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Magazine Chapter 12: Problem 29
If the gene for primase were mutated so that no functional primase was produced, what would be the effect on theta replication? On rolling-circle replication?
Short Answer
Expert verified
No functional primase halts theta replication and impairs lagging strand synthesis in rolling-circle replication.
Step by step solution
01
Understand the Role of Primase
Primase is an enzyme that synthesizes short RNA primers on a DNA strand during replication initiation. These primers are necessary because DNA polymerases, which extend the DNA chain, cannot start synthesis from scratch; they need a primer to add nucleotide bases.
02
Learn About Theta and Rolling-Circle Replication
Theta replication is commonly found in circular DNA molecules, such as bacterial chromosomes. It involves bidirectional replication from an origin of replication. Rolling-circle replication occurs in some viruses and plasmids, where a single-stranded DNA is nicked to form a template for continuous synthesis.
03
Analyze the Effect of Primase Mutation in Theta Replication
In theta replication, the lack of functional primase means that RNA primers cannot be synthesized. Without primers, DNA polymerase cannot begin DNA synthesis, halting the replication process.
04
Assess the Impact on Rolling-Circle Replication
Rolling-circle replication also requires RNA primers for the lagging strand synthesis. While the leading strand in rolling-circle replication can proceed initially because it primarily relies on nicked DNA, the replication of the complementary strand will be impaired due to the absence of primers, leading to incomplete replication.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Primase
Primase plays a crucial role in DNA replication. It is an enzyme responsible for synthesizing short RNA sequences called primers. These primers serve as starting points for DNA polymerases to begin DNA synthesis. Without primers, DNA polymerases cannot attach nucleotide bases to initiate the DNA strand copying process.
The activity of primase is vital because DNA polymerases have a limitation: they can only add nucleotides to an existing chain of DNA or RNA. Primase creates these essential primers by creating a short stretch of RNA, which provides a 3鈥�-OH group for the DNA polymerase to extend from, allowing the replication process to proceed smoothly.
The activity of primase is vital because DNA polymerases have a limitation: they can only add nucleotides to an existing chain of DNA or RNA. Primase creates these essential primers by creating a short stretch of RNA, which provides a 3鈥�-OH group for the DNA polymerase to extend from, allowing the replication process to proceed smoothly.
Theta Replication
Theta replication is a common form of replication seen in prokaryotes, particularly in circular DNA molecules such as bacterial chromosomes. This process starts at a specific region called the origin of replication and proceeds bidirectionally.
In theta replication, both leading and lagging strands are synthesized. For the lagging strand to be created, RNA primers synthesized by primase are critical. A mutation in the gene for primase, resulting in non-functional primase, would halt DNA strand synthesis. This is because RNA primers, necessary for starting DNA synthesis by DNA polymerase, would not be produced, leading to a stalled replication process.
In theta replication, both leading and lagging strands are synthesized. For the lagging strand to be created, RNA primers synthesized by primase are critical. A mutation in the gene for primase, resulting in non-functional primase, would halt DNA strand synthesis. This is because RNA primers, necessary for starting DNA synthesis by DNA polymerase, would not be produced, leading to a stalled replication process.
Rolling-Circle Replication
Rolling-circle replication is typically found in plasmids and some virus genomes. It starts with a nick in one of the DNA strands of a circular template. The un-nicked strand acts as a template for continuous replication, creating multiple copies of the circular DNA inside host cells.
There are two key phases in rolling-circle replication:
There are two key phases in rolling-circle replication:
- The leading strand synthesis, which can continue without interruption due to the continuous nature of the circular template.
- The lagging strand synthesis, which requires RNA primers synthesized by primase to enable DNA polymerase to function.
RNA Primers
RNA primers are short sequences synthesized by primase during DNA replication. These serve as the starting point for DNA polymerase to begin adding DNA nucleotides. The primers themselves are made of RNA, providing the essential 3鈥�-OH group that DNA polymerases require for process initiation.
In both theta and rolling-circle replication, RNA primers are indispensable. They act as placeholders for initiating DNA synthesis on the lagging strand. Without RNA primers, DNA polymerases would have no starting point, and replication could not properly occur, leading to incomplete DNA replication.
In both theta and rolling-circle replication, RNA primers are indispensable. They act as placeholders for initiating DNA synthesis on the lagging strand. Without RNA primers, DNA polymerases would have no starting point, and replication could not properly occur, leading to incomplete DNA replication.
DNA Polymerase
DNA polymerase is the enzyme responsible for synthesizing a new DNA strand by adding nucleotides to a pre-existing chain. It reads the template DNA strand and incorporates complementary nucleotides, extending the newly forming strand.
For DNA polymerase to function, it must have a primer to start with. This is why primers are so important; they provide the necessary 3鈥�-OH group for polymerase to add nucleotides. During both theta and rolling-circle replication:
For DNA polymerase to function, it must have a primer to start with. This is why primers are so important; they provide the necessary 3鈥�-OH group for polymerase to add nucleotides. During both theta and rolling-circle replication:
- In theta replication, both strands require primase-created RNA primers for DNA polymerase action on the lagging strand.
- In rolling-circle replication, DNA polymerase can continue on the leading strand without primers, but lagging strand synthesis requires them.
