The nucleotide is the basic unit of DNA and RNA. A nucleotide consists of a phosphate group, a five-carbon sugar molecule called deoxyribose, and one of four nitrogenous bases. The base attaches to the 2′ carbon atom on the ribose ring making it an adenine, cytosine (C), guanine (G) or thymine (T). The phosphate group and the deoxyribose sugar togetherare called a nucleoside.
A DNA or RNA molecule is formed when two strands of nucleotides bond to each other, creating a double helix shape with alternating molecules that contain an A:T base-pairing (adenine for thymine) and G:C base pairing. The nitrogenous bases in DNA always form a purine with another pyrimidine; adenine will always pair up with either cytosine (in the case of AT) or guanine (in the case of GC). In RNA, there are ribose instead of deoxyribose sugars found between the phosphate groups linking different nucleotides into chains and helices. And