Cloning

Quick Check: Describe how Dolly the sheep was produced. Include the names of the organisms involved and the role of each.

Dolly was produced using adult cell cloning (somatic cell nuclear transfer). A nucleus was removed from a mammary gland (body) cell of a Finn Dorset sheep — this is the donor, whose genome Dolly would share. An egg cell was taken from a Scottish Blackface sheep and its nucleus was removed (enucleated), leaving the cytoplasm. The donor nucleus was inserted into the enucleated egg cell. An electric shock was applied to stimulate the egg to begin dividing. The early embryo was then implanted into the uterus of a third sheep (also a Scottish Blackface) acting as the surrogate mother, who carried the pregnancy to term. Dolly was born genetically identical to the original Finn Dorset donor.

Quick Check: A botanist wants to produce 1,000 identical copies of a rare orchid as quickly as possible. Explain how tissue culture could be used to achieve this and state one advantage over taking 1,000 individual cuttings.

Tissue culture: a small sample of cells (e.g., from the meristem of the orchid) is taken and placed in a sterile growth medium (agar) containing the necessary nutrients and plant hormones. The cells divide and differentiate to form many small plantlets, all genetically identical to the original orchid. These are then grown on in controlled conditions until they are large enough to pot individually. Advantage over cuttings: tissue culture can produce thousands of plants from a tiny sample in a very short time, whereas taking 1,000 physical cuttings requires removing large amounts of tissue from the parent plant, potentially damaging or killing it (especially problematic with a rare specimen). Tissue culture also reduces disease risk because the medium is sterile.

Quick Check: Explain why a cloned organism may not be identical to the original organism in appearance and behaviour, even though it has the same DNA.

Although the clone has the same DNA sequence as the original, the physical appearance and behaviour of an organism (its phenotype) is determined by both genetics and the environment. Environmental factors — including temperature during development, diet, disease exposure, social interactions, and random developmental variation — influence which genes are switched on or off and how strongly they are expressed. For example, two cloned cats may have different coat colour patterns due to random X-chromosome inactivation in cells during early development, even though their DNA is identical. Identical twins (natural clones) often differ in weight, personality, and the diseases they develop, demonstrating that shared DNA does not guarantee identical phenotypes.