| lambda phage |
| One kind (species) of temperate bacteriophage.
|
| lambda dgal |
| A lambda phage carrying a gal(galactose)
bacterial gene and defective (d) for some phage function. |
| lac operon |
| An inducible operon including three loci
involved in the uptake and breakdown of lactose in Escherichia coli.
|
| lagging strand |
| In DNA replication, the strand that is synthesized
apparently in the 3' to 5' direction, but actually in the 5' to
3' direction by ligating short fragments synthesized individually.
Strand of DNA being replicated discontinuously. See also leading
strand. |
| lampbrush chromosomes
|
| Chromosomes of amphibian oocytes having
loops suggestive of a lampbrush. Large chromosomes found in amphibian
eggs, with lateral DNA loops, extending from chromomeres, producing
a brushlike appearance under the microscope. |
| lateral inhibition |
| The signal produced by one cell that prevents
adjacent cells from acquiring their fate. |
| lawn |
| A continuous layer of bacteria on the surface
of an agar medium. |
| leader peptide gene |
| A small gene within the attenuator control
region of repressible amino acid operons. Translation of a specific
leader peptide tests for the concentration of a specific amino acid
or set of amino acids in the cell. If the concentration is high
transcription is terminated in the attenuator, if low continued
leading to the production of enzymes for amino acid synthesis and
hence more amino acid. See leader transcript. |
| leader sequence |
| The sequence at the 5' end of an mRNA that
is not translated into protein. The length of untranslated mRNA
from the 5' end to the initiation codon AUG. |
| leader transcript |
| The mRNA transcribed by the attenuator region
of repressible amino acid operons. The transcript is capable of
several alternative stem-loop structures dependent on translation
of a short leader peptide gene. |
| leading strand |
| Strand of DNA being replicated continuously.
In DNA replication, the strand that is made in the 5' to 3' direction
by continuous polymerization at the 3' growing tip. See also lagging
strand. |
| leaky mutant |
| A mutant (typically an auxotroph) that results
from a partial rather than a complete inactivation of the wild-type
function. |
| leptonema (leptotene
stage) |
| The first stage of prophase 1 of meiosis
in which chromosomes become distinct. |
| lesion |
| A damaged area (a mutant site) in a gene,
a chromosome, or a protein. |
| lethal gene |
| A gene whose expression results in the death
of the individual expressing it. |
| lethal-equivalent alleles
|
| alleles whose summed effect is that of lethality
for example, four alleles each of which would be lethal 25% of the
time (or to 25% of their bearers), are equivalent to one lethal
allele. |
| Leu |
| Leucine (an amino acid). |
| leucine zipper |
| Configuration of a DNA-binding protein in
which leucine residues on two protein alpha-helices interdigitate
in zipper fashion to stabilize the protein. |
| leukemia |
| cancer of the bone marrow resulting in excess
production of leukocytes. |
| level of significance
|
| The probability value in statistics used
to reject the null hypothesis. |
| ligand-receptor interaction
|
| interaction between a molecule (usually
of an extracellular origin) and a protein on or within a target
cell. One type of ligand-receptor interaction can be between steroid
hormones and their cytoplasmic or nuclear receptors. Another can
be between secreted polypeptide ligands and transmembrane receptors.
|
| ligase |
| An enzyme that can rejoin a broken phosphodiester
bond in a nucleic acid. |
| LINE |
| Long interspersed element; a type of large
repetitive DNA segment found throughout the genome of Eukaryotes.
|
| line |
| A group of identical pure-breeding diploid
or polyploid organisms, distinguished from other individuals of
the same species by some unique phenotype and genotype. |
| linear tetrad |
| A tetrad that results from the occurrence
of the meiotic and postmeiotic nuclear divisions in such a way that
sister products remain adjacent to one another in an ordered manner
constrained within an ascus. |
| linkage |
| An association of gene loci on the same
chromosome. |
| linkage disequilibrium
|
| When the observed frequencies of haplotypes
in a population does not agree with haplotype frequencies predicted
by multiplying together the frequency of individual genetic markers
in each haplotype. |
| linkage equilibrium |
| When the observed frequencies of haplotypes
in a population agrees with haplotype frequencies predicted by multiplying
together the frequency of individual genetic markers in each haplotype.
|
| linkage group |
| A group of gene loci known to be linked;
a chromosome. There are as many linkage groups as there are homologous
pairs of chromosomes. See synteny. |
| linkage map |
| An abstract map of chromosomal loci, based
on recombinant frequencies. |
| linking number |
| The number of times one strand of a helix
coils about the other. |
| locus(Plural loci) |
| The position of a gene, DNA marker or genetic
marker on a chromosome. See gene locus. |
| lod score method |
| A technique (logarithmic odds) for determining
the most likely recombination frequency between two loci from pedigree
data. |
| lymphoma |
| cancer of the lymph nodes and spleen that
causes excessive production of lymphocytes. |
| Lyon hypothesis |
| The hypothesis, named after Mary Lyon who
stated it, suggesting that doseage compensation in mammals is by
inactivation of all but one X chromosome in cells with more than
one X chromosome. The Barr body, visible in some female mammalian
cells, is an inactivated X chromosome. |
| Lys |
| Lysine (an amino acid). |
| lysate |
| The contents released from a lysed cell.
|
| lysis |
| The breaking open of a cell by the destruction
of its wall or membrane. The rupture and death of a bacterial cell
upon the release of phage progeny. |
| lysogenic |
| The state of a bacterial cell that has an
integrated phage (prophage) in its chromosome. |
| lysogenic bacterium |
| A bacterial cell capable of spontaneous
lysis due, for example, to the release of a prophage from the bacterial
chromosome. |
