Featured Research

from universities, journals, and other organizations

Rewriting Textbooks On DNA Crossover

Date:
April 20, 2004
Source:
University Of California - Davis
Summary:
Key decisions in the genetic shuffling that occurs before eggs or sperm are formed are made earlier than thought, rewriting textbook genetics, according to recent papers from researchers at UC Davis, Harvard University and UC San Diego.

Key decisions in the genetic shuffling that occurs before eggs or sperm are formed are made earlier than thought, rewriting textbook genetics, according to recent papers from researchers at UC Davis, Harvard University and UC San Diego.

Related Articles


For sexual reproduction to occur, organisms have to form gametes (in animals, gametes are eggs or sperm) with half the usual number of chromosomes, so that when two gametes fuse during fertilization the offspring will have an equal genetic contribution from each parent. This process is called meiosis: Without it, the chromosome number would double with every generation.

Meiosis includes a crucial step in which DNA is broken and either repaired by "crossing over" with another chromosome or healed without a crossover. Each pair of chromosomes must have at least one crossover for meiosis to work. New research shows that the decision to make a crossover or not is made much earlier than previously thought, and sheds light on the molecular basis of this process.

Exchanging DNA

Two copies of each chromosome are present in each body cell. During meiosis, each chromosome lines up with its partner, and the DNA molecules are cut in several places. The partner chromosome DNA acts as a template to heal the breaks. This process, known as homologous recombination, can result in the exchange of chunks of DNA between chromosome arms -- a crossover. Or a break can be healed without exchanging DNA to give a non-crossover recombination.

Recombination stabilizes chromosome pairing, and crossovers are specifically required for the accurate distribution of chromosomes into the gamete cells, said Neil Hunter, assistant professor of microbiology at UC Davis. If the process fails, a gamete might end up with the wrong number of chromosomes, potentially leading to birth defects such as Down syndrome.

How chromosomes decide to make a crossover or non-crossover recombination has been "something of a mystery," Hunter said.

The textbook explanation has been that the linked DNA strands form structures called Holliday junctions that on paper can be processed in two different ways to create either a crossover or non-crossover. This model implies that the decision is made at a very late step, Hunter said.

Working in the brewer's yeast Saccharomyces cerevisiae, Hunter and colleagues Valentin Boerner and Nancy Kleckner of Harvard University, writing in the April 1 issue of Cell, show instead that the decision on whether or not to crossover is made at a much earlier stage: after the DNA is broken but before the ends of the breaks become stably intertwined with their partner chromosome. Once the decision is made, chromosomes are shepherded along to form Holliday junctions and then crossovers by a group of six proteins called the ZMMs.

In the same issue of Cell, Olga Mazina, Alexander Mazin and Stephen Kowalczykowski from UC Davis with Takuro Nakagawa and Richard Kolodner from the Ludwig Institute of Cancer Research at UC San Diego studied one of the ZMM proteins, Mer3, known to be important for crossover recombination to occur. They found that Mer3 unwinds the DNA double helix but works only in one direction relative to the broken DNA end. It blocks extension of the DNA strand in the opposite direction. Mer3 therefore helps to stabilize the Holliday junction structure and promotes crossover recombination, Hunter said.

The findings mean that the pathways to crossover and non-crossover recombination are distinct, and distinct from an early stage, Hunter said. That turns the textbook account of meiosis on its head, he said.

While researchers now have a better understanding of the process, how the decision is made remains a mystery, Hunter said.

"We're getting insights, but we're left with big questions," he said.


Story Source:

The above story is based on materials provided by University Of California - Davis. Note: Materials may be edited for content and length.


Cite This Page:

University Of California - Davis. "Rewriting Textbooks On DNA Crossover." ScienceDaily. ScienceDaily, 20 April 2004. <www.sciencedaily.com/releases/2004/04/040420013627.htm>.
University Of California - Davis. (2004, April 20). Rewriting Textbooks On DNA Crossover. ScienceDaily. Retrieved November 23, 2014 from www.sciencedaily.com/releases/2004/04/040420013627.htm
University Of California - Davis. "Rewriting Textbooks On DNA Crossover." ScienceDaily. www.sciencedaily.com/releases/2004/04/040420013627.htm (accessed November 23, 2014).

Share This


More From ScienceDaily



More Plants & Animals News

Sunday, November 23, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Ebola-Hit Sierra Leone's Late Cocoa Leaves Bitter Taste

Ebola-Hit Sierra Leone's Late Cocoa Leaves Bitter Taste

AFP (Nov. 23, 2014) The arable district of Kenema in Sierra Leone -- at the centre of the Ebola outbreak in May -- has been under quarantine for three months as the cocoa harvest comes in. Duration: 01:32 Video provided by AFP
Powered by NewsLook.com
Anglerfish Rarely Seen In Its Habitat Will Haunt You

Anglerfish Rarely Seen In Its Habitat Will Haunt You

Newsy (Nov. 22, 2014) For the first time Monterey Bay Aquarium recorded a video of the elusive, creepy and rarely seen anglerfish. Video provided by Newsy
Powered by NewsLook.com
Birds Around the World Take Flight

Birds Around the World Take Flight

Reuters - Light News Video Online (Nov. 22, 2014) An imperial eagle equipped with a camera spreads its wings over London. It's just one of the many birds making headlines in this week's "animal roundup". Jillian Kitchener reports. Video provided by Reuters
Powered by NewsLook.com
Could Your Genes Be The Reason You're Single?

Could Your Genes Be The Reason You're Single?

Newsy (Nov. 21, 2014) Researchers in Beijing discovered a gene called 5-HTA1, and carriers are reportedly 20 percent more likely to be single. 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:

Strange & Offbeat Stories


Plants & Animals

Earth & Climate

Fossils & Ruins

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