Chapter 11, Problem 25A

Summary Introduction

Introduction :

During cell division, the chromosomes separate and one of each of thesister chromatids going to opposite poles of the cell. As a result, eachnew cell has the correct number of chromosomes. Sometimes sister chromatids fail to separate properly during cell division. This condition is called nondisjunction. As a result of nondisjunction, the gametes will not have correct number of chromosomes. When one of these gametes fertilizes another gamete,the resulting offspring will not have the correct number of chromosomes.

Answer to Problem 25A

Correct answer :

The correct answer is option (C) The sister chromatids do not separate.

Explanation of Solution

Explanation/justification for the correct answer :

Option (C) Nondisjunction is a condition that occurs when the sister chromatids fail to separate during meiosis I or meiosis II. This result in incorrect number of chromosomes in the gametes. Thus, the correct answer is option (C).

The explanation for the incorrect answer :

Option (A). Cytokinesis does not occur properly. Cytokinesis is the division of cytoplasm. Division of cytoplasm is not associated with nondisjunction. Thus, this is an incorrect answer.

Option (B). The nucleoli do not disappear. The nucleoli and nuclear membrane disappear during prophase of cell division. Thus, this is an incorrect option.

Option (D). The chromosome do not condense properly. The condensation of chromosome does cause nondisjunction of chromosomes. Thus, this is an incorrect answer.

During cell division, the chromosomes separate and one of each of thesister chromatids going to opposite poles of the cell. As a result, eachnew cell has the correct number of chromosomes.Sometimes sister chromatids fail to separate properly during cell division. This condition is called nondisjunction. As a result of nondisjunction,the gametes will not have correct number of chromosomes. When one of these gametes fertilizes another gamete,the resulting offspring will not have the correct number of chromosomes.

Notice that nondisjunction can result in extra copies of a certain chromosome or only one copy of a particular chromosome in the

offspring. Having a set of three chromosomes of one kind is called trisomy. Having only one of a particular type of chromosome is called monosomy. Nondisjunction can occur in any organism in which gametes are produced through meiosis. In humans, alterations of chromosome number are associated with serious human disorders, which often are fatal.