12th Zoology



Cloning is an experimental technique wherein, a group of
genetically identical organisms is produced. Cloning of various
animals was has become possible due to knowledge gained in the field of
developmental biology and developmental genetics. It helped a lot, to
understand the genetic control over differentiation of cells and the
development of multicellular organisms.

Differentiation : In the development of multicellular animals the zygote
represents the progenitor cell of the future embryo. Multitudes of cells arise
from mitotic divisions of the fertilized egg cell. These cells later become
distinct cell types differing in form and function. This process is called

In the 1950s two embryologists R.Briggs and T.King developed a
technique called nuclear transplantation. The nuclei from frog egg cells are
taken out with a micropipette (enucleation) and replaced with nuclei taken
from the cells of an embryo the same animal. The recipient cells developed
into normal tadpoles and frogs with all the different cell types. The investigators,
with the above technique produced a number of genetically identical

Cells of early embryo which are capable of complete
development and producing the whole organisms are said to be totipotent However, experiments by J. Gurdon revealed that nuclei from older
embryos and tadpoles when transplanted to the enucleated egg cells
affected the developmental potential.

The older the individual from which the nuclei were taken, less the recipient egg cell was able to develop normally. His experiments also revealed that cells from different parts of the embryo differed in the degree of successful development attained after nuclear transplantation.

It is learnt that cells of embryos at a later stage of development switch over from totipotent state to pluripotent state. The later gives rise to development of specific tissues or organs.

Cloning of Sheep (Mechanism) :

Dr. Ian Wilmut has produced a cloned sheep called Dolly by nuclear
transplantation method. To produce cloned sheep he took the udder cell
which is a somatic cell with diploid number of chromosomes. An egg cell
was also removed from a donor sheep. The egg cell cannot grow into a new
sheep on its own because it only has half a set of chromosomes(n). The
body cell cannot grow into a new sheep on its own because it is not a
reproductive cell. So udder cell nucleus(2n) was removed. Similarly the egg
cell nucleus (n) was also removed. The nucleus of the somatic cell (Udder)
was injected into the enucleated egg. The egg after the nuclear transplantation comes to possess full set of chromosomes viz. the 2n
diploid. The egg was then transplanted back into the uterus of the sheep
from which it was removed. The egg also can be transplanted to a new
surrogate mother for development. The egg cell grew and developed into a
sheep (Dolly). This cloned sheep is genetically identical to the donor sheep,
which donated the diploid nucleus of its somatic cell and not the sheep which
donated the egg cell.

Ethical Issues, Merits and Demerits of cloning :-

1. Cloning of animals is considered as an unethical and unnatural
technique by some people.
2. It is feared that attempts to clone human may lead to the birth/production
of wrong persons.
3. Cloning cannot produce children like the children born to genetic mothers.
Variations in traits are bound to appear.
4. When organisms are created by cloning from somatic cells of the adult,
the longevity of the new born, disease tolerance capacity are some criteria to
be considered. Cloned animals have also developed diseases like arthritis.
5. Cloning also leads to wastage of egg cells. In the cloning of Cat, 200 egg
cells were used and 57 were implanted. Out of that only one cloned cat
survived to birth.
6. Cloned animals may have health problems. They may die at a much earlier
age than the rest of the species. So cloned animals from
somatic cells of adult, may have short life span.
7. Among the benefits of cloning, special mention should be made
regarding its role in biodiversity. Cloning will help to maintain biodiversity. It
can bring back even the animals which have become extinct recently and
safe guard all endangered species facing extinction.
8. Though human cloning has its own ethical problems, the
principle could be used to grow new organs from the cloned stem cells. Such
organ culture may solve transplantation problems, such as tissue incompatibility, tissue rejection, harmful immune reactions etc. Many human
lives could be saved.


Related Topics in Zoology:

Bio Zoology All Important Topics

  1. MODERN GENETICS Introduction and Scope

  2. Human Genetics – Karyotyping

  3. Karyotyping of Human chromosomes

  4. Genetic Engineering

  5. Tools Of Genetic Engineering

  6. Restriction enzymes – Recombinant DNA Technology

  7. DNA – Segmenting / Fragmenting

  8. Genetic Diseases

  9. Human Genome Project (HGP)

  10. Cloning

  11. Transgenic organisms

  12. Gene Therapy

  13. Bioinformatics

  14. Scope of Genetic Engineering – Scope of Bioinformatics

  15. Database

  16. Genome sequencing

  17. Protein structure


    1. Human population and explosion

    2. Population Explosion

    3. Growing Population and Environmental impacts

    4. Global warming – Green house effect

    5. Ozone layer depletion

    6. Prevention and Effect of Ozone depletions

    7. Waste management – Classification

    8. Management of hazardous wastes

    9. Management of non hazardous wastes

    10. Waste water treatment and management

    11. Biodiversity

    12. Conservation of Biodiversity

    13. Characteristics of a Bioreserve

    14. Energy crisis and its environmental impact

    15. Steps to be taken to resolve energy crisis

    16. Environmental impacts of Power Sources

    17. Poverty and environment

    18. Fresh water crisis and management

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