Thursday 25 January 2018

The Premature Death of Dolly the Sheep

Was it due to cloning or infection?

Dolly (July 5, 1996 - February 14, 2003) 
[Credit: Wikimedia Commons]

Cloning Dolly
A clone has the same DNA sequence as its parent and so they are genetically identical.  Dolly was the first mammal to be cloned from an adult cell.  This was a major scientific achievement then as it proved that the DNA from adult cells can be used to create an entire organism.  Before Dolly, several clones were produced in the lab from the DNA of embryos (frogs, mice, and cows).  

The Mechanics
Cloning from an adult cell is much more difficult than from an embryonic cell. Dolly was born after 277 attempts by scientists from the Roslin Institute in Scotland.

[Credit: ResearchGate]

1. An udder cell was acquired from a six-year-old Finn Dorset white sheep. 

2. The udder cell was 'reprogramed' to keep them alive but stop them from growing – it was achieved by altering the growth medium (the ‘soup’ in which the cells were kept alive). 

3. The cell was injected into an unfertilised egg cell which had its nucleus removed, and then fused by using electrical pulses.  [The unfertilised egg cell came from a Scottish Blackface ewe.]

4. To ensure the resulting cell develop into an embryo, they cultured it for 6 or 7 days to see if it divided and developed normally, before implanting it into a surrogate mother, another Scottish Blackface ewe.

5. From 277 cell fusions, 29 early embryos developed and were implanted into 13 surrogate mothers. Only one pregnancy went to full term, and the 6.6 kg Finn Dorset lamb 6LLS (alias Dolly) was born after 148 days.

What happened to Dolly?
Dolly lived a pampered existence at the Roslin Institute. She mated and produced normal offspring the normal way, showing that such cloned animals can reproduce. She was euthanised on 14 February 2003, aged 6½ years old.  Sheep can live to an age of 11 or 12, but Dolly suffered from arthritis in a hind leg joint and from sheep pulmonary adenomatosis, a virus-induced lung tumour that is common among sheep which are raised indoors.

The DNA in the nucleus is wrapped up into chromosomes, which shorten each time the cell replicates. This meant that Dolly’s chromosomes were a little shorter than those of other sheep her age and her early aging may reflect that she was raised from the nucleus of a 6-year old sheep. Dolly was also not entirely identical to her genetic mother because the mitochondria, the power plants of the cell that are kept outside the nucleus, were inherited from Dolly’s egg donor mother.

Shared by Azni Zainal Abidin
Guest Blogger

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