What forms the backbone of a polynucleotide strand of a nucleic acid?

Explanation

To answer the question, "What forms the backbone of a polynucleotide strand of a nucleic acid?" we can break down the solution into the following steps:

### Step-by-Step Solution:

1. **Understanding Polynucleotides**:
- A polynucleotide is a long chain of nucleotides linked together. Nucleotides are the building blocks of nucleic acids such as DNA and RNA.

2. **Components of Nucleotides**:
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