…but every sperm may not be great. Sometime last week I posted on the observation that older men are more likely to father kids who will go on to have problems with a variety of ailments. This observation is not all that surprising when you think about how sperm get made.
If you are a college graduate, it is very likely that you learned (or at least sat through a lecture on) how sperm get made. This would have happened in a biology course that you were forced to take as one of your “general education” requirements. “Wait a minute”, you may be saying, “I don’t remember that”. No. You don’t. Because in that lecture a professional biology professor, such as myself, demonstrating our professionability, used terms like “chromatid” and “telophase” and “centromere”, terms that made it sound like we were talking about equipment used during the Mars landing, and your eyes rolled up in your head and you passed out just about the time that we got to the money shot that made it all clear. And then you got that question wrong on the test. (It was question 25. The answer was “C”.)
Anyway, we won’t do that here. We’ll just stick to the essentials.
You probably remained conscious long enough to learn that the human body is made up of millions of teeny tiny cells. Every cell contains a collection of strands of genetic material. You could think of these strands like strings of beads, except that instead of lapis or quartzite or turquise or silver, every bead is a rolled-up set of instructions. If they were written in a language you could understand, the instructions would say things like “make red hair”, and “leave a cleft in the chin” and “when she eats delicious things, put the resulting fat on the hip area”. Amazingly, every single cell in your body contains all the instructions needed to make you what you are; but not all the instructions are being used in every cell, all the time. (This fact is why you keep hearing about stem cells, which are cells in which any or all of the instructions can be turned on at any time. But I digress.)
Ok, so down in that delicate and special region of a man¹ where sperm get made this is what happens: One particular set of cells is constantly making copies of themselves. To do this they have to first make copies of all the strands of genetic material; every bead has to be copied, and every bead has to be on the new string in the right order. We will not bother ourselves right now with exactly how this happens, although it is important and fascinating — for now, take my word for it: copying happens.
Once copying has happened, there are twice as many strands of genetic material in that weensy cell as there are supposed to be. It’s like a party where everyone has made their own date. Anyway, when copying is complete, half of the strands go to one end of the cell, half to the other. This results in two separate groups of genetic material, and each group has the same total number of strands, and the same number of beads on the strands. Then the cell divides down the middle, and voila! where there was one cell, there are now two cells!
Well. You know how men are. Never satisfied. Are twice as many cells as you started with good enough? Not when you’re makin’ sperm, baby. Once these two cells have separated from each other, the strands of genetic material in each cell migrate to opposite ends of the cell again; and the cells divide again. Except, in this round the strands didn’t copy themselves first. This means that each of the four resulting cells has half as many strands of genetic material in it as we had in the original cell. Then they all grow tails and, as graduate sperm, go looking for the exit, hoping to be let out in the vicinity of a likely egg².
If you have followed this thrilling narrative this far, you might have noticed some points at which mistakes can be made. (And, as in the rest of life, if mistakes can be made, mistakes will be made.) One of the key weak spots is the copying. Just like when you are making photocopies; the more copies you make, the more likely it is that mistakes will get made. If you photocopy a sheet of paper once, chances are good that you will position it correctly, close the lid carefully, and push the buttons in the right order. When you make a lot of copies, all kinds of things can happen. The machine can jam halfway through the job. The toner goes on darker on the first copies, and lighter on the last copies. Furthermore, the originals start to get worn out: they get wrinkled, they get thumbprints on them, they collect schmutz (that’s a technical term) that shows up on the copies as black spots.
So the observation that older men father children with a higher risk of later problems is consistent with the idea that the photocopy principle applies to sperm making. The more copies of the cells that make sperm a man makes, the greater the chance that at some point, mistakes will get made in the copying. And men make a LOT of copies — every time a man delivers the goods, about 100 million sperm get dropped off.
What’s most likely to explain the old dad/unhealthy child observation, is the accumulation of damage to the originals. Chance or chronic exposure to toxic chemicals or radiation, too much heat; any number of non-fatal insults to a man’s body can damage those instruction-carrying beads on the genetic strands in those sperm-making cells. The damage changes or garbles the instructions. When those damaged originals get copied, the messed-up instructions get copied too, and thus the resulting kid is built with the accumulated genetic mistakes. These mistakes are more likely to turn up in the sperm of older men just because they’ve been around for longer – they’ve had time to accumulate damage to the genetic material in the cells that are throwing off all the copies-that-will-be-sperm.
What I’d like to know now is: why are so many of the ailments the medicos have shown to be more likely in the kids of older dads brain-based? Autism, schizophrenia, bipolar disorder…are the genes for brain function somehow more vulnerable to the photocopy principle? Or are these ailments just easier to document than other types of copying errors?
¹ …and I know you know what I am talking about, I’m not using those words, or all sorts of weird traffic will show up here from the sad, sad people using Google.
² Should they encounter an egg, which also has only half the normal amount of genetic material, they strive to thrust themselves into it. The first sperm to succeed in penetrating the egg actually releases all its strands of genetic material into the egg, where they join those strands already in the egg, and presto! we are back to a single cell with as many strands of genetic material needed to make a whole human. Wild. I tell you, you couldn’t make this stuff up.