Name: eukaryotic dna copying at the replication fork
Uploaded: 2023-06-23T15:38:45.000Z
Duration: 1 min 29 s
Channel: JoVE

K12 Standards

Georgia Standards of Excellence

9-12

Biology

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Replication in Eukaryotes

Die DNA-Replikation in eukaryotischen Chromosomen beginnt an mehreren Replikationsursprüngen, die durch den Ursprungserkennungskomplex (ORC) identifiziert und gebunden werden.
Der ORC rekrutiert dann Helikasen, um die DNA abzuwickeln, und erzeugt eine Replikationsblase mit zwei Replikationsgabeln.
Die beiden Gabeln bewegen sich in entgegengesetzte Richtungen und zerstören die Nukleosomen vor ihnen. Diese Nukleosomen werden dann auf den Tochtersträngen wieder zusammengesetzt, wobei die Chromatinstruktur erhalten bleibt.
An jeder Gabelung bilden RNA-Primer die Stelle, an der die DNA-Polymerase den führenden Strang und die Okazaki-Fragmente des nacheilenden Strangs verlängert.
Ein RNase-Enzym entfernt dann diese Primer, und die DNA-Polymerase füllt die Lücken. Schließlich versiegelt die DNA-Ligase die Fragmente miteinander.
Wenn jedoch der letzte Primer aus dem nacheilenden Strang am Ende des linearen Chromosoms entfernt wird, erzeugt er eine überhängende Dehnung der Matrizen-DNA.
Ein Enzym namens Telomerase verlängert diese überhängende Dehnung mit nicht-kodierender DNA, um den Verlust kodierender DNA während nachfolgender Replikationszyklen zu verhindern.
Die Replikation wird fortgesetzt, bis die benachbarten Replikationsblasen verschmelzen und das gesamte Chromosom dupliziert ist.

replikation in eukaryoten

In eukaryotischen Zellen ist die DNA-Replikation hoch konserviert und streng reguliert. Mehrere lineare Chromosomen müssen vor der Zellteilung mit hoher Genauigkeit dupliziert werden, daher gibt es viele Proteine, die spezielle Rollen im Replikationsprozess erfüllen. Die Replikation erfolgt in drei Phasen: Initiierung, Verlängerung und Beendigung und endet mit zwei vollständigen Chromosomensätzen im Zellkern.

Viele Proteine orchestrieren die Replikation am Ursprung

Die eukaryotische Replikation folgt vielen der gleichen Prinzipien wie die prokaryotische DNA-Replikation, aber da das Genom viel größer ist und die Chromosomen linear statt kreisförmig sind, erfordert der Prozess mehr Proteine und weist einige wesentliche Unterschiede auf. Erstens erfolgt die Replikation bei Eukaryoten im Gegensatz zu Prokaryoten gleichzeitig an mehreren Replikationsursprüngen entlang jedes Chromosoms. Initiatorproteine erkennen und binden an diese Ursprünge und rekrutieren Helikaseproteine, um die DNA-Doppelhelix abzuwickeln. An jedem Ursprungspunkt bilden sich zwei Replikationszweige. Primase fügt dann kurze RNA-Primer zu den einzelnen DNA-Strängen hinzu, die als Ausgangspunkt für die Bindung der DNA-Polymerase dienen und mit dem Kopieren der Sequenz beginnen. DNA kann nur in der 5'- nach 3'-Richtung synthetisiert werden, sodass die Replikation beider Stränge von einer einzelnen Replikationsgabel in zwei verschiedene Richtungen erfolgt. Der führende Strang wird kontinuierlich synthetisiert, während der nacheilende Strang in kurzen Abschnitten von 100–200 Basenpaaren Länge, sogenannten Okazaki-Fragmenten, synthetisiert wird. Sobald der Großteil der Replikation abgeschlossen ist, entfernen RNase-Enzyme die RNA-Primer, DNA-Polymerase füllt die Lücken und DNA-Ligase dichtet die Lücken im neuen Strang ab.

Aufteilung der Replikationsarbeit auf Polymerasen

Die Arbeitsbelastung beim Kopieren von DNA in Eukaryoten ist auf mehrere verschiedene Arten von DNA-Polymerase-Enzymen aufgeteilt. Die wichtigsten Familien von DNA-Polymerasen in allen Organismen werden nach der Ähnlichkeit ihrer Proteinstrukturen und Aminosäuresequenzen kategorisiert. Die ersten entdeckten Familien wurden als A, B, C und X bezeichnet, die Familien Y und D wurden später identifiziert. Zu den Polymerasen der Familie B in Eukaryoten gehören Pol α, das auch als Primase an der Replikationsgabel fungiert, sowie Pol δ und ε, die Enzyme, die den größten Teil der Arbeit der DNA-Replikation am führenden bzw. nacheilenden Strang der Matrize erledigen. Andere DNA-Polymerasen sind für Aufgaben wie die Reparatur von DNA-Schäden, das Kopieren von Mitochondrien- und Plastiden-DNA und das Auffüllen von Lücken in der DNA-Sequenz auf dem nacheilenden Strang verantwortlich, nachdem die RNA-Primer entfernt wurden.

Telomere schützen die Enden der Chromosomen vor Abbau

Da eukaryotische Chromosomen linear sind, können sie an den Enden abgebaut werden. Um wichtige genetische Informationen vor Schäden zu schützen, enthalten die Enden der Chromosomen viele nichtkodierende Wiederholungen hochkonservierter G-reicher DNA, die Telomere. Ein kurzer einzelsträngiger 3'-Überhang an jedem Ende des Chromosoms interagiert mit speziellen Proteinen, die das Chromosom im Zellkern stabilisieren. Aufgrund der Art und Weise, wie der nacheilende Strang synthetisiert wird, kann eine kleine Menge der Telomer-DNA nicht bei jeder Zellteilung repliziert werden. Dadurch werden die Telomere im Laufe vieler Zellzyklen immer kürzer und können somit als Marker für die Zellalterung gemessen werden. Bestimmte Zellpopulationen wie Keimzellen und Stammzellen exprimieren Telomerase, ein Enzym, das die Telomere verlängert und es der Zelle ermöglicht, mehrere Zellzyklen zu durchlaufen, bevor sich die Telomere verkürzen.

Replikation in Eukaryoten

In eukaryotischen Zellen ist die DNA-Replikation hochkonserviert und streng reguliert. Mehrere lineare Chromosomen müssen mit hoher Genauigkeit dupliziert werden, bevor ...

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eukaryotic dna copying at the replication fork

Eukaryotic DNA copying happens at replication forks, where the two DNA strands separate and new strands are built. This process lets a cell make an exact copy of its genetic material before it divides.

The transcript explains that eukaryotic replication follows the same basic steps seen in other DNA replication systems, but it takes place in cells with a nucleus. The replication machinery works on each separated strand as a template, so the new DNA matches the original sequence.

This topic is important for high school biology because it connects DNA structure to cell division and inheritance. Understanding replication in eukaryotes helps students see how genetic information is passed on when body cells grow and reproduce.

Eukaryotic DNA Copying at the Replication Fork

eukaryotic-dna-copying-at-the-replication-fork

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replication in eukaryotes

Eukaryotic DNA copying happens at replication forks, where the two DNA strands separate and new strands are built. This process lets a cell make an exact copy of its genetic material before it divides. The transcript explains that eukaryotic replication follows the same basic steps seen in other DNA replication systems, but it takes place in cells with a nucleus. The replication machinery works on each separated strand as a template, so the new DNA matches the original sequence. This topic is...

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key terms and concepts

Learn how eukaryotic DNA is copied at replication forks, where strands separate and new DNA is built before cell division in nucleus-containing cells.

Eukaryotic DNA Copying at the Replication Fork | JoVE K12

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Eukaryotic DNA Copying at the Replication Fork

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