IB Biology SL

Revision Notes

3.4.4 Cloning

Natural Clones in Plants

  • Many plants can reproduce asexually using meristem cells, in a process called vegetative reproduction
  • Vegetative organs of plants include:
    • Root and shoot tips
    • Axillary buds (where leaves and the stem meet)
    • Vascular cambium (between xylem and phloem)
  • Naturally, over time a young, miniature plant (a plantlet) forms at these locations and remains attached to its parent plant
  • These plantlets are clones of their parent (as no other DNA has been introduced)
  • At maturity, the plantlet becomes detached from its parent and can live independently, when it is capable of photosynthesizing by itself
  • The new plants all have the same phenotype, so are uniform, making growing and harvesting easier
    • Plants that are hard to grow from seeds can be propagated, eg. orchids for the horticulture industry
  • Some plants have horizontal stems or runners that form over the soil surface, pointing sufficiently far away so that a new plant at that location will not be overshadowed by its parent, or in competition for water or soil nutrients
    • Roots form under the nodes of runners, called adventitious roots
    • The runner dies when the plantlet is self-sustaining
    • Strawberries, peppermint and spider plants reproduce in this way

Runners and adventitious roots, downloadable AS Level & A Level Biology revision notes
An example of asexual reproduction in plants with runners that form adventitious roots

Propagation techniques using vegetative reproduction

  • Many methods of propagation 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, rhizomesbulbs, suckers, and offsets
  • All modes of vegetative propagation contain modified stems that can generate meristematic 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
  • Gardeners and horticulturalists can use these techniques to propagate desirable species asexually, effectively and at less cost than utilising sexual reproduction techniques
    • This is done by taking cuttings and dividing up plants into different clumps or sections

Natural Clones in Animals

Asexual reproduction in animals

  • 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

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
  • Two embryos that form are identical, with the same genotype and develop in utero together
  • The result is the birth of identical offspring, always of the same gender, with identical phenotype
  • Because non-identical twins are formed from separate eggs and sperm, they are not considered clones


Exam Tip

Although identical (monozygotic) twins share the same genome at the moment when the embryo splits, identical twins are not clones in the true sense of the word. Because mutations occur with every cell cycle, Twin A will possess slightly different DNA base sequences to Twin B at the time of birth. The older the twins get, the more their genomes become dissimilar as mutations accumulate. They will still look very alike throughout their lives unless there are large differences in their environments as they grow up.

Production of Artificial Clones in Animals

Embryo twinning

  • The process of embryo twinning 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
  • 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

Embryo twinning 1, downloadable AS & A Level Biology revision notes Embryo twinning 2, downloadable AS & A Level Biology revision notesEmbryo twinning of cattle by splitting the embryo

Reproductive cloning

  • This is the method made famous by Dolly the sheep, cloned in Edinburgh, UK in 1996
  • Its 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 to be 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 eg. from an udder
    • 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 1, downloadable AS & A Level Biology revision notes Reproductive cloning of animals 2, downloadable AS & A Level Biology revision notes Reproductive cloning of animals.

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 a totipotent stem cell that 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

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