Circular molecules of DNA, such as plasmids and typical mitochondrial genomes, consist of two strands of DNA called the heavy strand (or H-strand) and the light strand (or L-strand). The two strands have different masses due to different proportions of heavier nucleic acids. While this difference is not known to have any functional significance, it can be used in the laboratory to segregate the strands of denatured DNA, and hence to analyze the strands separately.
Adenine and guanine (purines) are heavier than cytosine and thymine (pyrimidines) due to their extra ring. Because a purine always pairs with a pyrimidine, any excess of purines in one strand will occur with a corresponding excess of pyrimidines in the other strand and vice versa. Statistically, there is more likely to be such an imbalance than an exact 50/50 ratio. In addition, bias may arise due to differentials in the amount of protein-coding sequence on each strand, as codons do not all occur with equal frequency.
Famous quotes containing the words heavy and/or strand:
“Bees
Shaking the heavy dews from bloom and frond.
Boys
Bursting the surface of the ebony pond.”
—Wilfred Owen (1893–1918)
“The annals of this voracious beach! who could write them, unless it were a shipwrecked sailor? How many who have seen it have seen it only in the midst of danger and distress, the last strip of earth which their mortal eyes beheld. Think of the amount of suffering which a single strand had witnessed! The ancients would have represented it as a sea-monster with open jaws, more terrible than Scylla and Charybdis.”
—Henry David Thoreau (1817–1862)