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DNA replication is the process through which a DNA molecule makes a copy of itself. We will explore the enzymes involved in DNA replication, the concept of leading and lagging strands (Okazaki fragments), and walk you through the entire DNA replication stepbystep.

The first step in DNA replication is the separation of the double helix structure of the DNA molecule, which is carried out by DNA helicase. These separated strands serve as template strands in the DNA replication process.

Single stranded binding proteins (SSB) prevents the single template strands from reannealing, which allows the template DNA strands to stay separated and available for the replication process. RNA primer segments are placed on this unwound DNA by the enzyme primase.

DNA polymerase (III) arrives at the site of the RNA primers to begin replication. The DNA polymerase moves along the template strand, and adds bases that are complimentary to our DNA template strand in an antiparallel direction: DNA Polymerase reads the template strand in the 3' to 5' direction, but the new DNA strand is produced in the 5' to 3' direction.

Eventually, we differentiate into the leading and lagging strands of DNA. DNA polymerase binds to the leading strand, adding new complementary nucleotide bases to the strand of DNA in the 5’ to 3’ direction as helicase is unwinding the DNA. Chunks of DNA, called Okazaki fragments, are then added to the lagging strand also in the 5’ to 3’ direction. It is important to note that DNA replication always occurs in the 5' to 3' direction!

In the lagging strands, RNAse H (aka DNA Pol I) removes primers as DNA Polymerase (III) approaches. From there, DNA ligase combines the Okazaki fragments. This final step completes the DNA replication process.

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Instructor: Dave Carlson