WHAT ARE THE RISKS OF CLONING?
When we hear of cloning successes, we learn about only the few attempts that worked. What we don't see are the many, many cloning experiments that failed! And even in the successful clones, problems tend to arise later, during the animal's development to adulthood.
Cloning animals shows us what might happen if we try to clone humans. What have these animals taught us about the risks of cloning?
1. High failure rate
Cloning animals through somatic cell nuclear transfer is simply inefficient. The success rate ranges from 0.1 percent to 3 percent, which means that for every 1000 tries, only one to 30 clones are made. Or you can look at it as 970 to 999 failures in 1000 tries. That's a lot of effort with only a speck of a return!
Why is this? Here are some reasons:
2. Problems during later development
Cloned animals that do survive tend to be much bigger at birth than their natural counterparts. Scientists call this "Large Offspring Syndrome" (LOS). Clones with LOS have abnormally large organs. This can lead to breathing, blood flow and other problems.
Because LOS doesn't always occur, scientists cannot reliably predict whether it will happen in any given clone. Also, some clones without LOS have developed kidney or brain malformations and impaired immune systems, which can cause problems later in life.
3. Abnormal gene expression patterns
Are the surviving clones really clones? The clones look like the originals, and their DNA sequences are identical. But will the clone express the right genes at the right time?
In Click and Clone, we saw that one challenge is to re-program the transferred nucleus to behave as though it belongs in a very early embryonic cell. This mimics natural development, which starts when a sperm fertilizes an egg.
In a naturally-created embryo, the DNA is programmed to express a certain set of genes. Later on, as the embryonic cells begin to differentiate, the program changes. For every type of differentiated cell - skin, blood, bone or nerve, for example - this program is different.
In cloning, the transferred nucleus doesn't have the same program as a natural embryo. It is up to the scientist to reprogram the nucleus, like teaching an old dog new tricks. Complete reprogramming is needed for normal or near-normal development. Incomplete programming will cause the embryo to develop abnormally or fail.
4. Telomeric differences
As cells divide, their chromosomes get shorter. This is because the DNA sequences at both ends of a chromosome, called telomeres, shrink in length every time the DNA is copied. The older the animal is, the shorter its telomeres will be, because the cells have divided many, many times. This is a natural part of aging.
So, what happens to the clone if its transferred nucleus is already pretty old? Will the shortened telomeres affect its development or lifespan?
When scientists looked at the telomere lengths of cloned animals, they found no clear answers. Chromosomes from cloned cattle or mice had longer telomeres than normal. These cells showed other signs of youth and seemed to have an extended lifespan compared with cells from a naturally conceived cow. On the other hand, Dolly the sheep's chromosomes had shorter telomere lengths than normal. This means that Dolly's cells were aging faster than the cells from a normal sheep.
To date, scientists aren't sure why cloned animals show differences in telomere length.
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Need a refresher?
See the Tour of the Basics for an introduction to DNA, chromosomes and cells.