A technique that measures the rate of reassociation of complementary strands of DNA derived from a single source. The DNA under study is fragmented into pieces several hundred base pairs in length and then disassociated into single strands by heating. Subsequently, the temperature is lowered and the rate of reannealing (q.v.) is monitored. Reassociation of DNA is followed in the form of a cot curve, which plots the fraction of molecules that have reannealed against the log of cot. Cot values are defined as C0 × t, where C0 is the initial concentration of single-stranded DNA in moles of nucleotides per liter and t is the reannealing time in seconds. Typical cot curves are shown below. DNAs reannealing at low cot values (10−4–10−1) are composed of highly repetitive sequences, DNAs reannealing at cot values between 100 and 102 are moderately repetitive, and DNAs reannealing at higher cot values are nonrepetitive. See Chronology, 1968, Britten and Kohne; Alu family, delta T50H, mouse satellite DNA, repetitious DNA.
Reassociation kinetics For each of the DNA samples tested, the number of base pairs in the genome is indicated by an arrow on the logarithmic scale at the top of the graph. The poly-U + poly-A sample is a double helix of RNA, with one strand containing only A and the other strand only U. The mouse satellite DNA is a fraction of nuclear DNA in mouse cells that differs in its physical properties from the bulk of the DNA. The calf DNA represents only those sequences that are present in single copies per haploid genome. The denatured DNA samples were fragmented by mechanical shear to chain lengths of about 400 nucleotides and incubated at a temperature near 60°C. The fraction reassociated was measured by the decrease in UV absorption as double strands formed. Reprinted with permission from R. J. Britten and D. E. Kohne, 1968, Repeated sequences in DNA. Science 161: 529–540. © 1968 American Association for the Advancement of Science.
Subjects: Genetics and Genomics.