Links
[1] https://hootsgo.org/?q=node/6317
[2] http://biology.kenyon.edu/courses/biol63/watson_06.pdf
[3] http://academic.brooklyn.cuny.edu/biology/bio4fv/page/molecular%20biology/DNA-RNA-structure.html
[4] http://tigger.uic.edu/classes/phys/phys461/phys450/ANJUM04/
[5] http://www.whatislife.com/reader/dna-rna/dna-rna.html
[6] http://www.livescience.com/37247-dna.html