DNA or deoxyribonucleic acid, also known as your genome, is a long molecule that carries your genetic information. This information decides almost everything about you- your growth, development, characteristic features, behaviour too to some extent. Most of your DNA is present in the nucleus and some DNA is also present in the mitochondria.
The DNA is a double stranded molecule which has a unique double helix shape (that resembles a twisted ladder). The strand is formed by alternate sugar (deoxyribose sugar) and phosphate group. To every sugar molecule, one of the four chemical bases – Adenine(A), guanine (G), thymine(T) and cytosine(C). The two strands are held together with the hydrogen bonds that join the complimentary bases- Adenine bonds with thymine and guanine binds with cytosine. The sequence or order of these bases form the genetic code of the organism.
DNA can replicate and make copies of itself- each strand acts as a template for copying the exact sequence of the bases. This property is extremely important because during cell division, the new cell must have the exact copy of the genome as the old cell.
The human DNA has around 3 million bases and 99.9% of the sequence is same in all human beings- the remaining 0. 1% is what makes us unique.