Chapter 9, Problem 19STP

Summary Introduction

To describe:

The function of the centromere and to predict what might happen if there were no centromeres

Introduction:

Cells reproduce by a cycle of growing and dividing called the cell cycle. There are three main stages of a cell cycle; interphase, mitosis and cytokinesis.The second stage is mitosis in which cell’s nucleus and nuclear material divides. There are four stages in mitosis; prophase, metaphase, anaphase and telophase.

Answer to Problem 19STP

The centromeres help in attaching the sister chromatids which must remain joined till they are pulled apart into two daughter cells. Centromeres ensure that the sister chromatids segregate properly and each daughter cell receive one complete copy of DNA.

If centromeres were absent, the sister chromatids would not be held together and their separation would not be equal.

Explanation of Solution

Prophase is the first stage in which a dividing cell spends most of the time. The chromatin tightens and condenses into chromosomes. Chromosomes are shaped like an X. Each half of this X chromosome that was replicated in interphase is called sister chromatid. The sister chromatids are attached at the center with a structure called centromere. Towards the end of prophase, the spindle fibers attach to the sister chromatids of each chromosome on both sides of the centromere and then attach to opposite poles of the cell. This arrangement ensures that each new cell receives one complete copy of the DNA.

If centromeres were absent, the sister chromatids would not remain together and separate. There would be unequal distribution of chromosomes into the daughter cells.