# The average molar mass of one base pair of nucleotides in DNA is approximately 600 g/mol. The spacing between successive base pairs is about 0.34 nm, and a complete turn in the helical structure of DNA occurs about every 3.4 nm. If a DNA molecule has a molar mass of 4.5 × 10 9 g/mol, approximately how many complete turns exist in the DNA α -helix structure?

### Chemistry: An Atoms First Approach

2nd Edition
Steven S. Zumdahl + 1 other
Publisher: Cengage Learning
ISBN: 9781305079243

Chapter
Section

### Chemistry: An Atoms First Approach

2nd Edition
Steven S. Zumdahl + 1 other
Publisher: Cengage Learning
ISBN: 9781305079243
Chapter 21, Problem 134AE
Textbook Problem
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## The average molar mass of one base pair of nucleotides in DNA is approximately 600 g/mol. The spacing between successive base pairs is about 0.34 nm, and a complete turn in the helical structure of DNA occurs about every 3.4 nm. If a DNA molecule has a molar mass of 4.5 × 109 g/mol, approximately how many complete turns exist in the DNA α-helix structure?

Interpretation Introduction

Interpretation: The number of turns present in the α -helix structure of DNA is to be calculated.

Concept introduction: The alpha helix consists of a coiled polypeptide chain. The secondary structure of DNA is similar to the secondary structure of proteins which is an alpha helix structure held together by hydrogen bonds. The alpha helix in DNA consists of two right-handed polynucleotide chains that are coiled together by number of turns about the same axis resulting in alpha helical structure.

To determine: The number of turns present in the α -helix structure of DNA.

### Explanation of Solution

Explanation

The number of turns present in the DNA is calculated by knowing the number of base pairs present in the helix. Therefore, the numbers of base pairs are calculated as,

The given molar mass of each base pair is =600g/mol

And the given molar mass of the total DNA molecule =4.5×109g/mol

The number of base pairs is calculated by the formula,

The number of base pairs =Total molar mass of DNAmolar mass of base pairs

Substitute the value of the total molar mass of DNA and that of the mass of one base pair in the above expression.

the no.of base pairs  =total molar mass of DNAmolar mass of one base pair = 4.5×109g/mol600 g/mol = 7500000 base pairs

Now, the given spacing between the successive base pairs is 0

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