Kilobase: the kilobase unit for DNA base pairs
How can a length of DNA be expressed by counting its building blocks? The kilobase unit gives geneticists a thousand-pair scale: it represents the genetic information carried by 1000 pairs of base units in DNA's double helix. It can also express relative distance as the length of a DNA strand containing that many base pairs. The measurement thus joins an information quantity to a length defined through a count. Its object is DNA, and its counted components are pairs rather than an unpaired collection of building blocks. That paired structure gives the measure its particular biological setting.
DNA has the form of a double helix, with bases on one strand paired with bases on the other. The length of a segment or fragment is measured by the number of those base pairs. Biochemistry and genetics use kbp as a symbol for 1000 base pairs, while kb is a symbol for the kilobase unit. The abbreviations compress a longer expression built around the same paired structure. For relative distance, the counting runs along the strand; for genetic information, the unit refers to the information carried by the defined group. The pairs remain the common scale behind these two uses.
The first part of the name has a history beyond genetics. Kilo- is a metric prefix meaning 1000, formed by modifying chilioi, the Greek word for a thousand. Its symbol is the lowercase k. The kilobase unit applies that thousand-scale prefix to DNA's base pairs. Other measurements attach it to different quantities, from electric current to pressure. A kiloampere equals 1000 amperes, and a kilobecquerel equals 1000 atomic disintegrations per second. In each case the prefix gives the size of the grouping, while the rest of the name identifies the thing or process being measured.
In physics, a kilodalton is a mass unit equal to 1000 atomic mass units. It belongs to another measurement category from the information and relative-distance uses of the kilobase unit. A kiloelectronvolt is likewise a work or energy unit, equal to 1000 electronvolts. The same prefix can therefore accompany a count of DNA pairs, a mass, or an energy. Even familiar lengths can carry it: the kilofoot equals 1000 feet, exactly 304.8 meters. Telecommunications uses that combination of a metric prefix and an English unit to express cable lengths and transmission distances, extending the thousand-scale naming pattern into a practical engineering setting.
Computing gave kilo- another numerical use. In measurements of computer memory, the prefix often meant 1024 rather than 1000. A kilobyte is defined as 1000 bytes, but the name has also been used for 1024 bytes. A kilobit similarly means 1000 bits, or in some cases 1024 bits or 128 bytes. Information measurement consequently developed terminology in which a thousand-based name could accompany a different count. The naming convention had a second scale within computer usage. That variation led to a separate prefix for the binary quantity.
In 1998, the International Electrotechnical Commission adopted kibi- to replace kilo- for binary applications in computer science. Kibi- means 1024, and its name contracts "kilobinary." Its symbol is Ki-, with the capital letter chosen for uniformity with other binary prefixes such as Mi- and Gi-. A group of 1024 bytes of storage is a kibibyte, written KiB. The larger bit unit is supposed to be called a kibibit. These names give the binary scale its own vocabulary instead of relying on the metric thousand-prefix.
Data transmission adds time to the information quantity. A kilobit per second is a transmission-rate unit equal to 1000 bits per second, written kb/s or kbps. The slash form is the preferred symbol for this unit. Scientific time itself can use the thousand-prefix: a kilosecond is 1000 seconds, equal to sixteen minutes and forty seconds. It has uses in science even though it is absent from ordinary life. One day equals 86.4 kiloseconds, placing the less familiar scientific interval beside a familiar duration.
Chemistry uses a further measure for the activity of catalysts, substances that start or speed a chemical reaction. Most enzymes are proteins, but some catalytic RNA molecules also act as enzymes. One katal describes catalytic activity enabling a reaction to proceed at one mole per second. Pronounced "cattle," the unit is used especially in enzyme chemistry. It was added to the International System at the twenty-first General Conference of Weights and Measures in October 1999. Where genetic measurement counts paired building blocks, this chemical measure follows activity through time, giving enzyme chemistry a named unit for a reaction's enabled rate.