Figure 21.5A: Sugar phosphate backbone of common nucleotides

Figure 21.5B: Nucleosides (C=Cytosine)

Figure 21.5C: Nucleotides

Figure 21.6A: DNA double helix structure

Figure 21.6B: DNA Base pairing by hydrogen bond

The endosymbiotic theory concerns the origins of mitochondria and plastids (e.g. chloroplasts), which are organelles of eukaryotic cells. According to this theory, these organelles originated as separate prokaryotic organisms that were taken inside the cell as endosymbionts. Mitochondria developed from proteobacteria (in particular, Rickettsiales or close relatives) and chloroplasts from cyanobacteria. Mitochondrial and chloroplast genomes do not contain a full set of housekeeping genes, and lack many that other descendants of their speculative ancestors share, there must have been a loss of genes. However, some of these genes likely migrated to the nucleus, where analogues of these genes are now found.