A clone is a genetically identical copy of a living organism. Human clones are nothing new; they occur naturally in the recognizable form of identical twins. Not until the successful cloning of a sheep called Dolly, however, has the possibility of intentionally producing an identical copy of a human been considered seriously.
Production of a human clone would most likely utilize the same basic methodology that resulted in Dolly. A human egg would be retrieved from an individual, and its genetic material (DNA) would be removed and replaced with DNA derived from any adult human cell type. This would bypass the need for fertilization of the egg by the sperm in order to obtain a full complement of DNA. In a laboratory dish, the egg would then undergo several cell divisions. Placed into a uterus, the resulting embryo would grow and, with luck, develop until birth.
Although this cloning technique is conceptually and procedurally simple, its success rate has been extremely low. The birth of one Dolly, for instance, required the preparation of 277 re-nucleated eggs, followed by the implantation of 29 resulting best embryos. The low success rate can be attributed to the difference between the young DNA of a normally fertilized egg and the genetic material of the re-nucleated egg, which is mature and of defined destiny – it has already committed itself to a particular physiological role. In order for proper fetal development to occur, mature DNA must be coaxed into reverting to its youthful state, a complex process that will be difficult to achieve for the human species.

According to the passage, which of the following is a potential obstacle to human cloning?

A: Lack of adequately prepared human eggs
B: Predetermination of DNA function within the adult body
C: Shortage of funding for additional research
D: Inability to pinpoint specific biological relationships within an egg
E: Unpredictability of chemical reactions within the uterus

OA: B
I believe that answer lies in
"The low success rate can be attributed to the difference between the young DNA of a normally fertilized egg and the genetic material of the re-nucleated egg, which is mature and of defined destiny – it has already committed itself to a particular physiological role"

BUT I failed to de-code why that answer is.

After reading the question I went back and skimmed the passage passage, looking for something that would be much tougher in human cloning than in animal cloning. We finally see the reason in the last sentence of the passage:

"In order for proper fetal development to occur, mature DNA must be coaxed into reverting to its youthful state, a complex process that will be difficult to achieve for the human species.

So the right answer MUST have something to do with the difficulty of converting the DNA back to its youthful state. I look at the answer choices and B is the only one that has to do with the DNA. Bingo! B does in fact sum up the problem--DNA is predetermined to move forward to an "adult" state and thus does not remain in the youthful state needed for cloning.

_________________
Jamie Nelson
ManhattanGMAT Instructor