Process of Replication of DNA

In This Blogpost, You Will Learn Opposite Orientation, Leading Strand, Lagging Strand, And Enzymes Involved. Learn About The Opposite Orientation Of DNA Strands, Replication In The 5'-3' Direction, The Leading And Lagging Strands, And The Role Of Enzymes Like DNA Polymerase And DNA Ligase. Discover The Process Of DNA Replication, Including The Formation Of Okazaki Fragments And The Synthesis Of RNA Primers. Gain Insights Into The Intricate Mechanisms That Ensure Accurate DNA Replication.

Opposite Orientation of DNA Strand

DNA strands have opposite directions, with a sugar phosphate backbone that runs in different ways. Each strand has a 3' end and a 5' end. The numbers refer to the carbon atoms of the sugar in the nucleotides. At one end of the DNA strand, the sugar's 3' carbon atom is attached to an OH group, while at the other end, the sugar's 5' carbon has a phosphate group.

Replication Is in 5' - 3' Direction

The opposite orientation of the DNA strand is important during DNA replication. The enzymes responsible for linking DNA nucleotides to a growing daughter strand, known as DNA polymerase, only add nucleotides to the 3' end of the strand, not the 5' end. As a result, a daughter DNA strand can only grow in the 5'-3' direction.

Replication Fork

During replication, at a structure called the replication fork, DNA is unwound and unzipped to allow replication to occur.

Leading Strand

At the replication fork, only one of the new strands (daughter strands) runs in the 5'-3' direction. The template for this strand runs in the 3'-5' direction. The new strand running in the 5'-3' direction can be synthesized continuously and is called the leading strand. Therefore, replication is a continuous process for the leading strand.

Lagging Strand

However, when the parental strand running in the 5'-3' direction serves as the template, the synthesis of the new strand must also be in the 5'-3' direction. This means that synthesis has to start at the fork, outward from the point of replication. Although synthesis occurs in this direction, the new daughter strand ends up running from 5'-3' in the opposite direction to its template.

The new strand is synthesized in short segments as the fork opens up. These short segments are known as Okazaki fragments, named after the Japanese scientist who discovered them. In eukaryotes, these fragments are 100 to 200 nucleotides long, while in prokaryotes, they are 1000 to 2000 nucleotides long. Therefore, this process is called discontinuous replication. Discontinuous replication takes more time than continuous replication, which is why the new strand in this case is called the lagging strand.

RNA Primer

DNA polymerase can attach a nucleotide to the free 3' end of another nucleotide, but it cannot start the synthesis of a new DNA chain at the origin of replication. To initiate replication, an RNA polymerase lays down a short piece of RNA called an RNA primer (through the action of an enzyme known as primase) that is complementary to the DNA strand being replicated. Now, DNA polymerase can add DNA nucleotides in the 5'-3' direction. Later, during proofreading, DNA polymerase removes the RNA primer and replaces it with complementary DNA nucleotides.

DNA Ligase

Another enzyme called DNA ligase joins the 3' end of each fragment to the 5' end of another. As the DNA continues to unwind, new RNA primers are created, and DNA polymerase then moves forward 1000-2000 nucleotides toward the replication fork to begin making another Okazaki fragment.

Summary

The structure of DNA includes two strands with opposite orientations, each having a 3' end and a 5' end. During DNA replication, the replication fork is formed, where the DNA strands unwind and replication takes place. The leading strand is synthesized continuously in the 5'-3' direction, while the lagging strand is synthesized discontinuously in short segments called Okazaki fragments. An RNA primer is laid down to initiate replication, allowing DNA polymerase to add nucleotides in the 5'-3' direction. Later, the RNA primer is removed and replaced with DNA nucleotides. DNA ligase joins the fragments, and the process continues as the DNA unwinds further. The replication of DNA follows a strict 5'-3' directionality and involves multiple enzymes and processes.