Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Scientists Clarify a Mechanism of Epigenetic Inheritance

24.04.2008
“Silent” DNA stays that way as a result of RNA interference during chromosome replication
Although letters representing the three billion pairs of molecules that form the “rungs” of the helical DNA “ladder” are routinely called the human “genetic code,” the DNA they comprise transmits traits across generations in a variety of ways, not all of which depend on the sequence of letters in the code.

In some cases, rather than the sequence of “letters,” it is the physical manner in which DNA is spun around protein spools called histones and tightly packed into chromosomes that determines whether or with what intensity specific genes are expressed. A team of scientists at Cold Spring Harbor Laboratory (CSHL) has solved another in a series of mysteries about this critical mechanism of gene expression, described in a paper in the April 8 issue of Current Biology.

Inherited Clumping

According to CSHL professor Rob Martienssen, Ph.D., who led the research team, about a tenth of our DNA stands aloof, spending its time in tightly packed clumps called heterochromatin, and unwinding only to replicate when a cell divides. After copying, both of the resulting DNA molecules — to the surprise of many — have been observed to form reclusive clumps in the same places as the original one did.

This inherited clumping of DNA, which causes genes to be expressed in distinctive ways, is one of a series of phenomena that scientists call epigenetic. The same sequence of nucleotides in two people can produce different patterns of gene expression if the way the DNA is clumped happens to be different.

Probing Epigenetics in Yeast
“We have not understood epigenetic inheritance very well,” says Dr. Martienssen, a plant geneticist and one of the pioneers in the study of epigenetics. To explore this process, he and his team are studying the way DNA is packed in yeast, and how this packing can be transmitted across generations. The single-cell yeast organism is easy to study, in part because it lacks other epigenetic inheritance mechanisms, such as chemical modifications of DNA, that complicate the study of more complex animals and plants.

Long DNA molecules almost miraculously cram into cell nuclei that are almost a million times smaller than they are. They do so by wrapping around proteins called histones, which array themselves along the length of the DNA molecule like beads on a string. These DNA-wrapped histones then form larger arrays. The densely packed mass is then modified chemically by other proteins to form heterochromatin.
The dense packing of heterochromatin hides the DNA sequence from the cellular machinery that reads its genetic information, so the DNA in heterochromatin is “silenced.” The genes it contains are effectively turned off.

Surprisingly, the clumping persists even after cells divide, although, says Dr. Martienssen, “it’s always been a mystery how modifications of histones could be inherited.” A few years ago, however, his group and others solved this mystery. They found that histone modification is controlled by complicated cellular mechanisms broadly known as RNA interference, or RNAi.

In RNAi, RNA that is copied from particular regions of DNA interacts with various proteins to modify histones in the same regions. Because the RNA matches only the section of DNA that produced it, it “provides the specificity that you need to make sure that only that part of the chromosome gets these histone modifications,” Dr. Martienssen says. “If the whole chromosome were to get those histone modifications, you’d be dead.”

All in the Timing
These results raised a new puzzle, though: Since genes contained within heterochromatin are silenced, how can they give rise to the RNA molecules that help to modify histones? In new research, Martienssen’s team has now solved this puzzle by tracking the cells through their cycle of growth and division.

They found that the interfering RNA molecules appear only during the brief part of the cell cycle when DNA is replicating. This result, Martienssen says, “neatly accounts for the paradox about how ‘silent’ heterochromatin can be transcribed [into interfering RNA], because it’s transcribed only in a narrow window of the cell cycle.”
The researchers also found that RNAi varies strongly with temperature. They speculate that this variation is responsible for inherited traits such as vernalization, the well-known process by which certain plants must be exposed to low temperatures before they will flower. Indeed, Martienssen says, there is “a whole slew of epigenetic phenomena that are sensitive to temperature.”

“RNA Interference Guides Histone Modification during the S Phase of Chromosomal Replication” appears in the April 8, 2008, edition of Current Biology. The complete citation is as follows: Anna Kloc, Mikel Zaratiegui, Elphege Nora and Rob Martienssen. The paper is available online at:
http://www.current-biology.com/content/article/abstract?uid=PIIS0960982208003163.

Cold Spring Harbor Laboratory is a private, nonprofit research and education institution dedicated to exploring molecular biology and genetics in order to advance the understanding and ability to diagnose and treat cancers, neurological diseases and other causes of human suffering.

Jim Bono | EurekAlert!
Further information:
http://www.cshl.edu
http://www.current-biology.com/content/article/abstract?uid=PIIS0960982208003163

Further reports about: Chromosome DNA Genetic Histone Martienssen RNA epigenetic heterochromatin sequence

More articles from Life Sciences:

nachricht Cnidarians remotely control bacteria
21.09.2017 | Christian-Albrechts-Universität zu Kiel

nachricht Immune cells may heal bleeding brain after strokes
21.09.2017 | NIH/National Institute of Neurological Disorders and Stroke

All articles from Life Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Highly precise wiring in the Cerebral Cortex

Our brains house extremely complex neuronal circuits, whose detailed structures are still largely unknown. This is especially true for the so-called cerebral cortex of mammals, where among other things vision, thoughts or spatial orientation are being computed. Here the rules by which nerve cells are connected to each other are only partly understood. A team of scientists around Moritz Helmstaedter at the Frankfiurt Max Planck Institute for Brain Research and Helene Schmidt (Humboldt University in Berlin) have now discovered a surprisingly precise nerve cell connectivity pattern in the part of the cerebral cortex that is responsible for orienting the individual animal or human in space.

The researchers report online in Nature (Schmidt et al., 2017. Axonal synapse sorting in medial entorhinal cortex, DOI: 10.1038/nature24005) that synapses in...

Im Focus: Tiny lasers from a gallery of whispers

New technique promises tunable laser devices

Whispering gallery mode (WGM) resonators are used to make tiny micro-lasers, sensors, switches, routers and other devices. These tiny structures rely on a...

Im Focus: Ultrafast snapshots of relaxing electrons in solids

Using ultrafast flashes of laser and x-ray radiation, scientists at the Max Planck Institute of Quantum Optics (Garching, Germany) took snapshots of the briefest electron motion inside a solid material to date. The electron motion lasted only 750 billionths of the billionth of a second before it fainted, setting a new record of human capability to capture ultrafast processes inside solids!

When x-rays shine onto solid materials or large molecules, an electron is pushed away from its original place near the nucleus of the atom, leaving a hole...

Im Focus: Quantum Sensors Decipher Magnetic Ordering in a New Semiconducting Material

For the first time, physicists have successfully imaged spiral magnetic ordering in a multiferroic material. These materials are considered highly promising candidates for future data storage media. The researchers were able to prove their findings using unique quantum sensors that were developed at Basel University and that can analyze electromagnetic fields on the nanometer scale. The results – obtained by scientists from the University of Basel’s Department of Physics, the Swiss Nanoscience Institute, the University of Montpellier and several laboratories from University Paris-Saclay – were recently published in the journal Nature.

Multiferroics are materials that simultaneously react to electric and magnetic fields. These two properties are rarely found together, and their combined...

Im Focus: Fast, convenient & standardized: New lab innovation for automated tissue engineering & drug

MBM ScienceBridge GmbH successfully negotiated a license agreement between University Medical Center Göttingen (UMG) and the biotech company Tissue Systems Holding GmbH about commercial use of a multi-well tissue plate for automated and reliable tissue engineering & drug testing.

MBM ScienceBridge GmbH successfully negotiated a license agreement between University Medical Center Göttingen (UMG) and the biotech company Tissue Systems...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

“Lasers in Composites Symposium” in Aachen – from Science to Application

19.09.2017 | Event News

I-ESA 2018 – Call for Papers

12.09.2017 | Event News

EMBO at Basel Life, a new conference on current and emerging life science research

06.09.2017 | Event News

 
Latest News

Comet or asteroid? Hubble discovers that a unique object is a binary

21.09.2017 | Physics and Astronomy

Cnidarians remotely control bacteria

21.09.2017 | Life Sciences

Monitoring the heart's mitochondria to predict cardiac arrest?

21.09.2017 | Health and Medicine

VideoLinks
B2B-VideoLinks
More VideoLinks >>>