Overview of DNA Replication Structure
DNA replication is a semi-conservative process where the double helix unwinds, and each strand serves as a template for synthesizing a new complementary strand. It occurs during the S phase of the cell cycle and involves key enzymes like helicase, which unwinds the DNA, and DNA polymerase, which adds nucleotides to the growing strand. The process ensures each daughter cell receives an identical copy of the genetic material.
Key Steps in DNA Replication
Replication begins at origins of replication where the DNA strands separate, forming a replication fork. Primase synthesizes short RNA primers to initiate synthesis, and DNA polymerase extends these primers in the 5' to 3' direction. The leading strand is synthesized continuously, while the lagging strand forms in Okazaki fragments, which are later joined by DNA ligase. Topoisomerase relieves supercoiling ahead of the fork to prevent tangling.
Practical Example: Replication in Eukaryotic Cells
In human cells, multiple origins of replication allow simultaneous copying of the large genome. For instance, during cell division in skin cells, helicase unwinds DNA at thousands of sites, and polymerases work bidirectionally from each origin, completing replication in about 8 hours. This coordinated process exemplifies how replication supports rapid tissue repair and growth.
Error-Correction Mechanisms and Their Importance
DNA polymerase includes proofreading exonuclease activity, which removes mismatched nucleotides with 99% accuracy. Mismatch repair systems, like MutS and MutL in bacteria or MSH/MLH in eukaryotes, scan and correct errors post-replication, reducing the error rate to 1 in 10^9 bases. These mechanisms are crucial for preventing mutations that could lead to diseases like cancer, maintaining genomic stability across generations.