When the DNA is replicated, the DNA double helix is unwound to form the replication bubble. Explain the process and enzymes when the DNA bubble is formed
The formation of the replication bubble occurs in several steps and involves several enzymes:
1. Initiation: The DNA helicase (enzyme) binds to the origin of replication site and begins to unwind the double-stranded DNA.
2. Formation of the replication bubble: As the helicase moves along the DNA, it creates a replication bubble where the DNA strands are separated. DNA polymerase (enzyme) attaches to each of the separated strands and begins to add complementary nucleotides to form new strands.
3. Elongation: DNA polymerase adds nucleotides continuously to the leading strand, which runs in the 5’ to 3’ direction. It adds nucleotides in a discontinuous manner to the lagging strand, which runs in the opposite direction, in small sections known as Okazaki fragments.
4. Joining of fragments: The Okazaki fragments are eventually joined together by DNA ligase (enzyme), which seals any gaps between the fragments and creates a continuous strand.
Overall, the formation of the replication bubble allows for the efficient replication of DNA, with multiple replication bubbles forming along the DNA molecule to speed up the process.
When DNA replication begins, the DNA double helix needs to be unwound to expose the individual strands that can serve as templates for creating new complementary strands. This unwinding process occurs at specific sites called replication origins, where the DNA strands separate and form a replication bubble.
Here are the steps involved in the formation of a replication bubble:
1. Initiation: The process starts with the binding of a group of specialized proteins called initiator proteins to specific sequences within the DNA at the replication origin. These initiator proteins help to locally unwind the double helix and initiate the formation of the replication bubble.
2. Helicase action: Once the initiation is complete, an enzyme called helicase is recruited to the replication bubble. Helicase is responsible for actively unwinding the DNA double helix by breaking the hydrogen bonds between the paired bases. It moves along the DNA strand, separating the two strands and forming a Y-shaped structure known as the replication fork.
3. Single-strand binding proteins: To maintain stability and prevent the separated DNA strands from reannealing, single-strand binding proteins (SSBs) bind to the exposed single strands of DNA. SSBs help keep the DNA strands apart and protect them from degradation or damage.
4. Topoisomerase action: As the DNA strands are unwound and separated, the twisting strain ahead of the replication fork increases. To relieve this strain, an enzyme called topoisomerase (specifically topoisomerase II or DNA gyrase) helps by cutting the DNA strands, allowing some controlled rotation to relax the tension, and then resealing the DNA strands.
5. DNA polymerase: With the formation of the replication bubble and the unwinding of the DNA, the stage is set for DNA replication to occur. The primary enzyme involved in DNA replication is DNA polymerase. There are different types of DNA polymerases, but DNA polymerase III is the main enzyme responsible for synthesizing new DNA strands using the existing strands as templates. DNA polymerase III adds nucleotides to the growing new strand, following the base-pairing rules (A with T and C with G). It moves along the separated DNA strands within the replication bubble, creating two new daughter DNA molecules.
Overall, the formation of the replication bubble involves a coordinated action of various enzymes, including initiator proteins, helicase, single-strand binding proteins (SSBs), topoisomerase, and DNA polymerase, to ensure DNA replication proceeds accurately and efficiently.