Nov 29 Chapter 11 (continued)

3. Therapeutic and reproductive cloning

A. Cloning

Clones: genetically identical organisms

<Clones in nature>

- Organisms produced via asexual reproduction are clones.

            Examples: bacteria, plants

- Identical twins are clones.

Cloning: process of generating genetically identical organisms (reproductive cloning) or cells (therapeutic cloning).

 

B. Reproductive cloning

- Reproductive cloning of non-human species has potential to commercially produce clones with desirable traits.

- Cloning is a critical step for genetic engineering.

- Other hypothetical uses include: cloning of endangered species, cloning from extinct species???

 

<Clonal propagation of plants>

- When placed in appropriate conditions, differentiated plant cells can dedifferentiate and redifferentiate from single cells (Fig 11.3A, p213).

- This method is widely used for propagating plants for commercial purposes.

   

 

<Cloning of animals>

- Most animals do not reproduce asexually.

- Clones can be produced via nuclear transplantation. (Fig 11.3B, 213; Fig in p209)

            - destroy or remove the nucleus of an egg cell.

            - take a nucleus from an adult cell and transplant it to the egg cell.

            - implant the egg in a surrogate mother.

- First cloning of mammal: 1997, a sheep named Dolly was produced using this method.

- Not practical at this point because of a high failure rate.

 

<Human cloning?>

- Human cloning is theoretically possible but not practical at this point.

- Hypothetical uses: cloning for those without a child, cloning from deceased??? genetic engineering of human???

- Ethical problems

- Research on human reproductive cloning is currently banned in public research institutes.  However, this does not prevent a private company to conduct research. 

 

B. Therapeutic cloning

- Production of stem cells for therapeutic use.

Stem cells: cells which are capable of dividing and which can differentiate into many different types of cells.

- Early embryos contain stem cells (embryonic stem cells or ES cells).

- ES cells can be maintained as cell cultures.

- When placed in an appropriate condition (such as treatment with growth factors), ES cells can differentiate into desired cell types.

- Although it is still in the experimental stage, ES cells may provide treatments or even cure to currently untreatable diseases.

 

<Ethical problem>

- ES cells must be obtained from human embryos.

- Should embryos be treated as “life”?

- Each country has a strict guideline for stem cell research.

- The Canadian government allows the use of embryos which will be disposed otherwise (Ex. embryos that are made for the purpose of in vitro fertilization). 

- The US government does not fund stem cell research unless they use preexisting ES cell lines.

 

Adult stem cells

- Adult bodies contain stem cells to replace cells for normal body function.

Example: The bone marrow contains blood-forming stem cells, which give rise to different types of blood cells.

- In generally, it is believed that adult stem cells are capable of differentiating into much fewer types of cells. 

- Adult stem cells would make an ideal source of stem cells if they are made capable of differentiating into much more different cell types.

4. Genetic basis of cancer

- In normal cells, decision to divide is made according to developmental and environmental signals, such as growth factors.

- Tumor cells escape from the control mechanisms of the cell cycle.

- Cancer is caused by genetic mutations in somatic cells.  (Not inherited to offspring.)

- Mutations in either a proto-oncogene or a tumor-suppressor gene could result in cancer.

 

Proto-Oncogenes

- Proto-oncogenes are normal genes that have the potential to become oncogene (tumor-causing genes).

- Many are genes that normally stimulate cell growth.

- When mutated, proto-oncogenes can become oncogenes (Fig 11.15A, p223). ® tumor induction

 

Tumor-suppressor genes

- Genes normally inhibit cell division.

- When mutated, they may loose control of cell division (Fig 11.15B, p223). ® tumor induction

 

Signal transduction pathway and tumor induction

- Many proto-oncogenes and tumor-suppressor genes are normally involved in signal transduction pathway (Fig 11.16A, p224)

Multiple genetic changes are required for cancer

Example: development of colon cancer

                              

 

What causes cancer?

Carcinogens: cancer causing agents

- Most mutagens are also carcinogens.

- Prolonged exposures to carcinogens may cause cancer.

- Viruses can cause cancer. 

- When viruses are inserted into and excised from host DNA, they may leave small pieces of virus DNA.  This may result in mutation in host DNA.

            Example: cervix cancer, liver cancer

 

Cancer susceptibility

- Some individuals are more susceptible to cancer due to their genetic background.

Example 1: Heritable mutations in the BRCA1 gene increase the risk of developing breast cancer (genetic testing available).

Example 2: The Caucasian population is more susceptible to cancer causing effects of cigarette smoke than the oriental population.

 

Cancer prevention

- Minimize exposure to carcinogen.

- Some diets/habits are known to reduce cancer risks.