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What is DNA cloning?

When you hear the word "cloning," you might think of cloning entire organisms, like Dolly the sheep. However, cloning something means making a genetically identical copy of that something. In a molecular biology laboratory, what is most frequently cloned is a gene or another small fragment of DNA.


 

What is cloning?


Figure 1: Animal cells undergoing division.
Figure 1: Animal cells undergoing division.

Cloning refers to a diverse set of procedures that enable the creation of genetically identical replicas of a living organism. This genetic reproduction results in what is called a "clone," a copy possessing the same genetic information as the original organism. Throughout research, a wide range of biological materials has been successfully cloned, including individual genes, cells, tissues, and even entire organisms, as seen in the case of the sheep Dolly.


In nature, certain plants and unicellular organisms, such as bacteria, have the ability to produce offspring with identical genes through a process called asexual reproduction. In this type of reproduction, a new individual originates from a copy of a single cell of the parent organism.


In the case of humans and other mammals, there are also natural clones, better known as identical twins. These twins form when a fertilized egg divides, resulting in two or more embryos sharing an almost identical genetic composition with each other and distinct from their parents.


 

Types of cloning


Cloning exhibits various forms, from natural processes found in bacteria, plants, and fungi for reproduction, to advanced techniques like therapeutic cloning, enabling tissue regeneration and creating a variety of cells.


In this realm, there are three main types of cloning:


Genetic cloning is used to multiply copies of a specific DNA fragment of interest. This method is frequently observed in nature when our cells divide to form other genetically identical cells. In research, it is utilized to obtain multiple copies of specific genes.


On the other hand, reproductive cloning aims to produce a complete individual identical to an existing one, while therapeutic cloning aims to generate pluripotent stem cells for medical purposes.


Both forms of cloning, reproductive and therapeutic, share a key technique: Somatic Cell Nuclear Transfer (SCNT) (Figure 2). This laboratory technique involves taking an egg from which the nucleus has been removed and replacing it with the nucleus of a somatic cell. While both cloning modalities use SCNT, they differ in their focus and process. In reproductive cloning, the nucleus of a mature somatic cell from the animal to be cloned is extracted, nuclear transfer is performed, and the embryo is implanted into the uterus of a female, which will carry on the organism's development until the birth of an individual with the same genetic composition as the donor somatic cell.


Figure 2: Model of reproductive cloning through somatic nuclear transfer in a lamb.
Figure 2: Model of reproductive cloning through somatic nuclear transfer in a lamb.

In therapeutic cloning, a process consists of three fundamental phases. Initially, enucleation or the extraction of the nucleus from the recipient cell, known as an oocyte, is performed. Once the enucleated oocyte is obtained, the nucleus of interest from the donor cell is transferred. Subsequently, the oocyte is activated, and cellular genetic activity is reprogrammed. The final outcome of this process is the generation of embryonic stem cells capable of developing into pluripotent cells with the ability to transform into any cell type. This advancement enables the creation of cells for the specific purpose of regenerating or replacing damaged tissues, among other medical applications.


Figure 3: Therapeutic cloning model through somatic nuclear transfer in humans.
Figure 3: Therapeutic cloning model through somatic nuclear transfer in humans.

 

What animals have been cloned?


In the last five decades, scientists have conducted cloning experiments on a wide variety of animals using different techniques. In 1979, researchers successfully created the first genetically identical mice by splitting mouse embryos in a laboratory and then implanting the resulting embryos into the uteruses of adult female mice. Shortly after, the first genetically identical cows, sheep, and chickens were produced by transferring the nucleus from a cell taken from an early-stage embryo to an egg cell from which its nucleus had been removed.

Figure 4: Image of Dolly the sheep, the first successfully cloned mammal.
Figure 4: Image of Dolly the sheep, the first successfully cloned mammal.

However, it wasn't until 1996 that researchers achieved a pivotal milestone: successfully cloning the first mammal using a mature adult cell. After 276 attempts, Scottish scientists brought Dolly to life, the famous sheep created from a cell extracted from the udder of a six-year-old sheep.


This groundbreaking achievement opened the doors to cloning in various species, including cattle, sheep, cats, deer, dogs, horses, mules, oxen, rabbits, and rats. Furthermore, a significant milestone was reached by cloning an Indian macaque through embryonic division, further expanding our knowledge in the field of cloning and reproductive biology.




 

What are the potential disadvantages?


Reproductive cloning, despite being a fascinating technique, faces significant challenges. Inefficiency in developing healthy embryos is evident, with the vast majority of attempts failing. Dolly, the sole clone among 277 embryos, illustrates this low success rate.


Adverse health effects in cloned animals include organ defects, increased birth size, accelerated aging, and immune issues. Chromosome length, especially telomeres, is a concerning factor: clones might inherit shorter telomeres, shortening their lifespan. Dolly, cloned from an adult sheep cell, experienced this phenomenon and died prematurely at the age of six.


Regarding therapeutic cloning, using embryonic stem cells is seen as a promising path to treat human diseases by many researchers. However, some experts are concerned about the striking similarity between stem cells and cancer cells. Both cell types can proliferate indefinitely, and stem cells have been shown to accumulate mutations after 60 divisions, posing a cancer risk. Thus, understanding the relationship between stem cells and cancer cells is crucial before considering their use in medical treatments.


 

The ethics of cloning


Genetic cloning is widely accepted and employed in laboratories. However, reproductive and therapeutic cloning pose significant ethical dilemmas regarding their potential application in humans.


Reproductive cloning might create genetically identical beings, challenging social and religious values concerning human dignity and individual freedom. While some advocate for its use in infertile couples or to prevent hereditary diseases, others question its ethics.


Conversely, therapeutic cloning, promising for treating diseases, involves the destruction of embryos, sparking ethical controversy. Opponents deem obtaining embryonic stem cells ethically wrong, even if they benefit the ill or injured.





Authored by Irene Rodríguez.

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