Monday, October 28, 2013

Chromosome fusion

Comparison of human chromosome 2
with chimp chromosomes 2A and 2B.
Image source.
Genetics is one of the most rapidly developing sciences of our day.  Though it began with the work of Gregor Mendel in the 19th century, the field of genetics did not explode until the past few decades, when we began sequencing the genomes of plants, animals, and humans.  Geneticists have not yet explored all of the vast amounts of information contained in the DNA of life, and much is left to be discovered.  As a result, I consider genetics to be the front lines of the creation-evolution debate.

When discussing human evolution in the classroom, one of the most frequently cited lines of evidence is human chromosome number 2.  Chromosomes can be thought of as large packages of DNA and proteins within the nucleus of our cells that help keep everything organized, akin to how a filing cabinet keeps information organized in an office.  Every human, excluding rare and often dangerous mutations, has two pairs of 23 chromosomes, or 46 total.  Chimpanzees, thought to be our closest living relatives, have 24 pairs, or 48 total.  Chromosomes contain quite a bit of information, and the addition or loss of a chromosome typically has drastic consequences, such as Turner Syndrome, Down Syndrome, or death.  A zygote that lacks a pair of chromosomes likely won't develop at all.  Therefore, it is difficult to explain how the ancestors of humans could have lost a pair of chromosomes and yet survived.  The answer, evolutionists say, is in chromosome 2.  When you line up human chromosome 2 with two particular chimpanzee chromosomes, it appears as if the chimp chromosomes simply stuck together to form the human chromosome, which evolutionary geneticists say is exactly what happened, so the chimpanzee chromosomes were named "2A" and "2B," accordingly.  For a detailed explanation of the evolutionary view, and to read some of the banter between an evolutionist and a creationist, see Carl Zimmer's series of articles on the subject.

Aside from lining the chromosomes up side-by-side, several other evidences have been presented to demonstrate that this chromosome fusion event occurred.  Geneticists claim that the remnants of the centromere of chromosome 2B can be seen at the corresponding location in the human chromosome.  Each chromosome has one centromere, the "pinched" spot where the chromosome is handled during cell division, so it would indeed be odd for a chromosome to have two.  However, since only one is needed, evolutionists say that one centromere acquired enough mutations that it no longer functions, leaving the one now seen in human chromosome 2.  Jeffrey Tomkins and Jerry Bergman of the Institute for Creation Research, on the other hand, claim that the supposed former centromere site does not resemble other active centromeres, but more closely matches other non-centromere "alphoid sequences" found elsewhere in the genome.  The evolutionary view, meanwhile, would likely attribute this to extensive mutation.

The actual fusion site itself is said to resemble a short telomere.  Telomeres are long repetitive sequences at the ends of chromosomes that protect the important genes from being cut off.  The presence of a telomere-like sequence in the middle of a chromosome is therefore considered evidence that it was once two distinct chromosomes.  It should be noted that the sequence does not exactly resemble a telomere, and both evolutionists and creationists recognize this.  If a telomere were to find itself in the middle of a chromosome, it would serve no function and would therefore quickly acquire mutations.  Some creationist authors claim that there are too many mutations to account for in the 6 million years since the supposed split of humans and chimpanzees, but this claim does not appear to be well substantiated, as noted in Carl Zimmer's series of articles referenced earlier.

Thus, the ball was in the creationists' court until a couple weeks ago, when Jeffrey Tomkins published a new study on the alleged fusion site.  In his paper, Tomkins demonstrates that the hypothesized point of fusion actually lies within an active gene sequence which runs in the opposite direction.  See, DNA is composed of two strands, which unite to form what looks like a ladder.  When the DNA needs to be read, it is split in half, and one or the other half is read by proteins, but the halves are read in opposite directions, sometimes called the "positive" and "negative" directions.  When evolutionists discuss the telomere sequence at the fusion site, they typically describe it on the positive strand.  However, Tomkins shows that the gene at that site is read on the negative strand, so that it doesn't "look" like a telomere to the proteins.  This has apparently been recognized by other authors, but it was labelled a "pseudogene" and ignored, because it was thought to be useless.  Tomkins' study now shows that the gene is not inactive, but plays a role in RNA manipulation.  If that is the case, it should have a devastating effect on the theory of chromosome fusion, because an active site cannot build up mutations without becoming inactive.  If the site did not acquire mutations, then it apparently was never a telomere, leaving the chromosome fusion theory very little to go on.  Moreover, Tomkins found that the site is used as a binding site for proteins that copy the DNA strand, further emphasizing its usefulness.

If there is a weakness in Tomkins' paper, it is likely the lack of an exact role of the gene in question.  Evolutionists will likely try to demonstrate that it is, in fact, a pseudogene and plays no active role.  Even so, the fact that the gene extends across the fusion site will be difficult for them to deal with.  I suspect that this part of the debate is far from over.

1 comment:

  1. I think you may have to look into this issue a little deeper. My understanding of Tomkin's work is that it was shoddy and he purposefuly left out or misrepresented information in the studies. His work as been seen as completely bogus and dishonets. Nothing from Tomkins should be trusted.