Like a Virgin by Prasad, Aarathi (recommended reading txt) 📕

the mother’s and the father’s chromosomes came together in the fertilized egg, thus producing baby worm cells with four chromosomes. The contradiction puzzled him, and he did not publish any further work on the subject.

A year later, Van Beneden’s work came to the attention of the German biologist August Weismann. Alas, the esteemed professor was suffering from eye trouble. After many years of conducting his groundbreaking scientific experiments in chemistry, biology, and medicine, he could no longer look down the microscope for himself, and he was forced to turn his attention to theoretical questions. Still, he was not ready to give up experimentation altogether. Weismann asked his janitor to carry out the logistics of his experiments and assigned his students to do the microscopic analysis. His wife, Marie, would read scientific papers aloud to him, so that he could keep up with the latest ideas. Among the papers Marie recited was Van Beneden’s work on chromosomes in worm eggs. As Weismann listened to Van Beneden’s extensive observations, and Marie’s descriptions of the paper’s drawings, he came up with a theory: a very special division happened exclusively in the sex – or germ – cells, that is, in eggs and sperm.

For every cell in your body that isn’t an egg (for a woman) or a sperm (for a man), making a new cell is a simple case of copying the chromosomes, separating the copies into two lots, and then distributing the original set and its copy equally into two new cells. This process is called mitosis. Think of it as making a photocopy of some pages: you separate the original pages from the copied pages, keep the originals for yourself, and give the photostats to a colleague.

Weismann realized that a different division must also occur in order to make a sex cell. Whatever number of chromosomes there were in the original cells, these would need to be halved, resulting in that sex cell with only one set of chromosomes. He had discovered meiosis, a process that only ever happens in eggs and sperm, and the thing that makes sex exciting, in evolutionary terms; meiosis ups the ante in the grand gamble of reproduction. The word ‘meiosis’ is derived from the Greek for ‘diminution’, because, as Van Beneden and Weismann observed through their microscopes, duplicated DNA from a sex cell that is dividing is diminished by half in the new cells that are produced. (The devices available to them were not quite sophisticated enough to demonstrate that, in reality, meiosis achieves far more than that.)

Whereas most cells of our body contain one maternal chromosome and one paternal chromosome, each copied as precisely as possible, an egg or sperm must contain only one chromosome strand, and the copies of the chromosomes that end up in the egg or sperm are not simple duplicates of the strand in other cells. The process of meiosis physically shuffles and exchanges information between the two chromosome strands. To do this, the double helix of the chromosomes breaks, and the broken ends physically move across each other, swapping genes before the double helix re-forms. This is a much greater challenge than the usual process of cell division, because genes must be matched, sorted, scrambled, redistributed, and realigned. From this, a unique combination of genes is born. It is different to the gene combination found in either parent, different to the one inherited in every other body cell, and peculiar to the offspring that may be created when this sexual cell fuses with a mate’s. It is for this reason that no two children born to the same parents, unless they are identical twins developed from a lone fertilized egg, are genetically the same.

At the end of this complicated process, it is not just how the chromosomes are divided up that makes the egg especially unique. Inside the egg, there is also a cellular ‘soup’, which separates into two grossly unequal parts. The disproportionately smaller of the two parts helps to reduce the number of chromosomes until only one set of the two is left. Ultimately, that smaller part degenerates while the larger one sticks around to become the egg, ready and waiting to be fertilized.

This rudimentary, immature egg will then undergo a second meiosis, just as complicated as the first. Another unequal division of a cellular soup produces a tiny cell and a fully mature egg. Like the last one, this tiny cell is usually destroyed, but not always. In the fruit fly Drosophila melanogaster, this tiny cell sometimes ‘fertilizes’ the larger, mature egg to create virgin-born fly offspring, essentially using a part of the egg to stand in for sperm. In humans (and almost all vertebrate animals), right in the middle of this second meiosis, the egg stops dividing and enters a biological holding period, known as prophase I, in which it can remain for an extraordinarily long time. In frogs, this phase can last several years; in humans, several decades.

When a girl enters puberty and starts ovulating, the egg will resume its monthly meiosis. But there is another catch: the egg will be blocked from maturing further or transforming into an embryo until and unless some sperm show up. This block on development is dramatically named metaphase II arrest. Eggs need to be activated to start their dividing, and activation usually happens with fertilization – the fusion of sperm and egg. At least, that is, when things are proceeding normally.

Moment by moment in the course of your life, cells in your body are dying off. Before they do so, they divide and give rise to ‘replacement’ cells just like themselves – a skin cell divides into two new skin cells; a liver cell into two liver cells – which is how the body doesn’t dissolve into non-existence. But when eggs divide, they can give rise to