What is a Mitochondrion?

A mitochondrion is a cellular organelle that serves as the site for cellular respiration and energy production (ATP or Adenosine Triphosphate). It is a double-membrane-bound organelle and is only present in eukaryotic cells.

What are the Features of a Mitochondrion?

Some of the essential features of a mitochondrion are as follows:

• Mitochondria function as the power-generating unit of the cell, producing more than 90 percent of the energy in body cells. They are an important energy source for organs like the heart, liver, muscles, brain, and kidney. The organelles generate adenosine triphosphate (ATP) and a small amount of carbon dioxide during cellular respiration.
• These are rod-shaped, double-membrane bound organelles, and their diameter ranges from 0.5 micrometer to 1 micrometer.
• Mitochondria contain their own circular genome, and this DNA is inherited only from the mother.
• These organelles are capable of dividing independently when a cell requires more energy.
• Cytochrome c, present in the intermembrane space, triggers apoptosis when a cell receives an appropriate signal or when cellular constituents get damaged.

Mutation

A change or alteration in a deoxyribonucleic acid (DNA) sequence is known as a mutation. It can result from copying errors in the DNA sequence during cell division, ionizing radiation, mutagens (chemicals), or infections (especially by viruses).

The following are the major types of DNA mutation:

Point Mutation

• Point mutation refers to a change in a single nucleotide of DNA.
• The types of point mutation are substitution, insertion, and deletion.

Chromosomal Mutation

• A chromosomal mutation occurs in a large segment of DNA (chromosome).
• The types of chromosomal mutations are inversion, deletion, duplication, and translocation.

Copy Number Variation (CNV)

The phenomenon in which a segment or stretch of the genome (DNA) is repeated (duplicated, triplicated, or quadruplicated) such that the repetition number varies in different individuals is called copy number variation. The types of copy number variations include gene amplification and expanded trinucleotide repeat.

Do Mitochondria Contain DNA?

Yes, mitochondria contain DNA. Most of the eukaryotic DNA is packed in a chromosome within the nucleus of the cell. It contains a small circular chromosome found inside the mitochondrion and is most importantly inherited from the mother’s genome (through oocyte). In contrast, the nuclear genome is inherited from both parents. Males do not transmit mitochondrial DNA to their offspring. The genetic material of mitochondrion is also known as mitochondrial DNA or mt-DNA.

Mitochondrial Mutation

Inherited changes in mitochondrial DNA are known as mitochondrial mutations. Mutations in the mitochondrial DNA may reduce the ability of the mitochondria to generate energy, thereby impacting cell functioning. A mutation in the mitochondria can affect multiple organ systems and their functions.

Some of the problems associated with mtDNA (mitochondrial DNA) mutations include the following:

• Kidney failure
• Diabetes
• Muscle weakness
• Impaired vision and hearing
• Loss of cognitive and intellectual abilities

Genetic changes in the mitochondrion are also affected by somatic variants (non inherited factors). Somatic changes occur in the body cells (not in sperm or egg cells) and cannot transfer across generations. Some of the somatic variant problems associated with mtDNA (mitochondrial DNA) include the following:

• Heart disease
• Alzheimer's
• Parkinson's

Common Types of Mitochondrial DNA (mtDNA) Mutations

Mitochondrial mutations take place in two forms, i.e., point mutation and mtDNA rearrangements.

Point Mutation

mtDNA mutations are commonly inherited from the mother (maternal). This mutation can occur in proteins, transfer RNA (tRNA), or ribosomal RNA (rRNA) genes of mitochondria. Many researchers state that mt-tRNA (mitochondrial-transfer RNA) genes are associated with many diseases.

• Point mutations generally affect the mitochondrial protein-coding genes that are responsible for synthesizing the proteins of the respiratory chain complex (RC complex) of the mitochondria.
• Mutations in the tRNA (transfer RNA) encoding genes of the mitochondrial genome lead to less tRNA synthesis. Thus, the process of mitochondrial protein translation can be slowed down.
• Point mutations are highly heteroplasmic as well as recessive. They are considered as mitochondrial heterogeneity.

mtDNA Rearrangements

The major part of the mitochondrial DNA rearrangements occurs based on deletions of mitochondrial DNA, which may vary in size between 1.3 to 8-kilobases (i.e., partial deletions).

• mtDNA rearrangements are a significant cause of mitochondrial disease and some other age-related mitochondrial dysfunctions.
• The mtDNA mutation mainly affects the tissues of the brain and skeletal muscle.
• Different mtDNA rearrangements tend to accumulate in single cells, but the detailed nature of these rearrangements remains unknown.
• An accumulation of mtDNA rearrangement deletions mainly occurs in aged post-mitotic tissues in individuals who suffer from neurogenerative diseases and age-related mitochondrial dysfunction.
• Research has proven that mtDNA deletion mutations arise during the repair of damaged mtDNA.

Health Conditions Related to Chromosomal Changes

Mitochondria generate ATP (adenosine triphosphate) from energy molecules like NADH (nicotinamide adenine dinucleotide hydrogen) and FADH2 (flavin adenine dinucleotide). These ATP molecules are utilized by the cells to perform various functions. The cytoplasm of the cell contains mitochondria that have their own circular DNA. In human cells, mitochondrial DNA contains 16,500 base pairs of DNA. There are 37 genes present in mitochondrial DNA that play an essential role in cell functioning. Out of 37 genes, 13 are involved in preparing enzymes and oxidative phosphorylation. The remaining genes instruct the cells for the production of tRNA and rRNA to produce proteins.

Therefore, changing the structure, number, or count of mitochondrial DNA genes leads to several health effects. Mitochondrial mutations usually result in a disorder or disease in humans. For example, they may predispose individuals to Alzheimer's disease or Parkinson's disease.

Some of the most common diseases or disorders related to mitochondrial DNA mutations are:

Age-related Hearing Loss

Age-related hearing loss can begin from 30–40 years of age. Hearing impairment is common in patients with mitochondrial disorders that affect the normal function of the ears; hence it may hinder the transmission of auditory signals to the brain via the auditory nerve. Among aging people, mitochondrial diseases are associated with deletion mutations.

This damage results in the formation of oxygen-reactive species, which accumulate and damage or degrade the mitochondrial DNA and affect its functions. Due to less energy production, the cells which depend on high energy, such as the inner part of the ear, suffer damage. This damage disrupts the functioning of the ear.

Cyclic Vomiting Syndrome

• This syndrome most commonly affects children. It occurs due to damage or a mutation caused to the mitochondrial DNA.
• The symptoms of this syndrome include repetitive nausea, vomiting, and fatigue.
• These changes in the mtDNA affect the regular functioning of mitochondria. Some researchers have found that mtDNA damage also affects body functions such as digestion.

Cytochrome C Oxidase Deficiency

• Mutations in the three genes of the enzyme cytochrome c oxidase present in mitochondrial DNA lead to cytochrome c oxidase deficiency.
• A mutation in the cytochrome c oxidase gene (also called complex IV) results in this syndrome.
• Cytochrome c oxidase participates in a process called oxidative phosphorylation (formation of ATP in the presence of oxygen). Therefore, the damage to the cytochrome c oxidase impairs oxidative phosphorylation, affecting ATP production.
• Some researchers state that oxidative phosphorylation impairment also leads to cell death that affects the normal function of the brain, muscles, and heart cells.

Kearn’s Sayre Syndrome

• The syndrome occurs due to the deletion of a nucleotide sequence (1,000 to 10,000 bases) of mitochondrial DNA (mtDNA).
• This syndrome is associated with problems in the eyes, such as ophthalmoplegia and retinopathy.
• The deletion of a larger segment of mtDNA results in the impairment of protein synthesis and the lack of proteins needed for oxidative phosphorylation.
• As a result, the ATP generation gets affected, which leads to improper cell functioning.

Lebar Hereditary Optic Neuropathy

• The mutation in MT-ND1, MT-ND4, MT-ND4L, and MT-ND6 mitochondrial genes leads to Leber hereditary optic neuropathy.
• Damage to these genes affects the synthesis of proteins involved in large enzyme complexes knows as complex 1, which is used in oxidative phosphorylation.
• This syndrome affects the production of ATP by changing the single amino acid codes in MT-ND1, MT-ND4, MT-ND4L, and MT-ND6 genes.
• It is still unclear how Leber hereditary optic neuropathy damages the eye (impaired vision).
• Some researchers also state that this syndrome is linked to environmental, genetic, and other medical issues.

Leigh Syndrome

• Leigh syndrome is a progressive brain disorder caused by the mutation of several different genes present in mitochondrial DNA.
• It usually affects children. The symptoms associated with Leigh syndrome are muscle weakness and difficulty breathing.
• In Leigh syndrome, the mutated gene is MT-ATP6, which is a part of complex V. It is also involved in the production of adenosine triphosphate (ATP) during oxidative phosphorylation.
• Some other genes affect the production of tRNA (transfer RNA) molecules, which is also an essential molecule in mitochondrial protein production.

Mitochondria Complex III Deficiency

• An abnormal change in the MT-CYB gene causes mitochondria complex III deficiency.
• MT-CYB gene plays a major role in the production of cytochrome b, which is the component of complex III proteins and is involved in oxidative phosphorylation for ATP (adenosine triphosphate) generation.
• Mitochondria complex III is usually characterized by muscle weakness and pain. It affects the functioning of the liver, heart, kidneys, and brain.

The lack of cellular energy causes damage to the body parts. These mitochondrial disorders mainly damage or affect the ATP production pathway.

Some other mitochondrial disorders that occur due to mitochondrial DNA mutations include the following:

• Maternally inherited diabetes and deafness
• Mitochondrial encephalon myopathy, lactic acidosis, and stroke-like episodes
• Myoclonic epilepsy with ragged red fibers
• Non-syndromic hearing loss
• Pearson syndrome
• Progressive external ophthalmoplegia
• Cancer

Common Mistakes

Some students misunderstand that mitochondrial diseases are only inherited genetic conditions that take place in mtDNA (mitochondrial DNA).

• Mitochondrial diseases or disorders may be caused by mutations that are either acquired or inherited in mitochondrial DNA (mtDNA) or nuclear genome.
• The nuclear genome is the complete set of DNA present within an organism’s nucleus.

Context and Applications

This topic is significant in the professional exams for undergraduate, graduate, and postgraduate courses, especially for:

• Bachelor of Science in Human Biology
• Bachelor of Science in Bioinformatics
• Bachelor of Science in Molecular Biology and Genetics
• Bachelor of Science in Biotechnology
• Bachelor of Technology in Biotechnology
• Master of Science in Cellular and Molecular Biology
• Master of Science in Science in Biology
• Master of Science in Cell Biology
• Master of Science in Biological Science

Related Concepts

• Genetics
• Molecular Basis of Inheritance
• Molecular Biology
• Genetic Engineering
• Recombinant DNA Technology

Practice Problems

1. What is the approximate size of mitochondrial DNA?

a. 14,000 bp

b. 2,000 bp

c. 16,500 bp

d. 16,000 bp

Answer: c

Explanation: The DNA of mitochondria is circular and double-stranded. It is known to encode several functional proteins. It is made from approximately 16,500 base pairs.

2. Select the condition(s) that is/are not associated with mtDNA (mitochondrial DNA) mutation.

a. Diabetes

b. Huntington's disease

c. Sun and other types of radiation

d. Cancer

Answer: c

Explanation: Mitochondrial mutation is not associated sun and other types of radiation. Conditions like diabetes, Huntington’s disease and cancer are associated with the mutations of mitochondria in one way or another.

3. Which of the following is not an example of mitochondrial diseases?

a. Mitochondrial myopathy

b. Diabetes mellitus and defenses (DAD)

c. Leber hereditary optic neuropathy (LHON)

d. Patau syndrome

Answer: d

Explanation: Patau syndrome is a rare disease and is caused by the trisomy of chromosome number 13. It is not a mitochondrial disease. Mitochondrial myopathy, DAD, and LHON are all conditions caused by mitochondrial disturbance.

4. How many genes does mtDNA (mitochondrial DNA) contain?

a. 8

b. 16

c. 27

d. 37

Answer: d

Explanation: The small round DNA present inside the mitochondria contains 37 genes. These genes are concerned with carrying out the normal functioning of the cell's powerhouse. They provide instruction for processes like oxidative phosphorylation.

5. What is the primary consequence of mtDNA (mitochondrial DNA) mutation in the cell?

a. Decrease in energy metabolism

b. Decrease in DNA (deoxyribonucleic acid) replication

c. Increase in ATP (adenosine triphosphate) production in mitochondria

d. Decrease in ATP (adenosine triphosphate) production in mitochondria

Answer: d

Explanation: The primary consequence of mitochondrial DNA mutation will be a decrease in ATP production. Mitochondria are concerned with the generation of energy currency by oxidative phosphorylation, which will be disturbed in the case of mutation.