Eukaryotic genes

This reverses the selection pressures on bacteria whereas bacteria are oppressed by heavy selection pressure to lose genes, eukaryotes are under pressure to gain them. 

Besides duplicated protein-coding genes and tandemly repeated genes, eukaryotic cells contain multiple copies of other DNA sequences in the genome, generally referred to as repetitious DNA (see Table 10-1). Of the two main types of repetitious DNA, the less prevalent is simple-sequence DNA, which constitutes about 3 percent of the human genome and is composed of perfect or nearly perfect repeats of relatively short sequences. The more common type of repetitious DNA, composed of much longer sequences, is discussed in Section 10.3. 

In many cases it is possible to synthesize the product of a gene in a different organism, eg, bacteria, yeast, or higher eukaryote. Recombinant DNAs directing the synthesis of the gene product must contain information specifying a number of biochemical processes. 

Because different cell types in eukaryotic organisms express selected subsets of genes, RNA preparations from cells or tissues in which genes of interest are selectively transcribed are enriched for the desired mRNAs. cDNA... 

The field of DNA vaccination started when eukaryotic expression vectors were injected into the muscle of laboratory animals [2]. The authors observed protein expression for more than 2 months after injection and noted that no special delivery system was required to obtain this expression. Subsequently, it was demonstrated that antibodies can be induced simply by injecting plasmid DNA into the muscle of mice [3]. Subsequent studies found that the injection of expression plasmids also leads to the induction of a cytotoxic T-cell response. After injection, the DNA enters cells of the vaccinated host and the encoded gene becomes expressed. This eventually leads to the induction of a cellular cytotoxic T-cell, T-helper, and/or humoral (antibody) immune response. 

The eukaryotic expression cassette is the part of an expression vector that enables production of a protein in a eukaryotic cell. The cassette consists of a eukaryotic promoter for mRNA transcription, the gene and an mRNA termination and processing signal (Poly-A signal). 

Heterologous expression systems comprise prokaryotic organisms (e.g., E. coli) and eukaryotic cells (e.g., yeast, HEK293, Xenopus oocytes), which are used to functionally express foreign genes or cDNAs. 

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