Reproductive cloning is a biological procedure intended to produce a new organism that is genetically highly similar to a particular donor organism, usually with respect to the nuclear genome. In mammals, the best known method is somatic cell nuclear transfer, commonly abbreviated SCNT. In this procedure, the nucleus is removed from an egg cell, creating an enucleated egg, and the nucleus from a somatic cell of the donor organism is introduced into that egg.
The reconstructed cell is stimulated to begin embryonic development. If development progresses successfully, the embryo can be transferred into a surrogate female, where it may develop into an offspring. The resulting organism is not genetically identical in every respect to the donor because mitochondrial DNA generally originates from the egg cell, and epigenetic and developmental differences can arise. Reproductive cloning therefore means producing an organism from a donor genetic source through a cloning procedure rather than ordinary sexual reproduction.
Somatic cell nuclear transfer depends on the remarkable ability of an adult somatic cell nucleus to be reprogrammed. Most specialised body cells contain essentially the same nuclear genome, but different cell types express different genes because their chromatin states and regulatory programmes differ. During nuclear transfer, the egg cytoplasm provides molecular factors that can reset aspects of the transferred nucleus towards an embryonic state. The reconstructed embryo must then activate its own developmental programme and progress through early cell divisions.
This reprogramming is incomplete or inefficient in many cases, which helps explain why reproductive cloning has historically involved substantial developmental losses. The process demonstrates an important principle of developmental biology: cellular differentiation usually changes gene expression and cellular state rather than permanently removing most of the genetic information from the nucleus.
Dolly the sheep is the most famous example of mammalian reproductive cloning. She was produced using nuclear transfer involving a nucleus from an adult somatic cell and an enucleated sheep egg, followed by development in a surrogate. Her birth in 1996 was scientifically important because previous successful nuclear transfer experiments had often used nuclei from embryonic or less differentiated cells.
Dolly demonstrated that genetic material from an adult differentiated cell could be reprogrammed sufficiently to direct development of an entire mammal under the appropriate conditions. The experiment did not mean that every adult cell is naturally capable of producing an organism. Instead, it showed that the nucleus retained genetic information that could, under specialised experimental circumstances, be reprogrammed. Dolly subsequently lived for several years and became a major scientific and public symbol of cloning research.
Reproductive cloning differs from ordinary sexual reproduction because sexual reproduction combines nuclear genomes from two reproductive cells. In conventional mammalian reproduction, an egg and sperm contribute genetic material, and recombination during meiosis produces a genetically unique combination in the offspring. Reproductive cloning using SCNT instead derives the nuclear genome primarily from one donor somatic cell. This can preserve much of a donors nuclear genetic identity, but it does not recreate the donors complete developmental history.
The cloned offspring experiences a different embryonic environment and may encounter different nutritional, hormonal and environmental conditions. It therefore cannot be expected to have the same personality, behaviour or exact phenotype as the donor. Genetic similarity establishes a biological relationship but does not determine every aspect of an organisms development.
A major limitation of reproductive cloning is low efficiency. Many reconstructed embryos do not implant successfully, fail during pregnancy or develop abnormalities. Nuclear reprogramming must reset gene expression patterns appropriate for an embryo, and incomplete resetting can disturb development. Abnormal epigenetic states, placental development problems and other cellular abnormalities have been associated with unsuccessful cloning in experimental animals. The exact efficiency varies among species and techniques, and improvements in laboratory methods can alter outcomes. Reproductive cloning therefore involves considerably more biological uncertainty than ordinary reproduction in many experimental contexts. Animal welfare is consequently an important consideration in research involving cloning because unsuccessful attempts can involve embryo loss, pregnancy complications or health problems in surrogate animals and offspring.
Reproductive cloning has been investigated for several purposes, including research, preservation of particular animal genotypes and agricultural applications. It can potentially reproduce an animal with a valuable genetic composition, but it does not create genetic diversity and therefore cannot substitute for conventional breeding when population variation is important. Cloning can also be useful when a particular genotype is required repeatedly for research, because genetically similar animals can reduce some forms of genetic variation between experimental groups.
Conservation applications have been discussed for species in which suitable genetic material is available, but cloning cannot reconstruct a whole extinct population or restore the ecological relationships lost with extinction. Its usefulness depends heavily on the biology of the species, the availability of viable cells and the ability to support normal development.
Reproductive cloning should also be distinguished from embryo splitting. In embryo splitting, an early embryo is divided into separate embryos, somewhat analogous to the natural process that can produce identical twins. In SCNT, the nucleus of a somatic cell is transferred into an enucleated egg. These methods therefore create genetic similarity through different biological mechanisms. Reproductive cloning is also distinct from gene editing.
Gene editing changes DNA sequences, whereas reproductive cloning attempts to reproduce an organism from an existing genetic source. The two technologies can theoretically be combined, for example by editing cells before using them as nuclear donors, but they represent different technological processes. Similarly, reproductive cloning is not the same as cloning a DNA sequence or producing a cloned cell line.
Human reproductive cloning is a distinct subject from the established laboratory uses of molecular cloning, cellular cloning and animal reproductive cloning. Producing a cloned human would involve the same general biological challenge of nuclear reprogramming but would raise major medical, ethical, legal and social questions. Scientific concerns include developmental safety, low efficiency and the possibility of serious abnormalities.
Ethical considerations include the welfare and rights of children created through such a procedure, the use of embryos and surrogate pregnancy, consent and broader social implications. Laws and policies differ between jurisdictions, but human reproductive cloning is not an established clinical reproductive treatment. The scientific significance of animal reproductive cloning is therefore not evidence that human reproductive cloning is technically safe or medically established.
