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I'm surfing NCBI website -Nucleotide- to find some examples of real DNA sequences to use in my small homework project.

My question is related to the title of a DNA sequence below:

Sus scrofa mitochondrial DNA, D-loop region, isolate: Europeanwild boar 3

1,045 bp linear DNA

I thought a mitochondrial DNA is different than a linear/nuclear DNA, especially that a mitochondrial DNA is circular while a linear/nuclear DNA is linear. So what does the above title actually means (in simple terms)? Is it still linear or not?

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Well, Mitochondrial DNA CAN be linear (in some organism), see the wikipedia page on this

In most multicellular organisms, the mtDNA - or mitogenome - is organized as a circular, covalently closed, double-stranded DNA. But in many unicellular (e.g. the ciliate Tetrahymena or the green alga Chlamydomonas reinhardtii) and in rare cases also in multicellular organisms (e.g. in some species of Cnidaria) the mtDNA is found as linearly organized DNA. Most of these linear mtDNAs possess telomerase independent telomeres (i.e. the ends of the linear DNA) with different modes of replication, which have made them interesting objects of research, as many of these unicellular organisms with linear mtDNA are known pathogens.[19]

For human mitochondrial DNA (and probably for that of metazoans in general), 100-10,000 separate copies of mtDNA are usually present per cell (egg and sperm cells are exceptions). In mammals, each double-stranded circular mtDNA molecule consists of 15,000-17,000[20] base pairs. The two strands of mtDNA are differentiated by their nucleotide content, with a guanine-rich strand referred to as the heavy strand (or H-strand) and a cytosine-rich strand referred to as the light strand (or L-strand). The heavy strand encodes 28 genes, and the light strand encodes 9 genes for a total of 37 genes. Of the 37 genes, 13 are for proteins (polypeptides), 22 are for transfer RNA (tRNA) and two are for the small and large subunits of ribosomal RNA (rRNA). This pattern is also seen among most metazoans, although in some cases one or more of the 37 genes is absent and the mtDNA size range is greater. Even greater variation in mtDNA gene content and size exists among fungi and plants, although there appears to be a core subset of genes that are present in all eukaryotes (except for the few that have no mitochondria at all). Some plant species have enormous mtDNAs (as many as 2,500,000 base pairs per mtDNA molecule) but, surprisingly, even those huge mtDNAs contain the same number and kinds of genes as related plants with much smaller mtDNAs.[21]

https://en.wikipedia.org/wiki/Mitochondrial_DNA

In the case of your sequence, it only means that it has been entered in NCBI as a linear piece of DNA (it is usually the case when you sequence only a piece of a circular DNA for example) It does not mean that within the organism it is linear !

I hope this answers your question !

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