What is DNA?
Deoxyribonucleic Acid DNA is a nucleic acid polymer consisting of two long backbones, with nitrogenous bases connecting them (imagine a ladder). This structure is then twisted into a double helix.
The two back bones are composed of a pentose sugar (namely deoxyribose) with the nitrogenous bases facing inwards and connected to each other via hydrogen bonding. The sugars are in turn connected together by phosphate groups with phosphodiester bonds.
The function of DNA is long-term information storage. A brief synopsis is that DNA is used as a template to make mRNA (transcription) which directs the assembly of a protein in a ribosome (translation).
DNA must be replicated during cell reproduction, as each daughter cell resulting in mitosis will require a full genome. DNA replication is a nearly exact process, with error rates less than one-in-a-billion. This is due to the nitrogenous bases, which is where the information is stored. Four bases exist in DNA: adenine, guanine, cytosine and thymine (A,T,C and G). Remember the two legs of a ladder running anti-parallel to each other, with bases facing inwards. Each base is connected to the one across from it via relatively weak hydrogen bonding. This bonding however, allows the DNA to be replicated with remarkable exactness as A can only bond with T and C can only bond with G. As such, when DNA Polymerase enzymes polymerize a new strand of DNA, they "know" which order to put the bases in, thus conserving information. This is known as the "semi-conservative" model of DNA Replication.
The official definition of the word DNA is "deoxyribonucleic acid, a self-replicating material present in nearly all living organisms as the main constituent of chromosomes. It is the carrier of genetic information."