8.6
Replikation in Prokaryoten
Die DNA-Replikation besteht aus drei Hauptschritten: Initiierung, Verlängerung und Beendigung. Die Replikation in Prokaryoten beginnt, wenn Initiatorp…
In Prokaryoten wie E. coli beginnt die DNA-Replikation des zirkulären, doppelsträngigen Genoms am Ursprung der Replikation, genannt OriC.
Initiatorproteine binden an OriC und initiieren die DNA-Trennung. Dann wird eine Helikase auf jeden Strang geladen, um die DNA weiter abzuwickeln und eine Replikationsblase zu bilden.
Die abgewickelte DNA wird durch einzelsträngige DNA-Bindungsproteine oder SSBs stabilisiert.
Die Replikationsblase verfügt nun über zwei Replikationsgabeln auf jeder Seite, die bidirektional verlaufen.
An jeder Gabel synthetisiert die Multiprotein-Replikationsmaschinerie, einschließlich DNA-Polymerasen, gleichzeitig den führenden und den nacheilenden Strang.
Wenn das Abwickeln fortschreitet, wird die DNA vor der Gabel überwickelt und supergewickelt, wodurch eine Torsionsdehnung entsteht.
Topoisomerase-Enzyme vom Typ 1 helfen, diesen Stress abzubauen, indem sie eine Kerbe schaffen, durch die der andere Strang hindurchgehen kann, und dann die DNA wieder ligieren.
Während die Replikation entlang des Genoms fortschreitet, treffen sich die beiden Gabeln an den Terminations- oder ter-Stellen, an denen die DNA-Replikation abgeschlossen ist.
Die resultierenden Tochter-DNA-Moleküle bestehen aus einem Elternstrang und einem neu synthetisierten Strang nach dem semi-konservativen Modell der DNA-Replikation.
Q1: Where does DNA replication begin in prokaryotes?
In prokaryotes like E. coli, DNA replication begins at the origin of replication, called OriC. Initiator proteins bind to OriC and initiate DNA separation. Helicase is then loaded onto each strand to unwind the DNA further and form a replication bubble, which contains two replication forks that proceed bidirectionally around the circular chromosome.
Q2: What is the role of single-strand DNA binding proteins during replication?
Single-strand DNA binding proteins, or SSBs, stabilize the unwound DNA strands as the replication fork travels along the chromosome. These proteins prevent the separated template strands from re-annealing or forming secondary structures, maintaining them in a form accessible to DNA polymerases for synthesis of leading strand and lagging strand synthesis.
Q3: How do topoisomerases help relieve tension during DNA replication?
As DNA unwinds at the replication fork, the DNA ahead becomes overwound and supercoiled, creating torsional strain. Type one topoisomerase enzymes relieve this stress by creating a nick in one strand of the double-stranded DNA phosphate-sugar backbone, allowing the other strand to pass through, then ligating the DNA back together to restore integrity.
Q4: Why are Okazaki fragments necessary during prokaryotic DNA replication?
DNA polymerase can only synthesize DNA in the 5' to 3' direction. On the lagging strand template, which runs 3' to 5' toward the replication fork, synthesis must occur in short segments called Okazaki fragments, typically 1,000–2,000 base pairs long, rather than continuously like on the leading strand.
Q5: How fast can E. coli replicate its entire chromosome?
E. coli DNA polymerase III, responsible for the majority of replication, can polymerize approximately 1,000 base pairs per second. This remarkable speed allows the replication machinery at the two replication forks to duplicate the entire E. coli chromosome of 4.6 million base pairs in roughly 40 minutes under optimal conditions.
Q6: How can E. coli divide faster than it can replicate its genome?
Under favorable growth conditions, E. coli can divide every 20 minutes, faster than the 40 minutes needed for complete genome replication. Bacteria solve this by initiating another round of DNA replication from the origin of replication before the first round completes, so daughter cells receive a chromosome already undergoing replication and are prepared to divide again quickly.
Q7: What happens when the two replication forks meet during prokaryotic DNA replication?
As replication proceeds bidirectionally around the circular prokaryotic chromosome, the two replication forks eventually meet at termination or ter sites where DNA replication completes. The resulting daughter DNA molecules each contain one parental strand and one newly synthesized strand, following the semi-conservative model of DNA replication.