes a series of cellular divisions in a process called cleavage. This is followed by the formation of the blastula, a hollow sphere of cells. Gastrulation occurs next, where the embryo is divided into three germ layers: the endoderm, mesoderm, and ectoderm (from inside to outside). Hence, gastrulation is the stage in which this process first occurs. Neurulation and organogenesis occur after gastrulation.

Example Question #3 : Animal Development

Which of the following defines the process by which the entry of one sperm into an oocyte prevents other sperms from fertilizing the same egg?

Possible Answers:

Determination

Differentiation

The induction reaction

The cortical reaction

Correct answer:

The cortical reaction

Explanation:

Cortical reaction occurs in fertilization when a sperm enters the cytoplasm of an oocyte. The reaction causes a change in the zona pellucida that prevents polyspermy. Determination refers to the point at which a cell becomes committed to a certain developmental path during embryogenesis, and differentiation refers to the process by which that cell actually becomes a specialized tissue cell. Determination occurs very early during embryogenesis, whereas differentiation occurs later in the developmental process.

Example Question #4 : Developmental Stages And Events

A new species found by scientists exhibits indeterminate cleavage up to the third cleavage.  If all of the cells in the embryo are separated and incubated in healthy conditions on their own after the third cleavage, how many organisms will develop?

Possible Answers:

0

1

8

4

2

Correct answer:

8

Explanation:

After the third cleavage, the embryo would be composed of 8 cells. If every cleavage step up up to this point was indeterminate, then each cell still has the necessary components to develop into a complete organism. The result would be 8 genetical identical organisms.

Example Question #1 : Understanding Fertilization And Early Development

During gastrulation, the developing embryo undergoes a reorganization of the cells into a multi-layered organism, with each layer necessary to form distinct parts of the eventual fully-formed organism. Which of these primary layers will ultimately form the skin and the nervous system of the organism? 

Possible Answers:

Blastopore

Vegetal pole

Endoderm

Mesoderm

Ectoderm

Correct answer:

Ectoderm

Explanation:

The vegetal pole and the blastopore are not considered the primary layers formed early in gastrulation, nor will they give rise to skin and nervous system structures. The endoderm will ultimately form the digestive organs and lungs, and the mesoderm will form the muscles, kidneys, and bones. The correct answer is ectoderm, which will form skin and nervous system tissues. 

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