For the first time, scientists at Stanford University have sequenced single sperm cells.
But wait a minute, hasn’t human genome sequencing been going on for quite a while, now? And since all cells in our body have the same genome, what is the point of sperm cell sequencing?
Sperm Cells Are Half of a Normal Cell
Well, the answer is that almost all cells in our body have the same genome. All, except the reproductive cells. All non-reproductive cells in our body (called somatic cells) contain two copies of each chromosome, one from the father and one from the mother. Sperms (and egg cells from women) contain just one copy- the one that will be transmitted to the child. This halving of chromosomes in the reproductive cells is necessary to ensure that the child receives the right amount of genetic material.
Sons Have a Mosaic of Their Fathers’ DNA
During cell division in reproductive cells, the two chromosomes in a chromosome pair first interchange parts of each other in a process called recombination to produce two chromosomes that are a mosaic of the two parents. One of each mosaic pair then goes to the daughter sperm.
Sequencing will help shed light on this recombination process. Are there parts of the genome that are more susceptible to be recombined? What are the rates of mutation during this process? Mutation rates in humans are usually studied at a population level, this breakthrough will allow a closer look at the level of individual sperms by comparing the sperm DNA with the father’s.
Sequencing Single Cells on a Chip
To investigate these properties, researchers isolated sperm cells from a single semen sample. Individual cells are very hard to sequence because they have extremely small amounts of DNA. This experiment used a microfluidics chip to isolate 91 sperm cells from this sample and amplify each of their DNA (so they could generate enough DNA for the sequencing step). Each ‘sperm genome’ was then sequenced separately for a comparative analysis of their DNA.
Error in the recombination process has previously been linked to infertility and it is hoped that this could be a way to determine if the link is true, and if so, how. Adam Auton from the Albert Einstein College of Medicine in New York reckons the new technique could also be useful in studying cancer cells. “Every cancer is slightly different,” he says. If researchers can study single cells from a tumour, they may be able to get a better idea of which genes have been disrupted by mutations, and develop treatments to target them, he says.