megabase

Megabase, abbreviated Mb or Mbp, is a unit of length used in genetics to describe the size of DNA. It equals one million base pairs (bp). In double-stranded DNA, a base pair consists of two nucleotides on complementary strands, so one megabase corresponds to about 1,000,000 bp. The unit is widely used for reporting genome sizes, chromosome lengths, and the size of sequence assemblies.

Examples of typical genome sizes in megabases include the human haploid genome at about 3,200 Mbp (3.2 Gb) and the diploid genome at about 6,400 Mbp. Escherichia coli has roughly 4.6 Mbp, Drosophila melanogaster about 180 Mbp, and Oryza sativa (rice) around 430 Mbp. Bread wheat (Triticum aestivum) has a much larger genome, on the order of 15,000–17,000 Mbp (15–17 Gb). These values illustrate the wide range of genome sizes across taxa.

Relation to physical length: DNA has a rise of about 0.34 nanometers per base pair. Therefore, 1 Mbp corresponds to roughly 0.34 millimeters of DNA. Consequently, the human haploid genome would extend to about 1 meter when stretched, and the diploid genome to roughly 2 meters.

Notes: Megabase is not an official SI unit; bp and Mbp are informal units used in genomics. The terms Mb and Mbp are used variably, with Mb often denoting megabases in genome contexts and Mbp emphasizing base pairs.