Between each generation, organisms shuffle the chromosomes that they inherited from their parents in order to produce reproductive cells with unique combinations of alleles. This is a central goal of sexual reproduction because it promotes genetic diversity, thereby driving evolution. The chromosomal exchanges, called crossovers, are formed by a mechanism of DNA recombination that is initiated by the programed introduction of DNA double-strand breaks.

The laboratory will use a combination of biochemistry and molecular genetics methods in yeast to gain insights into the following fundamental questions:

  • What is the molecular mechanism of meiotic double-strand break formation?
  • What is the molecular mechanism of crossover formation?
  • How are these enzymatic activities related to the higher-order structure of meiotic chromosomes?

Double-strand break formation.

Meiotic double-strand breaks are formed by the enzyme Spo11, together with nine accessory proteins. Double-strand breaks are potentially dangerous because they can cause unwanted chromosomal rearrangements that can lead to diseases, like cancer. The activity of Spo11 is therefore highly controlled so that breaks only form in the right context. Using in vitro and in vivo approaches, we will seek to understand the mechanism of DNA cleavage by Spo11. How does Spo11 engage its DNA substrate? How is it activated? What are the roles of the accessory subunits? How do Spo11 and its partners communicate with one another, and how is double-strand break activity controlled?

Crossover formation.

After DNA cleavage, the breaks are repaired by homologous recombination. Recombination is an ancient mechanism found throughout the living world that serves to accurately repair broken DNA by using an intact copy as a repair template. The meiotic program has tweaked this mechanism for its specific purpose, which is to induce recombination between parental chromosomes (homologs) and promote the exchange of chromosomal fragments (crossovers). We seek to understand how the enzymatic recombination machinery drives the formation of crossovers.

Relationships between local events and chromosome organization.

The small-scale local events of double-strand break formation and recombination are intrinsically tied to large-scale features of meiotic chromosomes. Chromosomes undergo a series of structural changes as the cell progresses through meiosis: they first assemble as linear arrays of DNA loops that emanate from a proteinaceous axis, and as recombination progresses the homologs eventually pair along their length and form a structure called the synaptonemal complex. Double-strand break formation happens in the context of the loop-axis structure, and the later stages of recombination happen in the context of the synaptonemal complex. We aim to understand the relationships between DNA-level enzymatic events and chromosomal-level structural organizations.