Thursday, 8 August 2013


Research advances over the past decade have told us that, with a little work, we humans can clone just about anything we want, from frogs to monkeys and probably even ourselves!
Of all the reasons, cloning for medical purposes has the most potential to benefit large numbers of people. How might cloning be used in medicine?

what is cloning??

Have you ever wished you could have a clone of yourself to do homework while you hit the skate park or went out with your friends?
Imagine if you could really do that. Where would you start?
What exactly is cloning?
Cloning is the creation of an organism that is an exact genetic copy of another. This means that every single bit of DNA is the same between the two!
You might not believe it, but there are human clones among us right now. They weren't made in a lab, though: they're identical twins, created naturally. Below, we'll see how natural identical twins relate to modern cloning technologies.
How is cloning done?
You may have first heard of cloning when Dolly the Sheep showed up on the scene in 1997. Cloning technologies have been around for much longer than Dolly, though.
How does one go about making an exact genetic copy of an organism? There are a couple of ways to do this: artificial embryo twinning and somatic cell nuclear transfer. How do these processes differ?
1. Artificial Embryo Twinning
Artificial embryo twinning is the relatively low-tech version of cloning. As the name suggests, this technology mimics the natural process of creating identical twins.
In nature, twins occur just after fertilization of an egg cell by a sperm cell. In rare cases, when the resulting fertilized egg, called a zygote, tries to divide into a two-celled embryo, the two cells separate. Each cell continues dividing on its own, ultimately developing into a separate individual within the mother. Since the two cells came from the same zygote, the resulting individuals are genetically identical.
Artificial embryo twinning uses the same approach, but it occurs in a Petri dish instead of in the mother's body. This is accomplished by manually separating a very early embryo into individual cells, and then allowing each cell to divide and develop on its own. The resulting embryos are placed into a surrogate mother, where they are carried to term and delivered. Again, since all the embryos came from the same zygote, they are genetically identical.
2. Somatic Cell Nuclear Transfer
Somatic cell nuclear transfer, (SCNT) uses a different approach than artificial embryo twinning, but it produces the same result: an exact clone, or genetic copy, of an individual. This was the method used to create Dolly the Sheep.
What does SCNT mean? Let's take it apart:
Somatic cell: A somatic cell is any cell in the body other than the two types of reproductive cells, sperm and egg. Sperm and egg are also called germ cells. In mammals, every somatic cell has two complete sets of chromosomes, whereas the germ cells only have one complete set.
Nuclear: The nucleus is like the cell's brain. It's an enclosed compartment that contains all the information that cells need to form an organism. This information comes in the form of DNA. It's the differences in our DNA that make each of us unique.
Transfer: Moving an object from one place to another.
To make Dolly, researchers isolated a somatic cell from an adult female sheep. Next, they transferred the nucleus from that cell to an egg cell from which the nucleus had been removed. After a couple of chemical tweaks, the egg cell, with its new nucleus, was behaving just like a freshly fertilized zygote. It developed into an embryo, which was implanted into a surrogate mother and carried to term.


In What is cloning? we learned what it means to clone an individual organism. Given its high profile in the popular media, the topic of cloning brings up some common, and often confusing, misconceptions.
Misconception #1: Instant Clones!
Instant clones
Let's say you really wanted a clone to do your homework. After reviewing What is Cloning? and Click and Clone, you've figured out, generally, how this would be done. Knowing what you know, do you think this approach would really help you finish your homework...this decade?
A common misconception is that a clone, if created, would magically appear at the same age as the original. This simply isn't true. You remember that cloning is an alternative way to create an embryo, not a full-grown individual. Therefore, that embryo, once created, must develop exactly the same way as would an embryo created by fertilizing an egg cell with a sperm cell. This will require a surrogate mother and ample time for the cloned embryo to grow and fully develop into an individual.


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:
  • The enucleated egg and the transferred nucleus may not be compatible
  • An egg with a newly transferred nucleus may not begin to divide or develop properly
  • Implantation of the embryo into the surrogate mother might fail
  • The pregnancy itself might fail
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.
Ethical, legal and social issues.
There are several types of issues to consider as we think about cloning.
MoralsEthical issues are those that ask us to consider the potential moral outcomes of cloning technologies.

LegalLegal issues require researchers and the public to help policymakers decide whether and how cloning technologies should be regulated by the government.

SocialSocial issues involve the impact of cloning technologies on society as a whole.

Some questions to ponder.

The questions raised here have no clear right or wrong answer. Instead, your response will depend on your own set of values, as well as the opinions of those around you.
  • Who has the right to have children, no matter how they are created? Who doesn't? Why?
  • Is human cloning "playing with nature?" If so, how does that compare with other reproductive technologies such as in vitro fertilization or hormone treatments?
  • Does cloning to create stem cells, also called therapeutic cloning, justify destroying a human embryo? Why, or why not?
  • If a clone originates from an existing person, who is the parent?
  • What are some of the social challenges a cloned child might face?
  • Do the benefits of human cloning outweigh the costs of human dignity?
  • Should cloning research be regulated? How, and by whom?

No comments:

Post a Comment