Dark repair
An enzyme-catalyzed process for removal of thymine dimers from DNA and synthesis of a new DNA segment complementary to the undamaged strand.
Darwinian fitness (w)
The relative reproductive ability of individuals with a particular genotype.
Daughter chromosomes
Detached sister chromatids after they separate at the beginning of mitotic anaphase or meiotic anaphase II.
Deamination
Removal of an amino group from a nucleotide in DNA.
Degeneracy
In the genetic code, the existence of more than one codon corresponding to each amino acid.
Degradation control
The regulation of the rate of breakdown (turnover) of RNA molecules in the cell.
Deletion
A chromosomal mutation resulting in the loss of a segment of a chromosome and the gene sequences it contains.
Deoxyribonuclease (DNase)
An enzyme that catalyzes the degradation of DNA to nucleotides.
Deoxyribonucleic acid (DNA)
A polymeric molecule consisting of deoxyribonucleotide building blocks that in a double-stranded, double-helical form is the genetic material of all living organisms.
Deoxyribonucleotide
Any of the nucleotides that make up 4 DNA, consisting of a sugar (deoxyribose), a base, and a phosphate group.
Deoxyribose
The pentose (five-carbon) sugar found in DNA.
Depurination
Loss of a purine base (adenine or guanine) from a nucleotide in DNA.
Determination
Process early in development that establishes the fate of a cell, that is, the differentiated cell type it will become.
Development
Overall process of growth, differentiation, and morphogenesis by which a zygote gives rise to an adult organism. It involves a programmed sequence of phenotypic events that are typically irreversible.
Diakinesis
The final stage in prophase I of meiosis, during which the replicated chromosomes (bivalents) are most condensed, the nuclear envelope breaks down, and the spindle begins to form.
Dicentric bridge
A homologous chromosome pair in meiosis I in which one chromatid has two centromeres as the result of crossing-over within a paracentric inversion. As the two centromeres begin to migrate to opposite poles, a dicentric bridge stretching across the cell forms and eventually breaks.
Dicentric chromosome
A homologous chromosome pair in meiosis I in which one chromatid has two centromeres as the result of crossing-over within a paracentric inversion. As the two centromeres begin to migrate to opposite poles, a dicentric bridge stretching across the cell forms and eventually breaks.
Dideoxynucleotide (ddNTP)
A modified nucleotide that has a 3¿-H on the deoxyribose sugar rather than a 3¿-OH. A ddNTP can be incorporated into a growing DNA chain, but no further DNA synthesis can occur because no phosphodiester bond can be formed with an incoming nucleotide.
Dideoxy sequencing
A method for rapidly sequencing DNA molecules in which the DNA to be sequenced is used as the template for in vitro DNA synthesis in the presence of dideoxynucleotides (ddNTPs). When a dideoxynucleotide is incorporated into a growing DNA chain, no further DNA synthesis occurs, generating a truncated chain in which the terminal dideoxynucleotide corresponds to the normal nucleotide that occurs at that point in the sequence.
Differentiation
Series of cell-specific changes in gene expression by which determined cells give rise to cell types with characteristic structures and functions.
Dihybrid cross
A cross between two individuals of the same genotype that are heterozygous for two pairs of alleles at two different loci (e.g.,,).
Dioecious
Referring to plant species in which individual plants possess either male or female sex organs.
Diploid (2N)
A cell or an individual with two copies of each chromosome.
Diplonema
The stage in prophase I of meiosis during which the synaptonemal complex disassembles and homologous chromosomes begin to move apart.
Discontinuous trait
A heritable characteristic that exhibits a small number of distinct phenotypes, which commonly are determined by variant alleles at a single locus.
Dispersed repeated DNA
Repetitive DNA sequences that are distributed at irregular intervals in the genome.
Dispersive model
A model for DNA replication in which the parental double helix is cleaved into double-stranded DNA segments that act as templates for the synthesis of new double-stranded DNA segments, which are reassembled into complete DNA double helices, with parental and Ss Yy!Ss Yy progeny DNA segments interspersed. The results of the Meselson–Stahl experiment did not support this model.
DNA
A polymeric molecule consisting of deoxyribonucleotide building blocks that in a double-stranded, double-helical form is the genetic material of all living organisms.
DNA chip
An ordered grid of DNA molecules of known sequence—probes—fixed at known positions on a solid substrate, either a silicon chip, glass, or less commonly, a nylon membrane. Labelled free DNA molecules—targets— are added to the unlabelled fixed probes to analyze identities or quantities of target molecules. DNA microarrays allow for the simultaneous analysis of thousands of DNA target molecules.
DNA-dependent RNA polymerase
The more complete name for RNA polymerase, the enzyme responsible for transcription, the process of RNA synthesis using a DNA template.
DNA fingerprinting
Molecular analysis of DNA polymorphisms to identify individuals based on the unique characteristics of their DNA.
DNA helicase
An enzyme that catalyzes unwinding of the DNA double helix at a replication fork during DNA replication.
DNA ladder
Also known as DNA size markers, a set of DNA molecules of known size used in agarose gel electrophoresis experiments.
DNA ligase
An enzyme that catalyzes the formation of a phosphodiester bond between the 5’ end of one DNA chain and 3’ end of another DNA chain during DNA replication and DNA repair.
DNA markers
Sequence variations among individuals in a specific region of DNA that are detected by molecular analysis of the DNA and can be used in genetic analysis.
DNA microarray
An ordered grid of DNA molecules of known sequence—probes—fixed at known positions on a solid substrate, either a silicon chip, glass, or less commonly, a nylon membrane. Labelled free DNA molecules—targets— are added to the unlabelled fixed probes to analyze identities or quantities of target molecules. DNA microarrays allow for the simultaneous analysis of thousands of DNA target molecules.
DNA molecular testing
A type of genetic testing that focuses on the molecular nature of mutations associated with a particular disease.
DNA polymerase
Any enzyme that catalyzes the polymerization of deoxyribonucleotides into a DNA chain. All DNA polymerases synthesize DNA in the 5’ to 3’ direction.
DNA polymerase I (DNA Pol I)
One of several E. coli enzymes that catalyze DNA synthesis; originally called the Kornberg enzyme.
DNA polymorphism
Variation in the nucleotide sequence or number of tandem repeat units at a particular locus in the genome. Most commonly, this term is used for DNA markers, variations that are located outside of genes and that are detected by molecular analysis.
DNA primase
An enzyme that catalyzes formation of a short RNA primer in DNA replication.
DNA profiling
Molecular analysis of DNA polymorphisms to identify individuals based on the unique characteristics of their DNA. Also called DNA fingerprinting.
DNA typing
Molecular analysis of DNA polymorphisms to identify individuals based on the unique characteristics of their DNA. Also called DNA fingerprinting.
Domain shuffling
Proposed mechanism for evolution of genes with new functions by the duplication and rearrangement of exons encoding protein domains in different combinations. Also called exon shuffling.
Dominant
Describing an allele or phenotype that is expressed in either the homozygous or the heterozygous state.
Dominant lethal allele
An allele that results in the death of an organism that is homozygous or heterozygous for the allele.
Dosage compensation
Any mechanism in organisms with genotypic sex determination for equalizing expression of genes on the sex chromosomes in males and females.
Down syndrome
The presence of an extra copy of chromosome 21, which causes Down syndrome in humans. 2N+1.
Duplication
A chromosomal mutation that results in the doubling of a segment of a chromosome and the gene sequences it contains.
