Structure of Mitochondrial DNA

These scientific biological terms are not easy to understand, unless one is from a science background. Hence, before going ahead, one should know the terms, mitochondria and DNA.

Deoxyribonucleic acid (DNA) contains the genetic information, used in the development and functioning of living organisms. It is often called a blueprint, since it contains the genetic information, from which the components of cell, like proteins and RNA, can be constructed. Genes are DNA segments, that carry genetic information. They govern the physical traits of human body, like, eye color, hair color, bone thickness, build, etc.

Mitochondria are rod-shaped organelles, enclosed in a membrane, that are present in many eukaryotic cells. They are considered as powerhouses of the cell, because they take in nutrients and oxygen, and break them down into ATPs (adenosine triphosphate). This process is known as cellular respiration.

DNA is generally present in chromosomes within the nucleus, but mitochondria too have small amounts of their own, called mitochondrial DNA (mtDNA). This is present in the cytoplasm of the cell. It is a double-stranded circular molecule, which is inherited from the mother in all multicellular organisms.

It is because, during fertilization, all mtDNA comes from oocyte. There are about 2 to 10 transcripts of it in each mitochrondion. Compared to chromosomes, it is relatively smaller, and contains the genes in a limited number. Changes in it can cause maternally inherited diseases, which lead to faster aging process and geriatric disorders.

Mitochondrial DNA contains about 16,500 building blocks (base pairs), which represents a very small number compared to the total number present in cells. Out of the 37 genes, 13 are responsible for making enzymes, involved in oxidative phosphorylation, a process that uses oxygen and sugar to produce adenosine triphosphate (ATP).

The other 24 genes are responsible for making molecules, called transfer RNA (tRNA) and ribosomal RNA (rRNA). They help to accumulate amino acids into proteins.

mtDNA contains a host of enzymes, and is localized to the matrix. The proteins involved in the respiration are embedded within the mitochondrial inner membrane. Mitochondria have similar characteristics of plant chloroplasts. The plant chloroplasts contain their own DNA, which enables them to grow and reproduce independently.

In some metazoans, there are about 100 - 10,000 separate copies of mtDNA present per cell. Whereas in mammals, each molecule consists of about 16,500 base pairs, which encode the 37 genes; 13 for proteins, 22 for transfer RNA (tRNA), and one each for small and large subunit of rRNA.

The pattern is same in metazoans, albeit in some cases, one or more of the 37 genes may be absent, and the mtDNA size may be large. Some plants species have numerous ones (as many as 2,500,000 base pairs per molecule), but they still contain the same number of genes.

There are about 20,000 to 25,000 total genes in the human genome, and mitochondrial genes are among them.