The genome is the totality of genetic information in an organism. Nearly all the eukaryotic genome is carried on two or more linear chromosomes separated from the cytoplasm within the membrane of the nucleus. Diploid eukaryotic cells contain two homologues (divergent evolutionary copies) of each chromosome. Mutations, or genetic changes, frequently cannot be detected in diploid cells because the contribution of one gene copy compensates for changes in the function of its homologue. Whereas a gene that does not achieve phenotypic expression in the presence of its homologue is recessive, a gene that overrides the effect of its homologue is dominant. The effects of mutations can be most readily discerned in haploid cells, which carry only a single copy of most genes. Yeast cells (which are eukaryotic) are frequently studied because they can be maintained and analyzed in the haploid state.

Eukaryotic cells contain mitochondria and, in the case of plants, chloroplasts. Within each of these organelles is a circular molecule of DNA that contains a few genes whose function relates to that organelle. Most genes associated with organelle function, however, are carried on eukaryotic chromosomes. Many yeast contain an additional genetic element, an independently replicating 2-µm circle containing about 6.3 kbp of DNA. Such small circles of DNA, termed plasmids or episomes, are frequently associated with prokaryotes. The small size of plasmids renders them amenable to genetic manipulation and, after their alteration, may allow their introduction into cells.

Repetitive DNA, which occurs in large quantities in eukaryotic cells, is infrequently associated with coding regions and is located primarily in extragenic regions. These short sequence repeats (SSRs) or short tandemly repeated (STR) sequences occur in several to thousands of copies dispersed throughout the genome. The presence of SSRs and STRs is well documented, and some show extensive length-polymorphisms. Many eukaryotic genes are interrupted by introns, intervening sequences of DNA that are missing in processed mRNA when it is translated. Introns have been observed in archaebacterial genes but with a few rare exceptions are not found in eubacteria .

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مواضيع ذات صلة

Nucleic Acids and Their Organization in Eukaryotic, Prokaryotic, and Viral Genomes

The Sugar Arabinose and Arabinose Metabolism

A Mechanism for Induction

The DNA-binding Domain of lac Repressor

Targeting of Mitochondrial Proteins

Intra-Golgi Transport and Protein Processing

Cotranslational Protein Translocation Into the Endoplasmic Reticulum

The Isolation and Structure of Operator

The Migration Retardation Assay and DNA Looping

Repressor Binds to DNA: The Operator is DNA

Detection and Purification of lac Repressor

The Difficulty of Detecting Wild-Type lac Repressor