quadrants contain leading strands. left When DNA pol III on RNA primer, thus running out of template, it leaves a single- stranded gap between the last DNA nucleotide of the newly synthesized daughter strand and the first nucleotide of the RNA primer (Figure 7.19). The pol III, having very low affin- ity for these DNA-RNA single-stranded gaps, is then replaced by DNA polymerase I (DNA pol I), which has high affin- ity for such gaps (Figure 7.19,). The DNA pol I removes nucleotides of the RNA primer one with DNA nucleotides, beginning with the 5' nucleotide of the RNA primer and progressing in the 3' direction until all the RNA nucleotides in the primer have been replaced by DNA nucleotides complementary to the template strand. The daughter strands in the upper left and lower right quadrants shown in Figure 7.18 have a 5'-to-3' direction of elongation that runs opposite to the direction of movement of the replication fork. These daughter strands are elongated discontinuously, in short segments, each of which is initiated by an RNA primer. The discontinuously synthesized daughter strand is called the lagging strand. Thus in Figure 7.18, the lower right and upper left quadrants of the replication bubble contain lagging strands (see also step 5 of Figure 7.14) Reiji Okazaki detected the synthesis of short fragments of DNA in the replication of the lagging strand. He observed by one and replaces them Okazaki Figure 7.18 The replication bubble. Bidirectional expansion is driven by DNA synthesis at each replication fork. One repli- some containing two DNA pol III enzymes operates at each fork to replicate both daughter strands. Template DNA fragments Daughter DNA oric Replisome region Replisome region 1 3' 5 Replication fork 3 (location of Replication fork (location of one replisome) 3' 3'5' 5' 3' one replisome) 5' RNA primer Draw a second replication 5' bubble to the right of the 3' Leading strand Lagging strand T one illustrated. As these two Lagging strand 5' 3' replication bubbles expand toward one another, what kind of strand will each leading Leading strand 3' 5' 3' 3 5' 3' 5' i3' strand encounter when the bubbles make contact? 2 1 Okazaki fragments baot De 10 oric Bidirectional expansion of bubble d n0 in in