megabases

Megabase is a unit of length used in genetics and genomics to describe the size of DNA sequences. One megabase equals one million base pairs (1 x 10^6 bp). In double-stranded DNA this corresponds roughly to 2 million nucleotides. The term is commonly abbreviated as Mb or Mbp and is used to express the size of entire genomes, chromosomes, or large contigs.

Genome sizes vary widely. Bacterial genomes typically range from about 1 Mb to several Mb; the yeast genome is about 12 Mb; the human haploid genome is about 3,200 Mb (3.2 Gb), and a diploid human genome is about 6,400 Mb. Plant and animal genomes can be much larger due to repetitive content. In sequencing projects, megabases are used to report assembly contiguity and size; for example, an assembly might have an N50 of several Mb, or a chromosome might be described as tens to hundreds of Mb long. When comparing genomes, Mb is often used alongside Gb (gigabase) to convey scale.

Notes: the unit can vary by field; some use Mb to denote base pairs and Mbp to emphasize base pairs. In practice, references to a genome’s length in millions of base pairs should specify whether the value refers to haploid or diploid content and any relevant orientation or assembly context.