Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Rewriting Textbooks on DNA Crossover

19.04.2004


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.

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.

Andy Fell | UC Davis
Further information:
http://www.news.ucdavis.edu/search/news_detail.lasso?id=6992

More articles from Life Sciences:

nachricht A novel socio-ecological approach helps identifying suitable wolf habitats
17.02.2017 | Universität Zürich

nachricht New, ultra-flexible probes form reliable, scar-free integration with the brain
16.02.2017 | University of Texas at Austin

All articles from Life Sciences >>>

The most recent press releases about innovation >>>

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

Im Focus: Breakthrough with a chain of gold atoms

In the field of nanoscience, an international team of physicists with participants from Konstanz has achieved a breakthrough in understanding heat transport

In the field of nanoscience, an international team of physicists with participants from Konstanz has achieved a breakthrough in understanding heat transport

Im Focus: DNA repair: a new letter in the cell alphabet

Results reveal how discoveries may be hidden in scientific “blind spots”

Cells need to repair damaged DNA in our genes to prevent the development of cancer and other diseases. Our cells therefore activate and send “repair-proteins”...

Im Focus: Dresdner scientists print tomorrow’s world

The Fraunhofer IWS Dresden and Technische Universität Dresden inaugurated their jointly operated Center for Additive Manufacturing Dresden (AMCD) with a festive ceremony on February 7, 2017. Scientists from various disciplines perform research on materials, additive manufacturing processes and innovative technologies, which build up components in a layer by layer process. This technology opens up new horizons for component design and combinations of functions. For example during fabrication, electrical conductors and sensors are already able to be additively manufactured into components. They provide information about stress conditions of a product during operation.

The 3D-printing technology, or additive manufacturing as it is often called, has long made the step out of scientific research laboratories into industrial...

Im Focus: Mimicking nature's cellular architectures via 3-D printing

Research offers new level of control over the structure of 3-D printed materials

Nature does amazing things with limited design materials. Grass, for example, can support its own weight, resist strong wind loads, and recover after being...

Im Focus: Three Magnetic States for Each Hole

Nanometer-scale magnetic perforated grids could create new possibilities for computing. Together with international colleagues, scientists from the Helmholtz Zentrum Dresden-Rossendorf (HZDR) have shown how a cobalt grid can be reliably programmed at room temperature. In addition they discovered that for every hole ("antidot") three magnetic states can be configured. The results have been published in the journal "Scientific Reports".

Physicist Dr. Rantej Bali from the HZDR, together with scientists from Singapore and Australia, designed a special grid structure in a thin layer of cobalt in...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Booth and panel discussion – The Lindau Nobel Laureate Meetings at the AAAS 2017 Annual Meeting

13.02.2017 | Event News

Complex Loading versus Hidden Reserves

10.02.2017 | Event News

International Conference on Crystal Growth in Freiburg

09.02.2017 | Event News

 
Latest News

Biocompatible 3-D tracking system has potential to improve robot-assisted surgery

17.02.2017 | Medical Engineering

Real-time MRI analysis powered by supercomputers

17.02.2017 | Medical Engineering

Antibiotic effective against drug-resistant bacteria in pediatric skin infections

17.02.2017 | Health and Medicine

VideoLinks
B2B-VideoLinks
More VideoLinks >>>