Chapter 9, Problem 20A

Summary Introduction

Introduction:

A cell grows until it reaches its size limit, then it either stops growing or divides. Most cells undergo division. Cell division helps a cell to reproduce and makes the organism grow and heal certain injuries. 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 given graph shows a cell over the course of its cell cycle.

Answer to Problem 20A

Correct answer :

The correct answer is option C. S stage

Explanation of Solution

Explanation/justification for the correct answer:

Option C. S stage −the part labeled A is part of interphase. It is the stage in which a cell grows, carries out cellular activities and replicates. It makes copies of DNA to enter the next stage of the cycle. There are three stages in interphase; G₁, S and G₂. In S stage DNA is replicated. As per the graph DNA, molecules increase from 4 to 8 so there is replication of DNA going on in part A. Hence this is the correct option.

Explanation for incorrect answer:

Option A. prophase−This is the first stage of mitosis in which nuclear membrane disintegrates, nucleolus disappears, chromosomes condense and mitotic spindle begins to form between the poles of the cell. There is no replication of DNA. As per the graph DNA molecules increase from 4 to 8 so there is replication of DNA going on in part A. Hence, this is not the correct option.

Option B. G₁ stage- In G₁ stage, cell grows, carries out regular cell functions and prepares for synthesis of DNA. There is no replication of DNA. As per the graph DNA molecules increase from 4 to 8 so there is replication of DNA going on in part A. Hence this is not the correct option.

Option D. G₂ stage - In G₂ stage cell prepares for cell division. There is no replication of DNA. As per the graph DNA molecules increase from 4 to 8 so there is replication of DNA going on in part A. Hence this is not the correct option.