reproductive cloning: cloning with the goal of initiating a pregnancy and thus a new, living organism; e.g. human reproductive cloning, if successful, would result in a new human being--one who has (virtually) the same DNA as someone else [we will discuss this in the next lecture]
therapeutic (or embryonic) cloning: cloning with the goal of creating embryonic stem cells for use in the treatment of disease
Either form of cloning proceeds by way of…
somatic cell nuclear transfer (SCNT), a.k.a. nuclear somatic transfer (NST):
· step 1: A somatic (i.e. body) cell is taken from a donor.
· step 2: The DNA (46 chromosomes in humans) is removed from that cell.
· step 3: A female gamete (an egg or ovum) is harvested from a second donor and enucleated, i.e. its nucleus is removed. This removes the DNA (23 chromosomes in humans) contained in the nucleus.
· step 4: The DNA from step 2 is inserted into the enucleated egg; at this point in human cloning, the egg would contain a full set of 46 chromosomes, all from a single donor. It would be the functional equivalent of a human zygote (the single cell organism created at conception by the fusion of sperm and ovum).
· step 5: The resulting cell is allowed to mature for a few days, until it is a blastocyst (a pre-implantation embryo of about 128 cells).
This is nearly a form of asexual reproduction:
asexual reproduction (df.): reproduction requiring only one donor of genetic material rather than two.
Rather than having half of all of its chromosomes donated by one parent and the other half by another, all 46 of a human clone's chromosomes would come from the single donor of the DNA. It’s nearly, but not exactly, a form of asexual reproduction, because a small amount of the egg donor’s genetic material (mitochondrial DNA) is left behind on the mitochondria of the egg.
embryonic stem cells (df.): cells in a pre-embryo that have not yet differentiated into specific cell types, such as nerve cells, muscle cells, bone cells, blood cells, etc. They are pluripotent:
pluripotent (df.): having the potential to develop into many different types of body cell (the noun form of the word: “pluripotency.”)
Because of this, they can be harvested from a blastocyst (resulting in its destruction) and coaxed into developing into specific types of cell.
Because ESCs are pluripotent, they can be used in the treatment of disease. Scientists believe that they can be used to treat a wide range of diseases, including Alzheimer's, Parkinson's, cancer and diabetes.
By discarding hundreds of embryos to get one stem cell line, the researchers abused human life, said Richard Doerflinger with the U.S. Conference of Catholic Bishops.
"This is creating new human life solely to destroy it, and in an especially wasteful way," he said.
Researchers led by Shinya Yamanaka has developed a technique for reprogramming skin cells to behave as if they were embryonic stem cells—first in mice (2006) and then in humans (2007). These cells are known as induced pluripotent stem cells, or iPS cells.
This research suggests that we may eventually be able to derive all the benefits of therapeutic cloning without having to engage in the morally controversial destruction of human embryos.
But many also believe that research with human embryos needs to continue, as there is no guarantee that any one line of research will ever result in useful human therapies.
This page last updated 10/11/2010.
Copyright © 2010 Robert Lane. All rights reserved.