I read the human genome is 1.5 gigabytes in size. Thats actually not a lot; Photoshop probably takes more space. Mac OS takes 10+ gigabytes of space. Also, the genome is 1.5 gigabytes when counting base pairs as the alphabet, but the actual unit of information is a codon, which is made of 3 base pairs. That means that if the 64 codons are the alphabet, then the amount of information is (top of the hat calculation) 0.5 gigabytes. This is analogous to a "byte" in computer memory, composed of 8 bits. Byte -> codon, bit -> base pair.

Nevermind that some codons are start/stop signals and not part of a protein defining instruction. You can go further and say that the next level, formed by all the known proteins/chains of aminoacids, is a subset of all possible combinations of aminoacids, and forms a new alphabet. The language of proteins.

Anyway, a cell has 100 trillion atoms, and that structure is controlled by DNA containing 1.5 (or 0.5 depending on taste) gigabytes of instructions. That right there is pretty impressive. But then, the human body has 37.2 trillion cells, and you are telling me those 37.2 trillion cells, each containing 100 trillion atoms, are controlled by ~1 gigabytes of instructions? That doesn't add up.

In computers everything is 0 or 1, but after many layers that simple alphabet of {0, 1} can represent complex structures. I guess the same is possible with DNA, but I feel you'd need way more instructions to represent even the most basic of organs. So, I think I am missing something here. For basic structures, like tissue, veins, eyes, blood, hair; things that we share with animals, is that all encoded in DNA?

The complexity is rather enormous for a rather tiny amount of instructions, so there must be something else controlling organ and bone structure.

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    $\begingroup$ @user1136 this article suggest Junk DNA might not be junk after all: futurity.org/junk-dna-1728712 $\endgroup$ – user52117 Sep 2 '19 at 2:54
  • $\begingroup$ The genome is not a set of blueprints, it's more like a set of general rules for growth that are given to a machine. You also need a "mother' (or an egg or seed) to carry out those instructions. It's instructive to consider L-systems, and how very simple rules can reproduce the growth of many creatures. See e.g. and related works: en.wikipedia.org/wiki/The_Algorithmic_Beauty_of_Plants $\endgroup$ – jamesqf Sep 2 '19 at 4:12
  • $\begingroup$ Your question doesn't really seem like a question but an expression of disbelief. Biology is quite incredible so perhaps you can try reading some books on modern genetics and physiology. $\endgroup$ – WYSIWYG Sep 9 '19 at 9:58
  • $\begingroup$ Size of any "instructions," be-it in computer programming or biology, is a poor indicator of "complexity," if at all. The plant Paris japonica's genome is 50 times that of a human, I'm not sure which would you consider to be more "complex," or to what degree. In programming, Photoshop CC is 1.3GB in Windows but 800MB on Mac, and yet they function more or less identically, despite the almost 50% difference in size. $\endgroup$ – Gen Test Sep 9 '19 at 21:18

This is a VERY hard question to answer; I think mostly because it is very hard to think about how the DNA actually encodes information.

The first important thing to note (and also the only direct answer I can give to your question), is that

  1. (Yes) the human genome does encode all information to make a human being, but only in the context of a cell/organism that can read this genome.

Transferring this to the computer code analogy this means that if the genome is the source code for a program (or rather a couple of them), then the operating system (and also the actual hardware) that executes this code is the cell itself and the two parts have to match up in order to work together.

This is essentially a hen/egg problem: without the cellular machinery to read and operate on the genome the organism doesn't work, but at the same time the genome is the source of the everything that reads it.
If one would put the human genome into another (not closely related) species (or the egg cell of one), this would not form a human being, because the organism/egg cell can't read the instructions properly. (This is also the reason why hybrid animals like i.e. mules are often sterile - the genomes of their parental species are similar enough to be readable and make a working animal, but there are already enough differences to introduce problems).

This brings me to what i think is the actual core of what you're asking:

  1. How can so much complexity be encoded in so little space/bits/... ?

and I don't have a clear answer for that (and neither have I ever seen one). There are, however, a few things one needs to consider

Emergent behavior is when seemingly very simple and limited rules lead to very complex structures. This happens all the time in nature and our body plans are no exception, it is however very hard to see or understand and even harder to consciously design from scratch. Nowadays, machine learning algorithm can get complex results from initial simple instructions, but only because they run 'a couple' of optimization steps inbewteen - evolution has done the same thing for millennia.

A second thing to keep in mind, is that code can be re-used. Programmers already do that (a lot), but usually they try to keep the code readable and make a clean copy for a new purpose. Nature / the genome most of the time doesn't do that and its been re-using and re-purposing pathways on any level for millennia. The reason that arms and legs are built very similar, or that essentially all animals are symmetric is that it's just easier to (re)use the same set of instructions a couple of times with slight alterations than specifically writing everything down.

Finally (as almost always), Randal Monroe explained pretty well, why just knowing the 'source code' of the genome doesn't mean we actually understand how it's doing what it's doing:

xkcd's DNA comic

This answer's gotten pretty long now and I'm out of time to add sources - if anyone feels motivated to add something feel free to make/propose edits.

  • $\begingroup$ Nice answer, +1! Would really like to read some more about the "code reusing" point you made if you can provide some sources :D $\endgroup$ – another 'Homo sapien' Sep 3 '19 at 4:13
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    $\begingroup$ @another'Homosapien' one example on the cellular level are second messengers in signalling (esp. Ca2+) - there are only a handfull of different ones, but they are involved in many many different pathways across all celltypes. I'm pretty sure the same goes for (developmental) patterning signals/pathways, but I don't know good examples there. $\endgroup$ – Nicolai Sep 3 '19 at 9:25
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    $\begingroup$ Ah I did find one: the morphogen sonic hedgehog (yes ...) is involved in developement / shaping of nervous system, limbs, teeth, lungs and still has more functions. $\endgroup$ – Nicolai Sep 3 '19 at 9:29

First thing's first: life is an astounding display of emergent properties. I think Nicolai makes that clear in his answer. That is a primary consideration, for how a lot can be achieved with a little. I'd like to make a more philosophical, wishy-washy response to convince you that the genome really isn't all that central to life. Please pardon the metaphors in advance.

The problem we are faced with is that, as Patrick Bateson once quipped,

We can reconstruct a blueprint from a house, but not a recipe from a cake.

I'd also like to directly and overtly add that blueprints don't build houses.

In similar vain, you are assuming that the genome is responsible for complexity, when in fact it is only a collection of organized legible macromolecules. Sure, the genome is of paramount importance, and inheritance is crucial for evolution to produce novel attributes, but the kitchen of life depends on much more than just one its recipe books. And it is equally important to understand that genomes are more like recipe books rather than blueprints; you can make blueprints from existing buildings but you cannot make recipes from the cake. Life (and its complexity) is a little bit like cake in this regard.

I'd like to make an extreme case: the fact that the entire system of life requires liquid water means that the complexity of the whole biosphere and organisms is very much tangled with the properties of our sun and planet. This is absolutely a requisite and part-and-parcel of Earth life. The genome does not encode for the position or movement of stars but its genes are dependent on this arrangement. This complexity is, for the sake of simplicity, unrightfully overlooked (for practical and sensible reasons). Carl Sagan put it most succinctly:

If you wish to make an apple pie from scratch, you must first invent the universe.


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