Name: dna replication teams in the replisome
Uploaded: 2020-11-23T11:47:35.000Z
Duration: 3 min 1 s
Channel: JoVE

K12 Standards

Alabama Course of Study Science

9-12

Biology

from-molecules-to-organisms-structures-and-processes

From Molecules to Organisms: Structures and Processes

9-12.B.1.3

The Replisome

Die DNA-Replikation erfolgt durch eine hochgradig koordinierte Multiprotein-Assemblierung, die als DNA-Replikationsmaschinerie oder Replisom bekannt ist und die Effizienz der DNA-Replikation erhöht.
Die Kernkomponenten der Maschinerie sind Helikase, einzelsträngige DNA-Bindungsproteine, DNA-Primase, Gleitklemmen, ein Clamp-Loader und mehrere DNA-Polymerasen, die alle in der Nähe der Replikationsgabel miteinander verbunden sind.
Replikative DNA-Polymerasen haben eine Prozessivität von etwa 10 Nukleotiden, was der Anzahl der Nukleotide entspricht, die sie dem Tochterstrang hinzufügen können, bevor sie vom Matrizenstrang dissoziieren. 
Dies ist zu ineffizient, um ganze Genome in einem vernünftigen Zeitrahmen zu kopieren, und dieses Problem wird mit Hilfe von Schiebeklemmproteinen gelöst.
Wenn ATP mit dem Clamp-Loader-Protein assoziiert, binden sie an die Schiebeklemme und öffnen sie, so dass ihre ringförmige Struktur den Primer-Template-DNA-Komplex umgeben kann. Nach der Bindung hydrolysiert der Clamp-Loader das ATP zu ADP, wodurch sich der Clamp-Loader löst und sich die Clamp um die DNA schließt.
Dann bindet die DNA-Polymerase an die Clamp-Proteine und zusammen gleiten sie entlang der Matrizen-DNA, binden die DNA-Polymerase an den Strang und erhöhen ihre Prozessivität auf bis zu 1000 Nukleotide. 
Diese erhöhte Prozessivität ermöglicht es der DNA-Polymerase, eine kontinuierliche DNA-Replikation auf dem führenden Strang durchzuführen.
Auf der nacheilenden Strang-Matrize führt jedoch eine andere DNA-Polymerase eine diskontinuierliche DNA-Replikation auf eine Weise durch, die es den DNA-Polymerase-Molekülen ermöglicht, die führenden und nacheilenden Stränge gleichzeitig zu synthetisieren. Dieser Prozess wird manchmal als "Posaunenmodell" bezeichnet.
Der nacheilende Strang und sein Matrizenstrang bilden eine Schleife, wenn die DNA-Polymerase die Okazaki-Fragmentsynthese aus einem RNA-Primer initiiert. 
Die DNA-Schleife wächst aus beiden Richtungen, da die Helikase die DNA abwickelt und der nacheilende Strang synthetisiert wird.
Wenn die DNA-Polymerase auf den nächsten RNA-Primer trifft, löst sie sich vom Matrizenstrang. Währenddessen fügt die Primase dem nacheilenden Strang einen weiteren Primer hinzu, und die wachsende DNA-Schleife wird freigesetzt. 
Die Clamp- und Clamp-Loader-Proteine ermöglichen es der DNA-Polymerase, schnell wieder mit dem geprimten DNA-Template zu assoziieren.
Die Bildung und der Kollaps der Subsequenz der DNA-Schleife wiederholen sich mit der Synthese jedes neuen Okazaki-Fragments.

<ol>
	<li>O'Donnell, Michael, Lance Langston und Bruce Stillman. "Prinzipien und Konzepte der DNA-Replikation in Bakterien, Archaeen und Eukarya." Cold Spring Harbor Perspectives in Biology 5, Nr. 7 (2013): a010108.</li>
	<li>Alberts, Bruce. "DNA-Replikation und -Rekombination." Natur 421, Nr. 6921 (2003): 431.</li>
	<li>Kelch, Brian A., Debora L. Makino, Mike O'Donnell und John Kuriyan. "Clamp-Loader-ATPasen und die Evolution der DNA-Replikationsmaschinerie." BMC Biology 10, Nr. 1 (2012): 34.</li>
	<li>Hedglin, Mark, Ravindra Kumar und Stephen J. Benkovic. "Replikationsklemmen und Klemmlader." Cold Spring Harbor Perspectives in Biology 5, Nr. 4 (2013): a010165.</li>
	<li>Gardner, Andrew F. und Zvi Kelman. "Die DNA-Replikationsmaschinerie als therapeutische Ziele." Frontiers in Molecular Biosciences 6 (2019): 35.</li>
</ol>

das replisom

Die DNA-Replikation wird durch einen großen Komplex von Proteinen durchgeführt, die auf koordinierte Weise agieren, um eine hochpräzise DNA-Replikation zu erreichen. Zusammen wird dieser Komplex als DNA-Replikationsmaschinerie oder Replisom bezeichnet.

Die Synthese der Leit- und Verzögerungsstränge ist ein hochkoordinierter Prozess. Um dies zu erklären, wurde 1980 von Bruce Alberts das "Trombone-Modell" vorgeschlagen. Die Bildung der DNA-Schleife beginnt, wenn ein Primer auf dem Eltern-Verzögerungsstrang synthetisiert wird. Die Schleife wächst mit der Synthese eines Okazaki-Fragments und löst sich schließlich auf, wenn die Synthese des Fragments endet. Die DNA-Polymerase heftet sich dann an einen weiteren Primer und der gesamte Zyklus, von der Schleifenbildung bis zum Zusammenbruch, wiederholt sich. Das Replisom ermöglicht es, dass alle Aktivitäten als Replikationsmaschinenkomplex koordiniert werden.

Komponenten des Replisoms

Helikasen entwinden und trennen doppelsträngige DNA. Diese Enzyme sind bei Prokaryoten als ein einzelner Hexamerring und bei Eukaryoten als doppelter Hexamerring vorhanden. Eukaryotische Helikasen sind auf zusätzliche Proteine, Cdc45 und GINS, angewiesen, um zu funktionieren. Einzelstrang-DNA-bindende (SSB) Proteine verhindern, dass die DNA-Stränge sich wieder verbinden. Bei Prokaryoten bestehen SSBs aus einer einzigen Untereinheit, während sie bei Eukaryoten ein heterotrimeres Protein namens Replikationsprotein A (RPA) sind.

Primase fügt einen RNA-Primer hinzu, an dem die DNA-Synthese beginnen wird. Bei Prokaryoten ist die Primase als ein einzelnes Untereinheitenenzym namens DnaG vorhanden, das einen RNA-Primer von etwa 12 Nukleotiden synthetisiert. Bei Eukaryoten synthetisiert ein mehrteiliges Enzym, DNA-Polymerase-α-Primase, einen RNA-DNA-Hybridprimer von etwa 25 Nukleotiden. Zusätzlich zu Polymerase-α erweitert die replikative Polymerase die neu synthetisierte DNA. Während bei Prokaryoten eine einzige Art von replikativer Polymerase, DNA-Polymerase III, vorhanden ist, sind bei Eukaryoten zwei verschiedene Arten von replikativen Polymerasen, Pol ε und Pol δ, für die Synthese des Leit- bzw. Verzögerungsstrangs vorhanden.

Gleitklammerproteine halten die Polymerasen am DNA-Template fest. β-Klammer, ein homodimeres Protein, agiert als Klammer bei Prokaryoten. Bei Eukaryoten wird dieselbe Aufgabe von der proliferierenden Zellkernantigen (PCNA), einem homotrimeren Protein, durchgeführt. Der zentrale Pore der Gleitklammer ist positiv geladen, was es ihr ermöglicht, elektrostatische Wechselwirkungen mit dem negativ geladenen Phosphatrückgrat der DNA einzugehen. Die Klammer wird durch einen Klammerlader an die DNA angehängt. Diese pentamerischen Proteine gehören zur AAA+-Klasse der ATPasen. Der eukaryotische Klammerlader ist als Replikationsfaktor C bekannt, während der prokaryotische E. coli Klammerlader als γ-Komplex bekannt ist; jedoch wird angenommen, dass die Klammerproteine und Klammerlader evolutionäre Homologe sind, anders als viele andere Komponenten des Replisoms.

Das Replisom

Die DNA-Replikation wird durch einen großen Komplex von Proteinen durchgeführt, die in einer koordinierten Materie wirken, um eine hochgenaue DNA-Replikation zu erreichen. Zusammen ist dies ...

Education

Lehre

JoVE Core

Molecular Biology

Molekularbiologie

Unit 1

Einheit 1

DNA Replication

DNA-Replikation

KEY TERMS AND CONCEPTS

KERNBEGRIFFE UND KONZEPTE

dna replication teams in the replisome

DNA replication depends on the replisome, a coordinated complex of proteins that copies DNA with high accuracy. This DNA replication machinery brings many parts together so the leading and lagging strands can be made at the same time.

A key idea in this process is the trombone model, proposed by Bruce Alberts in 1980. In this model, a loop forms on the lagging strand when a primer is made. The loop grows as an Okazaki fragment is synthesized, then falls apart when that fragment is finished. DNA polymerase then moves to a new primer, and the cycle repeats.

Helicases start the process by unwinding and separating double-stranded DNA. In prokaryotes, helicase is a single hexamer ring, while in eukaryotes it forms a double hexamer ring. Eukaryotic helicases also need the extra proteins Cdc45 and GINS to work. Single-stranded DNA binding proteins keep the separated strands from reannealing. In prokaryotes, these are SSB proteins with one subunit. In eukaryotes, the equivalent protein is replication protein A, or RPA, which is a heterotrimer.

Primase adds the RNA primer that gives DNA synthesis a starting point. In prokaryotes, primase is the single-subunit enzyme DnaG, and it makes an RNA primer of about 12 nucleotides. In eukaryotes, DNA polymerase-alpha primase makes an RNA-DNA hybrid primer of about 25 nucleotides. After that, the replicative polymerase extends the new DNA strand. Prokaryotes use DNA polymerase III, while eukaryotes use Pol epsilon for the leading strand and Pol delta for the lagging strand.

Sliding clamp proteins hold the polymerases on the DNA template so copying can continue efficiently. The prokaryotic beta-clamp is a homodimer, and the eukaryotic clamp is proliferating cell nuclear antigen, or PCNA, which is a homotrimer. The clamp has a positively charged central pore that interacts with the negatively charged DNA backbone. A clamp loader places the clamp onto DNA. These pentameric AAA+ ATPases include the eukaryotic replication factor C and the E. coli gamma complex. The clamp proteins and clamp loaders are thought to be evolutionary homologs, unlike many other replisome components.

DNA Replication Teams in the Replisome

replication in prokaryotes

Prokaryotic DNA Replication Steps

Prokaryotic DNA replication copies the chromosome before a cell divides. In bacteria, this process starts at a specific DNA site called the origin of replication. From there, the cell builds new DNA strands using enzymes that match each base with its partner. The DNA double helix first opens so replication can begin. As the strands separate, each original strand serves as a template. New nucleotides are added in a set order, which helps the cell make an accurate copy of its genetic material. <br...

replication in eukaryotes

Eukaryotic DNA Replication Steps

Eukaryotic DNA replication copies the genome before a cell divides. In eukaryotes, this process happens in a controlled sequence so each new cell can receive genetic information. Replication begins at specific sites on the DNA called origins. The DNA is opened, and each original strand serves as a template, meaning it guides the building of a new complementary strand. This sets up two DNA molecules from one original molecule. The process then continues as enzymes add matching nucleotides to...

dna base pairing

DNA Base Pair Rules

DNA base pairing follows specific rules that help DNA carry genetic information. In DNA, the bases pair in a regular way, with adenine matching thymine and cytosine matching guanine. These pairs fit together because of their chemical shapes and bonding patterns. These base pairs also support accurate DNA replication. When DNA copies itself, each strand can guide the formation of a new partner strand using the pairing rules. Understanding base pairing helps explain how DNA stays organized and...

the dna replication fork

DNA Replication Stress and Fork Stalling

DNA replication stress and fork stalling can disrupt the copying of a cell’s genome. The replication fork is the Y-shaped site where DNA strands separate and new complementary strands are made at the same time. This linked unwinding and synthesis helps cells duplicate DNA efficiently and with fewer errors. In organisms with small circular DNA, such as E. coli, replication often begins at a single origin. That setup creates two replication forks that move in opposite directions from the opening...

proofreading

Proofreading in DNA Replication

Proofreading in DNA replication helps keep DNA copying accurate. It is a built-in check that reduces mistakes as new DNA is made. DNA polymerase is the enzyme that carries out this checking during replication. When it adds a wrong nucleotide, it can remove the error and replace it with the correct one. This correction step helps maintain the correct DNA sequence. Proofreading is an important part of DNA replication because it supports genetic accuracy. By catching mistakes as they happen, the...

dna helicases

Helicase Motor Proteins in DNA Unwinding

Helicase motor proteins help unwind DNA during key cell processes. They use energy from ATP hydrolysis, which is the breakdown of ATP to release usable energy. This movement lets helicases separate DNA strands when the cell needs access to the genetic code. These enzymes are involved in DNA replication, repair, recombination, and transcription. Helicases are found in all living organisms, but their structure, function, and movement can differ. In prokaryotes, the DnaB helicase moves along the...

the replisome

DNA replication depends on the replisome, a coordinated complex of proteins that copies DNA with high accuracy. This DNA replication machinery brings many parts together so the leading and lagging strands can be made at the same time. A key idea in this process is the trombone model, proposed by Bruce Alberts in 1980. In this model, a loop forms on the lagging strand when a primer is made. The loop grows as an Okazaki fragment is synthesized, then falls apart when that fragment is finished.

lagging strand synthesis

Okazaki Fragments in DNA Replication

DNA replication uses a leading strand and a lagging strand, and they are made at different rates. The leading strand starts first. The lagging strand starts later, moves more slowly, and is made in pieces. The two strands also differ in direction and starting steps. Leading strand synthesis follows the opening replication fork. Lagging strand synthesis goes in the opposite direction. The leading strand needs one RNA primer, while the lagging strand needs multiple RNA primers. After priming,...

mismatch repair

Fixing DNA Copying Errors

Mismatch repair helps fix DNA copying errors after replication. When DNA is copied, small mistakes can slip through. This repair system finds those mismatches and corrects them before they become permanent changes. The process depends on a group of mutator proteins. These proteins work together to detect the error, remove the incorrect section, and help replace it with the proper DNA sequence. In this way, mismatch repair supports the accuracy of genetic information. If the repair system does...

dna topoisomerases

How Topoisomerases Untwist DNA

DNA topoisomerases are enzymes that untwist DNA when it becomes overcoiled during cell processes such as DNA replication and transcription. They help regulate positive and negative DNA supercoiling without changing the nucleotide sequence. Clockwise overwinding creates positively supercoiled DNA, while counterclockwise underwinding produces negatively supercoiled DNA. Topoisomerases are grouped into two main types. Type I topoisomerases act on one strand of double-stranded DNA, and they...

telomeres and telomerase

How Telomerase Protects Chromosome Ends

Telomerase helps protect chromosome ends during eukaryotic DNA replication. When the final primer is removed, a single-stranded DNA fragment can remain at the end of the chromosome. That piece cannot be copied in the same way as the rest of the strand because there is no 3' end for new DNA to attach to. If this end piece is not replaced, chromosomal DNA is gradually lost each time a cell divides. The exposed single-stranded DNA can also trigger a DNA damage response from enzymes that detect it.

non-nuclear inheritance

Cytoplasmic Inheritance in Cells

Cytoplasmic inheritance explains how some traits are passed through cell structures outside the nucleus. These traits come from organelles such as mitochondria and chloroplasts. They do not follow the usual nuclear pattern of inheritance. Mitochondria and chloroplasts each contain their own DNA. Because of this, they can carry genetic information that affects cell function and certain visible traits. In high school biology, this topic helps explain why not all inherited traits come from...

animal mitochondrial genetics

Animal Mitochondrial DNA and Inheritance

Animal mitochondrial DNA has its own genome, separate from nuclear DNA. It is a double-stranded, closed circular molecule with about 20,000 base pairs. One strand is the heavy, or H-strand, because it is rich in guanine. The other is the light, or L-strand, because it is rich in cytosine. Compared with nuclear DNA, mitochondrial DNA has very little non-coding sequence. It also has no introns. Its genes are packed closely together, and some even overlap. The D-loop is the main non-coding...

comparing mitochondrial, chloroplast, and prokaryotic genomes

comparing-mitochondrial-chloroplast-and-prokaryotic-genomes

How Organelle Genomes Resemble Bacteria

Mitochondrial and chloroplast genomes still keep several traits from their bacterial ancestors. They also show changes that happened after these organelles evolved inside eukaryotic cells. Eukaryotic cells are cells with a nucleus, while prokaryotes are cells without one. Like prokaryotic genomes, mitochondrial and chloroplast genomes do not bind with histone-like proteins. They also do not show the complex chromosome-like packaging seen in eukaryotes. Mitochondria and chloroplasts divide by...

export of mitochondrial and chloroplast genes

export-of-mitochondrial-and-chloroplast-genes

How Organelles Pass Genes to the Nucleus

Eukaryotic cells can carry three genetic systems: nuclear, mitochondrial, and chloroplast. Over time, many genes from mitochondria and chloroplasts have moved into the nucleus. In some plant species, this gene transfer is still happening. This movement has affected large parts of the genome in some organisms. About 18% of the Arabidopsis thaliana nuclear genome is thought to come from the chloroplast's cyanobacterial ancestor. In yeast, around 75% of the genome is derived from the...

single-strand dna binding proteins

How SSB Proteins Protect DNA Strands

Single-strand DNA-binding proteins, or SSB proteins, help protect DNA during replication. When double-stranded DNA is unwound, the two strands separate. SSB proteins bind to the exposed single strands and keep them stable. These proteins bind in a sequence-independent manner. That means the order of the nitrogenous bases does not have to follow a specific pattern for SSB proteins to attach. Their binding straightens single-stranded DNA and makes it more rigid. This support may help DNA...

key terms and concepts

Explore how the replisome coordinates DNA replication, from helicase and primase to polymerases, sliding clamps, and the trombone model.

DNA Replication Teams in the Replisome | JoVE K12

molecular biology

JoVE Core Molecular Biology veranschaulicht die molekularen Prozesse der Zelle in prägnanten animierten Video-Lektionen. In Videos von Wissenschaftlern in Aktion wird die Anwendung dieser Konzepte zusätzlich durch reale Experimente in modernen Laboren demonstriert.

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9-12.B.1.1

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9-12.B.1.2

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9-12.B.1.2.a

DNA Replication Teams in the Replisome

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