DNA Replication SIMPLIFIED

It is known that somatic cells need to undergo mitosis/cell division for organism growth and development.

Before the cells go through mitosis, the DNA of the mother cell must be replicated so it can be prepared to create identical DNA for its daughter cells, called DNA replication.

The process of DNA replication forms two DNA double helices, each having one single strand from the mother DNA, and one new strand created from replication:

In the cell cycle, DNA replication occurs in a phase called “Interphase”, right before mitosis occurs:

In the previous blog, “The structure of DNA SIMPLIFIED”, we discussed how DNA is made up of two complimentary strands. This means that each strand has a 5’ end and a 3’ end, both strands having them in opposite directions:

During DNA replication, these two strands are separated by an enzyme called helicase, creating a shape called a replication fork. The two strands are held apart by single-stranded binding proteins, and topoisomerase is used to relax the tension of the DNA to prevent coiling:

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An enzyme called primase creates an area called a primer to indicate where DNA replication begins.

The primer is a notifier for where DNA polymerase, another enzyme, should begin to add complimentary nucleotides to the strand.

It is important to note that DNA polymerase only adds nucleotides in a 5’-3’ manner, and towards the replication fork. This means that the polymerase 5’-3’ motion only runs continuously on one of the DNA strands, called the leading strand

But what about the other strand?

In order for DNA polymerase to add nucleotides towards the replication fork, it would mean that it needs to add them in a 3'-5' manner. This cannot happen because polymerase only adds them 5'-3'; on the non-leading strand (or lagging strand), primase must make multiple primers for DNA polymerase to add on correct nucleotides towards the replication fork:

These multiple fragments are called Okazaki fragments, or lagging strands.

Afterwards, enzyme called exonuclease removes the primers before DNA polymerase fills those holes back in

Next, the enzyme DNA ligase connects the single new strands with the single older strands, forming 2 DNA double helixes

It should be noted that since DNA has such long strands, multiple replication forks must be made for efficient, faster DNA replication:

Confused? Watch this video for more information:

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