Human Mitochondrial DNA H & L Strands Explained

The rCRS (NC_012920 ) and the original CRS (J01415.1) are sequences are described as being the "L-strand". The following is an explanation of this terminology.

The Heavy Strand/ Light Strand nomenclature dates back to the 1970's and refers to the relative CsCl banding of the single strands of the mitochondrial DNA. The two strands have a G/C asymmetry which causes the sense strand to actually be "light" and the reverse complement to be "heavy" in terms of their relative molecular weights.

In the publication of the original Cambridge Reference hmtDNA sequence by Anderson et al in 1981 PDF, the authors state, "The sequence shown is that of the L strand which we define as the main coding strand containing the sense sequence of the rRNAs and most of the tRNAs and mRNAs. Thus, these RNAs are transcribed from (and therefore hybridize to) the H strand."

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Strand Asymmetry in the rCRS

AC_000021.2 #A #C #G #T Total # ~MW (kD) 5124 5181 2169 4094 16568 5063, "Light" 31% 31% 13% 25% 100%

AC_000021.2 Reverse Complement #A #C #G #T Total # ~MW (kD) 4094 2169 5181 5124 16568 5174, "Heavy" 25% 13% 31% 31% 100%