In the structure of DNA, each unit of a polynucleotide chain contains which combination of components bonded together?

Introduction / Context:
DNA deoxyribonucleic acid is a long polymer made of repeating units called nucleotides. Understanding what each nucleotide contains is essential for grasping DNA structure and function. This question asks which combination of components is found in each unit of a DNA polynucleotide chain.

Given Data / Assumptions:

• We are discussing DNA, not RNA.
• Each nucleotide includes a sugar, a nitrogenous base, and a phosphate group.
• The sugar in DNA is deoxyribose, while RNA contains ribose.
• The options mix and match these components in different ways.

Concept / Approach:
A DNA nucleotide is composed of three parts. The sugar is a five carbon sugar called deoxyribose. A nitrogenous base adenine, thymine, guanine, or cytosine is attached to the sugar, forming a nucleoside. A phosphate group is then attached to the sugar, usually at the five prime position, creating a nucleotide. Nucleotides join together through phosphate groups to form the polynucleotide chain, with sugar phosphate backbone and bases projecting inward.

Step-by-Step Solution:
Step 1: Recall that the three essential parts of a DNA nucleotide are sugar, base, and phosphate. Step 2: Identify that the sugar in DNA is deoxyribose, not ribose. Step 3: Note that each nucleotide must include at least one phosphate group to link to neighboring nucleotides in the chain. Step 4: Look at option A, which lists a nitrogenous base, deoxyribose sugar, and a phosphate group. This matches the standard description. Step 5: Option B includes ribose sugar, which is correct for RNA but not for DNA. Step 6: Options C, D, and E omit or mix components incorrectly. Step 7: Conclude that option A correctly describes the components of a DNA polynucleotide unit.

Verification / Alternative check:
Watson and Crick's model of DNA and modern biochemistry textbooks consistently show each nucleotide as deoxyribose linked to a base and a phosphate. The sugar phosphate backbone is a repeating pattern, and the bases are attached to the deoxyribose. The presence of deoxyribose distinguishes DNA from RNA, which supports the choice that specifically mentions deoxyribose rather than ribose.

Why Other Options Are Wrong:
Option B describes an RNA nucleotide with ribose, not a DNA nucleotide. Option C includes both deoxyribose and ribose sugars, which is not the structure of a single DNA nucleotide. Option D leaves out the sugar and is incomplete because the sugar links base and phosphate. Option E omits the phosphate group, which is essential for forming the backbone of the polynucleotide chain.

Common Pitfalls:
Students may confuse DNA and RNA components, especially the names of the sugars. It is helpful to remember that deoxy indicates DNA, while ribo indicates RNA. Another pitfall is forgetting the role of phosphate in linking nucleotides together and thinking that a base sugar unit alone is sufficient for a nucleotide, when it actually defines a nucleoside.

Final Answer:
Each unit of a DNA polynucleotide chain contains a nitrogenous base, deoxyribose sugar, and a phosphate group.