Biology
The map shown in Figure 3.40 accounts for the major RNA and protein products of the yeast mitochondrion. The most notable feature is the dispersion of loci on the map.
- The Chloroplast Genome Codes For Many Proteins And Rnas
KEY CONCEPTS:Chloroplast genomes vary in size, but are large enough to code for 50-100 proteins as well as the rRNAs and tRNAs. What genes are carried by chloroplasts? Chloroplast DNAs vary in length from 120-190 kb. The sequenced chloroplast genomes...
- The Human Genome Has Fewer Genes Than Expected
KEY CONCEPTS:Only 1% of the human genome consists of coding regions. The exons comprise ~5% of each gene, so genes (exons plus introns) comprise ~25% of the genome. The human genome has 30,000-40,000 genes. ~60% of human genes are alternatively spliced....
- How Did Interrupted Genes Evolve?
KEY CONCEPTS:The major evolutionary question is whether genes originated as sequences interrupted by exons or whether they were originally uninterrupted. Most protein-coding genes probably originated in an interrupted form, but interrupted genes that...
- Genes Show A Wide Distribution Of Sizes
KEY CONCEPTS:Most genes are uninterrupted in yeasts, but are interrupted in higher eukaryotes. Exons are usually short, typically coding for <100 amino acids. Introns are short in lower eukaryotes, but range up to several 10s of kb in length in higher...
- Organization Of Interrupted Genes May Be Conserved
KEY CONCEPTS:Introns can be detected by the presence of additional regions when genes are compared with their RNA products by restriction mapping or electron microscopy, but the ultimate definition is based on comparison of sequences. The positions of...
Biology
Mitochondrial DNA organization is variable
KEY CONCEPTS:
- Animal cell mitochondrial DNA is extremely compact and typically codes for 13 proteins, 2 rRNAs, and 22 tRNAs.
- Yeast mitochondrial DNA is 5× longer than animal cell mtDNA because of the presence of long introns.
Animal mitochondrial DNA is extremely compact. There are extensive differences in the detailed gene organization found in different animal phyla, but the general principle is maintained of a small genome coding for a restricted number of functions. In mammalian mitochondrial genomes, the organization is extremely compact. There are no introns, some genes actually overlap, and almost every single base pair can be assigned to a gene. With the exception of the D loop, a region concerned with the initiation of DNA replication, no more than 87 of the 16,569 bp of the human mitochondrial genome can be regarded as lying in intercistronic regions.
The complete nucleotide sequences of mitochondrial genomes in animal cells show extensive homology in organization (Boore, 1999). The map of the human mitochondrial genome is summarized in Figure 3.39. There are 13 protein-coding regions. All of the proteins are components of the apparatus concerned with respiration. These include cytochrome b, 3 subunits of cytochrome oxidase, one of the subunits of ATPase, and 7 subunits (or associated proteins) of NADH dehydrogenase (Anderson et al., 1981; for review see Clayton, 1984; Attardi, 1985; Gray, 1989).
The five-fold discrepancy in size between the S. cerevisiae (84 kb) and mammalian (16 kb) mitochondrial genomes alone alerts us to the fact that there must be a great difference in their genetic organization in spite of their common function. The number of endogenously synthesized products concerned with mitochondrial enzymatic functions appears to be similar. Does the additional genetic material in yeast mitochondria represent other proteins, perhaps concerned with regulation, or is it unexpressed?
The map shown in Figure 3.40 accounts for the major RNA and protein products of the yeast mitochondrion. The most notable feature is the dispersion of loci on the map.The two most prominent loci are the interrupted genes box (coding for cytochrome b) and oxi3 (coding for subunit 1 of cytochrome oxidase). Together these two genes are almost as long as the entire mitochondrial genome in mammals! Many of the long introns in these genes have open reading frames in register with the preceding exon (see 26.5 Some group I introns code for endonucleases that sponsor mobility). This adds several proteins, all synthesized in low amounts, to the complement of the yeast mitochondrion.
The remaining genes are uninterrupted. They correspond to the other two subunits of cytochrome oxidase coded by the mitochondrion, to the subunit(s) of the ATPase, and (in the case of var1) to a mitochondrial ribosomal protein. The total number of yeast mitochondrial genes is unlikely to exceed ~25.
- The Chloroplast Genome Codes For Many Proteins And Rnas
KEY CONCEPTS:Chloroplast genomes vary in size, but are large enough to code for 50-100 proteins as well as the rRNAs and tRNAs. What genes are carried by chloroplasts? Chloroplast DNAs vary in length from 120-190 kb. The sequenced chloroplast genomes...
- The Human Genome Has Fewer Genes Than Expected
KEY CONCEPTS:Only 1% of the human genome consists of coding regions. The exons comprise ~5% of each gene, so genes (exons plus introns) comprise ~25% of the genome. The human genome has 30,000-40,000 genes. ~60% of human genes are alternatively spliced....
- How Did Interrupted Genes Evolve?
KEY CONCEPTS:The major evolutionary question is whether genes originated as sequences interrupted by exons or whether they were originally uninterrupted. Most protein-coding genes probably originated in an interrupted form, but interrupted genes that...
- Genes Show A Wide Distribution Of Sizes
KEY CONCEPTS:Most genes are uninterrupted in yeasts, but are interrupted in higher eukaryotes. Exons are usually short, typically coding for <100 amino acids. Introns are short in lower eukaryotes, but range up to several 10s of kb in length in higher...
- Organization Of Interrupted Genes May Be Conserved
KEY CONCEPTS:Introns can be detected by the presence of additional regions when genes are compared with their RNA products by restriction mapping or electron microscopy, but the ultimate definition is based on comparison of sequences. The positions of...