Genome Complexity in Higher Organisms

The reassociation profiles of a number of different DNA s according to the Cot nomenclature proposed by Britten and Kohne (1968) are shown in Fig. 1. The relationship between the second-order reassociation 

The second class of rapidly reassociating DNA sequences make up between 20 and 25% of the mouse genome and reassociate at an average rate from a few to 100,000 times faster than would be expected for single copy sequences in a genome the size of a mouse. In RNA/DNA low Cot hybridization reactions, these repeated sequences are the major DNA component in the reaction. 

The third class of DNA comprises 60-70% of the mouse genome and consists of sequences which renatme at a rate which is consistent with the second-order kinetics predicted for the mouse genome, assuming each sequence is present only once per haploid genome. These are termed the nonrepeated, unique or single copy DNA base sequences. 

RNA/DNA hybridization experiments are appealing to the developmental biologist for such experiments are the only methods presently 

In light of the relatively low RNA concentrations, short reaction times, and relatively nonspecific reassociation conditions, the estimates of transcriptional diversity obtained with reiterated DNA sequences are difficult to interpret and probably provide overestimates of gene activity. The thermal stability of the RNA/DNA hybrids formed under these conditions indicate that there is incomplete base pairing, which is in contrast to the more precise pairing expected for true RNA transcript— DNA transcriptional site reassociation products (McCarthy and Church, 1970). The quantitative aspect of transcription during development in each species cannot be understood until the enormous sequence complexity of that mammalian genome can be described with some accuracy (Laird, 1971). 

---