The Genetic Basis of Development

From Single Cell to Multicellular Organism

When the primary research goal is to understand broad biological principles, the organism chosen for study is called a model organism.

• representative of larger group
• easy to grow
• have relatively short generation times
• small genomes (Ciana Fahnbulleh)

Campbell, Neil A. AP Edition Biology. 7th. San Francisco, CA: Pearson Benjamin Cummings, 2005.

The friut fly, mouse, and zebrafish are examples of model organisms.(Ciana Fahbnulleh)

friut fly, mouse, and zebrafish [Online image] Available www.nih.gov/science/models/, November 25, 2008

Embryonic development involves cell division, cell differentiation, and morphognesis.

A single-celled zygote produces cells of many different types in the embryonic development of most organisms. Each type has a different structure and corresponding function. The process of embryonic development must produce higher-level structures arranged in a particular way in three dimensions. During embryonic development, cells undergo cell differentiation, the process by which cells become specialized in structure and function. The various cells are organized into tissues and organs. The physical processes that give an organism its shape constitute morphogenesis. Morphogenesis is the development of body shape and organization. (Mroz)

Campbell, Neil A.. AP Edition Biology. 7th. San Francisco, CA: Pearson Benjamin Cummings, 2005.

Evidence for Genomic Equivalence

“The results of many experiments support the conclusion that nearly all the cells of an organism have the same genes.” (Dreyer)

Campbell, Neil A.. AP Edition Biology. 7th. San Francisco, CA: Pearson Benjamin Cummings, 2005.

Totipotency in Plants

An experimental approach for testing genomic equivalence is to see whether a differentiated cell can generate a whole organism. Such experiments were performed during the 1950s by F. C. Steward and his students at Cornell University, working with carrot plants. “These scientists found that differentiated cells taken from the root (the carrot) and placed in culture medium could grow into normal adult plants, each genetically identical to the “parent” plant. These results show that differentiation does not necessarily involve irreversible changes in the DNA. In plants mature cells can dedifferentiate and then give rise to all the specialized cell types of the mature organism. Any cell with this is said to be totipotent.”

Using one or more somatic cells from a multicellular organism to make another genetically identical individual is called cloning, and each new individual made in this way can be called a clone (from the Greek klon, twig). Plant cloning is now used extensively in agriculture. Indeed, if you have ever grown a new plant from a cutting, you have practiced cloning. (Dreyer)

Campbell, Neil A.. AP Edition Biology. 7th. San Francisco, CA: Pearson Benjamin Cummings, 2005.

 Transcriptional Regulation of Gene Expression During Development

The observable changes of tissues and organs of an embryo taking shape are the outcome of a cell's developmental history extending back to the first mitotic divisions of the zygote. These early changes only show at the molecular level. Before scientists knew about molecular changes in embryos, they referred to the events that lead to the observable differentiation of a cell as determination. Once an embryonic cell is irreversibly committed to its final state, it is referred to as determined. Tissue-specific proteins are found only in a specific cell type and give the cell its characteristic structure and function. Different cells make different proteins, usually by transcriptional regulation. Maternal molecules in the cytoplasm and signals from other cells directs transcriptional regulation. (Mroz)

Reproductive Cloning of Mammals

Scientists have been able to prove that all of the cells in an organism have the same DNA.  They take the DNA from a diploid cell of an organism and implant the nucleus into an egg cell that had its nucleus removed.  The scientists discovered that the offspring that came from the egg cell had the same DNA as nucleus donor.  The side effect of the cloning process is that, even if the offspring seems healthy when it is born, it has old cells.  The offspring normally have premature deaths, caused by conditions that only affect much older animals. (Archibald)

Chapter 21 Review Questions

Campbell, Neil A.. AP Edition Biology. 7th. San Francisco, CA: Pearson Benjamin Cummings, 2005.

Keller, Amy. "Chapter 21-Key Concepts" Avon High School, Avon, IN. 19 Nov 2008.

Problems Associated with Animal Cloning. 

In most nuclear transplantation studies only a small percentage of cloned embryos develop normally to birth. Like Dolly, many cloned animals exhibit defects. Cloned mice are prone to obesity, pneumonia, liver failure, and premature death. Scientists believe that even cloned animals that appear normal are likely to have subtle defects.

We have begun to learn possible reasons underlying the low efficiency of cloning and the high incidence of abnormalities. In the nuclei of fully differentiated cells, a small subset of genes is turned on and expression of the rest is repressed. This regulation often is the result of epigenetic changes in chromatin, such as acetylation of histones or methylation of DNA. Many of these changes must be reversed in the nucleus from a donor animal in order for genes to be expressed or repressed appropriately for early stages of development. Researchers have found that the DNA in embryonic cells from cloned embryos, often has more methyl groups than does the DNA in equivalent cells from uncloned embryos of the same species. This finding suggests that the reprogramming of donor nuclei is not always complete. Because DNA methylation helps regulate gene expression, misplaced methyl groups in the DNA of donor nuclei may interfere with the pattern of gene expression necessary for normal embryonic development. (Dreyer)

Campbell, Neil A.. AP Edition Biology. 7th. San Francisco, CA: Pearson Benjamin Cummings, 2005.

Keller, Amy. "Chapter 21-Key Concepts" Avon High School, Avon, IN. 19 Nov 2008.