Nucleic acids

DNA carries the hereditary information of the cell in the sequence of its bases.
A gene is a segment of DNA whose base sequence codes for the amino acid sequence of one polypeptide (protein); it is transcribed into mRNA, which is then translated.
So the gene works as the instruction manual for the synthesis of a protein.
A nucleoside (base + sugar) and a nucleotide (base + sugar + phosphate) are only building units of nucleic acids, and ribose is the sugar of RNA.
Hence the segment is the gene.

DNA contains two types of nitrogenous bases.
Purines: Adenine (A) and Guanine (G).
Pyrimidines: Cytosine (C) and Thymine (T).
In the double helix a purine always pairs with a specific pyrimidine on the opposite strand.
Adenine pairs with thymine (A : T) by two hydrogen bonds.
Guanine pairs with cytosine (G : C) by three hydrogen bonds.
Uracil is present in RNA in place of thymine; it is not a base of DNA.
So the complementary bases in DNA are adenine and thymine, guanine and cytosine.

Each nucleic acid consists of a pentose sugar, a heterocyclic base and phosphoric acid.
The sugar present in DNA is 2-deoxy-D(-)-ribose and the sugar present in RNA is D(-)-ribose.
These pentose sugars contain asymmetric (chiral) carbon atoms and belong to the D-series.
The heterocyclic bases (purines and pyrimidines) are planar aromatic rings and are not chiral.
The phosphate ester units are also not chiral.
So the chirality of DNA and RNA molecules is due to the presence of the D-sugar component.