The bonds that hold the two strands of DNA together come from a. the attraction of phosphate groups for each other b. strong bonds between nitrogenous bases and deoxyribose c. weak hydrogen bonds between the nitrogenous bases d. carbon-to-carbon bonds in the sugar portion of the nucleotides.

Step 1: Define the Problem

Identify which type of bond holds the two strands of DNA together.

Step 2: Break Down the Problem

Examine each option to determine which correctly describes the bonds between DNA strands.

Step 3: Apply Relevant Concepts

• Option a: Phosphate groups are part of the DNA backbone and do not attract each other to hold strands together.
• Option b: Strong bonds between nitrogenous bases and deoxyribose are not responsible for holding the two strands together; they are part of the same strand.
• Option c: Weak hydrogen bonds between nitrogenous bases are responsible for holding the two strands together.
• Option d: Carbon-to-carbon bonds are part of the sugar-phosphate backbone, not between the strands.

Step 4: Analysis, Verify and Summarize

• Hydrogen bonds are the correct type of bond that holds the two strands of DNA together, specifically between complementary nitrogenous bases (adenine with thymine, and guanine with cytosine).

Final Answer

The bonds that hold the two strands of DNA together come from c. weak hydrogen bonds between the nitrogenous bases.