Clamping Down On The Mystery Of Human DNA Replication

April 2, 2013

redOrbit Staff & Wire Reports Your Universe Online

Scientists at Penn State University have discovered how a vital step in the human DNA replication process the loading of molecular structures known as sliding clamps onto DNA molecules is performed.

The researchers say their work, the results of which were published in Tuesdays edition of the journal eLife, will help uncover some of the mystery surrounding this crucial part of the chemical replication process. This step had not previously been closely analyzed in human DNA replication, which is the basis for biological inheritance in all types of living organisms.

According to Mark Hedglin, a post-doctoral researcher in Penn States Department of Chemistry and one of the investigators behind the discovery, the sliding clamp is a ring-shaped protein which essentially encircles a DNA strand, latching around it much like a watch band. The sliding clamp then anchors polymerases enzymes involved in the synthesis of nucleic acids to ensure more efficient copying of the genetic material.

Without a sliding clamp, polymerases can copy very few bases the molecular letters that make up the code of DNA at a time. But the clamp helps the polymerase to stay in place, allowing it to copy thousands of bases before being removed from the strand of DNA, Hedglin explained in a statement.

However, because of the closed circular structure of those sliding clamps, another step in DNA replication the presence of a so-called clamp loader to latch and unlatch them at different stages of the process is required. Previously, researchers did not know exactly how the sliding clamp and the clamp loader interacted with one another, nor did they know exactly when the clamp was attached to or unattached from the DNA.

We know that polymerases and clamp loaders cant bind the sliding clamp at the same time, so the hypothesis was that clamp loaders latched sliding clamps onto DNA, then left for some time during DNA replication, returning only to unlatch the clamps after the polymerase left so they could be recycled for further use, Hedglin said.

In order to test their theory, they turned to a method known as Frster resonance energy transfer (FRET), which attaches fluorescent tags to human proteins and parts of DNA in order to monitor the interactions between them.

With those tags in place, Hedglin said he and his colleagues observed the formation of holoenzymes the active form of the polymerase involved in DNA replication, which consists of the polymerase itself along with any accessory factors that optimize its activity.

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Clamping Down On The Mystery Of Human DNA Replication