Question

Why do DNA have beta deoxyribose and not alpha deoxyribose in its structure?

Answer 1

Look at boththe structures above , it shows difference between beta and alpha deoxyribose sugars.
The difference is only in the orientation of the OH group. As you can see beta and alpha deoxribose sugars are otherwise same apart from the OH position.
Now , there is one anothe diagrambelow, that explains, how in a nucleotide ( monomer of a DNA) the nitrogen base attaches to the deoxyribose sugar. There are 4 nucleotides, but we have used only one for example here. The reaction occurs between the NH2 and OH group . These structures attach together via a hydrolysis reaction ( there is a loss of H2O molecule ). This reaction can only occur is the deoxyribose sugar is in beta orientation. If its in alpha orientation , the nitreogen base won't be able to react with deoxyribose sugar in order to make a nucleoside. Thus DNA has beta -deoxyribose sugar.