| B form DNA |
| A right-handed double-helical conformation
of DNA normally seen in solution described by Watson and Crick.
A second DNA conformation (A form) is seen in unhydrated DNA (fibres
or crystals of oligonucleotides). |
| bacillus |
| A rod-shaped bacterium. |
| back mutation |
| The process that causes reversion. A change
in a nucleotide pair in a mutant gene that restores the original
sequence and hence the original phenotype. |
| backcross |
| The cross of an individual with one of its
parents or an organism with the same genotype as a parent. |
| bacterial lawn |
| A continuous cover of bacteria on the surface
of a growth medium. |
| bacteriophage(phage) |
| A virus that infects bacteria. |
| balanced lethal system |
| An arrangement of recessive lethal alleles
that maintains a heterozygous chromosome combination. homozygotes
for any lethal-bearing chromosome perish. |
| balanced polymorphism |
| Stable genetic polymorphism maintained by
natural selection. |
| Balbiani ring |
| A large chromosome puff. The larger polytene
chromosomal puffs. See chromosome puff. |
| Barr body |
| A densely staining mass that represents
an inactivated X chromosome. Heterochromatic body found in the nuclei
of normal females but absent in the nuclei of normal males. |
| basal body |
| A short cylindrical array of microtubules
and other proteins, found at the base of a eukaryotic cilium or
flagellum, that organises the assembly of the axoneme (the bundle
of microtubules and other proteins forming the core of each cilium
or flagellum). |
| base analogue |
| A chemical whose molecular structure mimics
that of a DNA base; because of the mimicry, the analogue may act
as a mutagen. |
| Batesian mimicry |
| Form of mimicry in which an innocuous mimic
species gains protection by resembling noxious or dangerous model
species. |
| bead theory |
| The disproved hypothesis that genes are
arranged on the chromosome like beads on a necklace, indivisible
into smaller units of mutation and recombination. |
| beta-galactosidase |
| The enzyme that splits lactose into glucose
and galactose (coded by a gene (lac z) in the lac operon of Escherichia
coli). |
| beta-galactoside acetyltransferase
|
| An enzyme that is involved in lactose metabolism
and encoded by a gene (lac a) in the lac operon of Escherichia coli).
|
| beta-galactoside permease |
| An enzyme involved in concentrating lactose
in the cell (coded by a gene (lac y) in the lac operon of Escherichia
coli)). |
| bimodal distribution |
| A statistical distribution having two modes.
|
| binary fission |
| The process in which a parent cell splits
into two daughter cells of approximately equal size. Simple cell
division in single-celled organisms. |
| biochemical genetics |
| The study of the relationships between genes
and enzymes, specifically the role of genes in controlling the steps
in biochemical pathways. |
| biolistic |
| A method (biological ballistics) of transfecting
cells by bombarding them with microprojectiles coated with DNA.
|
| biological species concept
|
| Organisms are classified in the same species
if they are potentially capable of interbreeding and producing fertile
offspring. |
| bivalent |
| Structure, formed during prophase of meiosis
1, consisting of synapsed homologous chromosomes. Equivalent to
a tetrad of chromatids. |
| blastoderm |
| In an insect embryo, the layer of cells
that completely surrounds an internal mass of yolk. |
| blastomere |
| One of the cells formed by division of the
fertilized egg making up the blastula. |
| blastopore |
| The embryonic structure present during gastrulation
and formed by invagination of the endoderm. |
| blastula |
| An early developmental stage of vertebrate
embryos formed by cleavage of the fertilized egg, in which the embryo
consists of a single layer of cells surrounding a cavity (the blastocoel).
|
| blending inheritance |
| A discredited model of inheritance suggesting
that the characteristics of an individual result from the smooth
blending of fluid like influences from its parents. |
| blunt-end ligation |
| The ligating or attaching of blunt-ended
pieces of DNA by T4 DNA ligase. Used in creating hybrid vectors.
|
| bottleneck |
| A brief reduction in size of a population
which usually leads to random genetic drift. |
| brachydactyly |
| A human phenotype of unusually short digits,
generally inherited as an autosomal dominant. |
| branch migration |
| 1. The process in which a crossover point
between two DNA duplexes slides along the duplexes. |
| 2. The process by which a single invading
DNA strand extends its partial pairing with its complementary strand
as it displaces the resident strand from a DNA duplex. |
| breakage and reunion |
| The general mode by which recombination
occurs. DNA duplexes are broken and reunited in a crosswise fashion
according to the Holliday model. |
| breakage-fusion-bridge cycle
|
| Damage that happens to a dicentric chromosome
during each cell cycle. |
| bridging cross |
| A cross made to transfer alleles between
two sexually isolated species by first transferring the alleles
to an intermediate species that is sexually compatible with both.
|
| bud |
| A daughter cell formed by mitosis in yeast;
one daughter cell retains the cell wall of the parent, and the other
(the bud) forms a new cell wall. |
| buoyant density |
| A measure of the tendency of a substance
to float in some other substance; large molecules are distinguished
by their differing buoyant densities in some standard fluid. Measured
by density-gradient ultracentrifugation. |
| buoyant density of DNA |
| A measure of the density of DNA determined
by the equilibrium point reached by DNA after density gradient centrifugation.
|
| Burkitt lymphoma |
| A cancer of the lymphatic system manifested
by tumours in the jaw, often associated with a translocation bringing
a specific oncogene next to a novel regulatory element. |
