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 microSD 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 just 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 -- and it’s
also really, REALLY long.
If you stretched out a single molecule of your DNA? It would be two meters long!
So, 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 it 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 up the coil, and the genes there won’t
be expressed in that cell.
It’s part of the reason that, 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.
And while some studies in the past have suggested 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 not only contained epigenetic markers,
but they contained DIFFERENT epigenetic markers depending on whether the donor was lean or
obese.
Specifically, there were differences at 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 a year after.
Well, the differences in those sperm cells were found not in the structure of the DNA
or in the histones -- they were in a different, related kind of molecule, called small RNA.
And 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.
Then, in 1998, geneticists discovered that some RNA molecules play a huge role in silencing
your genes. They can 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 in the men in the study, they clipped onto different parts of the DNA, depending
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 maintain a healthy weight, don’t smoke, stay away
from alcohol and drugs -- because that could potentially affect the health of the child,
even before conception occurs.
And at least part of the reason is that these things can change epigenetic markers, which
then get passed on.
But according to the lead author on the study, these results tell us that prospective fathers
might want to take care of themselves, too.
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