kilobases

Kilobase is a unit of length used to measure nucleic acids. It equals 1,000 nucleotides for single-stranded DNA or RNA. For double-stranded DNA, length is commonly expressed as kilobase pairs (kbp), where 1 kbp equals 1,000 base pairs. Because a base pair consists of two nucleotides, 1 kbp corresponds to about 2,000 nucleotides in total.

In practical use, kilobases and kilobase pairs describe lengths of DNA fragments, plasmids, and chromosomes. DNA fragments used in cloning often range from a few kilobases to tens of kilobases. Bacterial chromosomes are typically about 1–6 megabases (Mb), while plasmids commonly span from a few kilobases up to roughly 300 kb. The human genome is about 3 gigabases (Gb), equivalently about 3,000 Mb or 3,000,000 kb. Gene lengths vary from a few hundred bases to several hundred kilobases, and many sequencing projects report assembly sizes in kb or Mb.

Notation and context: kbp is the precise unit for measuring double-stranded DNA length, while kb is commonly used in more general contexts. For larger scales, megabases (Mb) and gigabases (Gb) are standard terms. Accurate use of kb, kbp, and their higher-scale equivalents helps avoid ambiguity in genetic sequencing, cloning, and genome assembly discussions.