Question

reiji finds blood samples that have been exposed to extreme heat. when he tries to extract the dna for analysis, he finds that the double - stranded dna has separated into single strands. however, when he cools the samples slowly, some of the dna reforms the double helices, while some dna remains single - stranded. what determines whether the dna will re - anneal properly?
the length of the dna fragments
whether the complementary strand is present
the concentration of salt in the solution
whether the dna is from nuclear or mitochondrial sources
the age of the blood sample

Explanation:

DNA re - annealing depends on base - pairing. Complementary strands can hydrogen - bond to reform the double helix. Length, salt concentration, source (nuclear/mitochondrial), and sample age are less directly related to whether re - annealing occurs properly compared to the presence of the complementary strand.

Answer:

B. Whether the complementary strand is present