Chapter 6, Problem 20P

As Fig. 6.21 on p. 190 shows, DNA polymerase cleaves the high-energy bonds between phosphate groups in nucleotide triphosphates (nucleotides in which three phosphate groups are attached to the 5′ carbon atom of the deoxyribose sugar). The enzyme uses this energy to catalyze the formation of a phosphodiester bond when incorporating new nucleotides into the growing chain.

a.

How does this information explain why DNA chains grow during replication in the 5′-to-3′ direction?

b.

The action of the enzyme DNA ligase in joining Okazaki fragments together is shown in Fig. 6.23 on p.191. Remember that these fragments are connected only after the RNA primers at their ends have been removed. Given this information, infer the type of chemical bond whose formation is catalyzed by DNA ligase and whether or not a source of energy will be required to promote this reaction. Explain why DNA ligase and not DNA polymerase is required to join Okazaki fragments.