A biochemist isolated and purified molecules needed for DNA replication. When he added some DNA, replication occurred, but the DNA molecules formed were defective. Each consisted of normal DNA strand paired with numerous segments of DNA, a few hundred nucleotides long. What had the scientist probably left out of the mixture?

Explanation

Given the description of defective DNA molecules formed with normal DNA strands paired with numerous segments of DNA, the most likely component that the biochemist left out of the mixture is primers. Without primers to initiate DNA synthesis, DNA polymerase would be unable to properly replicate the DNA, resulting in the observed defective molecules. Primers are short sequences of RNA or DNA that provide a starting point for DNA synthesis. They are necessary for DNA polymerase to begin replication. Without primers, DNA polymerase cannot initiate the synthesis of new DNA strands, leading to defective replication products.