Cottrell College Science Awards - 2015
Optical Trapping Assay to Study Histone Replacement during Spermiogenesis
Despite considerable scientific progress, life is still a somewhat mysterious process. And life’s ability to reproduce is still fraught with unknowns. For example:
“Scientists have long thought about the problem of how DNA is folded into the nucleus,” notes Ashley Carter, assistant professor of physics at Amherst College. She has received a Cottrell College Science Award from Research Corporation for Science Advancement to study a critical aspect of vertebrate reproduction, specifically spermatogenesis.
“On one hand,” Carter says, “there is the question of how to get a two-meter-long DNA strand into a 10-micron-diameter cell nucleus. On the other hand, there is the question of how to fold the DNA, when highly folded regions of DNA are in general less transcriptionally active.”
“Transcription” is the first step by which DNA – the blueprint for a living organism – is expressed in the cell. It involves individual segments of DNA being copied into a closely related molecule, RNA, by the enzyme RNA polymerase.
“To strike a balance between compaction and activation, the cell employs a highly regulated and dynamic system to compact the DNA into the nucleus while maintaining transcriptional activity. Understanding how this complex system works is of interest across biochemistry, biology, medicine and physics,” Carter says.
Carter and her students will be looking at one aspect of the incredibly detailed and exacting process of sperm cell production in humans – histone replacement.
Composed of basic proteins, histones carry a positive charge. This makes it easy for these molecules to hook up with negatively charged DNA. Like any protein, histones become functional by folding into various shapes, some of which are more-or-less spool-like, and these forms take up, or compact, the thread-like DNA. In this spooled form, the DNA/histones are called “chromatins,” and overall they look a beaded necklace. Scientists call the individual beads “nucleosomes,” which consist of a core of positively charged histone proteins and the associated negatively charged DNA segment.
The histone proteins must eventually be replaced by protamines, which are small proteins found in the cell nucleus. Unlike histone molecules, protamines are rich in the amino acid arginine, one of the building blocks of protein. In other words, the body uses histone proteins as a tool to assemble other proteins essential to sperm cells.
Carter and her students aim to measure the physical mechanism underlying histone replacement. To do this they will used laser light to trap a tiny, manmade bead with attached nucleosomes and then add protamine molecules. They hope to monitor histone replacement by measuring changes in the length of the DNA in the nucleosomes.
“These measurements would be a pioneering first step toward discerning the physical nature of histone replacement,” Carter says.