Molecular Genetics

The relative information content of DNA can be described by its sequence complexity, a quantity obtained from DNA reassociation kinetics.

	bacterium	animal
genome size	0.004 pg	3 pg
genomes/12 pg	3000	4
specific seq. con.	750 x	x
k (rate constant)	750 y	y
Cot1/2	z	750 z

Facts

• Chemical complexity is the amount of DNA in a haploid cell, nucleus or organelle. The chemical complexity of a unit's DNA is also called its genome size.
• There are more different sequences in 3 pg animal nuclear DNA than in 0.004 pg of bacterial DNA. Conversely, in 12 pg of DNA there are many more copies of a genome if the genome is bacterial than if the genome is vertebrate. The animal DNA is more complex.
• Kinetic complexity is defined as the amount of DNA (in base pairs) required so that every sequence of a DNA is represented just once.
• For the calculation of complexity, E. coli DNA is used as a standard because it is readily available.
  C_ot _1/2 of X
  ------------------ x 4.2 x 10⁶ bp = complexity of X
  C_ot_1/2 of E.coli
• Repetition frequency, f, is defined as:
  f = (chemical complexity/ kinetic complexity )

Interpretations

• Genomes vary widely in size. Thus , the amount of DNA per cell or haploid nucleus also varies widely.
• Larger genomes reassociate at higher C_ot_1/2 values than do smaller ones

Further information

• C_ot _1/2 values used for complexity calculations are obtained from the rate constant, correcting for the contribution of that component to the total genome. For example, consider a component that is 45% of total and has an apparent C_ot _1/2 of 630. If it constituted 100% of the DNA, it would be 100/45 fold more concentrated. Thus, 0.45 x 630 = 283 (C_ot _1/2 corrected).
• For many organisms, the repetition frequency calculated for the kinetically identified unique DNA class is close to 1.0, indicating that these DNAs are truly unique in the genome.

This is page 112283 of Molecular Genetics by Ulrich Melcher, © 1997-2000