Homeobox Genes & Macroevolution

bob b

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HOMEOBOX GENES AND MACROEVOLUTION
(see Molecular Biology of the Cell, chapter 21)

Humans, worms and flies don't look very similar and they do not go through the same
developmental stages. Yet the genes that control their body shape and organization are
related in sequence. These genes all share a common sequence called the homeobox. This 180 nucleotide sequence codes for 60 amino acids found in these proteins. The rest of the proteins may be very different, but this 60 amino acid piece is crucial for their function.

The homeodomain is a helix turn helix DNA binding domain that recognizes a specific
DNA sequence. The homeodomain targets the remainder of the protein to regulate the gene expression of any genes with the appropriate recognition sequence in their control regions.

There are at least 50 homeobox genes in Drosophila. They fall into two main divisions, the complex superclass and the dispersed superclass. Those in the complex group are found in clusters, the dispersed group are solo genes.

One subset of these genes are called homeotic selector genes. In Drosophila, there are 8
genes arranged in a series along 650,000 base pairs of DNA. This whole region is called
the HOM complex. There are two smaller subsets of these genes in the HOM complex,
the antennapedia complex (5 genes) and the bithorax complex (3 genes). Other insects
have these genes all in one complex, so it looks as though the HOM complex became split in Drosophila.

Mutations in the 8 genes of the HOM complex cause large scale mutations in flies. A
mutation in bithorax causes a fly to have an extra set of wings. Mutation in antennapedia
causes a leg to grow where an antenna should be. These genes are not master switches for making wings or legs, but they specify position in the fly's body. The order of the genes on the chromosome is the same as the order of segments in the fly's body where they are expressed. The left most gene is expressed in the head, the right most gene is expressed in the abdomen. When a gene is deleted or mutated, the segment where it is normally expressed cannot tell where it is because its position clue is gone, so it behaves like the closest segment to it. That is why a bithorax mutation causes an extra set of wings. The segments adjacent to the bithorax segment dictated what should be made.

An amazing fact is that these HOM genes have clear homologs in vertebrates. These are
called hox gene clusters. Mice have four hox gene clusters on four different chromosomes.

These are called HoxA ,B, C and D. HoxB has all the same genes as HOM plus one more.

They are in exactly the same order. The other three segments are missing some of the
HOM genes, but they have some extra homeobox genes not in the HOM cluster.

And this final speculation:

"The HOM cluster seems to have arisen by gene duplication of a single homeobox gene long ago. This cluster then was duplicated in total four times in the lineage of vertebrates . Some additional gene duplication and deletion resulted in the present day set of Hox genes in mammals.".
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On the Derivation of Ulysses from Don Quixote (by David Berlinski)

I IMAGINE THIS story being told to me by Jorge Luis Borges one evening in a Buenos Aires cafe.

His voice dry and infinitely ironic, the aging, nearly blind literary master observes that "the Ulysses," mistakenly attributed to the Irishman James Joyce, is in fact derived from "the Quixote."

I raise my eyebrows.

Borges pauses to sip discreetly at the bitter coffee our waiter has placed in front of him, guiding his hands to the saucer.

"The details of the remarkable series of events in question may be found at the University of Leiden," he says. "They were conveyed to me by the Freemason Alejandro Ferri in Montevideo."

Borges wipes his thin lips with a linen handkerchief that he has withdrawn from his breast pocket.

"As you know," he continues, "the original handwritten text of the Quixote was given to an order of French Cistercians in the autumn of 1576."

I hold up my hand to signify to our waiter that no further service is needed.

"Curiously enough, for none of the brothers could read Spanish, the Order was charged by the Papal Nuncio, Hoyo dos Monterrey (a man of great refinement and implacable will), with the responsibility for copying the Quixote, the printing press having then gained no currency in the wilderness of what is now known as the department of Auvergne. Unable to speak or read Spanish, a language they not unreasonably detested, the brothers copied the Quixote over and over again, re-creating the text but, of course, compromising it as well, and so inadvertently discovering the true nature of authorship. Thus they created Fernando Lor's Los Hombres d'Estado in 1585 by means of a singular series of copying errors, and then in 1654 Juan Luis Samorza's remarkable epistolary novel Por Favor by the same means, and then in 1685, the errors having accumulated sufficiently to change Spanish into French, Moliere's Le Bourgeois Gentilhomme, their copying continuous and indefatigable, the work handed down from generation to generation as a sacred but secret trust, so that in time the brothers of the monastery, known only to members of the Bourbon house and, rumor has it, the Englishman and psychic Conan Doyle, copied into creation Stendhal's The Red and the Black and Flaubert's Madame Bovary, and then as a result of a particularly significant series of errors, in which French changed into Russian, Tolstoy's The Death of Ivan Ilyich and Anna Karenina. Late in the last decade of the 19th century there suddenly emerged, in English, Oscar Wilde's The Importance of Being Earnest, and then the brothers, their numbers reduced by an infectious disease of mysterious origin, finally copied the Ulysses into creation in 1902, the manuscript lying neglected for almost thirteen years and then mysteriously making its way to Paris in 1915, just months before the British attack on the Somme, a circumstance whose significance remains to be determined."

I sit there, amazed at what Borges has recounted. "Is it your understanding, then," I ask, "that every novel in the West was created in this way?"

"Of course," replies Borges imperturbably. Then he adds: "Although every novel is derived directly from another novel, there is really only one novel, the Quixote."
 

spaz

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Bob you are seriously daft.

They use nonlinear fomulas for statistical predictions based on what they do observe. It is not some speculative statements but based in complex math.
 

bob b

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spaz said:
Bob you are seriously daft.

They use nonlinear fomulas for statistical predictions based on what they do observe. It is not some speculative statements but based in complex math.

Garbage in garbage out.
 

bob b

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Johnny said:
What shall we infer by your silence?

I wasn't silent. I gave my opinion about the "handwaving".

BTW, what did you think of Berlinski's theory regarding the source of all novels? :chuckle:
 
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