Clones are groups of genetically identical organisms, derived from a single original parent cell

• Organisms that reproduce asexually will produce genetically identical clones
• Additionally, mechanisms exist whereby sexually reproducing organisms can produce clones (e.g. identical twins)

Cloning multicellular organisms requires the production of stem cells (differentiated cells cannot form other cell types)

Stem cells can be artificially generated from adult tissue using a process called somatic cell nuclear transfer (SCNT)

Natural Cloning

Many species can reproduce asexually and hence possess natural methods of cloning

• All bacteria, the majority of fungi and many species of protists reproduce asexually to produce genetic clones
• While most plants reproduce sexually, they also possess methods of asexual reproduction (vegetative propagation)
• Certain animal species can also reproduce asexually, via a variety of different mechanisms

Natural Animal Cloning Methods

Binary Fission

• The parent organism divides equally in two, so as to produce two genetically identical daughter organisms
• This method of cloning occurs in Planaria (flatworms) but is also common to bacteria and protists (e.g. euglena, amoeba)

Budding

• Cells split off the parent organism, generating a smaller daughter organism which eventually separates from the parent
• This method of cloning occurs in Hydra but is also common to many species of yeast

Fragmentation

• New organisms grow from a separated fragment of the parent organism
• This method of cloning is common to starfish and certain species of annelid worms

Parthenogenesis

• Embryos are formed from unfertilised ova (via the production of a diploid egg cells by the female)
• This method of cloning occurs in certain species of insect, fish, amphibians and reptiles

Natural Plant Cloning Methods

Plants have the capacity for vegetative propagation, whereby small pieces can be induced to grow independently

• This is because adult plants possess meristematic tissue capable of cellular differentiation (totipotent)

Virtually all types of roots and shoots are capable of vegetative propagation

• Garlic and onion bulbs are modified plant leaves – all the bulbs in a group are genetically identical
• Underground stems (e.g. potato tubers) can form new plants which are genetically identical to the parent plant
• Certain plants can form horizontal stems called runners (or stolons) that grow roots and develop into clones

Some plants (mainly algae, mosses and ferns) can reproduce asexually by producing spores

• Spores are also produced by certain types of bacteria and fungi

TWINS

Human Cloning Methods

Even human beings are capable of creating genetic clones through natural means

• Identical twins (monozygotic) are created when a fertilised egg (zygote) splits into two identical cells, each forming an embryo
• Non-identical twins (dizygotic) are created when an unfertilised egg splits into two cells and each is fertilised by a different sperm
• Identical twins will be clones of one another (genetically identical), while non-identical twins will share 50% of the same DNA

Artifical Cloning

Animals can be cloned at the embryo stage by breaking up the embryo into more than one group of cells

At a very early stage, embryonic cells retain pluripotency (meaning they can divide and become any type of tissue)

• These cells will differentiate to form all the different tissues comprising the organism

If these embryonic cells are separated artificially in the laboratory, each group of cells will form cloned organisms

• This separation of embryonic cells can also occur naturally to give rise to identical (monozygotic) twins

The separation of embryonic cells has to happen early in the developmental cycle, ideally around the 8 cell stage (morula)

• The separated groups of cells are then implanted into the uterus of a surrogate to develop into genetically identical clones

This method of cloning is limited by the fact that the embryo used is still formed randomly via sexual reproduction and so the specific genetic features of the resulting clones have yet to be determined

• Methods have been developed for cloning adult organisms using differentiated cells

What is therapeutic cloning? (human cloning)

• Therapeutic cloning is sometimes referred to as "somatic cell nuclear transfer" or "SCNT". SCNT involves removing the nucleus of an egg cell, replacing it with the material from the nucleus of a "somatic cell" (such as a skin cell) and stimulating this cell to begin dividing. This egg cell is never fertilized by sperm, and the genetic material within the cell is virtually identical to the genetic material extracted from the skin or other cell. Once the cells begins dividing, stem cells can be extracted from it 5-6 days later, just as they can be extracted from embryos created through in vitro fertilization. These stem cells - or the specialized cells derived from them - will be a genetic match to the patient. Therefore, the promise of SCNT is that the patient's body would accept these cells after transplantation.
• By using SCNT, scientists hope to understand how the protein factors in the egg cell cause these already specialized somatic cells to become stem cells. Once scientists learn how this cell "de-differentiation" occurs, they will no longer need to use egg cells.
• SCNT has nothing to do with sexual reproduction. It is a research technique to create cells that can be used to treat diseases such as Parkinson's Disease, diabetes, ALS, etc. The sole purpose of this technology is to address currently unmet medical needs.