(1) The derivatives of purine are called adenine and guanine, and the derivatives of pyrimidine are called thymine, cytosine and uracil.(2) During DNA replication, DNA polymerase incorporates unmethylated cytosine opposite to guanine, leading to an overall reduction in methylation of DNA.(3) In DNA the bases pair to form a ladder-like structure, with adenine paired with thymine and guanine with cytosine .(4) Isochores were defined as long genomic segments that are fairly homogeneous in their guanine and cytosine composition.(5) The nitrogen-containing units are the bases: adenine, cytosine , guanine, and uracil.
