Because of apoptosis,

cancer cells break away from a tumor and spread to other parts of the body

skin cells that form the embryonic webbing between chimpanzee fingers are destroyed

internal factors of the cell that transfer phosphate groups from one molecule to another in order to increase the energy of the receiving molecule

proteins attach to receptors in the cell membrane to initiate cell division

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.