How Your Unique DNA Begins: A Journey from Conception

One week after conception, you consist of a bunch of stem cells that can form any body part – they could become heart muscle cells, nerve cells, liver cells or anything else. At this stage they are still so flexible that they can even create more than one body. If the cells were to detach from one another, and form two separate cell clusters instead of one, they might develop into two complete people.

This is the most common way that identical twins occur, and since the placenta is already being formed, the twins will have to share it. Alternatively, the cells could have fallen apart a few days earlier, when they resembled a microscopic raspberry, in which case two vesicles will attach to the uterus and two embryos, each with their own placenta, will be created. About one third of identical twins begin like this.

Since identical, or monozygotic, twins originate from the same cell, they possess exactly the same DNA strands – they are natural clones. If one of the twins commits a crime, investigators will be unable to distinguish between them using a DNA analysis. This is because fingerprint patterns are in part shaped by the environment in the womb. The two twins occupy different spaces, and therefore experience different streams and pressures against their fingertips.

In addition, because the supply of nutrients from the placenta is not evenly distributed, one can grow slightly faster than the other. This means there will be small variations in the twins, even though their genes are exactly the same.

Twins can also result if the mother releases two eggs instead of one, and each becomes fertilised by its own sperm cell. These are called non-identical twins, and their DNA strands are no more similar than in normal sib-lings. But they are not exactly normal siblings. It appears that twins can exchange cells while in the womb, just as cells can be transferred to mothers. In this way, for example, they may end up having two blood groups – one that comes from themselves and another that comes from the twin sibling.

On rare occasions, the two cell clusters recombine before two bodies can be formed. If this happens with non-identical twins, then the child will grow up with two sets of DNA, a so-called chimera. Instead of all the cells having the same DNA strands, some of them will carry the DNA strands of the ‘twin’. Usually it never comes to light, but the phenomenon can sometimes lead to quite absurd situations.

Fairchild was a chimera. Before she was born, her cells had merged with a twin in the womb. When this happens, instead of each twin making its own complete body, the recombined cells become woven together and share the tasks between them. In this case, the cells that made the skin came from one twin; those that made the egg cells and cervix came from the other. Fairchild’s body was created by twin sisters – which made her the child’s mother and aunt at the same time.

Unless you have an identical twin, there’s not one person on the planet who has exactly the same DNA as you. When the sperm and egg cell merged during your conception, a unique code emerged. But the areas where your uniqueness is manifested are very small – most of the recipe is the same in all people, and these days it’s possible to look it up online.

AT THE START OF THE THIRD WEEK, the cell cluster which is soon to become you flattens and spreads out across the vesicle. Right now there’s nothing that even vaguely resembles a body – you look more like a little round plate. On each side of the plate are two fluid-filled sacs.

One of them becomes the foetal sac, which will enclose you and the little pool in which you’ll live for the next few months. The other will become the yolk sac, a round balloon with its cord fastened inside your stomach. The yolk sac creates your first blood cells; a job that your liver, spleen and your bone marrow will eventually take over. When it’s no longer needed, the yolk sac will shrivel up and become part of your intestines.

With birds and other egg-laying animals, the most important role for the yolk sac is to provide nutrition – they have no placenta to feed on, of course – so it’s packed with vitamins, minerals, fats and proteins. If you crack open a hen’s egg, in addition to the yellow yolk sac you may also notice some thin white threads keeping the yolk attached to the center of the egg. I’m a fertilized egg, a chick slowly emerges from a thin white plate on the surface of the sac. At first, it’s a barely visible speck, but after a few days red blood vessels coil themselves around the yolk. Soon after that, the yolk sac shrivels up and a living creature gradually emerges. Three weeks later the egg hatches and a new chick is ready to meet the world.

But at the start of the third week you take at least one important step forward from the plate stage. Over the course of a few crucial hours, you are given a front, a back, a top and bottom, and right and left sides. It’s one of the most critical periods in your whole development.

The first sign of this dramatic change is that the round plate becomes more of an oval shape. At the same time a thin strip appears. This is the beginning of your back, and it extends from the edge and towards the centre of the oval plate, where your head will pop up later on. If we were to zoom in on this strip, we’d see all the cells wandering down it towards a small pit at its centre. The cells dive into it, forming a new layer under the topmost one. Soon you will consist of two cell plates stacked one on top of the other.

Shortly after that, new cells arrive and spread themselves between the two plates, so that you end up with three layers of cells. These cells are no longer confused, needy newcomers with no idea where they are or what they re supposed to do. They have completed a rough division of labour. The cells on the top layer will form, among other things, skin, hair, nails, eye lenses, nerves and your brain. From the bottom layer you’ll get intestines, liver, trachea and lungs. And the middle layer will become your bones, muscles, heart and blood vessels.

As time moves on, each cell will become more and more specialized. Eventually, you will end up with over 200 different types. Their shape, size and characteristics will vary enormously. Round red blood cells will float around your body, carrying oxygen. Immune cells will patrol for intruders. Your ear will contain hairy sensory cells that dance to every sound you hear, and electrical signals will flicker and spark in your brain through the long threads of nerve cells.

Source : The Making of You: A Journey from Cell to Human by Katharina Vestre

Goodreads : https://www.goodreads.com/book/show/42121353-the-making-of-you

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