Читать книгу Cell Biology - Stephen R. Bolsover - Страница 97

DNA polymerase III cannot itself initiate the synthesis of DNA. The enzyme primase is needed to catalyze the formation of a short stretch of RNA complementary in sequence to the DNA template strand (Figure 4.1). This RNA chain, the primer, is needed to prime (or start) the synthesis of the new DNA strand. DNA polymerase III catalyzes the formation of a phosphodiester bond between the 3′‐hydroxyl group of the RNA primer and the 5′‐phosphate group of the appropriate deoxyribonucleotide. Several RNA primers are made along the length of the lagging strand template. Each is extended in the 5′ to 3′ direction by DNA polymerase III until it reaches the 5′ end of the next RNA primer. In prokaryotes the lagging strand is primed about every 1000 nucleotides whereas in eukaryotes this takes place every 200 nucleotides.