Harpers

330 / CHAPTER 36with the other strand of the DNA molecule accordingto the rules proposed originally by Watson and Crick(Figure 36–15). When an adenine deoxyribonucleosidemonophosphoryl moiety is in the template position, athymidine triphosphate will enter and its α phosphatewill be attacked by the 3′-hydroxyl group of thedeoxyribonucleoside monophosphoryl most recentlyadded to the polymer. By this stepwise process, thetemplate dictates which deoxyribonucleoside triphosphateis complementary and by hydrogen bondingholds it in place while the 3′-hydroxyl group of thegrowing strand attacks and incorporates the new nucleotideinto the polymer. These segments of DNAattached to an RNA initiator component are theOkazaki fragments (Figure 36–16). In mammals, aftermany Okazaki fragments are generated, the replicationcomplex begins to remove the RNA primers, to fill inthe gaps left by their removal with the proper basepaireddeoxynucleotide, and then to seal the fragmentsof newly synthesized DNA by enzymes referred to asDNA ligases.Replication Exhibits PolarityAs has already been noted, DNA molecules are doublestrandedand the two strands are antiparallel, ie, runningin opposite directions. The replication of DNA inprokaryotes and eukaryotes occurs on both strands simultaneously.However, an enzyme capable of polymerizingDNA in the 3′ to 5′ direction does not exist inany organism, so that both of the newly replicatedDNA strands cannot grow in the same direction simultaneously.Nevertheless, the same enzyme does replicateboth strands at the same time. The single enzyme replicatesone strand (“leading strand”) in a continuousmanner in the 5′ to 3′ direction, with the same overallforward direction. It replicates the other strand (“laggingstrand”) discontinuously while polymerizing the3′TPCA5′AGOHGURNA primerOHOHCOHUOHTGAACADNA templateTTTAGACGrowing DNA polymerCGAGTP P POHOH3′TGAAEntering TTPC5′Figure 36–15. The RNA-primed synthesis of DNA demonstrating the template function of thecomplementary strand of parental DNA.