Explain what 5' and 3' ends mean in regards to DNA structure?

5' and 3' ends describes the directionality of the DNA molecule. Essentially, the strand of a DNA molecule can have a 5' end and a 3' end. To understand what a 5' or 3' end is, we need to look at the molecular structure of DNA. DNA is a polymer of nucleotides, where each nucleotide is made up of a sugar (deoxyribose), a nitrogenous base, and a phosphate group. The deoxyribose sugar is a 5 carbon structure, where each carbon can be numbered 1-5. The base is always connected to Carbon 1 of the sugar and the phosphate group is connected to Carbon 5 of the sugar. The nucleotides are then connected to one another to form the polymer whereby the phosphate group of one nucleotide (on Carbon 5) connects to the next nucleotide sugar via Carbon 3. Therefore when the strand is built, the top nucleotide will have a free phosphate group on the Carbon 5 of the sugar, hence the 5' end, and the last nucleotide of the strand will have a free OH on the Carbon 3 of the sugar. Since DNA is made up of two antiparallel strands, each strand can have its own directionality. As they are antiparallel, they will run in opposite directions.