I am trying to learn about Plastid genetic engineering and saw this picture on PAGE 3-4 of the attached paper which I don't understand at all. I understand that it shows the names and color codes the function of the genes. But I have questions:
1. Why is it in a circle?
2. From the paper: The caption below the figure says "The gray arrows denote the direction of transcription for the two DNA strands of the (circularly mapping) genome, and the interior circle shows its tetrapartite structure. Abbreviations: IRA, inverted repeat A; IRB, inverted repeat B; LSC, large single-copy region; SSC, small single-copy region"
What is meant by a tetrapartite structure?
Article Engineering Plastid Genomes: Methods, Tools, and Application...
Hi,
The circle means that the genome is circular. The chloroplast derives from a cyanobacteria (with a circular genome). The tetrapartite structure means that you have two single-copy regions (a large one and a small one) separated by the inverted repeat regions (2 single-copy regions + 1 repeated (2 regions) = 4). Is this more clear?
The small single copy region is "small" relatively to the large one. It is always relative to the other single copy region.
Although most publications and textbooks show a circular form for plastid genomes, the molecular structure actually consists of unit-genome sized linear molecules and multi-genomic branched forms. Restriction digestion and sequencing predicts a circular map because for a gene sequence of ABC…XYZ, the map indicates that Z and A are adjacent. However, a linear head-to-tail concatemer, as is found for plastid genomes, is also consistent with this mapping pattern.
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