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Cloning is a common mechanism of reproduction of plant species or bacteria.
A clone can be defined as a population of molecules, cells or organisms that originated from a single cell and are identical to the original cell. In humans, natural clones are identical twins that originate from the division of a fertilized egg.
Dolly's great revolution, which paved the way for human cloning, was the demonstration for the first time that it was possible to clone a mammal, that is to produce a genetically identical copy from a differentiated somatic cell. To understand why this experience was amazing, we need to remember a little embryology.
The cell nucleus contains the 23 pairs of chromosomes.
We have all been a single cell, resulting from the fusion of a egg it is a sperm. This first cell already has at its core DNA with all the genetic information to generate a new being. DNA in cells becomes extremely condensed and organized into chromosomes. With the exception of our sex cells, the egg and sperm that have 23 chromosomes, every other cell in our body has 46 chromosomes. In each cell, we have 22 pairs that are equal in both sexes, called autosomes and a pair of sex chromosomes:
XX in females and XY in males. These cells, with 46 chromosomes, are called somatic cells.
Let us now return to our first cell resulting from the fusion of the egg and sperm. Immediately after fertilization, it begins to divide: one cell in two, two in four, four in eight, and so on. At least until the eight-cell phase, each is capable of developing into a complete human being. They are called totipotents. In the phase of eight to sixteen cells, the embryo's cells differentiate into two groups: a group of outer cells that will originate the placenta and embryonic attachments, and a mass of inner cells that will originate the embryo itself. After 72 hours, this embryo, now about one hundred cells, is called a blastocyst.
It is at this stage that the embryo implantation in the uterine cavity occurs. The inner cells of the blastocyst will originate the hundreds of tissues that make up the human body. They are called pluripotent embryonic stem cells. At some point, these somatic cells - which are still all the same - begin to differentiate into the various tissues that will make up the body: blood, liver, muscles, brain, bones, and so on. The genes that control this differentiation and the process by which it occurs are still a mystery.
What we do know is that once differentiated, somatic cells lose the ability to make any tissue. Descending cells of a differentiated cell will retain the same characteristics as those that originated them, that is, liver cells will originate liver cells, muscle cells will originate muscle cells, and so on. Although the number of genes and DNA is the same in every cell in our body, genes in differentiated somatic cells express themselves differently in each tissue, that is, gene expression is specific to each tissue. With the exception of genes responsible for the maintenance of cellular metabolism (housekeeping genes) that remain active in all cells of the body, only the genes important for maintaining this function will function in each tissue or organ. The others remain "silenced" or inactive.
Text adapted from Zatz, Mayana. "Cloning and stem cells". Cienc. Cult., jun. 2004, vol. 56, no. 3, pp. 23-27, ISSN 0009-6725.