DNA serves as the genetic material of most organisms. Generally, DNA is a double-stranded molecule that contains two antiparallel DNA strands held together by hydrogen bonds. During cell division, the complete DNA in the genome should be replicated, doubling the amount of DNA in the parent cell. DNA replication occurs in a semi-conservative manner where one of the DNA strands in the newly synthesized double-stranded DNA is an original strand. Hence, both strands should serve as a template in DNA replication. DNA polymerase is the enzyme responsible for DNA replication. It only synthesizes DNA in the 5’ to 3’ direction. However, since the double-stranded DNA is antiparallel, DNA synthesis should occur in both directions. Therefore, Okazaki fragments are formed during the synthesis of lagging template strand.
Key Areas Covered
1. What are Okazaki Fragments
– Definition, Features
2. Why are Okazaki Fragments Formed
– DNA Synthesis on the Lagging Strand
Key Terms: DNA Replication, Double-Stranded DNA, Lagging Strand, Leading Strand, Okazaki Fragments, Replication Fork
What is an Okazaki Fragment
Okazaki fragment is a short newly-synthesized DNA fragment on the lagging template strand formed during DNA replication. Therefore, Okazaki fragments are complementary to the lagging strand, which runs in the 5’ to 3’ direction. They form short double-stranded DNA sections that lie between 1,000 and 2,000 nucleotides in prokaryotes. In eukaryotes, Okazaki fragments are 100 to 200 nucleotides long. At the 5’ end of the Okazaki fragment, an RNA primer, which is approximately 120 nucleotides long, can be identified. An Okazaki fragment is shown in figure 1.
Figure 1: Okazaki Fragment
Okazaki fragments are ligated together by the action of DNA ligase after the removal of RNA primers, forming a continuous DNA strand.
Why are Okazaki Fragments Formed
DNA is a double-stranded molecule; one DNA strand is antiparallel to the other strand. Therefore, one strand runs in the 3’ to 5’ direction while the other runs in the 5’ to 3’ direction. The strand that runs in the 3’ to 5’ direction is known as the leading strand while the one that runs in the 5’ to 3’ direction is known as the lagging strand. The leading strand is so called because a continuous growth of the newly-synthesizing DNA strand can be observed on the leading strand. The DNA synthesis on the leading and lagging strands are shown in figure 2.
Figure 2: DNA Synthesis on Leading and Lagging Strands
Generally, DNA polymerase adds nucleotides in the 5’ to 3’ direction. Since the leading strand runs in the 3’ to 5’ direction, the enzyme can continuously add nucleotides to the growing strand on the leading strand. However, since the lagging strand runs in the 5’ to 3’ direction, the chain growth of the newly-synthesizing DNA strand is paused when it reaches the 5’ end of the strand. Then the synthesis of another DNA strand begins at the replication fork. The replication fork is the position on the DNA double-strand where the unwinding begins. Unwinding is critical in the synthesis of new DNA strands on the original strands. Once the replication fork moves forward on the DNA double-strand, DNA polymerase can add nucleotides onto the lagging strand. However, the synthesis is paused when it reaches 5’ end of the RNA primer of the already-synthesized DNA stretch. Hence, the DNA synthesis at the lagging strand is discontinuous and the resultant DNA stretches are known as Okazaki fragments.
Conclusion
Okazaki fragments are the short DNA fragments on the lagging strand formed during DNA replication. Since the lagging strands run in the 3’ to 5’ direction, the DNA synthesis on the lagging strand is discontinuous. It forms Okazaki fragments on the lagging strand that are ligated later by DNA ligase.
Reference:
1. “Okazaki Fragments.” Okazaki Fragments – Biology As Poetry, Available here.
Image Courtesy:
1. “DNA replication en” By LadyofHats Mariana Ruiz – Own work – renamed from File: DNA replication.svg (Public Domain) via Commons Wikimedia
2. “DNA replication (13080697695)” By Genomics Education Programme – DNA replication (CC BY 2.0) via Commons Wikimedia
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