Why are some animals harder to clone than others?

Cloning Pictures Cloning isn't a quick process that spits out duplicates like a copy machine. See more cloning pictures.
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­R­eproductive cloning, in which you create an exact ge­netic duplicate of an organism, isn't like photocopying pages in a book. The cloning process doesn't simply spit out replicas ad nauseum. Nevertheless, that concept seems to pervade the public's perception of the technology, fueling fears of clone armies and herds of cloned super-animals in the future.

Currently, there have been 15 documented species successfully cloned: cat, dog, horse, mule, sheep, guar, bull, mouse, goat, pig, horse, cow, deer, banteng and rabbit. Each time scientists successfully clone a new animal species, p­eople often recoil, viewing it as another step closer to human cloning. But worries about generating cloned people like snacks out of a vending machine are largely unfounded since, for one thing, many countries ban or strictly regulate research on human reproductive cloning. In fact, the United Nations attempted to pass a global ban on human reproductive cloning in 2005. Secondly, cloning is often a tedious process with a success rate less than 5 percent. Remember Dolly the sheep? That revolutionary animal clone followed 277 unsuccessful attempts.

­Cloning proponents argue that this science offers invaluable knowledge about how the human body works and the potential of extracting stem cells from cloned embryos. Yet after more than a decade following the cloning of Dolly, the odds of success have barely improved. This method of reproductive cloning has remained largely unchanged, and many scientists have acknowledged that cloning for reproductive purposes is rarely a practical venture [source: Kiem].

On top of that, animals' reproductive patterns can compound the challenges of cloning technology. Case in point: hard-to-clone animals like dogs. Who knew that man's best friend would also be one of the most difficult mammals to clone? The first cloned dog, an Afghan hound named Snuppy, took 1,095 failed embryos to get a winner. Researchers of the Snuppy project, at Seoul National University, used 123 surrogate dogs before one could carry a cloned puppy embryo to term in 2005 [source: Gorner].

­Scientists have tried for years to clone chickens and monkeys as well, with little progress. These cloning hurdles highlight the fact that cloning isn't a one-size-fits-all pattern for asexual reproduction. Just like your eyeglasses might not enhance a friend's vision, scientists have had to make adjustments with cloning procedures to fit the genetic nuances of different species.

Reproductive Cloning Isn't Easy

Bernann McKinney is the first person to buy a commercially cloned pet. Scientists at Seoul National University cloned her deceased pit bull terrier.
Bernann McKinney is the first person to buy a commercially cloned pet. Scientists at Seoul National University cloned her deceased pit bull terrier.
Kim Jae Hwan/AFP/Getty Images

Before we get into why dogs have proven to be one of the hardest mammals to clone so far, let's go over the basics of reproductive cloning. The most common type of reproductive cloning is called somatic cell nuclear transfer (SCNT). Let's say we plan to clone a male golden retriever named Archie. With SCNT, you begin by extracting an egg from a female golden retriever. You remove the nucleus from that egg, which contains the DNA. Then, you take a cell from Archie and remove that cell's nucleus as well. You inject the nucleus from Archie's cell into the donor egg. Next, apply electricity to the nucleus and egg to fuse the two together, mimicking the union of sperm and egg in natural reproduction. The electricity also stimulates cell division, forming an embryo. Finally, you implant that embryo into a female dog's uterus, thus impregnating her. When the surrogate dog gives birth, you have an adorable puppy clone of Archie.

But creating an Archie clone isn't as simple as it sounds. First, getting the egg from a female dog can be troublesome because dogs don't experience regular ovulation cycles like humans. Although hormones can jumpstart ovulation in humans, canines don't respond to such treatments [source: Gorner]. Not only must scientists wait for dogs to go into heat, the eggs stay mature for just a few hours, leaving a brief window of opportunity for extraction [source: Kiem]. If scientists do manage to extract an egg, a coating of fat makes it difficult to remove the nucleus. Consequently, since 2005, only around 40 dogs have been cloned [source: Kiem].

Although primates go through more regular ovulation cycles compared to canines, monkey cloning has remained a difficult feat for at least four years. After performing SCNT, the embryos still wouldn't properly develop. Then, in 2003, a group of scientists at the University of Pittsburgh School of Medicine made an important discovery while studying the cloned embryos of rhesus monkeys. They realized that during SCNT, monkey cells lose two key proteins involved in cell and chromosomal division [source: Graham].

A few years later in 2007, a team of American scientists realized that the dye and ultraviolet light, sometimes applied to the rhesus monkey egg during nucleus extraction, can harm it. By instead applying polarized light (light waves that vibrate on one plane) in a process the researchers coined Oosight, the eggs stayed healthy, allowing the scientists to finally clone the rhesus monkey embryos [source: BBC]. However, none of the cloned embryos they implanted in female monkeys survived to birth.

With the U.S. Food and Drug Administration's (FDA) recent safety approval of meat and milk from cloned cows, goats and pigs, livestock clones could be big business in the near future. But what about chickens? A handful of biotechnology companies have jumped on cloned chicken research, but haven't been able to make it happen. Because chickens lay their eggs externally, SCNT doesn't work. Once a hen lays an egg, the nutrients necessary for development are already inside the shell. Even the tiniest incision to extract and replace the nucleus for SCNT would prohibit the embryo from developing into a chick.

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­The time and expense researchers pour into every successful cloning story highlights the delicate nature of the science. Combine that with each species' unique reproductive system, and it makes sense that the list of cloned animals isn't too long. Overall, when it comes to baby making, it looks like nature has a leg up on technology.

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Sources

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  • "Cloned Turkey On Your Thanksgiving Table?" University Of Arkansas, Fayetteville. ScienceDaily. November 25, 2003. (Sept. 4, 2008)http://www.sciencedaily.com­/releases/2003/11/031125072324.htm
  • Fiester, Autumn. "Researchers Create First Cloned Dog." The Washington Post. Aug. 4, 2005. (Sept. 4, 2008)http://www.washingtonpost.com/wp-dyn/content/discussion/2005/08/03/DI2005080301689.html
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  • Graham, Sarah. "Monkey Cloning Failures Cast Doubt on Feasibility of Human Reproductive Cloning." Scientific American. April 11, 2003. (Sept. 4, 2008)http://www.sciam.com/article.cfm?id=monkey-cloning-failures-c
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  • Keim, Brandon. "Cloned Puppies: Sure, They're Cute, But at What Cost?" Wired. Aug. 19, 2008. (Sept. 4, 2008)http://www.wired.com/science/discoveries/news/2008/08/dog_cloning
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  • "Wanna clone a cat? It's not easy." Associated Press. MSNBC. Nov. 9, 2004. (Sept. 4, 2008)http://www.msnbc.msn.com/id/3404434/

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