What is attached to the 3' end of a sugar in a nucleotide?

In a nucleotide, the 3' end of the sugar is indeed attached to a free -OH group. This hydroxyl (-OH) group is crucial for the formation of phosphodiester bonds when nucleotides link together to form nucleic acid polymers like DNA and RNA.

During DNA or RNA synthesis, the free -OH group on the 3' carbon of one nucleotide reacts with the phosphate group of the incoming nucleotide. This reaction is essential for forming the backbone of the nucleic acid chain and allows for the sequential addition of nucleotides. The presence of the -OH group at the 3' end is a defining characteristic of the structure of nucleotides and plays a significant role in the mechanisms of replication and transcription, where an ongoing supply of nucleotides is required for the synthesis process.

Therefore, recognizing the significance of the -OH group at the 3' end is fundamental in understanding how genetic material is structured and replicated.