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Drosophila Melanogaster Genoma Circular

Figure 3.6. Diagram of the circular DNA molecule from the mitochondria of Drosophila yakuba. The outside circle shows the open reading frames (URF1 to URF6 and URF 4L) that code for subunits of the respiratory chain NADH dehydrogenase and of the genes coding for cytochrome b, cytochrome c oxidase subunits I, II, and III, and ATPase subunits 5 and 6. The origin and direction of replication are indicated by O and R. The variable A+T region is shaded. The arrows indicate the direction of gene transcription. The tRNA genes are crosshatched and indicated by their single-letter amino acid codes. lrRNA and srRNA are the large and small rRNA genes. The numbers on the inside of the outer circle are the numbers of apparently noncoding nucleotides that occur between the genes. The innermost circles indicate restriction fragments produced with the enzymes indicated (from Clary and Wolstenholm 1985).

Figure 3.6. Diagram of the circular DNA molecule from the mitochondria of Drosophila yakuba. The outside circle shows the open reading frames (URF1 to URF6 and URF 4L) that code for subunits of the respiratory chain NADH dehydrogenase and of the genes coding for cytochrome b, cytochrome c oxidase subunits I, II, and III, and ATPase subunits 5 and 6. The origin and direction of replication are indicated by O and R. The variable A+T region is shaded. The arrows indicate the direction of gene transcription. The tRNA genes are crosshatched and indicated by their single-letter amino acid codes. lrRNA and srRNA are the large and small rRNA genes. The numbers on the inside of the outer circle are the numbers of apparently noncoding nucleotides that occur between the genes. The innermost circles indicate restriction fragments produced with the enzymes indicated (from Clary and Wolstenholm 1985).

Most eggs and somatic cells contain hundreds or thousands of mtDNA molecules, so a new mutation can result in a situation in which two or more mtDNA genotypes coexist within an individual (heteroplasmy). Heteroplasmy, however, is apparently a transitory state in germ cells. Thus, the majority of individuals are effectively haploid with regard to the number of types of mtDNA transmitted to the next generation.

Mitochondrial DNA evolves faster than single-copy nuclear DNA because mitochondria are relatively inefficient in repairing errors during DNA replication or after DNA damage. In Hawaiian Drosophila, mtDNA appears to evolve about three times faster than the genes of nuclear DNA (Moritz et al. 1987). Because mtDNA does not code for proteins involved directly in its own replication, transcription, or translation, mtDNA has a large number of length mutations and transitions.

Mitochondrial DNA can be amplified easily from mitochondria by the polymerase chain reaction (PCR) (see Chapter 8) because there are multiple copies in each cell. Mitochondria are easier to purify from cells than a specific segment of nuclear DNA. Mitochondria have a specific buoyant density and high copy number within cells. Isolation of mitochondria by centrifugation is relatively easy, making mtDNA a useful subject for systematics or population genetics studies, as will be described in Chapters 12 and 13.

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