Cloning

 

Cloning

                                    Dolly the sheep 1996-2003

Image courtesy Wikimedia Commons

I have been thinking about cloning, not as a pastime, you understand, but as a concept.

National Geographic says, ’Cloning is a technique scientists use to make exact genetic copies of living things. Genes, cells, tissues, and even whole animals can all be cloned.

Some clones already exist in nature. Single-celled organisms like bacteria make exact copies of themselves each time they reproduce.’

Parthenogenesis, sometimes known as ‘virgin birth,’ is a complex subject and a form of cloning. Asexual reproduction only requires one parent organism, a female, and resultant offspring are genetically identical. Although it may seem an efficient method of increasing numbers, it is also likely that their vulnerability is enhanced because there is decreased genetic diversity. Any inherent weaknesses in the parent will be replicated in the offspring.

Possibly the largest animals to reproduce asexually are the Hammerhead shark and the Zebra shark. In 2001, a captive hammerhead shark gave birth to a normal live female. She had had no contact with a male for at least three years.

Just look at those eyes! 

Image courtesy Wikimedia Commons

Similarly, a zebra shark in Australia gave birth to three live babies in 2017, although she had been separated from her mate since 2012. Genetic testing proved that the young sharks carried DNA only from their mother.  

Asexual reproduction can take other forms than parthenogenesis.

In fragmentation, which occurs in starfish, the organism breaks into several parts, each part developing into a complete new organism. Sea stars may reproduce sexually and asexually. In some instances, a starfish will deliberately break off a part and regenerate that part, while the broken part will grow a whole new body.

Fission is similar but even simpler. The organism divides itself into two new bodies, each identical. Fission is the commonest form of asexual reproduction.

Other organisms reproduce themselves by budding. The organism grows a ‘bud’ or extension that eventually separates from the original to become an independent creature. For example, Hydras are freshwater creatures that demonstrate asexual budding. They have been in existence for about 200 million years, so were alive in the Late Triassic period when the first, frequently small, two-legged dinosaurs appeared.

Other animals that can reproduce asexually include Komodo dragons, whiptail lizards, pythons, wasps and ants.

Earthworms are interesting. Probably every schoolboy knows – at least, in the dark ages before sophistication, they did – that an earthworm cut in half will become two earthworms. Earthworms cannot reproduce asexually of their own volition. Earthworms are hermaphrodites, carrying male and female sex organs, and can self-fertilise, although that is not the norm.

There are no instances of mammals reproducing parthenogenetically.  

Reproductive cloning begins in a laboratory. The first successfully cloned animal was Dolly the sheep, in 1996. She died when she was seven. Many animals have been cloned since, but they often show cardiac anomalies, or limb and facial aberrations. Cloning has a success rate of about 20% (Columbia University, NY, 2018) That means that 80% of attempts fail because of abnormalities.

Therapeutic cloning or somatic cell nuclear transfer (SCNT) creates human embryonic stem cells. This is known as ‘cloning for therapeutic purposes.’ Though promising, research is still in its infancy and fears of creating humans in the laboratory are unfounded.