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:
“The sin of my ingratitude even now
Was heavy on me. Thou art so far before
That swiftest wing of recompense is slow
To overtake thee. Would thou hadst less deserved,
That the proportion both of thanks and payment
Might have been mine! Only I have left to say,
More is thy due than more than all can pay.”
—William Shakespeare (1564–1616)
“Dust rises from the main road and old Délira is stooping in front of her hut. She doesn’t look up, she softly shakes her head, her headkerchief all askew, letting out a strand of grey hair powdered, it appears, with the same dust pouring through her fingers like a rosary of misery. She repeats, “we will all die”, and she calls on the good Lord.”
—Jacques Roumain (1907–1945)