Genetics Terms Starting With N
Genetics Glossary: N
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Natural Selection
/ NACH-er-ul suh-LEK-shun / · Latin: natura (nature) + selectio (choice)
Natural Selection is the process by which heritable traits that increase an organism's reproductive success become more common in a population over successive generations.
Natural selection requires heritable variation in traits, differential reproductive success linked to that variation, and sufficient time for allele frequency changes to accumulate. Depending on which end or range of the trait distribution is advantaged, selection can be directional, stabilizing, or disruptive. Directional selection is well documented at the lactase persistence locus, where alleles supporting adult milk digestion rose to high frequency in some pastoralist populations after dairying became culturally and nutritionally important.
Selection acts only on existing variation and cannot generate new alleles on its own.
Darwin and Wallace independently proposed the theory of natural selection in 1858, but it took nearly a century before the molecular basis of heritable variation was understood well enough to fully validate the mechanism.
Evolution is not driven by natural selection alone. Genetic drift, gene flow, and mutation also change allele frequencies, and in small populations drift can overpower selection entirely.
The peppered moth (Biston betularia) population in industrial England shifted from predominantly pale to dark forms during the 19th century as soot darkened tree bark, making pale moths more visible to predators. By the height of industrialization near Manchester, dark melanic moths comprised more than 90 percent of the local population, a shift documented by Bernard Kettlewell in the 1950s.
Non-Coding DNA
/ non KOH-ding dee-en-ay / · English: non + coding + DNA
Non-Coding DNA is genomic sequence that does not encode protein, encompassing regulatory elements, introns, repetitive sequences, and genes for functional RNA molecules such as microRNAs and long non-coding RNAs.
Although early research dismissed much non-coding DNA as functionless, the ENCODE project reported biochemical activity across a large fraction of the human genome, while later critiques emphasized that biochemical activity does not automatically prove selected biological function. Non-coding DNA includes promoters, enhancers, silencers, insulators, telomeres, centromeres, and the templates for small interfering RNAs and ribosomal RNAs. Mutations in non-coding regulatory regions are increasingly recognized as important drivers of disease and evolution, sometimes producing stronger phenotypic effects than mutations within protein-coding exons.
Comparative genomics has revealed that many non-coding sequences are more conserved across vertebrate species than the surrounding coding regions, indicating strong selective pressure to maintain their function.
Ultraconserved elements, a set of 481 DNA segments identified in 2004 by Bejerano and colleagues, are non-coding sequences that are 100 percent identical across the human, mouse, and rat genomes, a level of conservation exceeding that of most protein-coding genes.
Non-coding DNA is not synonymous with non-functional DNA. Regulatory non-coding sequences control when, where, and how much genes are expressed, and mutations in these regions cause numerous human diseases including certain cancers and developmental disorders.
A single nucleotide change in a non-coding enhancer element near the HMGA2 gene on chromosome 12 is associated with increased adult height in genome-wide association studies spanning hundreds of thousands of individuals. This variant does not alter any protein sequence, demonstrating that regulatory DNA can have measurable phenotypic effects without encoding a protein.
Building Blocks of Nucleic Acids →Non-disjunction
/ non-dis-JUNK-shun / · Latin: non (not) + disjunctio (separation)
Non-disjunction is the failure of homologous chromosomes or sister chromatids to separate correctly during meiosis or mitosis, producing cells with abnormal chromosome numbers.
Non-disjunction during meiosis I prevents homologous chromosomes from separating, producing gametes with two copies or zero copies of a chromosome. Failure during meiosis II or mitosis results from sister chromatids remaining joined rather than moving to opposite poles. Offspring of non-disjunction events in gametes may have trisomy, with one extra chromosome, or monosomy, with one missing chromosome, leading to developmental disorders or lethality.
Turner syndrome, caused by monosomy X, and Klinefelter syndrome, caused by XXY trisomy, are two survivable outcomes of meiotic non-disjunction in humans.
The risk of meiotic non-disjunction increases dramatically with maternal age, which is why trisomy 21 and other chromosomal trisomies are more common in children born to older mothers.
Difference Between Chromosome and Chromatid →Non-disjunction is not caused by DNA mutations. It is an error in the mechanical process of chromosome segregation during cell division, not a change in the nucleotide sequence of any gene.
Non-disjunction of chromosome 21 during the formation of an egg cell produces a gamete with two copies of chromosome 21, and fertilization by a normal sperm produces a trisomy 21 zygote. Trisomy 21, the chromosomal basis of Down syndrome, occurs in approximately 1 in 700 live births, making it the most common chromosomal trisomy compatible with survival to term.
Nonsense Mutation
/ NON-sens myoo-TAY-shun / · English: nonsense + Latin: mutatio (change)
Nonsense Mutation is a point mutation that converts an amino acid-coding codon into a premature stop codon, causing translation to terminate early and producing a truncated, usually nonfunctional protein.
Premature stop codons introduced by nonsense mutations prevent synthesis of the full-length protein and often trigger nonsense-mediated mRNA decay, reducing mRNA levels as well as protein output. The severity of the effect depends on where in the coding sequence the stop codon appears, with early truncations usually causing complete loss of function. Nonsense mutations account for roughly 11 percent of all described single-gene disease alleles, making them one of the most clinically significant categories of point mutation.
Duchenne muscular dystrophy and certain forms of cystic fibrosis are among the diseases frequently caused by nonsense mutations in their respective genes.
Aminoglycoside antibiotics can suppress nonsense mutations by causing the ribosome to read through premature stop codons, a mechanism being explored as a therapy for genetic diseases including Duchenne muscular dystrophy and Hurler syndrome.
A nonsense mutation is not the same as a missense mutation. A missense mutation changes one amino acid to a different amino acid, while a nonsense mutation changes an amino acid codon to a stop codon, typically eliminating protein function entirely.
Building Blocks of Proteins →A nonsense mutation converting an arginine codon to a stop codon at position 213 of the p53 tumor suppressor protein is one of the most frequently observed TP53 mutations in human cancers, appearing in tumors of the lung, colon, and bladder. This single-base change eliminates the final 80 or more amino acids of the protein, destroying its DNA-binding domain and abolishing its tumor-suppressive activity.
Nuclear DNA
/ NOO-klee-er dee-en-ay / · Latin: nucleus (kernel) + DNA abbreviation
Nuclear DNA is the genetic material located in the cell nucleus, organized into linear chromosomes and constituting the vast majority of a eukaryotic cell's total genetic information.
Human nuclear DNA spans about 3.2 billion base pairs distributed across 23 chromosome pairs, encoding roughly 20,000 protein-coding genes alongside extensive regulatory and non-coding sequence. Unlike the circular mitochondrial genome, which is present in hundreds to thousands of copies per cell and spans only about 16,569 base pairs in humans, nuclear DNA is linear and packaged with histone proteins into chromatin. Nuclear DNA is inherited biparentally, with half contributed by each parent, giving it far greater discriminating power for individual identification than maternally inherited mitochondrial DNA.
This biparental inheritance also means that nuclear loci undergo recombination each generation, shuffling allele combinations in ways that mitochondrial DNA does not.
A single human diploid nucleus contains about two meters of DNA packed into a space only about six micrometers wide. Across the whole body, total DNA length depends on how many nucleated cells are counted, so popular distance comparisons vary widely and should be treated as estimates rather than fixed facts.
Cells contain DNA outside the nucleus. Mitochondria and, in plants, chloroplasts also carry their own distinct circular DNA molecules, inherited through separate pathways and tracing back to ancient bacterial endosymbionts.
Forensic scientists prefer nuclear DNA for genetic profiling because its biparental inheritance and high variability across 20 or more STR loci provide discriminating power exceeding one in a quadrillion for unrelated individuals. Mitochondrial DNA, by contrast, is shared identically among all maternally related relatives, making it far less useful for distinguishing individuals within a family.