Do you remember Dolly the sheep? The first mammal to ever be cloned. If you think a sheep was impressive, researchers in China have now cloned the first monkey, and not just that... They are twins! Meet Zhong Zhong and Hua Hua.
Figure 1: Zhong Zhong and Hua Hua, identical monkeys.
It took 20 years since Dolly, but now two identical long-tailed macaques have been cloned. During this time 23 mammalian species have been cloned. Some examples besides sheep are mouse, cattle, pig, cat, dog, and rat. While it is technologically impressive that we as humans have the ability to clone animals, why though?
Besides bragging rights, cloning is useful because of two key concepts in science.
- When you are doing experiments you want to have one variable parameter. All the other parameters should stay constant so that whatever you measure is due to the change of that one variable parameter only.
- The sample size should be large enough to give a reliable result.
If you are doing physiological research or testing new treatments on animals, and let’s say you are using 100 animals, then all those 100 animals will have a different genetic makeup. So much for keeping your parameters constant right? The solution is to use 100 animals that have an identical genetic makeup: Clones. The reason why the successful cloning and growth of these monkeys is so important is because monkeys are primates like us. They are physiologically comparable and this makes them highly valuable for research purposes.
Method of cloning
The monkeys were cloned with a technique called Somatic Cell Nuclear Transfer (SCNT). To explain how this technique works it is first good to know what a somatic cell is and what a nucleus is.
- A somatic cell is a non-reproductive cell, which means any cell beside sperm or egg cells.
- A nucleus is a compartment in the cell that holds the genetic code or DNA.
It sounds like the technique transfers the nucleus of a somatic cell. But to where? First, you take a fresh egg cell and take the nucleus of the egg cell, secondly, you transfer the nucleus of the somatic cell to the empty egg cell. Simple copy and paste right? Now the new egg cell can be placed in a surrogate mother monkey, who can give birth to the cloned baby monkey (Figure 2).
Figure 2: The cloning procedure of SCNT – Zhen et al. 2018.
Actually, it is a lot more difficult than it seems. The hard part is to successfully get the surrogate mother pregnant. The authors found a way to approach this. I can tell you they injected H3K9me3 demethylase Kdm4d mRNA and did a treatment with histone deacetylase inhibitor trichostatin A before they placed the cell in the surrogate mother, which improves blastocyst development and increases pregnancy rates. But really, what does that even mean? Let me unpack the previous sentence:
In essence, the somatic cell nucleus needs to be reprogrammed to fit and develop into an egg cell. Reasons, why this wouldn’t happen are called ‘undefined reprogramming defects’. One study by Matoba et al. (2014) found that H3K9me3 causes such defects. The mRNA added by Liu et al. removes H3K9me3 and increases the chance of successful pregnancy.
DNA hypermethylation, which is a process that alters the activity of DNA segments, causes errors in DNA expression. A study by Kishigami et al. (2005) showed that DNA hypermethylation can be reduced by a treatment with trichostatin A.
It is an amazing technological achievement that the monkeys have been cloned. It is even more impressive when you realize that this technique can also be used to clone humans. Although, ethical objections will prevent that in the near future. I haven’t really discussed the ethical issues in this post because I wanted to focus more on the technology.
What do you think though? Cloning humans? Yes, or no?
This post was a discussion based on the following paper:
Liu Z, Cai Y, Wang Y, Wang Z, Poo M, Sun Q, et al. Cloning of Macaque Monkeys by Somatic Cell Resource Cloning of Macaque Monkeys by Somatic Cell Nuclear Transfer. Cell [Internet]. :1–7
National Geographic also made a nice video of the news: