`alpha`-helix : protein :: double helix : ………….

Explanation

To solve the analogy given in the question, we need to identify the relationship between the terms provided.

1. **Understanding the first part of the analogy**: The term "alpha-helix" refers to a specific structural feature of proteins. In proteins, the alpha-helix is a common secondary structure that is stabilized by hydrogen bonds between the backbone atoms in the polypeptide chain.

2. **Identifying the corresponding term for the second part**: The second part of the analogy is "double helix." The double helix is a well-known structure associated with nucleic acids, specifically DNA. DNA consists of two strands that coil around each other, forming a helical structure.

3. **Establishing the relationship**: Since "alpha-helix" is a structural feature of proteins, we need to find out what "double helix" is a structural feature of. The double helix is a characteristic structure of DNA.

...