Question

differential gene expression ?

Answer 1

Differential Gene Expression

If the genome is the same in all somatic cells within an organism (with the exception of the above-mentioned lymphocytes), how do the cells become different from one another? If every cell in the body contains the genes for hemoglobin and insulin proteins, how are the hemoglobin proteins made only in the red blood cells, the insulin proteins made only in certain pancreas cells, and neither made in the kidney or nervous system? Based on the embryological evidence for genomic equivalence (and on bacterial models of gene regulation), a consensus emerged in the 1960s that cells differentiate through differential gene expression. The three postulates of differential gene expression are as follows:

1.

Every cell nucleus contains the complete genome established in the fertilized egg. In molecular terms, the DNAs of all differentiated cells are identical.

2.

The unused genes in differentiated cells are not destroyed or mutated, and they retain the potential for being expressed.

3.

Only a small percentage of the genome is expressed in each cell, and a portion of the RNA synthesized in the cell is specific for that cell type.

The first two postulates have already been discussed. The third postulate—that only a small portion of the genome is active in making tissue-specific products—was first tested in insect larvae. Fruit fly larvae have certain cells whose chromosomes become polytene. These chromosomes, beloved by Drosophilageneticists, undergo DNA replication in the absence of mitosis and therefore contain 512 (29), 1024 (210), or even more parallel DNA double helices instead of just one (Figure 4.13A,Figure 4.13B). These cells do not undergo mitosis, and they grow by expanding to about 150 times their original volume. Beermann (1952)showed that the banding patterns of polytene chromosomes were identical throughout the larva, and that no loss or addition of any chromosomal region was seen when different cell types were compared. However, he and others showed that in different tissues, different regions of these chromosomes were making organ-specific RNA. In certain cell types, particular regions of the chromosomes would loosen up, “puff” out, and transcribe mRNA. In other cell types, these regions would be “silent,” but other regions would puff out and synthesize mRNA.



Figure 4.13

Polytene chromosomes. (A) Polytene chromosomes from the salivary gland cells of Drosophila melanogaster. The four chromosomes are connected at their centromere regions, forming a dense chromocenter (arrowhead). The DNA has been stained red with propidium (more...)

The idea that the genes of chromosomes were differentially expressed in different cell types was confirmed using DNA-RNA hybridization (Figure 4.13C). This technique involves annealing single-stranded pieces of RNA and DNA to allow complementary strands to form double-stranded hybrids. While some mRNAs from one cell type were also found in other cell types (as expected for mRNAs encoding enzymes concerned with cell metabolism), many mRNAs were found to be specific for a particular type of cell and were not expressed in other cell types, even though the genes encoding them were present (Wetmur and Davidson 1968). Thus, differential gene expression was shown to be the way a single genome derived from the fertilized egg could generate the hundreds of different cell types in the body. The question then became, How does this differential gene expression occur? The answers to that question will be the topic of the next chapter. To understand the results that will be presented there, however, one must become familiar with some of the techniques of molecular biology that are being applied to the study of development. These include techniques to determine the spatial and temporal location of specific mRNAs, as well as techniques to determine the functions of these messages.