DNA helicase unwinds DNA by binding to the laggingstrand template at each replication fork and moving in the 5 b 3 direction along the strand

Unwinding

Chapter 11, topoisomerases control the supercoiling of DNA. In replication, DNA gyrase reduces torsional strain (torque) that builds up ahead of the replication fork as a result of unwinding (see Figure 12.12). It reduces torque by making a double-stranded break in one segment of the DNA helix, passing another segment of the helix through the break, and then resealing the broken ends of the DNA. This action removes a twist in the DNA and reduces the supercoiling.

Primers All DNA polymerases require a nucleotide with a 3'-OH group to which a new nucleotide can be added. Because of this requirement, DNA polymerases cannot initiate DNA synthesis on a bare template; rather, they require a primer—an existing 3'-OH group—to get started. How, then, does DNA synthesis begin?

An enzyme called primase synthesizes short stretches of nucleotides (primers) to get DNA replication started. Primase synthesizes a short stretch of RNA nucleotides (about 10-12 nucleotides long), which provides a 3'-OH

group to which DNA polymerase can attach DNA nucleo-tides. (Because primase is an RNA polymerase, it does not require an existing 3'-OH group to which nucleotides can be added.) All DNA molecules initially have short RNA primers imbedded within them; these primers are later removed and replaced by DNA nucleotides.

On the leading strand, where DNA synthesis is continuous, a primer is required only at the 5' end of the newly synthesized strand. On the lagging strand, where replication is discontinuous, a new primer must be generated at the beginning of each Okazaki fragment (I Figure 12.13). Primase forms a complex with helicase at the replication fork and moves along the template of the lagging strand. The single primer on the leading strand is probably synthesized by the primase-helicase complex on the template of the lagging strand of the other replication fork, at the opposite end of the replication bubble. __

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