DNA is duplicated by a process called replication. DNA replication is semiconservative replication which means that each of the DNA strands from the original DNA molecule is used as a template to create the new strands. This creates two DNA molecules, each with one original strand and one copied strand. During replication, the two DNA strands of the double helix are unzipped by a helicase enzyme. DNA replication begins as soon as the DNA strands begin to be separated. This creates a Y shaped structure referred to as a replication fork.
There is a directional orientation of DNA strands that is important when describing DNA replication. The 5’end of a DNA strand has a phosphate group attached, whereas the 3’ end has a hydroxyl group (OH) attached. The DNA double helix is referred to as antiparallel since one strand is oriented in the 3’to 5’ direction and the paired strand is oriented in the 5’ to 3’ direction. DNA can only be synthesized in the 5’to 3’ direction. This creates an issue at the replication fork as one strand is oriented 3’ to 5’ (leading strand) and one strand is oriented in the 5’ to 3’ (lagging strand).
As the two strands are being separated, a primer binds to the template DNA strand allowing the polymerase enzyme to begin creating another DNA strand by adding nucleotides in the 5’ to 3’ direction that are complementary to the bases of the template strand. The leading strand is copied continuously. This is not possible on the lagging strand as the 3’ end of the template strand has not yet been separated from the leading strand. In this case, replication occurs in short fragments. As the new area is freed a new RNA primer is placed for the polymerase to bind and continue copying until it hits the previously synthesized DNA. This happens in short fragments called Okazaki fragments.
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