replication of DNA in prokaryotes and its mechanism, steps and enzymes involved in the process
In prokaryotes, replication of DNA occurs in a process called semiconservative replication, meaning that each newly synthesized DNA molecule contains one strand from the original DNA molecule and one newly synthesized strand. The process involves several steps and requires the activity of various enzymes. Here is an overview of the mechanism, steps, and enzymes involved in DNA replication in prokaryotes:
1. Initiation:
- The replication of DNA initiates at a specific site called the origin of replication (oriC).
- The protein initiator DnaA binds to the oriC and opens up a short stretch of DNA, creating a replication bubble.
- Helicase (DnaB) then binds to the unwound DNA strands and separates them, forming a replication fork.
2. Elongation:
- DNA polymerase III (Pol III) binds to the unwound DNA at the replication fork.
- Primase (DnaG) synthesizes RNA primers that are complementary to the DNA template strands.
- DNA polymerase III adds nucleotides to the 3' end of the RNA primers, synthesizing new DNA strands in the 5' to 3' direction.
- The leading strand is synthesized continuously in the same direction as the replication fork movement.
- The lagging strand is synthesized discontinuously in short fragments called Okazaki fragments, which are later joined together.
3. Termination:
- DNA replication proceeds bidirectionally along the DNA template until two replication forks meet at a specific site called the termination site (ter).
- Tus protein (termination utilization substance) binds to the ter sites and blocks the replication fork movement.
4. Enzymes involved:
- DnaA: Initiator protein that binds to the oriC and helps in the initiation of DNA replication.
- Helicase (DnaB): Unwinds the DNA double helix at the replication fork.
- Primase (DnaG): Synthesizes RNA primers that are complementary to the DNA template strands, providing the necessary starting point for DNA synthesis.
- DNA polymerase III (Pol III): Main DNA polymerase responsible for DNA synthesis during replication.
- DNA polymerase I (Pol I): Removes RNA primers and replaces them with DNA nucleotides.
- DNA ligase: Joins the Okazaki fragments on the lagging strand by catalyzing the formation of phosphodiester bonds.
- Topoisomerases: Relieves the tension caused by unwinding DNA during replication.
This mechanism ensures accurate replication of the entire DNA molecule in prokaryotes during each cell division, allowing for the transmission of genetic material to daughter cells.
The replication of DNA in prokaryotes is a complex process that involves several steps and enzymes working together. Let's break it down:
1. Initiation: The first step in DNA replication is the initiation. It starts at a specific sequence called the origin of replication (oriC) in prokaryotes. The enzyme responsible for initiating the replication process is known as the DNA helicase (DnaB). It unwinds and separates the DNA strands, creating a replication bubble.
2. Elongation: Once the DNA strands are separated, the process of elongation begins. The enzyme responsible for synthesizing new DNA strands is DNA polymerase III (Pol III). It adds nucleotides to the growing DNA chain, following the template provided by the parent DNA strand. However, the DNA polymerase requires a primer to start the synthesis. This is where the enzyme DNA primase (DnaG) comes in. It synthesizes short RNA primers, which serve as starting points for DNA synthesis.
3. Leading Strand Synthesis: In one direction of the replication fork, the DNA synthesis occurs continuously and is called the leading strand synthesis. The DNA polymerase III adds nucleotides in the 5' to 3' direction, matching the exposed DNA template strand. The leading strand only requires one primer at the origin of replication.
4. Lagging Strand Synthesis: In the other direction of the replication fork, the DNA synthesis occurs discontinuously and is called the lagging strand synthesis. Since the DNA polymerase III synthesizes DNA in the 5' to 3' direction, the lagging strand is synthesized in fragments called Okazaki fragments. The DNA primase adds RNA primers at regular intervals, and then DNA polymerase III adds nucleotides to each primer, synthesizing short Okazaki fragments.
5. Primase Activity: Another enzyme called primase (DnaG) is responsible for continuously synthesizing RNA primers on the lagging strand to initiate the synthesis of each Okazaki fragment.
6. DNA Polymerase I: Once the DNA polymerase III completes elongation, the primers on both the leading and lagging strands need to be replaced with DNA. DNA polymerase I (Pol I) removes the RNA primers and replaces them with DNA.
7. DNA Ligase: The individual Okazaki fragments are joined together to create a continuous DNA strand. DNA ligase seals the gaps between the fragments by forming phosphodiester bonds, creating a complete and intact daughter strand.
In summary, DNA replication in prokaryotes involves initiation at the origin of replication, DNA strand separation by helicase, DNA synthesis by DNA polymerase III, RNA primer synthesis by DNA primase, leading strand synthesis, lagging strand synthesis with the formation of Okazaki fragments, replacement of RNA primers by DNA polymerase I, and finally, the ligation of fragments by DNA ligase.
It's important to note that this process is slightly different in eukaryotes, where multiple origins of replication are involved, and additional enzymes and proteins participate in the replication process.
The replication of DNA in prokaryotes, such as bacteria, involves several steps and enzymes. Here is a step-by-step explanation of the process:
Step 1: Initiation
- DNA replication begins at specific DNA sequences called origins of replication.
- In prokaryotes, there is usually a single origin of replication per chromosome (circular DNA).
- An initiator protein called DnaA binds to the origin and promotes the unwinding of the DNA double helix.
Step 2: Unwinding of the DNA helix
- An enzyme called helicase, specifically DnaB in prokaryotes, binds to the unwound DNA at the origin.
- Helicase uses ATP energy to separate the DNA double helix by breaking the hydrogen bonds between the complementary bases.
Step 3: DNA stabilization and prevention of re-annealing
- Single-stranded DNA binding proteins (SSBPs) bind to the separated DNA strands.
- SSBPs stabilize and protect the single-stranded DNA from spontaneous secondary structures or re-annealing.
Step 4: Priming for DNA synthesis
- An enzyme called primase synthesizes a short RNA primer complementary to the DNA template.
- The primer provides a starting point for DNA synthesis.
- In prokaryotes, the RNA primer is typically about 10 nucleotides long.
Step 5: DNA synthesis
- DNA polymerase III is the main enzyme involved in DNA synthesis in prokaryotes.
- DNA polymerase III adds new nucleotides to the template strand in a 5'-to-3' direction.
- The leading strand is synthesized continuously in the same direction as the replication fork movement.
- The lagging strand is synthesized discontinuously as Okazaki fragments in the opposite direction of the replication fork movement.
Step 6: Okazaki fragment processing
- DNA polymerase I removes the RNA primers from the Okazaki fragments and replaces them with DNA nucleotides.
- This process is called primer removal and gap filling.
- DNA ligase then joins the adjacent Okazaki fragments by sealing the gaps between them.
Step 7: Termination
- Replication continues bidirectionally until the two replication forks meet at a termination sequence.
- Termination involves the Tus protein binding to a specific sequence, which halts the progression of the replication forks.
Enzymes involved:
- DnaA: Initiator protein that binds to the origin of replication.
- Helicase (DnaB): Unwinds the DNA double helix.
- Primase: Synthesizes RNA primers.
- DNA polymerase III: Main enzyme for DNA synthesis.
- DNA polymerase I: Involved in primer removal and gap filling.
- DNA ligase: Joins the Okazaki fragments by sealing gaps.
- Tus: Protein involved in the termination of replication.
Please note that this is a simplified explanation of DNA replication in prokaryotes, and there are additional proteins and factors that contribute to the process.