Course: SAT 170 Introductory Biotechnology

Unit 8 Reading and Multimedia Assignment

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- Table of Contents (Selected)
- Unit 8 Key Terms (Selected)

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“Mammalian cloning through reproduction,” by Boundless, CC BY SA 3.0

https://www.boundless.com/biology/biotechnology/stem-cells-and-cloning/mammalian-cloning-through-reproduction/

Reading Assignment: Mammalian Cloning

Cloning

Reproductive Cloning

Reproductive cloning is a technology used to generate an animal that has the same nuclear DNA as another animal. Dolly was created by reproductive cloning technology. The donor nucleus was taken from a mammary cell, which had been partially starved in culture to stop cell division and induce de-differentiaion of the cell. The genetic material from the nucleus was transferred to an egg without nucleus. This fusion resulted in an early embryo. The reconstructed egg was treated with an electric current in order to stimulate cell division and transferred to the uterus of a female host where it continued to develop until birth. While many embryos were implanted in surrogate mothers, only Dolly developed properly (see above figure).

Animals created using nuclear transfer technology are not truly an identical clone of the donor animal. Only the clone's chromosomal or nuclear DNA is the same as the donor. Some of the clone's genetic materials come from the mitochondria in the cytoplasm of the enucleated egg, which contain its own DNA. Acquired mutations in mitochondrial DNA are believed to play an important role in the aging process. In additon, clones may look different from their parents due to epigenetic reprogramming.

Success of reproductive cloning proved that the genetic material from a specialized adult cell could be reprogrammed to generate an entire new organism. However, some scientists believe that errors in the reprogramming process cause the high rates of death, deformity, and disability observed among animal clones.

Embryo Cloning

Therapeutic cloning, also called "embryo cloning", is the production of human embryos for use in research. The goal of this process is to harvest stem cells, which are pluripotent cells that can be used to generate any type of specialized cell in the human body. Stem cells are extracted from blastocyst. The extraction process destroys the embryo, which raises various ethical concerns. Many researchers hope that stem cells can be used to serve as replacement cells to treat many diseases, as well as in organ transplants.

In November 2001, scientists from Advanced Cell Technologies (ACT) announced that they had cloned the first human embryos for the purpose of advancing therapeutic research. However, their results were limited in success. Although this process was carried out with eight eggs, only one was able to divide into six cells before stopping.

In 2007, Oregon National Primate Research Center (ONPRC) scientists were the first to utilize cloning to reproduce primate embryos and then extract embryonic stem cells. However, due to the inefficiency of animal cloning (only about one or two viable offspring for every 100 experiments) and the limited understanding of reproductive cloning, many scientists and physicians strongly believe that it would be unethical to attempt to clone humans. Not only do most attempts to clone mammals fail, about 30% of clones born alive are affected with "large-offspring syndrome" and other debilitating conditions. Several cloned animals have died prematurely from various complications. The same problems would be expected in human cloning.

Somatic epigenetic inheritance, particularly through DNA methylation and chromatin remodeling, is very important in the development of multicellular eukaryotic organisms. The genome sequence is mostly static, but cells differentiate into many different types that perform different functions and respond differently to the environment and intercellular signalling. Thus, as individuals develop, morphogens activate or silence genes in an epigenetically heritable fashion, giving cells a "memory." In mammals, most cells terminally differentiate, with only stem cells retaining the ability to differentiate into several cell types ("totipotency" and "multipotency"). In mammals, some stem cells continue producing new differentiated cells throughout life, but mammals are not able to respond to the loss of some tissues.

In order for an animal cloned from a differentiated cell to develop normally, many of the genes that are repressed by methylation must be expressed again during the embryo's development. Most cloned embryos do appear to have higher levels of methylation than normal embryos, similar to a differentiated cell. This methylation is thought to cause developmental defects.

Genomic imprinting is an epigenetic process that involves methylation and histone modifications in order to achieve monoallelic gene expression without altering the genetic sequence. These epigenetic marks are established in the germline and are maintained throughout all somatic cells of an organism. Imprinting may cause problems in cloning, with clones having DNA that is not methylated in the correct position. It is possible that this is due to a lack of time for reprogramming to be completely achieved.