kilobasescale

Kilobase scale refers to lengths of nucleic acids in the thousands of base pairs, typically expressed in kilobases (kb). One kilobase equals 1,000 base pairs for double-stranded DNA and is used to describe the size of genes, plasmids, genomic fragments, and sequencing libraries. The kilobase scale covers a broad range, from about 1 kb up to several megabases (Mb); many bacterial genes and plasmids fall within a few to tens of kilobases, while whole genomes or large genomic regions span hundreds of kilobases to megabases.

Measurement and use: Fragment sizes are estimated by gel electrophoresis, pulsed-field gel electrophoresis, or sequencing library size selection; long-read sequencing technologies such as PacBio and Nanopore can read kilobase-length fragments. Assembly metrics such as N50 are often reported in kilobases, and researchers describe contig lengths in kb. The kilobase scale is convenient for describing gene lengths, plasmid sizes, and subchromosomal regions.

Common examples: The pBR322 plasmid is about 4.3 kb; the lambda phage genome is approximately 48.5 kb; typical bacterial genes are 1–3 kb. Human genes vary widely in length, and transcripts can span several kilobases. Historically, sequencing reads ranged from hundreds to thousands of bases, while long-read platforms now deliver tens to hundreds of kilobases per read.

Relation to other scales: 1 kilobase equals 0.001 megabase; a megabase is 1,000 kilobases. The kilobase scale is used for describing gene lengths, plasmid sizes, and genomic fragments, whereas the megabase scale is used for whole genomes and chromosomal regions.