EUKARYOTIC GENES Remember that eukaryotic genes are linear rather than circular. These are imbedded into nucleosomes, which pack the DNA together. The genome size does not relate to the actual size of the organism; small organisms can have much larger genomes than humans, for example. In humans, thee are about 20,000 different transcribing genes in the genome. Eukaryotic cells also have mitochondrial genomes and plants have chloroplast genomes. These two other genomes are much smaller so that, in humans, the number of base pairs in mitochondrial DNA is just about 16,500 base pairs. Gene regulation in eukaryotes is more complex than it is in bacteria. The main expression occurs at the level of transcription—particularly at the start of transcription. There are proteins in eukaryotic genes that will modulate the activity of RNA polymerase. These proteins and the regulation of transcription is, of course, different in each type of cell in a multicellular organism. This is a necessity because different cell types need to make different proteins and enzymes. There are different regulatory proteins in the different types of cells. Methylation of DNA also adds to cellular complexity. With bacteria and eukaryotes, there are cis-acting genes or sequences, which are genes that are located together or adjacent to one another. Genes are transcribed by RNA polymerase II; each gene has two promotor elements. The first is called the TATA box and the second is called the INR sequence. These bind general transcription factors. There are also sequences called enhancers, which can be located far upstream from the actual genes that get transcribed. These enhancer sequences allow for the more efficient transcription of the genes. Enhancers can also be located downstream from transcribed gene sites. Without an enhancer, the gene will only be transcribed at a low level. Enhancers bind to proteins that change the activity of RNA polymerase. Because of looping of the DNA molecule, the enhancer does not have to be near the promotor site. This allows transcription factors bound to a specific enhancer to act in similar ways to the promotor sites. This basically means that enhancers are really no different from promotor sites and other regulatory sequences on cis-acting genes.
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