Heavy strand

Although transcription of the heavy strand of mtDNA initiates from the heavy strand promoter (PH) and proceeds completely around the circular mtDNA molecule, this full-length transcript is only made about 10% of the time. The other 90% of the time a truncated transcript from the heavy strand promoter terminates after the 16S rRNA gene. How does this happen The nucleotides marked with an asterisk in Figure 8-8 form a transcription termination signal that, when bound by a mitochondrial termination factor, causes the mitochondrial RNA polymerase to dissociate from the mtDNA and cease transcription. It is proposed that alterations in the DNA sequence of this tRNA gene therefore not only may affect the role of the tRNA in protein synthesis but... 

The Meselson-Stahl experiment resulted in a single band of DNA on CsCl gradient centrifugation after one round of cell division. Its density was intermediate between heavy and light DNA. This could only occur if the newly synthesized dsDNA contained an old light strand and a new heavy strand. 

Comparing the density of single-stranded DNA from these cells (by separating the strands at pH 12) revealed that cells grown in for one generation contained equal amounts of light and heavy strands. 

Gillum A.M. and Clayton D.A.(1979). Mechanism of mitochondrial DNA replication in mouse L-cells RNA priming during the initiation of heavy-strand synthesis. J. Mol. Biol. 135 353-368. 

FIGURE 10.3 The experimental evidence for semiconservative replication. Heavy DNA labeled with forms a band at the bottom of the tube, and light DNA with N forms a band at the top. DNA that forms a band at an intermediate position has one heavy strand and one light strand. 

The semiconservative replication of DNA means that a newly formed DNA molecule has one new strand and one strand from the original DNA. The experimental evidence for semiconservative replication comes from density-gradient centrifugation (Figure 10.3). If replication were a conservative process, the original DNA would have two heavy strands and all newly formed DNA would have light strands. 

Fig. 8.10. The organisation of the mitochondrial genome from the domestic fowl (Desjardins Morals, 1990). The genes for the tRNAs for each amino acid are indicated by the single amino acid code. Genes for proteins are indicated thus Cyt b, cytochrome b ND, NADH dehydrogenase CO, cytochrome oxidase ATPase, adenosine triphosphate synthetase. In each, the subunit number is given as the suffix. The two concentric circles represent the L- (light) and H- (heavy) strands.

Species Tissue Reference" Circular duplex (gm/ml) Buoyant densities Light strand Heavy strand H-L (alkaline) A (mg/ml)... 

Hybridization-competition experiments have shown the existence in 29-infected ceils of at least two classes of viral RNAs, early and late. The use of the two separate strands of 29 DNA has indicated that early and late RNAs are transcribed, respectively, from the light and heavy strands and that most species of early RNA are also transcribed at late times. ... 

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