This morning I received an email from my boss. Subject line: “good news!” I think it takes a special kind of scientist to get very excited about the arrival of a semen sample.
You see, my lab specializes in finding mutations involved in a specific disorder: tuberous sclerosis complex (TSC). We have already found a mutation in this particular patient, and by sequencing the DNA from his semen, we can assess the frequency of that mutation in his sperm, and thus, how likely he is to pass it on.
But first, we have to get the DNA out of the sperm.
Extracting DNA from cells is a geneticists bread and butter, but this is semen we’re talking here: I wanted to be sure I didn’t have to handle it again. As it turns out, it was a good thing I did my reading, because spermatazoa are so well protected from the environment, that normal lysis protocols will not work on them. They’re basically the armored knights of cells when it comes to cracking them open.
The reason why these nuts are so tough to crack (pardon the pun) is because the DNA is linked by disulfide bonds, which my Bio 251 prof refers to as “molecular staples.” These bonds are inordinately strong, and they hold the DNA the proteins are bound on tightly together.
This special design made me curious about the manner in which DNA is packaged into sperm cells, so I investigated further. When normal cells undergo mitosis – cell division – DNA is wrapped around histones, sort of like thread around a spool. This is called chromatin, which is super compact. The aggregation of those chromatin structures forms the traditional chromosomes you might see from a karyotype.
Sperm DNA though, replaces most of the histones with different proteins (protamines), allowing the DNA to be packed even tighter. The resulting structure is so compact, it needs to be stapled together with disulfide bonds for the whole thing to be crammed into a sperm cell.
This is the molecular way of data compression. That is to say, DNA in spermatazoa are .zip files of your genome.