What is Cloning in Biology?

Cloning simply means creating exact replicas or copies. Cloning in biotechnology refers to the process of creating identical copies of either DNA fragments, cells or organisms. The organism which has the identical genetic make-up and the morphological attributes of the source organism is called a clone, while the process is called cloning.

Types of Cloning

Based on the origin of the process, we can have two main types of cloning, i.e. natural cloning and artificial cloning:

1. Natural Cloning: Reproduction through asexual and vegetative means result in the development of clones, and thus are examples of natural cloning methods.
   a. Plants generally produce clones by vegetative means, while some lower organisms and bacteria reproduce asexually to produce clones.
   b. Few examples of asexual reproductive methods are fission, budding, reproduction by asexual spores, fragmentation, etc.
   c. Few examples of vegetative reproductive methods are reproduction by rhizomes, runner, corms, bulbs, tubers, offset, stolon, etc.
   d. In all these methods/ways, the new organisms (offsprings) formed are the clones of the parent. 
2. Artificial Cloning: Artificial cloning primarily refers to the biotechnological process of creating clones of either DNA fragments or genes or cells or organisms for various purposes. Various techniques have been developed and used based on the entity to be cloned.

Types of Natural Cloning

1. Reproductive Cloning in Horticulture: Cloning of an entire organism is known as reproductive cloning.
   a. In plants, this happens naturally by vegetative methods.
   b. Grapevine propagation by cuttings is an age-old example of organism cloning in horticulture.
   c. Apomixis is another natural way of creating genetically identical organisms.
   d. Apomixis is defined as reproduction without fertilization.
   e. Replacement of seeds by plantlets and replacement of flowers by bulbils are considered types of apomixis.
   f. Some vascular plants like dandelions and some grasses form seeds asexually by apomixis and form clonal populations which are genetically identical to the parent plant.
2. Organism Cloning by Parthenogenesis: Natural organism cloning exists in nature in some animals which reproduce by parthenogenesis.
   a. Parthenogenesis is the development of an organism without fertilization of gametes.
   b. Such organisms that develop through parthenogenesis are genetically identical to the mother.
   c. Few examples of animals where reproduction by parthenogenesis takes place are insects like honeybees, wasps, some crustacean species, nematodes, and lizards (komodo dragon).
3. Cellular Cloning: Creating a population of cells from a single-parent cell is referred to as cellular cloning.
   a. Unicellular organisms like bacteria and yeast exhibit the simplest form of cellular cloning, and in this case, just a single-cell inoculation is required into the nutrient medium.

Types of Artificial Cloning

1. Molecular Cloning: It is also referred to as gene cloning or DNA cloning.
   a. The branch of biology that deals with such molecular cloning is called genetic engineering or recombinant DNA (rDNA) technology.
   b. A fragment of DNA or gene is replicated into a number of identical copies.
   c. DNA fragments containing whole genes or part of genes or other regulatory elements such as promoters are amplified.
   d. The cloned products are used in genetic engineering, from sequencing the fragments to protein production.
   e. There are two approaches to perform molecular cloning.
   f. One approach uses live organisms like bacteria and plasmids. DNA fragments are inserted into natural or artificial cloning vectors, and then these vectors are replicated in host organisms like bacteria, animal or plant cells.
   g. Another approach of amplifying the DNA fragments is more techno-chemical and does not need living organisms.
   h. The polymerase chain reaction (PCR) is the most common technique used for molecular cloning, and it relies on the thermal cycling of DNA fragments and enzyme driven DNA replication.
2. Cellular Cloning: The cellular cloning of multicellular cells is more challenging.
   a. Tissue culture techniques are used to select a single cell with desirable characteristics by exposing it to mutagens or drugs.
   b. Then, with single-cell suspension culture, the clones are cultured,  separated and propagated to get cell lineage.
   c. The cells of such a lineage are the clones of the original parent cell.
3. Reproductive Cloning
   a. When the whole organism is cloned, it is referred to as reproductive cloning.
   b. An identical copy or clone of an entire multicellular organism is made in this process.
   c. Breakthrough in reproductive cloning came when Dolly was cloned using a technique called somatic cell nuclear transfer (SCNT).
   d. To explain this technique briefly, the nucleus from a somatic cell of an adult donor is removed and inserted into an enucleated (whose nucleus has been removed) egg cell or early blastocyst cell.
   e. Once the nucleus is inside the egg cell or blastocyst cell, the cell is stimulated with mild electric current for division.
   f. As the cell divides, it develops into an embryo. This cloned embryo is a genetically identical copy of the original organism.
   g. Reproductive cloning has been successfully performed in a number of species.
   h. Few examples of organisms where reproductive cloning is successful are tadpole, zebrafish, pig, cat, rat, mice, mule, horse, dog, Pashmina goat, macaque monkey, etc.
4. Therapeutic Cloning
   a. Cloned embryos are used to get stem cells from them for research and therapeutic usage.
   b. Stem cells can be dedifferentiated into more than \(200\) types of cells.
   c. Cloned stem cells are genetically identical to the patient and hence can be grafted in the patient’s body without risk of rejection by the immune system.
   d. Embryos are not implanted into the uterus.
   e. Examples of diseases treated with cloned stem cells are Alzheimer’s disease, Parkinson’s disease, diabetes mellitus, spinal injury, etc.
   f. Cloned stem cells are also used to study normal or abnormal embryo development and to know if drugs are toxic or cause any birth defects.

History of Animal Cloning

Early experiments on reproductive cloning began some \(40\) years ago through a process known as embryo splitting. In this procedure, a single two-celled stage embryo was split manually into two cells, and then each cell was grown as an identical embryo.  In \(1924,\) Hans Spemann and his student Hilde Mangold performed some experiments of somatic cell nucleus transfer (SCNT) in amphibian embryos. This was considered the first step towards animal cloning.

In \(1996,\) Ian Wilmut and his team announced the successful cloning of a sheep, Dolly. This was a major breakthrough. Dolly was cloned using the same technique of somatic cell nuclear transfer (SCNT). The cloning of Dolly was significant because she was the first mammal to be cloned successfully using an adult somatic cell. The birth of Dolly was also significant because it demonstrated that a nucleus could be dedifferentiated and redesigned to develop into a new organism.

Story of Dolly the Sheep

1. Ian Wilmut developed the cloning technique (SCNT) and successfully cloned the first mammal, a sheep named Dolly.
2. Scottish Blackface sheep was the egg cell donor, and \(6\) -year-old Finn-Dorset sheep was the nuclear cell donor.
3. Scottish Blackface sheep was the surrogate mother for the cloned embryo.
4. Dolly was born on \({{\rm{5}}^{{\rm{th}}}}\) July \(1996\) at Roslin Institute in Scotland.

Fig: SCNT procedure used in the cloning of Dolly

Why Human Cloning is Difficult?

1. Reproductive human cloning faces a lot of technical difficulties.
2. The success rate of SCNT is very low.
3. During the cloning of Dolly, the success rate was \(29\) embryos per \(277\) tries.
4. In macaque monkeys, out of \(100\) cloned embryos implanted in \(50\) females, none survived.
5. Reproductive cloning using SCNT is considered harmful as nuclear damage is involved while enucleating and inserting the donor nucleus.
6. The cloned embryos rarely make the full term of gestation and may get birth defects.
7. Maintaining human embryo viability after SCNT is challenging, and embryos die at an early stage.
8. Some scientists argue that current technological advances are not enough to clone primates and humans.

Human Cloning Ethics

1. Human cloning is banned in many countries for religious, psychological, and social reasons.
2. The cost of obtaining human egg cells can be very high as this is not volunteer work.
   Surrogate motherhood (part of reproductive cloning) is also under many ethical and legal controversies.
3. As of \(2018, 70\) countries have legally banned reproductive human cloning. However, therapeutic cloning is permitted only in few countries.

Summary

Cloning means creating identical copies. Biotechnologically, cloning refers to creating identical copies of DNA fragments, cells or animals. Cells and animals cloned are genetically identical to the source organism. Cloning can be of two types, i.e. natural and artificial. Propagation by vegetative and asexual reproductive methods are be considered natural cloning methods. Artificial cloning primarily refers to the biotechnological process of creating clones. However, human reproductive cloning has technological limitations, and the current techniques are not sufficient to create a human clone. It is also legally banned in many countries and is related to many ethical controversies.

Frequently Asked Questions (FAQs) on Cloning

Q.1. Can humans be cloned?
Ans: Reproductive human cloning is not possible as of now due to technological limitations and ethical controversies. It is also legally banned in many countries. However, therapeutic human cloning for obtaining stem cells is being practised for research purposes only.

Q.2. Why is human cloning banned?
Ans: Human cloning is banned mainly for religious controversies associated with it. It is also feared that human clones could be abused in many ways. Seventy countries have legally banned human cloning.

Q.3. What is cloning?
Ans: The organism which has the identical genetic make-up and the morphological attributes of the source organism is called a clone, while the process is called cloning.

Q.4. When was the first human cloned?
Ans: The first hybrid human clone was developed in \(1998\) by scientists at Advanced Cell Technology, USA. They created a hybrid clone by taking the nucleus from man’s leg cells and inserting it into a cow’s egg cell from which the nucleus was removed. This embryo died after \(12\) days.

Q.5. How is DNA cloning done?
Ans: DNA cloning is done either by rDNA technology (genetic engineering) or by PCR technique. Reproductive and therapeutic cloning (whole new organisms are produced) is done primarily by a technique called somatic cell nuclear transfer (SCNT).