Cloning of Animals

The process of cloning, or producing exact genetic duplicates, is an application of molecular genetics that continues to raise controversy. It has recently become possible to clone not only DNA molecules, but also entire complex organisms such as sheep. One method by which animal cloning may be accomplished is demonstrated here. Cloning requires two types of cells: a donor cell containing the complete genome of the animal of interest, and a recipient unfertilized egg cell. In this case, sheep stem cells, which have a unique capacity to become almost any type of body tissue, are used as donor cells. The stem cells are collected from the sheep to be cloned and cultured in vitro, or artificially, in the presence of a specially formulated growth serum, shown as the green liquid. Then, cells to be used in cloning experiments are grown at a reduced concentration of nutrients to induce entrance into a state of dormancy, in which cell growth and division is halted. It has been shown that, for cloning to work properly, both donor and recipient cell must be at synchronized states in the cell cycle or in other words, if the recipient is in a dormant state, the donor cell must also be inactive. This induction into a state of non-division is represented here by the removal of one cell from the nutrient media; this cell will act as the DNA donor in this experiment. When the donor cells are ready for use, an unfertilized egg cell is harvested from a sheep. Holding the cell in place with a needle, an instrument called a micropipette is used to remove the nucleus, and with it all of the cell’s DNA. As a result, the egg enters a state of dormancy, which can then be inserted in place of the nucleus. Usually an electric pulse, not shown here, is used to induce the egg to fuse with the donor stem cell and accept its genetic material as a new nucleus. Next, the cell is activated with protein media, shown as the green liquid, to promote growth. The stem cell within the egg divides rapidly into many cells; one is removed and again deactivated by growth in a serum that is low in nutrients. The nucleus of a second unfertilized sheep egg is then removed by the same technique as before, and the stem cell harvested from the first egg is fused with the second egg. The fused cell is then activated with protein media as before, and the cells again multiply rapidly. However, this time no cells are removed and an embryo is allowed to develop. Finally, the embryo is implanted into the womb of another sheep and allowed to develop until birth – if all goes well, the result is a healthy lamb that is genetically identical to the donor animal. It is speculated that this technology may be applied to human beings as well as to sheep, as an infertility treatment alternative. However, many of the clones generated in laboratories at the present time have shown health problems such as rapid aging and physical deformities. These issues, as well as ethical concerns, have caused many people to object to cloning experiments involving humans. Clearly, a great deal of thought, discussion, and refinement of experimental techniques should occur before any attempt is made to clone a human being.

Question 1: What type of cells are required for cloning?
  1. An egg cell and a sperm cell.
  2. A donor cell containing the complete genome of the animal of interest, and a recipient, unfertilized egg cell.
  3. A donor cell containing the gene of the animal of interest, and a recipient, unfertilized egg cell.
  4. A donor cell and a host cell.


Answer 1 Answer: (b) A donor cell containing the complete genome of the animal of interest, and a recipient unfertilized egg cell.

Question 2: Why sheep stem cells are used as donor cells?
  1. Because they are larger than other sheep cells.
  2. Because they can take up new DNA better than human cells.
  3. Because they can take up new DNA better than other sheep cells.
  4. Because they have a unique capacity to become almost any type of body tissue.


Answer 2 Answer: (d) Because they have a unique capacity to become almost any type of body tissue.

Question 3: Where are stem cells are artificially cloned?
  1. In vitro
  2. In vivo
  3. In chicken
  4. In sheep


Answer 3 Answer: (a) In vitro

Questions 4: What does a “state of dormancy” mean?
  1. When cell growth and division is halted.
  2. When a cell is asleep.
  3. When a sheep is getting ready for surgery.
  4. When cells stop elongating.


Answer 4 Answer: (a) When cell growth and division is halted.

Question 5: An unfertilized egg cell is harvested from a sheep and its DNA material is removed by a ____________, an electric pulse is used to induce the _____________to fuse with the _________ and accept its genetic material as a new nucleus.
  1. micropipette, egg, donor stem cell
  2. gel, stem cell, egg
  3. tweezer, egg, donor sperm cell
  4. tweezer, egg, donor stem cell


Answer 5 Answer: (a) micropipette, egg, donor stem cell