0.2 Eukaryotic genetics  (Page 5/9)

The enhancer for the promoter of the gene for the delta chain of the gamma/delta T-cell receptor for antigen (TCR) is located close to the promoter for the alpha chain of the alpha/beta TCR (on chromosome 14 in humans). A T cell must choose between one or the other. There is an insulator between the alpha gene promoter and the delta gene promoter that ensures that activation of one does not spread over to the other.

Example: The enhancer for the promoter of the gene for the delta chain of the gamma/delta T-cell receptor for antigen (TCR) is located close to the promoter for the alpha chain of the alpha/beta TCR (on chromosome 14 in humans). A T cell must choose between one or the other. There is an insulator between the alpha gene promoter and the delta gene promoter that ensures that activation of one does not spread over to the other.

Another example: In mammals (mice, humans, pigs), only the allele for insulin-like growth factor-2 (IGF2) inherited from one's father is active; that inherited from the mother is not — a phenomenon called imprinting.

The mechanism: the mother's allele has an insulator between the IGF2 promoter and enhancer. So does the father's allele, but in his case, the insulator has been methylated. CTCF can no longer bind to the insulator, and so the enhancer is now free to turn on the father's IGF2 promoter.

Link to a discussion of imprinting.

Many of the commercially-important varieties of pigs have been bred to contain a gene that increases the ratio of skeletal muscle to fat. This gene has been sequenced and turns out to be an allele of IGF2, which contains a single point mutation in one of its introns. Pigs with this mutation produce higher levels of IGF2 mRNA in their skeletal muscles (but not in their liver).

This tells us that:

• Mutations need not be in the protein-coding portion of a gene in order to affect the phenotype .
• Mutations in non-coding portions of a gene can affect how that gene is regulated (here, a change in muscle but not in liver).

Mutations in non-coding portions of a gene can affect how that gene is regulated (here, a change in muscle but not in liver).

For consideration of regulation elements in detail, such as GAL genes in S. cerevisiae ( PDF ), Transcription regulation in S. cerevisiae ( PDF ), and Global transcriptional profiling ( PDF - 1.4 MB ), click PDF files from MITOPENCOURSEWARE respectively.

Lecture 27. tetrad analysis in fungi

In general, tetrad is the products of a single meiosis in all eukaryotic diploid organisms from simplest ones such as Saccharomyces cerevisiae to complex organisms like human beings. Tetrad analysis is a genetic dissection involving tetrads and based on movement laws of chromosomes in meiosis. Theorically tetrad analysis can be carried out in all eukaryriotes. However, technically tetrad analysis can easily and Mutations in non-coding portions of a gene can affect how that gene is regulated (here, a change in muscle but not in liver).

The yeast Saccharomyces cerevisiae has been a very important genetic tool. It has been used in genetic studies for many decades as one of the best characterized eukaryotic organisms. Since it is very small and unicellular, large numbers of the yeast can be grown in culture in a very small amount of space, in much the same way that bacteria can be grown. However, yeast has the advantage of being a eukaryotic organism, so the results of genetic studies with yeast are more easily applicable to human genetics. It reproduces abundantly and quickly, producing more haploid cells. They can also mate with an appropriate strain, later undergoing karyogamy and growing as a diploid. The diploid can undergo meiosis to form ascospores, recombinant haploid progeny unlike either parent. Mitosis and meiosis can be more easily studied in these organisms. Lee Hartwell, from the Fred Hutchison Cancer Research Center in Seattle, won the Nobel Prize in Medicine in 2001 for his pioneering work on the mitosis genes in S. cerevisiae. He shared the prize with R. Timothy Hunt and Paul M. Nurse of the Imperial Cancer Research in London, who work on another yeast, Schizosaccharomyces pombe. The genes they discovered and characterized in the yeast as a model organism have led to some important discoveries in fighting cancer in humans.