L Leucine in proteins lactose operon The cluster of three genes that code for the enzymes
involved in utilization of lactose by Escherichia
coli lagging strand The strand of the double helix which is copied in a
discontinuous fashion during DNA replication. LAN Local Area Network. A network that connects computers in
a small, defined area, such as the offices in a single wing
or a group of buildings. lariat Refers to the lariat-shaped intron RNA that results from
splicing a GU-AG intron. lattice in crystallography, a network of unit cells. Often
called, a crystal lattice. leader segment The untranslated region of an mRNA upstream of the
initiation codon. leading strand The strand of the double helix which is copied in a
continuous fashion during DNA replication. Leaky mutation a mutation that results in partial loss of a
characteristic legacy database databases that were designed and built using technology
that is not currently the main database technology within an
organization. These databases often have to be integrated
into any new database system. Data warehouses are composed
of legacy databases. length abridgement The gradual shortening of pseudogenes during evolution
that is caused by an excess of deletions over insertions (6-4) lesion A dimer between two adjacent pyrimidine bases in a
polynucleotide, formed by ultraviolet irradiation. lethal mutation A mutation that results in death of the cell or
organism leucine zipper A dimerization domain commonly found in DNA-binding
proteins (transcription factors for example). library An ordered collection of clones (i.e. cloned DNA from a
particular organism), whose relationship to each other can
be established by physical mapping. Compare with genomic
library, arrayed library). In programming, a library is a predefined object file
that contain the machine language instructions for various
operations commonly required by programs. A collection of
tools written by other programmers that perform specific
operations. ligation the formation of a phosphodeester bond to link two
adjacent bases separated by a nick in double-stranded DNA.
The process catalyzed by the enzyme ligase. LINE Long interspersed nuclear element. A type of genome-wide
repeat, often with transposable activity link linkage The proximity of two or more markers (e.g., genes, RFLP
markers) on a chromosome; the closer together the markers
are, the lower the probability that thewy will be separated
during DNA repair or replication processes (binary fission
in prokaryotes, mitosis or meiosis in eukaryotes) and hence
the greater the probability that they will be inherited
together. linkage map A map of the relative positions of genetic loci on a
chromosome, determined on the basis of how often the loci
are inherited together. Distance is measured in centimorgans
(cM). linker DNA A DNA that links nucleosomes: the "string" between the
"beads" in the beads-on-a-string model for chromatin
structure. linker histone A histone, such as H1, that is located outside of the
nucleosome core octamer. lint A UNIX program that can detect bugs, portability
problems, and other possible errors in C programs. local alignment An optimal alignment that includes the most similar local
region or regions. Local alignments may include only short
portions of the sequences that were used to calculate the
alignment. Local align ments are especially useful for
distantly related sequences, although thewy work equally
well for closely related ones. localize Determination of the original position(locus) of a gene
or other marker on a chromosome localized repeated sequence Tandemly arrayed, repetitive sequences, usually made up
of short simple repeated motifs (e.g. satellite DNA) locus (pl. loci) The position on a chromosome of a gene or other
chromosome marker; also, the DNA at that positon. The use of
locus is sometimes restricted to mean regions of DNA that
are expressed. locus control region A DNA sequence that maintains a functional domain in an
open, active configuration. lod score A statistical measure of linkage as revealed by pedigree
analysis. login Sign on and identify yourself to the system. At the
beginning of each terminal session, the system propmts a
user for their username and a password. log-odds A scoring system in which the values are the logarithm of
the relative probability (odds) of a comparison being due to
homology or being due to chance. long branch attraction The artifactual placement of rapidly evolving sequences
with other rapidly evolving sequences and slowly evolving
sequences with other slowly evolving sequences in
phylogenetic trees. The placement is independent of the true
phylogenetic relationships. long patch repair A nucleotide excision repair process of Eschericia coli
that results in excision and resynthesis of up to 2kb of
DNA. loss-of-function mutation A mutation that reduces or abolishes a protein's
activity. low complexity region A region of a nucleic acid or protein sequence with
highly biased residue composition, or consisting of many
short, near-perfect repeats. lowly repetitive gene Genes appearing in only a few copies in the haploid
genome. ls UNIX command for listing the contents of a directory LTR element A type of genome-wide repeat typified by the presence of
long terminal repeats (LTRs). lysogenic pathway The type of bacteriophage infection that involves
integration of the phage genome into the host DNA
molecule. lytic pathway The type of bacteriophage infection that involves lysis
of the host cell immediately after the initial infection,
with no integration of the phage DNA molecule into the host
genome.