Nonrepetitive DNA

The DNA in a eukaryotic genome can be divided into different sequence classes. These are unique-sequence, or nonrepetitive, DNA and repetitive-sequence DNA. In the haploid genome, unique-sequence DNA generally includes the single copy genes that code for proteins. The repetitive DNA in the haploid genome includes sequences that vary in copy number from two to as many as 10 copies per cell. 

Nonrepetitive DNA makes up 40% of the genome. Most of this portion is of unknown function, and only about 5% of it is composed of genes which code for proteins. 

Under the conditions used in most of the hybridization experiments, only DNA sequences hybridize which exist in many copies ( repetitive sequences, Britten and Kohne, 1968). Nonrepetitive DNA sequences hybridize only at extremely high RNA-DNA ratios (Gelderman et al, 1971). But under these conditions the specificity of the DNA-RNA interaction is lower thus, sequences which are similar but not identical also hybridize. Finally, it should be mentioned that the amount of RNA synthesized per genome in very early stages of sea urchin embryos is larger than in later stages (Brandhorst and Humphreys, 1971). Whether more species with different base sequences or more copies of the same species are synthesized is unknown. 

More Than Half the DNA in Eukaryotic Organisms Is in Unique or Nonrepetitive Sequences... 

Eukaryotic DNA can be divided into three types based on the overall nature of the sequence highly repetitive, moderately repetitive and nonrepetitive. 

Telomeres. The DNA sequences at the chromosome ends have a TG-rich strand, such as the (TTGGGG)5o 7o Tetrahymemy and the (TTAGGG) of both human and trypanosome chromosomes. The complementary DNA strand is CA-rich. The S. cereaisiae telomers have 350 base pairs containing the sequences (TGj 3 / C3 3A) as well as one or more copies of a 6.7-kb nonrepetitive sequence and other elements. In many species the repetitive telo-meric sequences have 3 poly(G) tails at the ends of the DNA molecules. These tails are able to form G quartet structures (Fig. 5-26 and Chapter 5, Section C,4). A variety of telomere-binding proteins have been isolated. " Some of these bind to G quartet struc-... 

Bonner and Widholm (1967) carried out interesting experiments on hybridization of chromosomal RNA from organs of die pea plant with nuclear pea RNA. They found that RNA from different organs is partially complementary to different segments of the nuclear DNA. This was to be ejq)ected. However, in their experiments these workers did not divide the DNA into repetitive and nonrepetitive sequences, so that their findings still cannot be accepted as proof of the hypothesis put forward above. 

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