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Leading and Lagging Strand Synthesis


Since DNA is synthesized from 5’ to 3’, the template DNA must be read from 3’ to 5’ (remember the antiparallel structure of the double helix). This causes a problem at the replication fork: one strand can be copied continuously as the fork extends, but the other strand must be copied discontinuously. This discontinuous replication occurs in the following manner (see Figure 5.4): 
  1. An RNA primer (a short stretch of nucleotides) attaches to one strand via complementary base pairing. The primer is synthesized by a primase enzyme.
  2. The DNA polymerase begins synthesizing a new, complementary strand in the 5’ to 3’ direction. The strands made between RNA primers are known as Okazaki fragments.
  3. The primers are degraded and the Okazaki fragments are linked together by the enzyme DNA ligase. 
The strand that is synthesized continuously is called the leading strand. The other strand, synthesized discontinuously, is referred to as the lagging strand.

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