Most of the biological reactions and molecules show chirality. Why?
Biological molecules are much larger than the molecules we have been talking about in organic chemistry so far. However, they are really just long chains of simpler building block molecules such as the nucleoside (cytosine) shown above on the left. There are three chiral centers in the molecule, indicated with an asterisk (*). Cytosine is one of the four so-called "nucleic acid bases" that are bonded "head-to-toe" to make up a strand of DNA. Two single strands come together to make the famous DNA double helix. From the point of view of this class, what is interesting about the DNA structure is chirality. Because the building blocks have multiple chiral centers and are found as single enantiomers in nature, the DNA structure itself is chiral and exists as a single enantiomer.
The helix is a so-called "right-handed" helix. Like the threads on a screw, helices are chiral, and this form of DNA, called the "B-form" helix is the most commonly seen structure, and it is a chiral helix because the nucleic acid bases are chiral.