What is meant by anterior and posterior region in our body?

"Anterior" means 'nearer to the front'.  The anterior region of the body
is the region that includes the parts that are nearer to the front, like
toes and eyes.

"Posterior" means 'further back in position'.  The posterior region of the
body is the region that includes the parts whose position is further back,
like buttocks and heels.

The basic repeating unit of nucleic acids are known as nucleotides. a nucleotide consists of three distinct chemical groups.