Strands of DNA are joined byMultiple Choicehydrogen bonds.covalent bonds.ionic bonds.phosphodiester bonds.

1. ### Break Down the Problem

We need to identify the type of bonds that join strands of DNA. The options provided are:

1. Hydrogen bonds
2. Covalent bonds
3. Ionic bonds
4. Phosphodiester bonds

2. ### Relevant Concepts

In DNA:

• Hydrogen bonds form between the nitrogenous bases of nucleotides on opposite strands (A-T and C-G pairs).
• Covalent bonds link the sugar and phosphate groups in the DNA backbone.
• Ionic bonds are not typically involved in the structure of DNA.
• Phosphodiester bonds are a type of covalent bond that specifically connects the sugar and phosphate groups in a single strand of DNA.

3. ### Analysis and Detail

• In DNA, hydrogen bonds primarily connect complementary bases on opposite strands (Adenine to Thymine, and Cytosine to Guanine).
• Covalent bonds are found throughout the sugar-phosphate backbone; however, they do not directly join two strands.
• Ionic bonds are not significant in the context of DNA structure.
• Phosphodiester bonds refer to the specific covalent bonds that connect nucleotides within a single strand.

4. ### Verify and Summarize

The main type of bond joining the two strands of DNA through complementary bases is hydrogen bonds. Conversely, phosphodiester bonds and covalent bonds refer to connections within a single strand.

Final Answer

The strands of DNA are joined by hydrogen bonds.