45kilobase

45kilobase, more commonly written as 45 kilobases or 45 kb, is a unit of length used in genomics to describe a span of double-stranded DNA. One kilobase equals 1,000 base pairs, so 45 kb corresponds to about 45,000 base pairs, or roughly 90,000 nucleotides in double-stranded DNA. The term is used to quantify the size of DNA fragments, contigs in genome assemblies, plasmids, or viral and phage genomes.

In genomic and molecular biology contexts, 45 kb is considered a mid-sized fragment. It is larger than many regulatory regions, promoters, or operons, but smaller than whole bacterial chromosomes and some viral genomes. Some bacteriophages and larger plasmids have genomes or inserts in this range; for example, the lambda phage genome is about 48.5 kb. Plasmids can also carry inserts of this order, depending on the cloning system and host.

Technologies for working with 45 kb fragments include sequencing, restriction mapping, and physical size estimation methods such as pulsed-field gel electrophoresis and optical mapping. In cloning and genetic engineering, 45 kb inserts are substantial enough to encompass multiple genes or operons, and specialized vectors—such as bacterial artificial chromosomes (BACs) and certain cosmids—are often used to maintain fragments of this size or larger.

Overall, 45 kb serves as a practical reference point for describing moderate-length DNA segments in bacterial, phage, and plasmid contexts, as well as for planning sequencing and cloning strategies.