Featured Research

from universities, journals, and other organizations

Mechanism Of Epigenetic Inheritance Clarified

Date:
April 24, 2008
Source:
Cold Spring Harbor Laboratory
Summary:
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.

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 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.”

Journal reference: Anna Kloc, Mikel Zaratiegui, Elphege Nora and Rob Martienssen. “RNA Interference Guides Histone Modification during the S Phase of Chromosomal Replication” appears in the April 8, 2008, edition of Current Biology.


Story Source:

The above story is based on materials provided by Cold Spring Harbor Laboratory. Note: Materials may be edited for content and length.


Cite This Page:

Cold Spring Harbor Laboratory. "Mechanism Of Epigenetic Inheritance Clarified." ScienceDaily. ScienceDaily, 24 April 2008. <www.sciencedaily.com/releases/2008/04/080422151826.htm>.
Cold Spring Harbor Laboratory. (2008, April 24). Mechanism Of Epigenetic Inheritance Clarified. ScienceDaily. Retrieved April 17, 2014 from www.sciencedaily.com/releases/2008/04/080422151826.htm
Cold Spring Harbor Laboratory. "Mechanism Of Epigenetic Inheritance Clarified." ScienceDaily. www.sciencedaily.com/releases/2008/04/080422151826.htm (accessed April 17, 2014).

Share This



More Health & Medicine News

Thursday, April 17, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Could Even Casual Marijuana Use Alter Your Brain?

Could Even Casual Marijuana Use Alter Your Brain?

Newsy (Apr. 16, 2014) A new study conducted by researchers at Northwestern and Harvard suggests even casual marijuana use can alter your brain. Video provided by Newsy
Powered by NewsLook.com
Thousands Of Vials Of SARS Virus Go Missing

Thousands Of Vials Of SARS Virus Go Missing

Newsy (Apr. 16, 2014) A research institute in Paris somehow misplaced more than 2,000 vials of the deadly SARS virus. Video provided by Newsy
Powered by NewsLook.com
Formerly Conjoined Twins Released From Dallas Hospital

Formerly Conjoined Twins Released From Dallas Hospital

Newsy (Apr. 16, 2014) Conjoined twins Emmett and Owen Ezell were separated by doctors in August. Now, nearly nine months later, they're being released from the hospital. Video provided by Newsy
Powered by NewsLook.com
Ebola Outbreak Now Linked To 121 Deaths

Ebola Outbreak Now Linked To 121 Deaths

Newsy (Apr. 15, 2014) The ebola virus outbreak in West Africa is now linked to 121 deaths. Health officials fear the virus will continue to spread in urban areas. Video provided by Newsy
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:

Breaking News:
from the past week

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile: iPhone Android Web
Follow: Facebook Twitter Google+
Subscribe: RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins