Describe how nucleotides and hydrogen bonds affect the structure of dna.

Strands of DNA are composed of the sugar and phosphate portions of the nucleotides, while the middle portions are made of the nitrogenous bases. The nitrogenous bases on the two strands of DNA pair up, purine with a pyrimidine (A with T, G with C), and are held together by weak hydrogen bonds. Internal and external hydrogen bonds maintain the DNA molecule. The two strands of DNA stay together by H bonds that occur between similar nucleotide base pairs. Two hydrogen bonds occur among the adenosine and the thymine base pairs, and between the cytosine and the guanine, there are three.