Chapter 12, Problem 21A

Summary Introduction

To infer:

How complementary base pairing is responsible for semi conservative replication.

Introduction:

Before a cell divides it must copy its chromosomes so that the new cells have same number of chromosomes as the parent cell. The process by which DNA in the chromosome is copied is called DNA replication. Semi conservative replication is a mechanism by which each strand of DNA serves as a template to synthesize new DNA molecule.

Answer to Problem 21A

During replication of DNA, each strand serves as a template to synthesize new DNA molecule. A new strand is made from the original strand of DNA by complementary base pairing.

Replication of DNA produces two molecules from one. Each molecule has one original strand and one new strand. The original DNA is partly conserved in the new DNA molecule.

Hence, complementary base pairing is responsible for semi conservative replication.

Explanation of Solution

Each DNA molecule consists of two strands of nucleotides joined together like a zipper t the nitrogenous bases. Both the strands have complementary base pairs. Adenine on one strand pairs with thymine on the other strand. Similarly cytosine on one strand pairs with guanine on the other strand. During replication, the DNA unzips and the nucleotides that are floating freely in the cell attaches to the unzipped chains. On the unzipped strand where there is adenine, thymine will attach. The original DNA molecule continues to unzip till every base finds its pair and a new molecule of DNA is formed.

The two DNA molecules are identical to the original DNA molecule.

This method of replication is called semi conservative replication as one strand of DNA in the new molecule is from the original DNA molecule. So the DNA is partly conserved in the new molecule of DNA. A new strand is made from the original strand of DNA by complementary base pairing.