I have been ploughing through Genes vii, the standard undergraduate text on modern genetics. For the first time in my life, I understand the appeal of Michael Behe’s arguments: everything in biology is unbelievably complicated when you look at it up close.
Behe derives from this the idea that God must be responsible. I just doubt that anything as complicated as a worm, let alone as complicated as I am, actually exists.
This morning I came across a new problem. There are three things that everyone ought to know about genetics: that DNA replicates by base-pairing, which can match two different strands exactly; that genes are exchanged between chromosomes at mitosis; and that different genes have different base sequences. All this is clearly laid out in the textbook, along with a great many illustrations which make it schematically exact.
But it seemed to me, thinking about that stuff this morning, that genes could never be properly exchanged, since the base-pairing rules would mean that different alleles could not match up with each other’s complementary strands when they swapped chromosomes.
What happens in crossover is that two double strands of DNA lie next to each other: call them AB and ab. A and B spiral round each other; so do a and b. Then the two strands come partially come unwound, and rewind so that portions of a are wrapped up with B and portions of b with A. The problem that occurred to me was that the bits that are swapped could not pair up properly with the recipient strands unless they were identical with the bits they were replacing. But if they are identical, then there can be no exchange of genes, or at least the exchange would make no difference.
It turrns out that what happens is that the recipient chromosome is actually broken in both strands, so when the new, differing bit is wound on, it is used as a template to make the replacement section of the complementary strand. The original is presumably chewed up by a passing enzyme. So there’s an exchange of pattern but not an exchange of molecules.