7.2
Replicação em Eucariotos
Nas células eucarióticas, a replicação do DNA é altamente conservada e fortemente regulada. Múltiplos cromossomos lineares devem ser duplicados com al…
A replicação do DNA em cromossomos eucarióticos inicia-se em múltiplas origens de replicação, que são identificadas e ligadas pelo complexo de reconhecimento de origem, ou ORC.
O ORC então recruta helicases para desenrolar o DNA, produzindo uma bolha de replicação com dois garfos de replicação.
As duas bifurcações se movem em direções opostas e interrompem os nucleossomos à sua frente. Esses nucleossomos são então remontados nas fitas filhas, mantendo a estrutura da cromatina.
Em cada bifurcação, os primers de RNA fornecem o local para a DNA polimerase alongar a fita principal e os fragmentos de Okazaki da fita atrasada.
Uma enzima RNase remove esses primers e a DNA polimerase preenche as lacunas. Finalmente, a DNA ligase sela os fragmentos.
No entanto, à medida que o último primer é removido da fita atrasada no final do cromossomo linear, ele produz um trecho saliente do DNA molde.
Uma enzima chamada telomerase estende esse trecho saliente com DNA não codificante para evitar a perda de DNA codificante durante os ciclos de replicação subsequentes.
A replicação continua até que as bolhas de replicação adjacentes se fundam e todo o cromossomo seja duplicado.
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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.