Rewriting Textbooks on DNA Crossover

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.