Six Cloned Calves at their First Birthday Party - "Moo-oo-oo!
It has been reported that Dolly, the cloned Scottish sheep, was born old. Her cells show signs of progeriapremature aging. This raised the obvious question: does cloning older adults generate aged offspring? As a result of studies reported today in Science , the answer appeaars to be a resounding "No!". On the contrary, six calves, created by cloning a decrepit cow, show in every way a youthfulness not even present in other calves the same age.
"Cells also betray their age through the wear and tear on their telomeres, the regions at the tips of their chromosomes. Telomeres cap off the ends of each chromosome and keep their genetic threads from fraying and disappearing with each tug that happens when the cell divides itself again. Since most mammalian telomeres can't repair themselves, they are usually slowly worn away over time--shorter telomeres are often found in older cells. Scientists spotted this telltale sign of maturity in Dolly. Her chromosomes were shorter than those of normal sheep of the same age, suggesting that she had inherited the "age" of her genetic mother and was old before her time.
"By contrast, chromosomes from the cow sisters are the picture of youth. Telomeres from the clones are actually longer than those from normal cows of the same age, and in most cases even longer than those from newborn calves. Far from being prematurely aged, cells from the cow clones appear to have recaptured and even prolonged their youth, lengthening their lifespan beyondthat expected for their chronological age.
"To create the cow clones, the researchers used cells that were near the end of this lifespan, with only a few bouts of cell division left. Surprisingly, Lanza and the others discovered that the cloning process seemed to restore the "nine lives" of these cells in the six cows. Instead of being zero to four division cycles away from the end of their lives, cells taken from the cows were more than 90 cycles away from their end.
"Why are Dolly and the cow clones so different in this respect? The Science authors suggest a number of reasons, including differences in cloning techniques. Rather than creating clones from cells at the end of their lifespan, as was the case with the cows, Dolly's creators used cells that had been starved and sent into a resting state. Differences in the original donor cells used--mammary cells for Dolly and fibroblast (connective tissue) cells for the cows--may also play a role.
"'Previous studies have indicated that there may be variation in how different cell types repair telomeres, which could make the choice of donor cell significant,' says Lanza."
What is highly significant about this is that it suggests that there may be some way to totally arrest aging, and to restore previously-old cells to vibrant youth. The cells of the body gradually age, dividing about 90 times before the reach the Hayflick Limit at which they are no longer capable of dividing again. As they age, unrepaired genetic errors gradually accumulate, and wrinkles, gray hair, eyeglasses, and liver spots become the visible manifestations of these genetic mishaps. Given this, have you ever wondered why babies are born young rather than at the ages of their parents? After all, they are formed from 20-, 30-, or 40-year-old sex cells donated by their parents.(Granted that the sex cells themselves may be fresh, the tissue that created the sex cells would nevertheless have undergone many divisions, and would be expected to be 20, 30, or 40 years down the road toward its Hayflick Limit.) I had thought that, perhaps, sex cellsgametesmight somehow be insulated from the aging that occurs when somatic cells divide. But cloned animals, like the calves above, are gestated from somatic cells rather than gametes. And we know that their parental somatic cells are aged. (Even more significant than the fact that infants are born brand-new is the fact that organisms' genomes are quite resistant to mutation. If they weren't, the genetic identities of species would be lost through mutation after a few generations. What is it that keeps this germ plasm pure generation after generation?) Apparently, something happens that rejuvenates the genomes of fetal cells between the times that gametes are created and the time when an embryo appears. Are the two daughter cells formed by the mitosis of a zygote already rejuvenated? When in the fertilization and gestation cycle does this rejuvenation occur? And would it be possible to create the chemical conditions that effect this rejuvenation within the body as a whole, so that somatic cells are rejuvenated?
It's worth noting that cancer cells are immortal, although they mutate rapidly, with drug-resistant and radiation-resistant mutations occurring about once in every 100,000 cell divisions. These mutations then breed true, except for the 1 in the 100,000th cell that mutates again. It's also worth noting that cancer cells become badly damaged, often dropping back to fewer than the normal complement of human chromosomes.
Unicellular organisms such as amoebae and paramecia also seem to be immortal.
DNA (Desoxy-ribose-Nucleic Acid) is designed in the form of a double helix so that if one strand of the DNA is damaged, the undamaged codon on the other strand of the helix can be used as a template to repair the damaged codon. (If two codons are different rather than identical at any point along a gene, it's obvious that the DNA has been changed at that point.) This works as long as both strands haven't been altered. However, there are situations in which, allegedly, both strands of the DNA are modified. For example, x-rays affect only one of the two complementary codons, so that about ¾ths of the damage caused by x-rays can be repaired. However, neutrons allegedly damage a DNA locus on both its strands, and the damage is unrepairable. And yet, in spite of this, the genetic blueprint of these calves was somehow restored to a pristine state. And this is true in general when an organism reproduces. If both sides of a DNA locus have been damaged, how does the cell know what it should be? Of course, damage to a codon will almost surely lead to a molecule that isn't quite one of the four nucleic-acid molecules that are permissible as valid codons. It might be possible to restore the damaged molecule to the nucleic acid molecule whose chemical structure is closest to that of the damaged molecule.
The only alternative that strikes my mind is that of an interaction with the genome of another cell in which, presumably, the DNA isn't damaged at the same locus.
Additional information may be found at ScienceDaily, and at Advanced Cell Technology, Inc.
The six cloned cows are shown in the photograph below. Thus far, they have refused to comment regarding the controversy swirling around their youthful existence, except as noted below the photograph.