It’s easy. All you need to know about molecular biology is that it’s all about DNA. And you don’t need to know what that stands for, or anything else about DNA except this. It’s a really long molecule, like a chain with millions of links. Actually make that a double chain. And there are 4 different kinds of links that go by 4 different letters. Those letters are A, C, G, and T, and it doesn’t matter where they come from either. And now here is the secret to all of molecular biology: the A link on one chain always pairs up with a T link on the other chain. Always. You will not find an A opposite another A, or a C or G. Only T. Got that? You can remember it if you just think of the most important symbol these days: @. Which of course stands for “at” or in Molecular Biologese AT.
Now you might ask if A and T are always so bound up with each other, what about C and G? What do you think they are doing? Take a guess. YES, they have a thing going also, what else would they be doing? So, since A and T are always matched, so are C and G. And that, boy and girls, is all there is to molecular biology. Let’s review:
1.The A on one strand (chain) of DNA always and only bonds to a T on the other strand.
2 The C on one strand of DNA always and only bonds to a G on the other strand.
That’s it. The rest is details. Once you know the two statements above, its easy to understand how DNA replicates, what a gene is and how it works, how genes make proteins, what a mutation is, how evolution works, and why identical twins are clones, but everybody else is genetically unique. Pretty simple, eh?
Of course, we know where the Devil lives, right? Right. Those pesky details. And there are lots of them. The peskiest of all the details in biology, the thing that drives biology grad students insane, is this: biology (and that includes molecular biology) is full of lies.
That isn’t a value judgement, or an expression of hostility – it’s the truth. Almost every statement in biology is a lie. Or more precisely, it isn’t true. This is not the case in physics or chemistry. We can say that force is equal to mass times acceleration and that is true all the time, everywhere. The volume of a gas depends on its volume and temperature. Fact. No wiggle room.
In biology there are no such statements. Nothing you can say in biology is always true. Not even what I just told you about A and T always pairing up is true. Sometime A will form a bond with a G or a C, or even (gasp) another A. Yes, it happens, and so when I said always and only, I lied. I am after all a biologist, and we all lie all the time. (Actually, that isn’t true either.)
So how could I dare to say something that isn’t true? How can I say that A only bonds with T, when in fact sometimes it doesn’t? Shameful, I admit. So let’s rephrase. Under the vast majority of normal circumstances, A does only pair with T. But sometimes circumstances in biology are not normal. And mistakes are made. When that happens, and an A matches up with a G, or a T finds itself opposite a C, we have something called a mutation. In fact that is the definition of one kind of mutation. (In biology there are always several different kinds of everything).
Are mutations good or bad? Ha ha, what a question. Clearly if you asked that, you are not a biologist. Nothing is good or bad in biology = everything depends on something else. Mutations can be very bad; they can lead to diseases like cancer or genetic diseases like cystic fibrosis or to increased susceptibility to a host of diseases. A mutation can make you stupider or shorter or uglier than you would otherwise be. Of course, it could also make you smarter, taller or prettier. Mutations are also essential for evolution – if there were no mutations we would still be very primitive bacteria and wouldn’t be having this conversation.
Speaking of evolution, some people sometimes ask why evolution is still a theory and not a law. A law is a statement of a scientific truth in mathematical terms. Laws can have exceptions, sometimes, but laws don’t do well in biology (there are a total of maybe three biological laws), because the number of exceptions to anything we might say about how biology works are too many to be dealt with. Too many lies, in other words. So, while a lot of people have come up with a lot (thousands, actually) of mathematical laws of evolution, none of these are all-encompassing, because none of them fit all of the various and diverse situations that apply to evolution.
Getting back to molecular biology, let’s take a look at how the basic fact of AT and GC base pairing (I might not have mentioned this. but those links named A, C, G, and T are actually called “bases”) works. When a cell is ready to divide into two cells, the DNA double strand (which is wound around into a helix) separates into two individual strands. And then each A on one strand attracts a T, each C attracts a G, and a new strand is built up with all the correct matching bases. Since this happens to both of the original strands, what we end up with is two double strands, each of which is identical to the original double strand. Pretty neat, eh?
When James Watson and Francis Crick solved the puzzle of the DNA structure in 1953, they realized right away that the obligatory base pairing and the double helix solved the biochemical mystery of how DNA replicates to form two perfect copies of itself. Thus the basis of inheritance was discovered.
There is more. (A lot more, actually.) The genetic code, which is the way the genes (DNA) make all the characteristics of the cell, called the phenotype (the proteins), also relies on base pairing. But that is a very long and complex story that we should save for later. For now, just practice saying AT… AT… AT. And maybe also practice a little bit of lying, if it doesn’t already come naturally. And you will be on your way to becoming a real molecular biologist.