Biology
- Mitochondria Evolved By Endosymbiosis
How did a situation evolve in which an organelle contains genetic information for some of its functions, while others are coded in the nucleus? Figure 3.41 shows the endosymbiosis model for mitochondrial evolution, in which primitive cells captured bacteria...
- 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...
- 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...
- 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
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.
The situation is generally similar to that of mitochondria, except that more genes are involved. The chloroplast genome codes for all the rRNA and tRNA species needed for protein synthesis. The ribosome includes two small rRNAs in addition to the major species. The tRNA set may include all of the necessary genes. The chloroplast genome codes for ~50 proteins, including RNA polymerase and ribosomal proteins. Again the rule is that organelle genes are transcribed and translated by the apparatus of the organelle.
About half of the chloroplast genes code for proteins involved in protein synthesis. The endosymbiotic origin of the chloroplast is emphasized by the relationships between these genes and their counterparts in bacteria. The organization of the rRNA genes in particular is closely related to that of a cyanobacterium, which pins down more precisely the last common ancestor between chloroplasts and bacteria.
Introns in chloroplasts fall into two general classes. Those in tRNA genes are usually (although not inevitably) located in the anticodon loop, like the introns found in yeast nuclear tRNA genes (see 24.14 Yeast tRNA splicing involves cutting and rejoining). Those in protein-coding genes resemble the introns of mitochondrial genes (see 26 Catalytic RNA). This places the endosymbiotic event at a time in evolution before the separation of prokaryotes with uninterrupted genes.
The role of the chloroplast is to undertake photosynthesis. Many of its genes code for proteins of complexes located in the thylakoid membranes. The constitution of these complexes shows a different balance from that of mitochondrial complexes. Although some complexes are like mitochondrial complexes in having some subunits coded by the organelle genome and some by the nuclear genome, other chloroplast complexes are coded entirely by one genome.
- Mitochondria Evolved By Endosymbiosis
How did a situation evolve in which an organelle contains genetic information for some of its functions, while others are coded in the nucleus? Figure 3.41 shows the endosymbiosis model for mitochondrial evolution, in which primitive cells captured bacteria...
- 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...
- 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...
- 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...