Nucleic acids including DNA are composed of nucleotide units that are made up of three parts: a nucleobase, a sugar, and a phosphate (Figure 2.1). The nucleo-base or 'base' imparts the variation in each nucleotide unit while the phosphate and sugar portions form the backbone structure of the DNA molecule.
The DNA alphabet is composed of only four characters representing the four nucleobases: A (adenine), T (thymine), C (cytosine), and G (guanine).
Basic components of nucleic acids: (a) phosphate sugar backbone with bases coming off the sugar molecules, (b) chemical structure of phosphates and sugar molecules illustrating numbering scheme on the sugar carbon atoms. DNA sequences are conventionally written from 5 to 3.
The various combinations of these four letters, known as nucleotides or bases, yield the diverse biological differences among human beings and all living creatures. Humans have approximately three billion nucleotide positions in their genomic DNA. Thus, with four possibilities (A, T, C, or G) at each position, literally trillions of combinations are possible. The informational content of DNA is encoded in the order (sequence) of the bases just as computers store binary information in a string of ones and zeros.
Directionality is provided when listing a DNA sequence by designating the '5'-end' and the '3'-end.' This numbering scheme comes from the chemical structure of DNA and refers to the position of carbon atoms in the sugar ring of the DNA backbone structure (Figure 2.1). A sequence is normally written (and read) from 5' to 3' unless otherwise stated. DNA polymerases, the enzymes that copy DNA, only 'write' DNA sequence information from 5' to 3', much like we read words and sentences from left to right.
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