**Question:** True about mitochondrial DNA:
(A) Linear
(B) Circular
(C) Transmitted by mother only
(D) Transmitted by both parents
(E) Contains less genes than nuclear DNA

**Core Concept:** Mitochondrial DNA (mtDNA) is a type of genetic material found within mitochondria, which are organelles responsible for producing energy in the form of adenosine triphosphate (ATP) within cells. Mitochondria are unique in that they possess their own DNA distinct from the nuclear DNA found in the cell nucleus.

**Why the Correct Answer is Right:** Mitochondrial DNA is indeed circular and non-recombining, which means that it does not undergo genetic recombination like nuclear DNA. This makes mtDNA easier to study and understand the inheritance patterns.

**Why Each Wrong Option is Incorrect:**

A. Linear mtDNA is incorrect as it is circular, not linear.
B. Circular mtDNA is correct as explained above.
C. Mitochondrial DNA is not exclusively transmitted by the mother, as mtDNA inheritance follows a unique pattern known as maternally inherited diseases. However, the statement "transmitted by mother only" is partially true but not entirely correct. Both parents contribute to the mtDNA pool, but mitochondria divide without a nuclear envelope, allowing the maternal mtDNA to predominate in the offspring.
D. Mitochondrial DNA is not transmitted by both parents in equal quantities. The inheritance pattern of mtDNA is unique, but it is not equally inherited from both parents.
E. Mitochondrial DNA contains approximately 16,569 base pairs, while nuclear DNA can contain millions of base pairs. However, the comparison of the number of genes between these two types of DNA is misleading since they serve different functions in the cell.

**Clinical Pearls:**

- Understanding the unique inheritance patterns of mitochondrial diseases is essential for clinical diagnosis and management.
- Mitochondrial disorders can have systemic involvement and impact multiple organ systems, often presenting with non-specific symptoms which can lead to misdiagnosis if not thoroughly considered.
- Mitochondrial DNA inheritance, unlike nuclear DNA, demonstrates a maternal pattern; however, both parents contribute to the mtDNA pool.
- Mitochondrial diseases often require a multidisciplinary approach in diagnosis and management, involving clinical, laboratory, and genetic evaluation.