chrM

chrM refers to the mitochondrial chromosome, a circular double-stranded DNA molecule located in the mitochondria. In humans, the mitochondrial genome is about 16,569 base pairs long and exists in multiple copies per mitochondrion and per cell; mitochondria are inherited almost exclusively from the mother.

The mtDNA encodes 37 genes: 13 protein-coding genes essential components of the oxidative phosphorylation system, 22 transfer RNAs, and 2 ribosomal RNAs (12S and 16S rRNA). The genetic code differs slightly from the nuclear code. The genome is compact with no introns; most of the genome is coding.

The genome is circular with two strands, heavy (H) and light (L). The D-loop noncoding control region contains promoters and the origin of replication for the heavy strand. The gene arrangement is conserved among vertebrates, and the majority of genes are encoded on the H-strand with a subset on the L-strand.

mtDNA is maternally inherited; heteroplasmy can occur, producing variable phenotypes. The mutation rate in mtDNA is high relative to nuclear DNA, making it useful in forensic, evolutionary, and population genetics studies, including haplogroup analysis and maternal lineage tracing. In genomic databases and browsers, the mitochondrial chromosome is designated chrM (or MT in some builds).

The standard human mitochondrial reference sequence is NC_012920.1, with a genome length of 16,569 base pairs.