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Sunday, January 06, 2008

Mitochondrial DNA is Closed circular

009. Mitochondrial DNA is :

1. Closed circular.

2. Nicked circular.

3. Linear.

4. Open circular.

Answer

1. Closed circular.

Reference

The Molecular Biology of Plant Mitochondria edited by Charles S Levings III, Indra K Vasil

Ganong 22nd Edition Page 10

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Discussion

Small circular DNA occur in two forms

  1. The closed, supercoiled conformation
  2. The open, nicked form

Explanation

Mitochondrial DNA (mtDNA) is a small (16.5kb), closed-circular molecule of DNA that exhibits a strict maternal transmission and is present within cells in multiple copies.

Comments

In addition, many of the proteins responsible for the maintenance, replication and transcription of the mitochondrial genome are nuclear encoded and as such the genetic defect in patients with mitochondrial disorders may occur in either the mitochondrial or nuclear genome

Tips

The presence of many hundreds or thousands of copies of mtDNA within each cell plays an important role in the expression of a pathogenic mutation in terms of a biochemical and clinical phenotype. Homoplasmy describes the situation where all mtDNA copies are identical within a cell. In contrast to this, the majority of patients with pathogenic mtDNA mutations harbour a mixture of wild-type and mutant mtDNA within each cell, the phenomenon of heteroplasmy [Lightowlers et al, 1997]. For any heteroplasmic mutation, the proportion of mutant mtDNA molecules in a cell typically exceeds a critical threshold before a biochemical defect is apparent. At mitosis, mutant and wild type mtDNA is believed to be randomly segregated to each of the daughter cells, affecting both mitochondrial disease expression and inheritance as it will lead to different percentage levels of mutant mtDNA between tissues, whole organs and even individuals. Since higher levels of heteroplasmic, pathogenic mtDNA mutations accumulate in post-mitotic tissues such as skeletal muscle (which is often a clinically-affected tissue), the majority of diagnostic tests for mtDNA mutations are usually performed using DNA extracted from this tissue. Nevertheless, some mutations can reliably be detected in blood.

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