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Sperm cells are the smallest cells produced by the human body at only 50 micrometers long. But a single sperm cell contains a complete copy of all the data found on 23 chromosomes, which can be hundreds of megabytes of information. Even your fancy new micro SD card can't compete with that kind of storage density. And thanks to a study published this week in the journal Cell Metabolism, we now know that sperm carries not only genetic information, but epigenetic information as well.

Your DNA contains most of the instructions for making you, organized into genes, and all those genes together make up your genome. But even though each cell gets a complete copy of that genome, it only needs to use certain parts, and it figures out which parts based on your epigenetic information.

It needs the help because your DNA is an incredibly complex molecule. It's also really really long, two meters long if you stretched it out all the way. How do you squeeze a two meter long molecule inside the nucleus of a cell? You wrap the DNA tightly around a long filament of protein called histone. 

Coiling the molecule into a spring like that compresses it down, but this also introduces a new problem: your DNA is wrapped so tightly around the histone that your cell can't access most of your genetic information anymore. So in addition to histone and DNA you also have epigenetic markers. "Epigenetic" literally means "above your genes", and those epigenetic markers sit right on top of your DNA.

They tell the cell where to coil the DNA tighter and where to let it unspool. Unspool the DNA and the cell can read the genes in that location. Tighten it up and the genes there won't be expressed in that cell. It's part of the reason why, say, a neuron becomes a neuron and not a muscle cell. Those cells carry different epigenetic data. The neuron reads the parts of your DNA that tell it how to be a neuron, and not the parts that would tell it how to be a muscle cell.

That's how powerful epigenetic markers can be, and they can change based on how you live your life. Your weight, diet, stress levels, and even your moods can alter those markers. We also know that they're inherited, but for a long time geneticists thought that sperm cells were too small to contain both the genome and the epigenetic information of the donor. 

While some studies in the past have suggested that this might not be the case for mice, there hasn't been much research into the human side of things. Sperm cells were believed to be inert carriers of DNA, so scientists just never really looked into it. Until now.

In this study, a group of researchers from the University of Copenhagen were specifically looking at the differences in the sperm cells between lean and obese men. And what they discovered was that those sperm cells contained not only epigenetic markers, but they contained different epigenetic markers depending on whether the donor was lean or obese. Specifically, there were differences of the gene regions associated with appetite control.

The team compared sperm cells from 13 lean men and 10 obese men and then tracked 6 men who were going through weight-loss surgery to see how the weight loss affected their sperm. On average, five thousand changes to sperm DNA were found between samples from before the surgery and after, and the differences in sperm cells weren't found in the structure of the DNA or the histones, they were in a different, related kind of molecule called small RNA.

Turns out that small RNA can also act as an epigenetic marker by telling the cell which genes to turn on and off. For decades, scientists believed that RNA molecules only functioned as messenger molecules, carrying information to different parts of your cells. But then in 1998 geneticists discovered that RNA molecules play a huge role in silencing your genes. They prevent genes from being expressed, making them an epigenetic marker. 

They detect certain groups of instructions in your DNA and then just clip onto them so the cell can't access them. And when it came to the men in the study, they clipped onto different parts of the DNA depending on if they were lean or obese.

Women who want to have a child hear a lot of health recommendations both before and after they get pregnant, things like maintaining a healthy weight, don't smoke, stay away from alcohol and drugs, because that could affect the health of the child even before conception occurs. And at least part of the reason is because these things can change epigenetic markers, which get passed on. But now, according to the lead author on this study, the results tell us that the prospective fathers might want to take care of themselves too.

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