7.2
Replicación en eucariotas
En las células eucariotas, la replicación del ADN está altamente conservada y estrictamente regulada. Se deben duplicar múltiples cromosomas lineales…
La replicación del ADN en los cromosomas eucariotas se inicia en múltiples orígenes de replicación, que están identificados y unidos por el complejo de reconocimiento de origen, o ORC.
A continuación, el ORC recluta helicasas para desenrollar el ADN, produciendo una burbuja de replicación con dos horquillas de replicación.
Las dos horquillas se mueven en direcciones opuestas e interrumpen los nucleosomas que tienen delante. A continuación, estos nucleosomas se vuelven a ensamblar en las hebras hijas, manteniendo la estructura de la cromatina.
En cada bifurcación, los cebadores de ARN proporcionan el sitio para que la ADN polimerasa alargue la hebra principal y los fragmentos de Okazaki de la hebra rezagada.
A continuación, una enzima ARNasa elimina estos cebadores y la ADN polimerasa rellena los huecos. Finalmente, la ADN ligasa sella los fragmentos.
Sin embargo, a medida que se elimina el último cebador de la hebra rezagada al final del cromosoma lineal, produce un tramo sobresaliente del ADN molde.
Una enzima llamada telomerasa extiende este tramo sobresaliente con ADN no codificante para evitar la pérdida de ADN codificante durante los ciclos de replicación posteriores.
La replicación continúa hasta que las burbujas de replicación adyacentes se fusionan y todo el cromosoma se duplica.
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Q1: What is DNA replication in eukaryotes and why is it essential?
DNA replication in eukaryotes is the process of copying the entire genome before cell division. It ensures each daughter cell receives an identical set of genetic instructions. This process is fundamental to maintaining genetic continuity across generations and is tightly regulated by the cell cycle control system to prevent errors.
Q2: How does the replication process begin at the DNA level?
DNA replication begins at specific sites called origins of replication where the double helix unwinds. Enzymes called helicases break the hydrogen bonds between base pairs, creating a replication fork. DNA polymerase then synthesizes new strands by adding complementary nucleotides to each template strand.
Q3: What are the key differences between leading and lagging strand synthesis?
The leading strand is synthesized continuously in the 5' to 3' direction as the replication fork advances. The lagging strand is synthesized discontinuously in short fragments called Okazaki fragments, also in the 5' to 3' direction but opposite to fork movement. Both strands require DNA polymerase but follow different synthesis patterns.
Q4: How do eukaryotes manage replication of their large genomes?
Eukaryotes have multiple origins of replication distributed across their chromosomes, allowing simultaneous replication at many sites. This parallel replication significantly reduces the time needed to copy the entire genome. Molecular factors affecting cell division coordinate the timing and regulation of these multiple replication events.
Q5: What mechanisms ensure accuracy during eukaryotic DNA replication?
DNA polymerase possesses proofreading ability, removing incorrectly paired nucleotides immediately after incorporation. Mismatch repair systems scan newly replicated DNA for errors and correct them post-replication. These quality control mechanisms maintain replication fidelity at approximately one error per billion nucleotides, ensuring genetic stability.
Q6: How is DNA replication regulated in the eukaryotic cell cycle?
Replication occurs exclusively during S phase of the cell cycle, controlled by checkpoint mechanisms and regulatory proteins. Once replication completes, re-replication is prevented until the next cell cycle begins. This regulation ensures DNA replicates exactly once per cell division cycle.
Q7: What role do telomeres play in eukaryotic DNA replication?
Telomeres are repetitive DNA sequences at chromosome ends that shorten with each replication cycle due to the end-replication problem. Telomerase, an enzyme with reverse transcriptase activity, extends telomeres in certain cell types. This mechanism protects chromosome integrity and limits cellular replication potential.