How big is the human mitochondrial genome and how many genes does it carry?

It is a circular molecule of about 16,569 base pairs encoding 37 genes: 13 that specify respiratory-chain proteins, 22 transfer RNAs, and 2 ribosomal RNAs.

Are all mitochondrial proteins encoded by mtDNA?

No. Only 13 polypeptides are encoded by mtDNA; the great majority of mitochondrial proteins are encoded by nuclear genes, made in the cytoplasm, and imported into the organelle.

Mitochondrial DNA Structure and Organization

Human mitochondrial DNA is a small, double-stranded circular molecule of roughly 16,569 base pairs that encodes a compact set of genes essential for oxidative phosphorylation. Its dense, almost intron-free organization, its two compositionally distinct strands, and a short non-coding control region set it apart from the much larger nuclear genome.

Definition

Mitochondrial DNA (mtDNA) is the circular, double-stranded, multicopy genome resident in mitochondria; in humans it is about 16.6 kilobases long and encodes 13 respiratory-chain polypeptides, 22 transfer RNAs, and 2 ribosomal RNAs, with a single major non-coding control region.

Scope

This topic describes the physical form, gene content, and organizational features of the mammalian mitochondrial genome, drawing on the complete human sequence determined in 1981. It covers the heavy and light strands, the 37 genes (13 protein-coding, 22 tRNA, 2 rRNA), the control region (D-loop), and the genome's economical, overlapping arrangement. It does not cover replication mechanics or inheritance, which are treated in sibling topics.

Core questions

What is the size and physical form of the human mitochondrial genome?
Which genes does mtDNA encode and which mitochondrial proteins are instead made from nuclear genes?
What distinguishes the heavy strand from the light strand?
What is the control region (D-loop) and what does it contain?
How does the genetic economy of mtDNA differ from the nuclear genome?

Key concepts

Circular double-stranded genome (~16.6 kb)
Heavy (H) and light (L) strands
37 genes: 13 protein-coding, 22 tRNA, 2 rRNA
Non-coding control region / displacement loop (D-loop)
Compact, near-intronless organization with overlapping genes
Variant mitochondrial genetic code
Revised Cambridge Reference Sequence (rCRS)

Mechanisms

The two strands of mtDNA differ in base composition: the guanine-rich heavy strand and the cytosine-rich light strand, a property exploited historically to separate them and reflected in their distinct gene complements. The 1981 complete human sequence (Anderson and colleagues) showed that almost the entire genome is coding, with genes abutting or slightly overlapping, few or no introns, and some reading frames sharing nucleotides; the only substantial non-coding segment is the control region, which harbors origins and promoters that govern replication and transcription. The genome encodes 13 subunits of the oxidative phosphorylation complexes plus the 22 tRNAs and 2 rRNAs needed for their translation inside the organelle, while the remaining roughly 1,500 mitochondrial proteins are encoded in the nucleus and imported. The parallel mouse sequence (Bibb and colleagues, 1981) confirmed this economical organization as a general mammalian feature.

Clinical relevance

The compact organization of mtDNA means that even small sequence changes can disrupt genes critical to energy metabolism, and the location of a variant within coding, tRNA, or control regions helps explain the biochemical consequences described in clinical genetics. This entry describes genome structure for educational orientation and is not a basis for individual genetic interpretation or care.

History

The structure of mtDNA was defined when Anderson and colleagues published the complete sequence of the human mitochondrial genome in 1981, one of the first whole genomes ever sequenced; this work established the gene catalogue, the strand asymmetry, and the variant genetic code. The contemporaneous mouse sequence confirmed the organization across mammals. The reference sequence was later corrected and is now used as the revised Cambridge Reference Sequence against which human variants are reported.

Key figures

Frederick Sanger
Sjef Anderson
David A. Clayton
Bart G. Barrell

Seminal works

anderson-1981
bibb-1981

Frequently asked questions

How big is the human mitochondrial genome and how many genes does it carry?: It is a circular molecule of about 16,569 base pairs encoding 37 genes: 13 that specify respiratory-chain proteins, 22 transfer RNAs, and 2 ribosomal RNAs.
Are all mitochondrial proteins encoded by mtDNA?: No. Only 13 polypeptides are encoded by mtDNA; the great majority of mitochondrial proteins are encoded by nuclear genes, made in the cytoplasm, and imported into the organelle.