Lipopolysaccharide - a compound containing lipid bound to a polysaccharide; often a component of microbial cells walls.

Liposome - a microscopic artificial membrane vesicle consisting of a spherical phospholipid bilayer Liposomes can be incorporated into living cells and used to transport relatively toxic drugs into diseased cells, where they can exert their maximum effect DNA molecules may be entrapped in, or bound to the surface of, the vesicles, and subsequent fusion of the liposome with the cell membrane will deliver the DNA into the cell Liposomes have been used to develop an efficient transfection procedure for Streptomyces bacteria.

Liquefaction - enzymatic digestion (often by a-amylase) of gelatinised starch to form lower molecular weight polysaccharides.

Liquid media - media without a solidifying agent.

Liquid membranes - thin films made up of liquids (as opposed to solids) which are stable in another liquid (usually water) Thus the liquid must not dissolve in the water, but nevertheless must be prevented from collapsing into a lot of small droplets.

Liquid nitrogen - nitrogen gas condensed to a liquid with a boiling point of about 196C Very commonly the medium in which containers of genetic material are stored.

Litmus - a pH indicator paper (range 45-83) impregnated with an extracted lichen pigment It turns red in acidic and blue in alkaline solutions However, the use of litmus paper as an indicator is not a precise method of pH measurement

Live vaccine - a living, nonvirulent form of a microorganism or virus that is used to elicit an antibody response that will protect the inoculated organism against infection by a virulent form of the microorganism or virus Also a living, nonvirulent micro organism or virus that express a foreign antigenic protein and is used to inoculate humans or animals The latter organisms are also called a live recombinant vaccines.

Locus - a site on a chromosome.

Lod score - the logarithm of the odds of linkage between two loci Calculated from pedigree data, as the log (to base 10) of the ratio of the probability of the observed pedigree assuming linkage with a specified recombination fraction q, to the probability of the observed pedigree assuming no linkage, ie, recombination fraction = : 05 A lod score (also called a Z value) is thus calculated as z = log10 {Prob(data I q)/ : Prob( data I 05) } A lod score of + 300 (which is odds of 1000: 1) or greater is regarded as acceptable evidence for linkage; -200 (which is the log: of 1 :100) or less indicates that no linkage exists.

Logarithmic phase - the steepest slope of the growth curve; the phase of vigorous growth, during which cell number doubles every 20 -30 minutes.

Long template - a DNA strand that is: synthesised during the poly-merase chain reaction and has a primer sequence at one end but is extended beyond the: site that is complementary to the second primer at the other end.

Long terminal repeat - a string of bases that occurs at each end of the genome of a retrovirus that has become integrated into the host genome Involved in the integration process

Long day plant - plant requiring short nights before flowering is initiated.

Loop bioreactors - fermenters in which the fermenting material is cycled between a bulk tank and a smaller tank or loop of pipes The circulation helps to mix the materials and to ensure that gas injected into the fermenter is well distributed in the liquid There actors are also very useful for photosynthetic fermentations, where they allow the photosynthesizing organisms to be passed along a large number of small pipes, where the light can get to them easily, rather than inside a single volume, where only the organisms near the edges get much light.

Luteinising hormone - a pituitary hormone which causes growth of the yellow body of the ovary and also stimulates activity of the interstitial cells of the testis.