The fourth post below this one compared cellular evolution to corporate mergers. However, the devil’s always in the details, isn’t it? So let’s try to get to the semi nitty gritty here. You can also think of cellular evolution as a computer upgrade where chips (endosymbionts) are plugged into a computer motherboard or mainframe (cell nucleus). How advanced the upgrade is depends upon how sophisicated or complex the chip is. In other words, the chip itself may have been upgraded prior to insertion into the motherboard. Computers are basically nothing more than transistors, simple on/off switches. How smart they become depends upon how many of these things get installed. It’s the same thing with cells. The more complex the endosymbiont that gets installed, the better the upgrade. So the next question should be straight forward: What is the mechanism of installation? Well, we might want to revisit earlier posts to assist in explaining how this might occur: Hiding in Plain Site. In lieu of this, however, let me provide a stand alone explanation: In the computer world, a bit is simply a transistor. It takes eight of these to produce a byte which is the smallest piece of information that a computer can use to generate text. In the case of cells, a bit is a single DNA nucleotide like A, T, G, and C. I won’t bother you with chemical details here. It takes three of these nucleotides to generate the smallest piece of information that a cell can use to determine how to translate the DNA code into usable “text”. Let’s just let it go at that before we fall into a molecular biological black rabbit hole from which there will be little chance of escape.
Ok, so now what? Well, before we can plug anything in we need to generate a port, and not all ports are created equal and for good reason. Plug the wrong thing in and you can put a hex on the whole dang thing (exploited from Will Smith, Independence Day.) Put another way, you could potentially corrupt the whole computer. In the case of cells, you could potentially cause cancer. So what kind of “ports” do we find on nuclear DNA? Again, let’s avoid as many pitfalls as possible lest we fall into that rabbit hole. As you probably know, DNA is double stranded. It manages to do this because those magical letters, A, T, G, and C are capable of pairing up with each other, but only in one way: A only pairs with T, and G only pairs with C. So if one DNA strand has the following sequence: AGGTA it will only pair up with a strand having the sequence TCCAT. So this is kind of like dovetail joints in woodworking. The more of them you have, the stronger the bonding of the two strands. However, these joints are flexible and reversible. In other words, they can “breathe”. Therefore, the more of them there are, the less likely they are to come apart because not all of them breathe at the same time. Under normal conditions, these kinds of “ports” require roughly 100 base pairs in order to provide a semi stable connection between nuclear DNA and endosymbiotic DNA. Because of the size of chromosomes during cell division, these connections are quite delicate and easily disrupted by physical movement. Therefore, reinforcements are required beyond simple base pairing. One form of reinforcement is proteins like histones which bind up the DNA and protect it from physical disruptions. However, during DNA replication and RNA production (don’t ask), a portion of the DNA must be uncovered for these processes to occur. Under these conditions it must be easy for DNA to separate into single strands and come back together again once the process is over. However, if connections between endosymbiotic and nuclear DNA only rely on base pairing, there is a great risk that the endosymbiotic DNA will separate and become lost. This can be avoided by coupling the two DNA compartments together using covalent chemical bonds (again, don’t ask) which are much stronger than base pairing bonds. Now the DNA has become branched. The branch that is created is the very crux of this blog. This is where switching can occur, much like on a railroad track. Take a simple circular virus, for example. It base pairs with a port on the nuclear DNA, but instead of fusing with it, it forms a four strand switch that is linked together by covalent bonds. Whew! This is getting heavy. Time for a picture! This switch is binary, just like a transistor. The circle can either fuse with the main DNA or it can be released and lost based upon which covalent bonds are disrupted on the switch. Here, have a pic.
Ok, don’t freak out, I’ll explain, ok? The two rectangular shapes labeled 1 (see A) are complementary “circles” of double stranded DNA. The coiling has been removed for clarity. The rungs in these ladders represent base pairing between the two strands. Remember what I just said about based pairing? As the rungs open in Fig. B, some of them begin to base pair with the strands beneath, forming a four stranded structure (Fig. C). This is the “railroad”switch I was talking about. When it is in one position, the strands fuse together as in Fig D and E. Note that if the structure in Fig. D occurs, information has been lost within the switch which makes this irreversible. However, no information has been lost in the structure in Fig. E. It can either fuse the two circles together or it can reverse the process and release both of them.
This picture is also the crux of this blog because it explains how this kind of a switch can be used to add complicated endosymbiotic DNA to the cell nucleus using a simple port. However, such a port is also a two-edged sword. If the wrong DNA gets incorporated into it; this could corrupt the cell machinery, eventually leading to cancer. Apoptosis is one way to prevent this from happening.
Next Post. Thing 1 and Thing 2.