13.5
Replication in Prokaryotes
DNA replication has three main steps: initiation, elongation, and termination. Replication in prokaryotes begins when initiator proteins bind to the s…
Prokaryotic DNA replication ensures that genetic material is accurately passed to daughter cells during cell division.
It begins when initiator proteins bind to the origin of replication, a specific DNA sequence, and form a complex.
This binding locally unwinds the DNA at the origin, creating a small opening in the double helix.
Then, the enzyme DNA helicase binds to the complex and unwinds the DNA further, forming a replication bubble.
The newly exposed strands are stabilized by single-stranded DNA-binding proteins.
The replication bubble now contains two replication forks that move in opposite directions.
Ahead of the forks, topoisomerase enzymes bind to the DNA and reduce torsional strain as the molecule unwinds.
Once the strands are separated, another enzyme, primase, synthesizes an RNA primer, which is a short stretch of RNA that is complementary to the DNA sequence.
The primer provides a starting point where DNA polymerase can add nucleotides that match the template strand and create a new DNA strand during elongation.
DNA polymerase III synthesizes DNA in the five-prime to three-prime direction, so synthesis of this strand, called the leading strand, proceeds continuously.
The other strand, called the lagging strand, has the opposite orientation to the direction of replication fork movement. DNA polymerase can add nucleotides only in the 5’ to 3’ direction. To overcome this constraint, DNA is synthesized in short pieces called Okazaki fragments. Each fragment starts from a new RNA primer and grows in the direction opposite to the movement of the replication fork.
The RNA primers are then removed by DNA polymerase I and replaced with DNA. The DNA fragments are joined together by the enzyme DNA ligase, forming a continuous strand.
Termination occurs when the two replication forks converge at specific Ter sites, where termination proteins bind and halt further replication, ensuring complete duplication of the DNA.
This process produces two identical circular DNA molecules. Each molecule contains one original strand and one newly synthesized strand, and is described as semiconservative.
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Q1: What happens when initiator proteins bind to the origin of replication in prokaryotes?
Initiator proteins bind to the origin of replication, a small DNA region with a specific base sequence, creating a complex that initially separates the DNA strands. This complex prepares the DNA for unwinding by helicase and subsequent replication machinery to begin synthesizing new DNA strands.
Q2: Why does prokaryotic DNA synthesis occur differently on the leading and lagging strands?
DNA polymerase synthesizes DNA only in the 5' to 3' direction. The leading strand has the same orientation as the replication fork, allowing continuous synthesis. The lagging strand has opposite orientation, requiring synthesis in short pieces called Okazaki fragments that are later joined together by DNA ligase.
Q3: What role do single-stranded DNA binding proteins play during prokaryotic replication?
Single-stranded DNA binding proteins stabilize the newly separated DNA strands after helicase unwinds the double helix by breaking hydrogen bonds between complementary bases. These proteins prevent the single strands from re-annealing or forming secondary structures, keeping them available as templates for DNA polymerase.
Q4: How does topoisomerase reduce torsional strain during DNA replication?
As helicase unwinds the DNA double helix, it creates torsional strain ahead of the replication fork. Topoisomerase enzymes bind to the DNA and break one side of the phosphate-sugar backbone, allowing the helix to unwind more rapidly and relieving this tension during the replication process.
Q5: What is the role of RNA primers in prokaryotic DNA replication?
Primase synthesizes short RNA primers complementary to the DNA template sequence. These primers provide the 3'-OH group that DNA polymerase requires to begin adding nucleotides. After replication, RNA primers are removed and replaced with DNA by enzymes like DNA polymerase I, then joined by DNA ligase.
Q6: Why is prokaryotic DNA replication considered a semiconservative process?
During semiconservative replication, the original double-stranded DNA molecule unwinds, and each original strand serves as a template for a new complementary strand. The result is two DNA molecules, each containing one original strand and one newly synthesized strand from the replication process.
Q7: How can E. coli divide every 20 minutes when DNA replication takes 40 minutes?
E. coli initiates a new round of DNA replication from the origin of replication before the previous round completes. Daughter cells receive chromosomes already undergoing replication in eukaryotes, allowing them to divide again quickly without waiting for complete genome duplication to finish.