DNA is repository of genetic information. DNA codes for proteins through mRNA. The process by which mRNA is synsized on DNA template is called transcription. The genetic message in mRNA is later on translated into sequence of amino acids in proteins.

Watson and Crick made following background Information available for developing a model of DNA: 
$?$ Pairing between two strands of polynucleotide chains. 
$?$ Base pairing of polynucleotide chains is complementary in nature. 
$?$ If base sequence of one strand is known, n base sequence of anor strand can be predicted. 
$?$ If each strand from a DNA acts as a template. n both daughter DNAs would be similar to mor DNA. 

Contributions of Watson and Crick: 
$?$ A simple model of DNA was available because of m. 
$?$ Genetic implications of DNA replication could be easily understood. 
$?$ The model brought revolution in understanding of biology at a molecular level.