• Clones are groups of genetically identical organisms, derived from a single original parent cell
• A cloned cell is a cell that is?genetically identical?to the cell that it originated from
• Sexual reproduction produces a?zygote?when gametes fuse
• In a single birth, this zygote is not cloned and will itself reproduce sexually as an adult
  • Identical twins are?clones?of each other?as they are formed from one zygote splitting into two parts, which each develops into an embryo
• Clones form?naturally?and?artificially
• The simple gardening technique of?taking plant cuttings?relies on cloning
• Other organisms are manipulated to?form multiple clones?when grown commercially eg. large-scale growth of crop plants, to ensure a uniform crop and good crop yields
• This ensures that?desirable characteristics?appear in the phenotypes of every organism

• Many plant species and some animal species have?natural?methods of cloning
• Asexual reproduction is?much less common in animals?than in plants
• Some small animals reproduce asexually by?parthenogenesis?eg. aphids
• The other naturally occurring incidence of cloning in animals is?identical twins

Natural cloning in plants

• Many methods of cloning?do not require seeds?as it is not sexual reproduction that is occurring, it is?asexual reproduction
• A well as runners, plants can propagate asexually using?tubers, rhizomes,?bulbs, suckers,?and?offsets
• All modes of natural plant cloning contain?modified stems?that can generate?meristem?tissue
• Potato?tubers?are swollen modified roots that form?eyes?on their surface
  • Eyes can sprout new growth (called 'chitting')
  • The starch stored in the tuber fuels the early growth of the new plant
• Ginger forms?rhizomes, a modified stem that grows horizontally underground
  • New growth stems from nodes in the rhizome, forming?new stems?and?adventitious roots
  • The section used in cookery is the rhizome
• Onions and garlic form?bulbs?that can grow adventitious roots underground and leafy shoots above ground
• Suckers?are growths that appear from the root systems of many trees and shrubs, which can provide meristematic tissue for vegetative propagation
  • Examples are?poplars, cherries?and?plums
• Offsets?are small, virtually complete daughter plants that have been asexually produced on the mother plant
  • Examples are?tulips?and?lilies

An example of natural cloning in plants with runners that form adventitious roots

Identical twins

• An?egg is fertilised by a sperm?as in a singleton birth
• This forms a?zygote
• The single zygote undergoes a few cell cycles (mitotic divisions) to become an?embryo
  • This is why identical twins are referred to as?monozygotic
• At the embryo stage, the?embryo splits in two; the exact causes of this kind of split are not well understood
• The two embryos that form are?identical?(have exactly the same genotype) and develop?in utero?(i.e. in the uterus) together
• The result is the birth of?identical offspring, always of the same gender, with identical phenotypes
• Because?non-identical twins?are formed from separate eggs and sperm, they are?not clones

Identical twins are natural clones when a zygote splits into two parts and each part develops into a separate embryo

• The process of?embryo twinning?(sometimes called splitting or fragmentation) produces offspring that are?clones of each other?but not of their parents
• It has been a?routine procedure?carried out to?boost yields of livestock?and?promote desirable characteristics?since the 1980s
• The key step is the?deliberate division of the embryo?into two half embryos
• Both halves contain cells that are?pluripotent
  • The embryo is split at around the eight-cell stage
• These are then inserted into a?surrogate mother?for gestation and birth
• The surrogate gives birth to?identical twins
• In some cases, embryos are split into?single identical cells, each of which can be implanted into a?separate?surrogate mother animal
• Although embryo twinning guarantees desirable characteristics in the offspring, it is?not possible to predict how many offspring?will be produced?within a herd of livestock, something of vital importance to a farmer

Embryo twinning of cattle by splitting the embryo

Therapeutic cloning

• This is a technique designed to use cloned cells to?replace dead or damaged cells?that cause a loss of function in an individual
• Embryos are?cloned as in reproductive cloning,?but the embryos are removed and subdivided
• Each individual embryo cell is pluripotent and can be cultured and?artificially differentiated?into any type of specialised cell
• In theory,?any specialised cell?can be derived by this method
  • Crucially, specialised?cells with the same genome as the sufferer?can be cloned and replaced
• An example is replacing specialised brain tissue in sufferers of?Parkinson's Disease
• At present, there is?a lot of potential?for therapeutic cloning but?little clinical progress?has been made

• Methods have been developed for cloning adult animals using?differentiated cells
• It would be?desirable to clone a differentiated cell?because by then it would be easy to assess the organism's characteristics and whether any of its traits were desirable enough to clone
  • Pluripotent cells can develop into any specialised cell, but it is difficult to predict whether, once differentiated, they will display desirable characteristics
• Differentiated cells are more difficult to clone because?certain genes have been permanently switched off?(those genes will never be transcribed again) as the cell has developed its specialised role
• However, pioneering work on?Xenopus?(the African clawed frog) in the 1950s involved:
  • Removal of nuclei from?Xenopus?tadpoles'?somatic cells
  • It was significant that the somatic cell was already?fully?differentiated?eg. a skin cell
  • Insertion of these nuclei?into enucleated?Xenopus?egg cells?(eggs whose own nuclei had been destroyed by UV radiation treatment)
• This resulted in embryos that grew, divided and differentiated into?fully-functioning live tadpoles, and ultimately, adult?Xenopus?frogs
• Whilst this work was successful, cloning mammals from differentiated cells proved much?more difficult
• The work on?Xenopus?frogs?laid the platform?for the first mammal cloned by nuclear extraction
• It wasn't until the 1990s that the first large mammal,?Dolly the sheep,?was cloned successfully

Cloning of?Xenopus?frogs by nuclear transfer from a differentiated cell

Production of cloned embryos produced by somatic cell nuclear transfer

• This is the method made famous by?Dolly the sheep, cloned in Edinburgh, UK in 1996
• Its full name is?Somatic Cell Nuclear Transfer?(SCNT)
• Dolly made headlines as being the?first livestock animal to be created from a clone
• Three separate animals are required:
  • The?animal being cloned?(by donating a cell)
  • The female to?donate an egg?cell
  • The?surrogate mother
• How the procedure is carried out:
  • The animal to be cloned?donates a somatic (body) cell
    • In Dolly's case, this was an udder cell
  • The egg cell is extracted from the egg donor and?enucleated?(its nucleus is removed by suction and discarded)
  • The nucleus from the udder cell is?injected into the enucleated egg?cell
  • The hybrid zygote cell is now treated to?encourage it to divide?by mitosis
  • The embryo is?implanted into the surrogate mother?for gestation and birth

Reproductive cloning of animals by somatic cell nuclear transfer