遗传分析-第二十一章Extranuclear Genes.doc

Chapter21 Extranuclear GenesKey Concepts Chloroplasts and mitochondria each contain multiple copies of their own unique “chromosome” of genes. Generally, organelle DNA—and any variant phenotype encoded therein—is inherited through the maternal parent in a cross. In mixtures of two genetically different mitochondrial DNAs or chloroplast DNAs, it is commonly observed that a sorting-out process results in descendant cells of one type or the other. In “dihybrid” organelle mixtures, recombination can be detected. Organelle genes encode mainly organelle translation components and components of energy-producing systems. Most organelle-encoded polypeptides unite with nucleus-encoded polypeptides to produce active proteins, which function in the organelle. Introduction By far the larger proportion of the DNA of eukaryotic organisms is found in the nuclear chromosomes. However, two types of organelles, mitochondria and chloroplasts (Figure 21-1), each contain a unique type of “chromosome” of genes that encode specific functions of that organelle. The mitochondrial chromosome is called mtDNA, and the chloroplast chromosome is cpDNA. The functions of mitochondrial genes are directed at making ATP (“chemical energy”) by oxidative phosphorylation, which takes place in the mitochondrion itself. Chloroplast genes are ultimately concerned with making ATP by photosynthesis. The number of genes in organellar chromosomes is small relative to the number in the nucleus. For example, the human nuclear genome consists of 3,000,000 kb of DNA containing about 100,000 genes, whereas human mtDNA is only 17 kb and has only 37 genes. In any one organism, a gene found in an organelle chromosome is generally not found in the nuclear chromosomes, although a few may be present in the nucleus as inactive pseudogenes. In structure and function, organelle genes show many generic similarities with nuclear genes, but there are enough differences in their action and inheritance to make them worthy of s