Promiscuous DNA

Content
Organization of Genetic Material 3.  Split Genes, Overlapping Genes and Pseudogenes
Split genes or interrupted genes 
Discovery and nature of split genes
R-loop mapping and restriction mapping of interrupted genes
Structure of chicken ovalbumin split gene
Split genes in fungal mitochondria
Split genes in chloroplasts
Intron of one gene may contain exon of another gene
Exon sequences are conserved, but intron sequences vary
Introns with coding sequences
Overlapping genes
Pseudogenes
Promiscuous DNA


Promiscuous DNA
(Movement of DNA between mitochondria, chloroplasts and nucleus)
During 1982-84, several evidences suggesting transfer of DNA from chloroplasts to mitochondria and from both these organelles to the nuclear genome, have been reported. In a report from Plant Breeding Institute, Cambridge, U.K. in 1983, it was suggested that apparent translocation of DNA sequences from chloroplasts to mitochondria has taken place in corn. This conclusion was based on the discovery that a 12 kilobase segment of mitochondrial DNA (mtDNA) was homologous to a section of chloroplast DNA (cpDNA) of same organism, corn. During the same period, a report from Texas, U.S.A., described the same phenomenon in yeast, where a segment of mtDNA showed similarity with a segment in nuclear DNA, suggesting flow of DNA from mitochondria to nucleus. A similar report of transfer of mtDNA to nucleus in sea urchin also came from California Institute of Technology. In 1984, more reports of promiscuous DNA came from different places in materials like mungbean, spinach, corn and peas, suggesting that movement of DNA sequences among organelles and between organelles and nucleus must be very common and not exceptions. The movement of DNA among mitochondria chloroplasts and nucleus are shown in Figure 29.15.
 
Exchange of genetic material between nuclear genome, chloroplast genome and mitochondrial genome. Thick arrows indicate transfer of promiscuous DNA for which evidence is available.
Fig. 29.15. Exchange of genetic material between nuclear genome, chloroplast genome and mitochondrial genome. Thick arrows indicate transfer of promiscuous DNA for which evidence is available.

The evolutionary implications of promiscuous DNA are far reaching and it is speculated that chloroplasts and mitochondria, before invading eukaryotic cell were free living primitive prokaryotes. The endosymboint hypothesis suggests that in course of time these entrapped organisms lost their independence by losing genetic information to nuclear genome. Thus the promiscuity of DNA gave further support to endosymboint hypothesis, but exchange of DNA between mitochondria and chloroplasts is surprising, since they are independent of each other. This may be partly explained on the basis of great variation and bigger size of mtDNA, relative to cpDNA which is tightly constrained in size and sequence even between species. While mtDNA can tolerate foreign DNA and other disturbances, chloroplasts are sensitive to any such variation or modification. This is confirmed by evidence produced at Carnegie Institute of Washington, Stanford, U.S.A., where it was shown that chloroplast sequences from a variety of plants are scattered throughout mitochondrial genomes, and some of these transfers are believed to be rather recent.

The mechanism of this transfer of so called promiscuous DNA is not known but two possibilities are considered, (i) Some kind of a vector or transposon or transducing phage may be involved, (ii) The two organelles might have undergone fusion at some stage. This second mechanism involving fusion is preferred on the basis of electron microscopic observations. However, more than one mechanisms may be involved in such DNA transfers.