Overview
All organisms must pass an intact genome onto their progeny, so we are interested how chromosomes are faithfully inherited when cells divide. The centromere is the position on a chromosome where it attaches to the mitotic spindle, facilitating correct segregation. The protein complex that creates a microtubule binding site at the centromere is termed the kinetochore. We study chromosome properties that specify centromere location and function using the model plant Arabidopsis thaliana. Arabidopsis has key advantages for studying centromeres:
- facile genetics
- centromere DNA structure that is similar to most plants and animals (megabases of short tandem repeats)
- simple karyotype (haploid # = 5), allowing us to visualize individual kinetochores easily
Specific Projects
We are studying the following chromosome features that distinguish centromeres:
1. The centromere-specific histone CENH3
Centromeres in many eukaryotes are marked epigenetically by a centromere-specific version of histone H3 (CENH3), which replaces conventional H3 in centromeric nucleosomes. In several cases, centromere tandem repeat DNA is dispensable for centromere function, providing that CENH3 nucleates a functional centromere. We are developing new methods for quantifying CENH3 nucleosomes at centromeres. CENH3 evolves much more rapidly than conventional H3, and we are investigating the functional consequences of this rapid evolution.
2. Centromeric heterochromatin
Centromeres are typically embedded in repeat- and transposon-rich chromosome regions with extensive transcriptional silencing i.e. heterochromatin. Arabidopsis heterochromatin mutants are well-characterized, and we are using these resources to study how changes in gene silencing and in chromatin modifications affect centromere function.
3. Centromere DNA
In most plants and animals, centromere DNA is composed of megabases of short tandem repeats. Like the centromere-specific histone, these sequences evolve very rapidly. The size of the repeat array and high degree of similarity between repeats make centromere DNA difficult to study with conventional genetic tools. In collaboration with Ian Korf and Bob Stupar, we are using bioinformatics, shotgun sequencing and cytogenetics to characterize centromere DNA from a very wide range of eukaryotes. By comparing centromere DNAs from many genomes, we hope to discover principles that govern their function and evolution.
Chromosome spread from metaphase II photographed by Ravi. DNA is stained with DAPI, and five pairs of sister chromatids can be seen in each cell.
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