When you consider how babies are made, you bump up against a basic math problem. No need for calculus here, or even the mental gymnastics of carrying the one. It’s a problem of doubling.
A mother has 23 pairs of chromosomes. A father also has 23 pairs of chromosomes. When sperm meets the egg, you would expect the embryo, and thus the fetus, and thus the baby to have 46 pairs of chromosomes. And when that baby grows up, one of its eggs, filled with 46 pairs of chromosomes, might meet-cute a sperm, carrying its own 46 pairs of chromosomes, and the result would be a baby whose cells hold onto 92 pairs of chromosomes.
Given humanity’s long history on this planet and its propensity to make babies, our cells would have, by now, exploded with chromosomal goo. This surplus of DNA would be unmanageable from a containment perspective, let alone what it would do to the body. We simply do not need that much DNA. When we have one extra chromosome, such as in trisomy 21, the effects are appreciable. What would happen if we had dozens upon dozens of extra chromosomes, or hundreds?
In order to figure out nature’s answer to this quandary, someone needed to crawl under the metaphorical bed sheets and figure out just how babies are made. That man was Oscar Hertwig, and the procreators that captured his attention were spiny globules we call sea urchins.
A sea change in understanding fertilization
Hertwig was born in 1849 in Friedberg, Germany. He and his brother Richard, one year his junior, attended university together, and Oscar’s curiosity was later piqued by the question of just what happened when a sperm cell and an egg cell met. Two main theories were fighting it out in academic circles at the time. The first was that the sperm touched the egg and basically vibrated against it, which spurred the egg into becoming an embryo. In this scenario, sperm was a sort of alarm clock loudly buzzing and prodding the egg to wake up and fulfill its destiny.
The second hypothesis was that the sperm penetrated the egg and their raw materials mixed and became the embryo.
Hertwig saw the evidence that had been published for this second theory but was not happy with it. When he heard that his brother was leaving for the Mediterranean to work on a research project with the preeminent biologist they had both studied under, Oscar decided to join them. And he made an important observation while there: sea urchins, which can be found on the Mediterranean seashore, had transparent embryos.
With the veil of fertilization thus lifted, Hertwig was able to witness sea urchin sperm entering a sea urchin egg cell and fusing with its nucleus, which contains the chromosomes. Sperm was no alarm clock; it mixed with the egg. Once a single sperm cell had penetrated the egg, a force field called the vitelline membrane was erected by the egg to block out the remaining sperm cells.
This is now common knowledge, but what Hertwig didn’t know then was exactly what was happening to the contents of the sperm and egg when they met… and what strange chromosomal dance had taken place before the sperm and egg were even created. Because that would prove to be the answer to the problem of DNA doubling up all the time.
Swapping legs
Let’s start with sperm, the simpler of the two.
Inside the testis of an adolescent male, we look for a regular cell that is about to become a sperm cell. We race past the complex structures that bring this dynamic cell to life and enter its nucleus, the command center of the cell which contains the chromosomes. We focus on chromosome 1. There are two of them, joined at the hip. Let’s call them Dwayne and Matthew. Matthew was inherited from the teenage boy’s mom (hence the “M" name), while Dwayne was a gift from his dad. They are, in a manner of speaking, fraternal twins: the same age, with similar enough features, but with important differences.
Cells, which contain Dwayne and Matthew and nearly two dozen other pairs of chromosomes, divide, like a soap bubble being pinched into two. In order to make sure that the cells that result from this division both carry Dwayne and Matthew, Dwayne and Matthew need to be copied.
And so, through the magic of molecular biology, Dwayne and Matthew, joined at the hip, find themselves next to Dominic and Marc, also joined at the hip and looking like clones of Dwayne and Matthew. We have now doubled the amount of DNA inside this cell in preparation for that bubble pinch.
But before this can happen, a critical event takes place. It does not happen in any other cell of the boy’s body, but strictly in those that are to become sperm cells.
Matthew, originally inherited from Mom, and Dominic, a copy of Dwayne which was inherited from Dad, get together. It’s not exactly love, but more akin to one of Dr. Frankenstein’s experiments. A part of Matthew goes to Dominic and vice versa, creating new chromosomes: Matthinic and Domew.
What happens next is simple. Dwayne and the new chromosome Matthinic move to one end of the cell, while Domew and Marc shuffle over to the other side. The cell splits into two, and these cells then each split into two, creating four sperm cells.
There is a name for this entire process of a cell swapping material between Mommy’s and Daddy’s chromosomes after it has doubled up its DNA, then twice halving its content. It’s called meiosis (pronounced “my-OH-sis”) and it means a “lessening.” It takes place in cells that are meant to become sex cells, i.e. sperm and egg in humans.
Figure 1: A simplified illustration of meiosis, using the development of sperm cells as an example. 1) Each chromosome consists of two halves, one inherited from Daddy (labelled Dwayne here) and the other inherited from Mommy (labelled Matthew), and identical copies of them are made at the beginning of this process (Dominic and Marc). 2) One of the halves inherited from Mommy swaps material with one of the halves inherited from Daddy. 3) This creates new recombinant chromosomes. 4) Eventually, each one of these chromosomes finds itself in a single sperm cell. Created with BioRender.com.
Cells which contained both Dwayne and Matthew now only contain either Dwayne or a modified version of Matthew. Where once there were two chromosomes 1 in the same cell, there is now only one. The same process concurrently affects chromosomes 2 to 22 and the sex chromosomes as well. Sperm cells thus have half as much DNA as any other cell in the body.
Meiosis also takes place in females, though the process stops-and-goes over the course of many years to eventually create egg cells that also contain half as much DNA as any other cell. (To be more accurate, some cells in our body actually contain even more DNA than regular cells, for example up to half of our liver cells.)
This is why, when a sperm penetrates an egg, we do not get 46 pairs of chromosomes.
The 23 single chromosomes of the sperm are matched with the 23 single chromosomes of the egg, and this creates a new combination of 23 pairs of chromosomes in the embryo. Only Dwayne becomes part of the embryo, or Marc, or Matthinic, or Domew, or any one of a near-infinite combination of Matthew and Dominic chromosomes, or of Dwayne and Marc recombined chromosomes—all depending on which sperm wins the race.
And the egg they will fertilize will contain May, or Dawn, or Darilyn, or Marlene, or any combination of Marilyn and Darlene and of May and Dawn.
And these recombinations—creating Matthinics and Darilyns—are critical events because they engender genetic diversity. Without these DNA swaps, the same chromosomes would get passed down, intact, from generation to generation. Instead, a mother’s and father’s genetic contributions are scrambled inside their children when the latter start making sexual cells. This diversity benefits us: it gives us a better chance to survive when our environment changes and helps reduce the chances of our children inheriting certain genetic diseases. And this perk is not just for us. Meiosis takes place in plants and animals more broadly.
One of the core principles of toxicology is that it’s the dose that makes the poison. In a way, it holds true in genetics. We could not withstand a growing accumulation of chromosomes from generation to generation. Meiosis is thus key in keeping the number of chromosomes constant and helping ensure diversity.
Take-home message:
- Without a special process in place, every child would receive all of its mother’s and father’s DNA and thus end up with twice as much DNA as each parent
- What actually happens is that sperm cells and egg cells wind up with half of the DNA of regular cells through a process called meiosis
- During meiosis, equivalent chromosomes inherited from different parents swap parts of each other, which contributes to genetic diversity
