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

La réplication de l'ADN est effectuée par un assemblage multiprotéique hautement coordonné, connu comme machine de réplication de l'ADN, ou réplisome, qui augmente l'efficacité de la réplication de l'ADN. Les principaux composants de la machine sont l'hélicase, les protéines de liaison de l'ADN simple brin, l'ADN primase, les pinces coulissantes, un chargeur de clamp, et plusieurs ADN polymérases, qui sont tous associés les uns aux autres près de la fourche de réplication. Les ADN polymérases réplicatives ont une processivité d'environ 10 nucléotides, qui est le nombre de nucléotides qu'elles peut ajouter au brin fille avant de se dissocier du brin modèle.Cela n'est pas assez efficace pour copier des génomes entiers dans un délai raisonnable, et ce problème est résolu à l'aide de protéines à pince coulissante. Lorsque l'ATP s'associe à la protéine chargeur de clamp, ils se lient et ouvrent la pince coulissante, de sorte que sa structure en anneau puisse entourer le complexe d'ADN amorce-matrice. Une fois lié, le chargeur de clamp hydrolyse l'ATP en ADP, ce qui provoque la disassociation du chargeur de clamp et la fermeture du clamp autour de l'ADN.L'ADN polymérase se lie ensuite aux protéines clamp. Et ensemble, ils glissent le long de l'ADN modèle, rattachant l'ADN polymérase au brin et augmentant sa processivité jusqu'à 1 000 nucléotides. Cette processivité accrue permet à l'ADN polymérase d'effectuer une réplication continue de l'ADN sur le brin avancé.Toutefois, sur le modèle brin retardé, une autre ADN polymérase effectue une réplication discontinue de l'ADN de manière à permettre aux molécules d'ADN polymérase de synthétiser simultanément les brins avancés et retardés. Ce processus est parfois décrit comme le modèle du trombone. Le brin retardé et son brin modèle forment une boucle lorsque l'ADN polymérase initie la synthèse de fragments Okazaki à partir d'un amorce d'ARN.La boucle d'ADN se développe dans les deux sens lorsque l'hélicase déroule l'ADN et que le brin retardé est synthétisé. Lorsque l'ADN polymérase rencontre l'amorce d'ARN suivante, elle se détache du brin modèle. Pendant ce temps, la primase ajoute une autre amorce au brin retardé.Et la boucle d'ADN en croissance est libérée. Les protéines clamp et chargeur de clamp permettent à l'ADN polymérase de se réassocier rapidement au modèle d'ADN amorcé. La formation et l'effondrement subséquent de la boucle d'ADN se répète avec la synthèse de chaque nouveau fragment d'Okazaki.

<ol>
	<li>O’Donnell, Michael, Lance Langston et Bruce Stillman. « Principes et concepts de la réplication de l’ADN chez les bactéries, les archées et les eucaryas. » Cold Spring Harbor Perspectives in Biology 5, n° 7 (2013) : a010108.</li>
	<li>Alberts, Bruce. « Réplication et recombinaison de l’ADN. » Nature 421, n° 6921 (2003) : 431.</li>
	<li>Kelch, Brian A., Debora L. Makino, Mike O'Donnell et John Kuriyan. « Les ATPases à chargeur de pince et l'évolution de la machinerie de réplication de l'ADN. » BMC Biology 10, n° 1 (2012) : 34.</li>
	<li>Hedglin, Mark, Ravindra Kumar et Stephen J. Benkovic. « Pinces de réplication et chargeurs de pinces. » Cold Spring Harbor Perspectives in Biology 5, n° 4 (2013) : a010165.</li>
	<li>Gardner, Andrew F. et Zvi Kelman. « La machinerie de réplication de l’ADN en tant que cibles thérapeutiques. » Frontières des biosciences moléculaires 6 (2019) : 35.</li>
</ol>

le réplisome

La réplication de l'ADN est assurée par un vaste complexe de protéines qui agissent de manière coordonnée afin d'obtenir une réplication de l'ADN de haute fidélité. L'ensemble de ce complexe est connu sous le nom de machinerie de réplication de l'ADN ou de réplisome.

La synthèse des brins principaux et secondaires est un processus hautement coordonné. Pour l'expliquer, le « modèle du trombone » a été proposé par Bruce Alberts en 1980. La formation d'une boucle d'ADN commence lorsqu'une amorce est synthétisée sur le brin parent retardataire. La boucle se développe avec la synthèse d'un fragment d'Okazaki et se dissout finalement lorsque la synthèse du fragment se termine. L'ADN polymérase s'attache alors à une autre amorce et le cycle complet, de la formation de la boucle à son effondrement, se répète. Le réplisome permet de coordonner toutes les activités en tant que complexe de la machinerie de réplication.

Composants du replisome

Les hélicases se déroulent et séparent l'ADN double brin. Ces enzymes sont présentes sous la forme d’un seul cycle hexamère chez les procaryotes et sous la forme d’un double cycle hexamère chez les eucaryotes. Les hélicases eucaryotes dépendent de protéines supplémentaires, Cdc45 et GINS, pour fonctionner. Les protéines de liaison à l'ADN simple brin (SSB) empêchent les brins d'ADN de se ré-hybrider. Chez les procaryotes, les SSB sont constitués d’une seule sous-unité, tandis que chez les eucaryotes, il s’agit d’une protéine hétérotrimérique connue sous le nom de protéine de réplication A (RPA).

La primase ajoute une amorce d'ARN à l'endroit où la synthèse de l'ADN commencera.  Chez les procaryotes, la primase est présente sous la forme d'une enzyme à sous-unité unique appelée DnaG, qui synthétise une amorce d'ARN d'environ 12 nucléotides.  Chez les eucaryotes, une enzyme à plusieurs sous-unités, l'ADN polymérase-α primase, synthétise une amorce hybride ARN-ADN d'environ 25 nucléotides. En plus de la polymérase-α, la polymérase réplicative étend l'ADN nouvellement synthétisé. Alors qu'un seul type de polymérase réplicative, l'ADN polymérase III, est présent chez les procaryotes, deux types différents de polymérases réplicatives, Pol ε et Pol δ, sont présents chez les eucaryotes, pour la synthèse du brin principal et du brin secondaire, respectivement.

Les protéines Sliding Clamp maintiennent les polymérases attachées à la matrice d’ADN. La pince β, une protéine homodimère, agit comme une pince chez les procaryotes. Chez les eucaryotes, la même tâche est accomplie par l’antigène nucléaire cellulaire en prolifération (PCNA), une protéine homotrimère. Le pore central de la pince coulissante est chargé positivement, ce qui lui permet d'avoir des interactions électrostatiques avec le squelette phosphate chargé négativement de l'ADN. La pince est fixée à l’ADN par un Clamp Loader. Ces protéines pentamères appartiennent à la classe AAA+ des ATPases. Le chargeur de pince eucaryote est connu sous le nom de facteur de réplication C, tandis que le chargeur de pince procaryote E. coli est connu sous le nom de complexe γ ; cependant, on pense que les protéines clamp et les chargeurs clamp sont des homologues évolutifs contrairement à de nombreux autres composants du réplisome.

Le réplisome

La réplication de l’ADN est effectuée par un grand complexe de protéines qui agissent de manière coordonnée pour obtenir une réplication de l’ADN haute fidélité. Ensemble, ce ...

Education

Enseignement

JoVE Core

Molecular Biology

Biologie moléculaire

Unit 1

Unité 1

DNA Replication

Réplication de l'ADN

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

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 Biologie Moléculaire illustre les processus moléculaires au sein de la cellule grâce à des leçons vidéos claires et concises. Les applications de ces concepts sont ensuite démontrées grâce à des expériences concrètes en laboratoire.

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9-12.B.1.1

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a. Use a model to describe the structure and sequence of DNA, including nucleotide structure, base pairing, and the structure of the helix.

9-12.B.1.2.a

DNA Replication Teams in the Replisome

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