How DNA strands separate

Cornell researchers have answered a fundamental question about how two strands of DNA, known as a double helix, separate to start a process called replication, in which genes copy themselves.

Credit: Chris Pelkie and Daniel Ripoll/Cornell Theory Center


This image shows a DNA double helix (green and purple strands) being separated by a helicase enzyme (green globule) at the junction where the two strands fork. To show that helicases actively separate the two strands of DNA, the researchers attached one end of a DNA strand to a microscope cover slip and attached the end of the other DNA strand to a micron-sized plastic bead. The bead was then trapped in a tightly focused laser beam (red), which allowed the researchers to measure the motion of the helicase as it unwound the DNA.

The research, published in the current issue of the journal Cell, examined the role of an enzyme called a helicase, which plays a major role in separating DNA strands so that replication of a single strand can occur.

Scientists have known that helicases bind to the area of a double helix where the two strands fork away from each other, like the free ends of two pieces of thread wound around each other. The forked area opens and closes very rapidly. But scientists have debated whether helicases actively separate the two strands at the fork or if they passively wait for the fork to widen on its own.

The research found that the helicase appears to actively exert a force onto the fork and separate the two strands.

"A simple passive unwinding mechanism does not explain our data," said Michelle Wang, associate professor of physics and the paper's senior author.

"Defects in helicases are associated with many human diseases, ranging from predisposition to cancer to premature aging," said co-author Smita Patel, a biochemistry professor at the Robert Wood Johnson Medical School in Piscataway, N.J. "Helicases are involved in practically all DNA and RNA metabolic processes."

The researchers made their discovery by anchoring one end of one of the strands in a double helix to the surface of a microscope cover slip. The end of the other strand was attached to a micron-sized plastic bead. They then focused a laser beam on the tiny bead and trapped the bead in place within the beam of light. This setup allowed the researchers to measure the position and force on the bead, creating a very precise sensor of the helicase motion. As the helicase moved toward the fork and the double helix unwound, the tension on the two strands lessened. Using statistical mechanics models, the researchers could then compare actual measurements of movement with predictions based on both active and passive scenarios.

"The unwinding has to have some active component to it, and based on our data, we can tell you exactly how active it is," said Wang. "Basically, it is an active unwinding motor."

While helicases unwind very rapidly in cells, in test tube experiments the unwinding is much slower. The researchers believe that helicases work with other enzymes, where "accessory proteins are helping the helicase out by destabilizing the fork junction," said Wang.

Original Source: Cornell University
Cornell researchers determine how an enzyme plays a key role in gene copying
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Prions key in Alzheimer's disease

Proteins which cause mad cow disease may also protect against Alzheimer's disease, UK researchers say. Prions naturally present in the brain appear to prevent the build up of a key protein associated with the condition.

In laboratory tests, beta amyloid, the building block of Alzheimer's "plaques", did not accumulate if high levels of the prions were present. The findings could lead to new treatments, the Proceedings of the National Academy of Sciences reported.

In variant Creutzfeldt-Jakob disease (vCJD), the human version of mad cow disease, the normal version of the prion protein present in brain cells is corrupted by infectious prions causing it to change shape, resulting in brain damage and death. But little is known about purpose of the normal prion proteins.

Due to the similarities between Alzheimer's and diseases such as variant CJD, researchers at the University of Leeds, looked for a link.

Plaque formation

They found that in cells in the laboratory, high levels of the prions reduced the build-up of beta-amyloid protein, which is found in the brains of people with Alzheimer's disease. In comparison, when the level of the prions was low or absent, beta amyloid formation was found to go back up again, suggesting they have a preventive effect on the development of the condition.

The researchers also looked at mice who had been genetically engineered to lack the prion proteins and again found that the harmful beta-amyloid proteins were able to form.

Study leader Professor Nigel Hooper said they now needed to look at whether ageing had an affect on the ability of the prion proteins to protect against Alzheimer's.

"Until now, the normal function of prion proteins has remained unclear, but our findings clearly identify a role for normal prion proteins in regulating the production of beta-amyloid and in doing so preventing formation of Alzheimer's plaques.

"Whether this function is lost as a result of the normal ageing process, or if some people are more susceptible to it than others we don't know yet."

He said although they needed to learn more, theoretically if a treatment could be designed to mimic the effect of the prions it could halt the progression of the disease.

Professor Clive Ballard, director of research at the Alzheimer's Society said this was the first time a link had been made between prions and Alzheimer's. "These are early findings, which suggest prion proteins may have a regulatory effect on the development of beta amyloid." He added: "This provides the foundations for a novel approach to finding new therapeutic targets in Alzheimer's disease."

Original Source BBC health news 19 June 2007
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