Question

List the proteins that unwind DNA during in vivo DNA synthesis. How do they function?

Short answer

Answer: The main proteins involved in unwinding DNA during in vivo DNA synthesis are helicase, single-strand binding proteins (SSB), and topoisomerase. Helicase unwinds the double-stranded DNA by separating the two strands and creating a replication bubble. Single-strand binding proteins (SSB) bind to the single strands of DNA separated by helicase to prevent them from reannealing and maintain their stability. Topoisomerase relieves the supercoiling generated during the unwinding process by breaking the phosphodiester bonds in the DNA strands, allowing them to rotate, and then resealing the bonds.

Step-by-step solution

List the proteins involved in unwinding DNA

The main proteins involved in unwinding DNA during in vivo DNA synthesis are helicase, single-strand binding proteins (SSB), and topoisomerase.

Describe the function of helicase

Helicase plays a crucial role in unwinding the double-stranded DNA. It binds to the DNA at the replication fork and moves along the DNA strand in the 5' to 3' direction, using the energy from ATP hydrolysis. As helicase moves, it separates the two DNA strands, creating a replication bubble.

Describe the function of single-strand binding proteins (SSB)

Single-strand binding proteins (SSB) bind to the single strands of DNA that have been separated by helicase. SSBs prevent the separated DNA strands from reannealing, thereby maintaining the stability of the single-stranded DNA and allowing other proteins, such as DNA polymerase, to synthesize a new complementary DNA strand.

Describe the function of topoisomerase

Topoisomerase is another essential protein involved in DNA unwinding. As helicase separates the DNA strands, it generates positive supercoiling in the DNA ahead of the replication fork. Topoisomerase acts to relieve this supercoiling by breaking the phosphodiester bonds in one or both DNA strands, allowing the strands to rotate and then resealing the broken bonds. This process reduces the torsional stress on the DNA molecule and helps in the unwinding process.

Key concepts

Helicase Function

Helicase is a fascinating enzyme that plays a vital role in DNA replication. It can be thought of as the molecular motor of the cell. During DNA replication, helicase binds to the origin of replication on the DNA molecule. From here, it moves along the strands in the 5' to 3' direction. As it moves, helicase uses energy obtained from ATP hydrolysis to break the hydrogen bonds between the two strands of DNA.

This action effectively "unzips" the double helix structure, separating the strands to form a replication fork. The separated strands create what is known as a replication bubble, serving as the template for the synthesis of new DNA strands. Notably, helicase's ability to unwind DNA quickly is crucial because the separation must happen continuously and rapidly for successful replication.

Without helicase, the DNA strands could remain tightly wound, preventing the cell from accessing the genetic code necessary for replication and division.

Single-Strand Binding Proteins

Once helicase has unwound the DNA, single-strand binding proteins, often abbreviated as SSBs, step in to stabilize the now exposed single-stranded regions. These proteins are critical because, once the DNA strands are separated, they have a natural tendency to reattach to each other or form secondary structures through hydrogen bonding.

SSBs bind to the single-stranded DNA near the replication fork, blocking the strands from rebinding. In doing so, they maintain the separation necessary for the DNA polymerase and other enzymes to perform their tasks effectively. Additionally, by stabilizing these single strands, SSBs prevent any secondary structures that could otherwise disrupt the replication process.

• Maintain strand separation
• Prevent secondary structure formation
• Assist other replication enzymes

Overall, SSBs ensure that the entire replication mechanism functions smoothly and precisely.

Topoisomerase Role

Topoisomerase is another key player in the unwinding of DNA and serves a very specialized function during replication. As helicase separates the strands, the unwinding action can cause an increase in positive supercoiling ahead of the replication fork. This tension can potentially hinder further unwinding and may even become detrimental if not managed.

Topoisomerase addresses this issue by introducing temporary breaks in the DNA strands. It makes nicks in one (or both) of the DNA strands, allowing the supercoils to relax. Subsequently, these "nicked" strands can pass over one another, thus relieving the accumulated tension. After this process is completed, topoisomerase then reseals the breaks, ensuring the integrity of the DNA is maintained.

• Reduces torsional stress
• Prevents excessive supercoiling
• Facilitates smooth DNA unwinding

In essence, topoisomerase is indispensable for maintaining proper DNA structure during replication.