DNA, Gene Structure, and DNA Replication - Insect Molecular Genetics - page 8

Figure 1.3 Bases in DNA are purines (adenine and guanine) or pyrimidines (thymine and cytosine).
Uracil is substituted for thymine in RNA.
Individual nucleotides are linked together by phosphodiester bonds to form
polynucleotides ( Figure 1.4 ). Polynucleotides have chemically distinct ends. In
Figure 1.5 , the top of the polynucleotide ends with a nucleotide in which the
triphosphate group attached to the 5 ′ -carbon has not participated in a phospho-
diester bond. This end is called the 5 ′ or 5 ′ -P terminus. At the other end of the
molecule, the unreacted group is not the phosphate, but the 3 ′ -hydroxyl. This
end is called the 3 ′ or 3 ′ -OH terminus. This distinction between the two ends
(5 ′ and 3 ′ ) means that polynucleotides have an orientation that is very impor-
tant in many molecular genetics concepts and applications.
Polynucleotides can be of any length and have any sequence of bases. The
DNA molecules in chromosomes are probably several million nucleotides long.