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6.4:

La forca di replicazione del DNA

JoVE Core
Molecular Biology
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JoVE Core Molecular Biology
The DNA Replication Fork

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Nella cellula, la replicazione del DNA inizia sempre in posizioni specifiche sul DNA definite l’origine della replicazione. All’inizio della replicazione, l’enzima DNA elicasi lega e si muove lungo il filamento del DNA, svolgendo e separando il DNA. La struttura risultante a Y formata dai filamenti separati del DNA assomiglia ad una forcella a due punte e diventa un sito di replicazione del DNA;pertanto, questa struttura è nota come fork di replicazione.Le molecole separate di DNA a singolo filamento sono inclini a formare anse a doppio filamento o a riavvolgersi con l’altro filamento. Per evitare che ciò accada, le proteine Single Strand DNA Binding, o SSB, si legano ad un DNA a singolo filamento e ne inibiscono il riavvolgimento. I singoli filamenti esposti del DNA, possono fungere da modelli per la sintesi di filamenti complementari figli.

6.4:

La forca di replicazione del DNA

An organism’s genome needs to be duplicated in an efficient and error-free manner for its growth and survival. The replication fork is a Y-shaped active region where two strands of DNA are separated and replicated continuously. The coupling of DNA unzipping and complementary strand synthesis is a characteristic feature of a replication fork.   Organisms with small circular DNA, such as E. coli, often have a single origin of replication; therefore, they have only two replication forks, one in each direction leading away from the location of the initial opening.  In organisms with large genomes, the replication of DNA is not done from a single point of origin but in many distinct, localized replication forks. 

The unhindered progression of the replication fork is necessary for complete DNA replication and genome stability; however, the replication fork is often stalled by internal or external factors that can slow or stop its progression, resulting in replication stress. Replication stress causes genomic instability, which is a hallmark of diseases like cancer. Genomic instability is characterized by genomic alterations and increased frequency of harmful mutations. The movement of the replication fork can stop due to several reasons. For example, the drug hydroxyurea depletes the pool of nucleotides available for incorporation into the daughter strand, stalling the replication fork. Other problems that may hinder the progression of the replication fork include DNA lesions, a collision between a replication fork and a transcription complex, and defects in the enzymes involved in DNA replication.

The cell has a variety of repair mechanisms to reinitiate the stalled replication fork.  S-phase checkpoints do not allow cells to begin mitosis until DNA repair is complete. Additionally, fork repriming can restart DNA synthesis by bypassing a DNA lesion or block. Despite these robust mechanisms, sometimes stalled forks cannot be reinitiated, which leads to the collapse of a fork, halting DNA replication.

Suggested Reading

  1. Mazouzi, Abdelghani, Georgia Velimezi, and Joanna I. Loizou. "DNA replication stress: causes, resolution and disease." Experimental Cell Research 329, no. 1 (2014): 85-93.
  2. Saada, Anissia Ait, Sarah AE Lambert, and Antony M. Carr. "Preserving replication fork integrity and competence via the homologous recombination pathway." DNA Repair 71 (2018): 135-147.
  3. Waga, Shou, and Bruce Stillman. "The DNA replication fork in eukaryotic cells." Annual Review of Biochemistry (1998): 721-751.
  4. Boddy, Michael N., and Paul Russell. "DNA replication checkpoint." Current Biology 11, no. 23 (2001): R953-R956.