All about Cloning

  • Strictly speaking, cloning is the creation of a genetic copy of a sequence of DNA or of the entire genome of an organism.
  1. The copied material is called a clone.
  2. Geneticists have cloned cells, tissues, genes and entire animals. 
  • In the latter sense, cloning occurs naturally in the birth of identical twins and other multiples.
  • But cloning can also be done artificially in the laboratory via embryo twinning or splitting: an early embryo is split in vitro so that both parts, when transferred to a uterus, can develop into individual organisms genetically identical to each other.
  • In the cloning debate, however, the term ‘cloning’ typically refers to a technique called somatic cell nuclear transfer (SCNT).
  • SCNT involves transferring the nucleus of a somatic cell into an oocyte from which the nucleus and thus most of the DNA has been removed. (The mitochondrial DNA in the cytoplasm is still present).
  • The manipulated oocyte is then treated with an electric current in order to stimulate cell division, resulting in the formation of an embryo.
  • The embryo is (virtually) genetically identical to, and thus a clone of the somatic cell donor.
  • Dolly was the first mammal to be brought into the world using SCNT.
  • Dolly, however, was not 100% genetically identical to the donor animal.
  • Genetic material comes from two sources: the nucleus and the mitochondria of a cell. Mitochondria are organelles that serve as power sources to the cell.
  • They contain short segments of DNA. In Dolly’s case, her nuclear DNA was the same as the donor animal; other of her genetic materials came from the mitochondria in the cytoplasm of the enucleated oocyte.
  • For the clone and the donor animal to be exact genetic copies, the oocyte too would have to come from the donor animal (or from the same maternal line – mitochondria are passed on by oocytes). 
  • In biomedical research, cloning is broadly defined to mean the duplication of any kind of biological material for scientific studies, such as a piece of DNA or an individual cell.
  • With the advent of recombinant DNA technology in the 1970s, it became possible for scientists to create transgenic clones—clones with genomes containing pieces of DNA from other organisms.
  • Cloning happens all the time in nature—for example when a cell replicates itself asexually without any genetic alteration or recombination. 
  • Prokaryotic organisms (organisms lacking a cell nucleus), such as bacteria and yeasts, create genetically identical duplicates of themselves using binary fission or budding.
  • In eukaryotic organisms (organisms possessing a cell nucleus) such as humans, all the cells that undergo mitoses, such as skin cells and cells lining the gastrointestinal tract, are clones; the only exceptions are gametes (eggs and sperm), which undergo meiosis and genetic recombination.
  1. Mitosis is the division of a cell into two daughter cells that are genetically identical to the parent cell.
  2. Meiosis is the division of a germ cell into four sex cells (e.g. egg or sperm), each with half the number of chromosomes of the parent cell.
  3. Mitosis is a means of asexual reproduction, whereas meiosis is necessary for sexual reproduction.

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