5' - 3' direction

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5' - 3' direction refers to the orientation of nucleotides of a single strand of DNA or RNA. The 5' and 3' specifically refer to the 5th and 3rd carbon atoms in the deoxyribose/ribose sugar ring. The phosphate group attached to the 5' end of one nucleotide and the hydroxyl group at the 3' end of another nucleotide have the potential to form phospodiester bonds, and hence link adjacent nucleotides. This linkage provides the sugar-phosphate backbone that gives DNA its structural rigidity. Any single strand of DNA/RNA will always have an unbound 5' phosphate at one end and an unbound 3' hydroxyl group at the opposite end. DNA is always read in the 5' to 3' direction, and hence you would start reading from the free phosphate and finish at the free hydroxyl group.

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