Building Myself from Scratch
I was watching a sixties movie last week. Halfway through the black-and-white classic I remembered having heard from my father’s mouth that this was an outstanding movie. I was surprised that I had that insignificant and meaningless (to me then) one sentence film review in my memory because I must have been under 10 then. The movie really was very well made. It wasn’t a popular movie then and isn’t remembered much now either. It suited my taste and style though. I asked myself if I had acquired that taste from my father. This could be my wishful thinking. Yet it always amazes me to think how that half set of genes of each of my parents eventually shaped me to this day. How this 60+ kilos of my body with all its fully functioning organs emerged from that single tiny cell formed about 38 weeks before my birth?
Developmental Biology is the subject that seeks to answer this precise question. The subject has now become richer with input from genetics. The eyes that helped me visualise the movie, for instance, is an organ made as per the code in a number of genes. Just for assigning the colour of the eyes some sixteen genes are responsible. The proteins that constitute each of the parts of the eye are manufactured as per the instructions written in the genes. Raw materials required for protein making are obtained through life processes as we grow. This fundamental process works similar to how a builder assembles materials based on a building plan.
If I were to plan a kitchen, I will work out where to place the cooktop, microwave, oven, fridge, bench space and sink. The key guiding factors will be the position of water plumbing and gas inlet. Once determined, I will draw the layout on the plan area marked for the kitchen. I will do similar exercise for other rooms — bathroom, lounge and bedroom. With all major features drawn up, let’s say, I have a complete plan of the house. I give the plan to the builder. He builds the house. Done. In roughly the same amount of time a human being comes out of gestation inside the mother’s womb.
Building a human however is way more complicated. Imagine that the building plan is lying inside a brick which multiplies to make the complete house. If that was not daunting enough, consider further that your builder too lives inside the brick and has to build the house inside out. Because all you have in the beginning is just one brick — the fertilised cell. Just having the plan is not enough in this case. Having a toolkit that works in isolation is not enough either. What you need is a toolkit that interacts with the plan live. Or a plan that interacts with the toolkit live. Whichever way you want to look at it, one thing is for sure that it is a highly interwoven mix of planning and building.
That special cell has what are called toolkit genes. So do the other copies of the cell that populate the space around it during the course of development. For instance, there is a specific toolkit gene which is responsible for the decision to begin making an eye at a certain time and at a specific place in the body. That decision is made within a month of conception when this gene — Pax6 — switches on and tells the developing cells, “Here, right at this place please make an eye.” The cells oblige by activating further genes to make specific proteins and by dividing in such a fashion that a few months before we leave the mother’s womb, we have a fully functional pair of eyes.
Similar other toolkit genes provide go/no-go decision to the process of making of organs such as limbs and heart. Fine, we have the recipes for making organs. But how does the developing embryo know where to make which body part? What if it puts the heart in the head? That doesn’t normally happen thanks to another set of toolkit genes — a more fundamental one called Hox genes.
Hox genes associate identity of a body segment and decide which body parts go where by turning on or off specific genes. Like other genes they too make proteins. But they make proteins of a special kind called transcription factors which determine if the next gene in line should be allowed to express or kept shut off. That way they regulate the process of protein making at the very start. There is in fact a cascade of gene regulation in the whole process of human development from the fertilised cell to a full adult human. These are called gene regulatory networks. It’s like you get to choose between recipes of various types of dough and depending on what dough you end up making, you are presented with specialised recipes of pizzas, rolls and pies.
These regulatory genes not only operate in human beings but in all animals. If that seems logical to you, how about this: the toolkit genes we discussed above are present in the tiniest of animals too — in mice as well as fruit flies! And they are similar in all of these!! That adds a whole new dimension to the science of Developmental Biology. Now we are talking of evolution of the process of development. A new discipline is born: Evolutionary Developmental Biology better known by its cooler short form Evo-Devo. How I came into being isn’t much different after all from how an earthworm is formed. The basic toolkit was available at least half a billion years ago. Nature doesn’t reinvent the wheel. It just makes it a little more sophisticated at every turn.
Vestre, Katharina (2019) The Making of You. Wellcome Collection. (for sweet little introduction)
Carroll, Sean B. (2005) Endless Forms Most Beautiful. W. W. Norton & Company. (for in-depth study)