What background information did Watson and Crick have made available for developing a model of DNA? What was ir contribution? 

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

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