Submitted by ihaveredhaironmyhead t3_106bz01 in askscience
When the chromosomes are lined up and then pulled to the poles of the cell, how does this process happen so carefully and exactly with no neurology guiding anything? If the brain doesn't control this process what ultimately does? The DNA itself is being copied and pulled around. How does this organized process happen?
Chemical interactions? I still don't get how the spindles know which chromosomes to grab and what to do with them.
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There are protein complexes which identify the centromeres of the chromosomes to be targeted by the spindle apparatus. The protein complexes bind specifically to the centromeres of the chromosomes. Is this sort of what you were looking for?
I think what foxes was saying is that these interactions are passive, not controlled by a nervous system or anything.
What's another example of a passive interaction? Maybe this would help me understand. To me the entire process of cell division is mysterious. I don't understand how something requiring careful arrangement can be non-neurological.
I think on a certain level everything happens this way. A virus has no neurological system but it is replicated by a series of chemical reactions. If it doesn’t work it doesn’t continue to exist. Same for cell division. When it doesn’t work the cell dies - or becomes cancer and kills the host.
It is hard to conceive of the fact that this system has evolved by these chemicals literally bumping into each other for billions of years.
The information in the DNA is what contains the instructions for our body, correct? Does the DNA also contain instructions for how the cell works? Or is this a separately evolved system that works based on molecules bumping into each other? Can I conceive of the cell as almost a separate life form that exists inside of me?
Your question is more accurately a chemistry question than a biology question. More specifically physical chemistry. It is entirely a matter of molecules bumping into each other. Molecules have different types of surface area made up of different atom types --your carbon, oxygen and nitrogen mostly in biology. There are either favorable "sticky" interactions or unfavorable repulsive interactions with the surface types of other molecules.
Most intuitive example is how oil and water don't mix, that's unfavorable thermodynamics. But sugar and water do mix, that's favorable thermodynamics.
Evolution used these rules to build cells that function. If two molecules need to find each other to function, they evolved "sticky" parts that lock together.
This is remarkable to me. I don't know why this isn't talked about more. Every inch of us is composed of cells - yet the function of these cells (you could almost say the function of "you") is entirely a chemical process based on random interactions. The spindle emerging and grabbing hold of chromosomes and arranging them in the middle and splitting them into different sides - this intelligent looking process is fundamentally the same as pouring oil into water. Do I have that right?
Yes. The laws of physics are always working on the larger scale and the smaller scale. There's a lot of reasons why a car works, internal combustion engine, friction of the tires, blah blah blah, but ultimately it's fundamental physics.
Physical chemistry is the hardest undergraduate level class. There's a lot more to it than oil and water. But oil and water is the reason cell membranes exist.
The centrosome in an interphase cell uses its radiating microtubules to sense the peripheries of the cell and place itself roughly in the center. Like pushing rods out in every direction to end up in the middle. During mitosis, the centrosome divides and each new baby centrosome does something similar, except they're also pushing each other away. So they end up towards each side of the cell.
The microtubules projecting from each centrosome each bind a chromatid at the other end, at the kinetochore (a big protein complex that sits on a particular sequence of DNA). Microtubules can bind on each side, so both centrosomes get connected to one side of the pair of sister chromatids. They can then do the same pushing thing, and since they're equally good at pushing, the chromatids all end up roughly at the mid-line of the cell. Then it's just a matter of cell cycle regulation - once everything is ready, the anaphase-promoting complex is activated, and activates separase to cleave the proteins holding the sister chromatids together. Then each centrosome can pull its set of chromosomes to its side of the cell in preparation for cytokinesis.
This is a great description of what happens, but where are the instructions for all of this located? It's been explained to me that it's nothing but molecules bumping into each other but it sounds so complex it's hard to understand that.
The information is stored in DNA. Proteins are made based on the DNA sequence, and the shape of those proteins dictates their function. Like how tubulin makes microtubules and so on.
So the shape of the centromeres dictates they behave in this complex manner of arrangement? Is it like a child's toy where squares fit into squares and circles fit into circles? I still struggle to understand how this can happen with zero guidance from a brain. It's just the shape of the molecules? What you described they do is so complicated.
> Is it like a child's toy where squares fit into squares and circles fit into circles?
That's pretty much how it works for putting things together. When the proteins have to do more, they have to change shape. But that still depends on how they're put together, they're like little machines with like springs, levers etc.
That's like asking how does the carburetor know how much air and gas to admit and how to mix them or how a loom knows what threads go where. The carburetor and loom do what they do because that's what that machine does when it's running. Because the physics will be followed, they will not make smoothies and they will not sort coins. Inside the cell are little machines that evolved to do exactly what they do because of chemistry (which is really just physics) and it literally took millions of years and countless generations for them to evolve to this astounding level of complexity. The spindle apparatus is no more intelligent than an engine cam or a loom shuttle but it does what it does when it has the proper inputs, a whole lot of helper machines around it and a source of power.
The actual names of the little machines that guide the spindles are Aurora Kinase A and B and a couple other proteins. They are like jigsaw puzzle pieces that only fit in the specific places that they're supposed to fit.
Watch this video for a general overview of cellular machinery and please excuse the anthropomorphism, it's irresistible when you see some of these little buggers work.
If I was a creationist (I'm definitely not) this is definitely what I would cite as the strongest evidence of a creator. It truly boggles the mind that random interactions of molecules can lead to something like a cell which is way more than the sum of its parts - and it's driven by the same forces that pull the rock from your hand to the floor. Going into my biology education I thought the cell was just a house to protect fragile DNA. But the cell is really what you are... The DNA contains instructions to build the first cell of your life but after that the DNA is not involved in cellular division. It's just chemistry and ultimately physics. Is that right? It makes me think we have no free will and everything is just what happens when you have a big Bang and let billions of years happen.
I have read the last two comment threads and try to summarize them, but first there are few concepts which are essential to know.
Chance is even more important than you might think: If you are interested look up "Entropy, Ludwig Blotzmann, James Maxwell "
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Now to mitosis:
In the pro(meta)phase the centrosomes will "shoot" out thousands of microtubules and some of them will bump into the kinetochores of the chromosomes and attach. Now, in the metaphase, the chromosomes get pulled (there are motorproteins which can walk along the microtubules) along the microtubules and since they get pulled from both sides they will gather in the middle. In the anaphase proteins, which cut the linkage between the chromosomes, are released and the single chromosomes get pulled to the poles of the cell.
This wikipedia article https://en.wikipedia.org/wiki/Spindle_checkpoint gives nice overview of this.
Just remember that all this is regulated by many complex interactions in a cascade like manner (Condition X is met because the concentration of molecule A is high/low and thus more of protein XY is released which itself promotes different reactions)
Foxs-In-A-Trenchcoat t1_j3hiwqc wrote
"Weak" chemical interactions like dispersion forces. Chemical interactions control all molecular positions and functions in a cell. Behind this, it's all controlled by Gibb's free energy, enthalpy, and entropy.
These forces partition molecules to certain locations to control whether or not they function. Membraneless organelles are a fairly new discovery where people are seeing how important noncovalent interactions are.
Ultimately, every chemical reaction or noncovalent interaction that has ever happened from the beginning of time had to happen because it was thermodynamically favorable.