C₀t Curve 

 A C₀t curve describes the sequential complexity of a DNA sample. It describes the kinetics of reassociation and reannealing of a sample of double-stranded DNA that has been (1) fragmented to small pieces, approximately 400 nucleotides, (2) denatured to single strands, and (3) then permitted to reanneal. The extent to which two complementary DNA strands reassociate is proportional to C₀t. C0 is the original molar concentration of the complementary strands of DNA, and t is the incubation time. C₀t1/2 is the value of C0t required for completing half of the reannealing reaction. If the sequences of all initial DNA fragments were identical, complementary fragments encounter each other readily and the mixture reanneals readily at a low value of C₀t. At the other extreme, reannealing is very slow and occurs only at high C₀t values if the original DNA contains many different sequences, so that the concentrations of complementary strands are very low and they encounter each other only infrequently. The value of C₀t_1/2 is a measure of the sequential complexity of the original DNA.

Such studies on genomic DNA were one of the first indications that genomes often contain repetitive DNA. Such DNA segments are present in higher concentrations than those segments present in only one copy per genome, and this fraction of the DNA reanneals at a correspondingly lower C0t value. The results indicate that different sequences are present in widely varying frequencies throughout the genome, and only a fraction occurs only once per genome.