Clones They're Not
You may have read a recent news report stating that the Mars lander Phoenix had found a compound on Mars that might rule out the possibility of life on Mars. When I learned that the compound was a perchlorate I thought to myself, “Hey, I worked with a perchlorate in my lithium battery work at Bell Labs and I’m still around decades later!” Apparently influenced by such reports, NASA issued a news release on August 5 in which Michael Hecht, JPL’s lead scientist for the wet chemistry lab on Phoenix, is quoted as saying, "Finding perchlorates is neither good nor bad for life, but it does make us reassess how we think about life on Mars." Hecht also notes that the results are based on only two teaspoons of Martian soil. I imagine that we could sample two teaspoons of soil here on Earth in some locations and conclude our planet could not support life. For example, take a spoonful of Chile’s Atacama Desert and you’ll find perchlorate.
If you watched the opening ceremony for the Beijing Olympics you may have seen some perchlorates doing their thing in the fireworks displays. In my lithium battery work, lithium perchlorate was the salt used in the electrolyte of the batteries. Actually, NASA wants to confirm the presence of perchlorate using their Thermal and Evolved-Gas Analyzer (TEGA), which heats the soil sample and analyzes the gases driven off. So far, they’ve found oxygen but no chlorine. (Perchlorates have four atoms of oxygen for each atom of chlorine.) The oxygen could have come from perchlorate and the absence of chlorine doesn’t rule out perchlorate. I assume that the chlorine could still be tied up in a compound remaining after evolution of some or all of the oxygen. I’m sure NASA will have more to say about perchlorates.
Now that we’ve established that life is neither ruled in or out on Mars, let’s take a look at cloning, which relates to life here on Earth. We’ve cloned various animals, the late Dolly being the first, but cloning of human beings remains strictly out of bounds for ethical, religious and scientific reasons. When it comes to humans, it has been pointed out that cloning of humans takes place quite naturally in the case of the birth of identical twins. Identical twins are formed when a fertilized egg splits in two and the resulting two eggs go on to develop into fetuses that end up being identical twins. Because the twins derived from the same egg it has been assumed that they are indeed identical, with the same DNA.
Over the years, I’ve read of all kinds of studies involving sets of identical twins, including twins that had been separated early in life. Some of the findings indicate remarkable similarities in the lives of such twins even in old age. On the other hand, other studies show marked differences in susceptibilities to various diseases such as Parkinson’s or diabetes. With recent advances in decoding of DNA, it was bound to happen that someone would ask the question as to whether or not identical twins do indeed share identical genomes with the same genes and the rest of their DNA. The answer, as I found in an article by Charles Choi in the May 2008 issue of Scientific American, is a surprising “No”. The article, titled “Copy That”, deals with the work of geneticists Jan Dumanski and Carl Bruder of the University of Alabama at Birmingham and colleagues from Sweden and The Netherlands published in the March issue of American Journal of Human Genetics.
We’ve talked before about DNA and the AGCT “letters” (actually bases) in the DNA code. Some diseases have been found to be associated with mutations resulting in a change in a single letter of the DNA. These are known in the trade as single- nucleotide polymorphisms, or SNPs (pronounced “snips”). On the other hand, some differences in DNA involve missing or extra copies of genes, known as “copy number variation”, or (CNV). On the University of Alabama Web site, I found that CNV can also relate to not just complete genes but also to smaller segments of DNA.
The researchers looked at 19 sets of identical twins. In nine of the pairs, one twin had Parkinson’s or a similar neurological problem. Sure enough, in each set of twins one twin had either missing genes or extra copies of genes compared to the sibling. This suggests that perhaps the missing or extra genes are associated with the disease. However, at this early stage, I gather that no particular missing gene or CNV can be pinned down as culprit in susceptibility to neurological diseases.
The real surprise came when the researchers looked at 10 pairs of identical twins with no obvious differences between them. In one of these pairs, one twin was missing a whole section of a chromosome rich with genes while the other twin had those genes. Not onl that, but preliminary work suggests that eight others of these 10 sets of twins had copy number variations within the sets.
But that wasn’t all. When the researchers looked at blood cells from the twin that was missing that whole section of one chromosome, they found another startling thing. I don’t know about you, but I’ve been under the impression that every cell in my body has the same DNA. Well, in this case, the twin missing the section of a chromosome only showed this deletion in about three fourths of the blood cells sampled. This finding indicates that this genetic loss happened sometime later in the twin’s life (his age wasn’t given). So much for the assumption that all our cells must have the same DNA!
The study shows that our DNA can change appreciably during our lives and that our genome is not the stable, unchanging entity that was once thought. Perhaps, in retrospect, I shouldn’t be surprised. After all, especially we senior citizens have been subject to all manner of X-rays of various types, not to mention all the environmental pollutants and additives in our foods. What seems surprising to the authors of the twin study is the scope of genetic change that takes place, even in identical twins.
Finally, old Bortrum just can’t resist sharing with those golfers out there his score of 31 in our Old Guard golf league. OK, it’s just a 9-hole par 3 course but I did par five out of the six longest holes ranging from 100 to 159 yards. Hey, when you’re 80, you’re lucky to be able to walk the course, let alone shoot just 4 over par!
Allen F. Bortrum