Molecular Biology Terms Starting With N
Molecular Biology Glossary: N
Jump to Molecular Biology Term
Non-Coding RNA
/ non KOH-ding ar-en-ay / · English: non-coding + RNA
Non-Coding RNA is an RNA molecule transcribed from DNA that functions without being translated into a protein.
Non-coding RNAs include transfer RNAs, ribosomal RNAs, microRNAs, small nuclear RNAs, small nucleolar RNAs, long non-coding RNAs, and telomerase RNA. Ribosomal RNA forms the catalytic and structural core of the ribosome, transfer RNA carries amino acids during translation, and small nuclear RNAs participate directly in pre-mRNA splicing. Large portions of the human genome are transcribed into non-coding RNAs, although the biological significance of many low-abundance transcripts remains debated.
Well-characterized non-coding RNAs regulate chromatin, guide enzymes to target sequences, control mRNA stability, and organize nuclear bodies, showing that lack of protein-coding capacity does not imply lack of function.
TERC, the RNA component of human telomerase, is a 451-nucleotide non-coding RNA that provides the template used to extend chromosome ends. Mutations in TERC can cause dyskeratosis congenita by impairing telomere maintenance.
Non-coding RNA is useless because it does not produce protein. Many non-coding RNAs are essential molecules that catalyze translation, guide splicing, regulate chromatin, or silence gene expression.
U1 small nuclear RNA base-pairs with the 5-prime splice site of pre-mRNA during early spliceosome assembly in human cells. This RNA is 164 nucleotides long, and even short disruptions in its pairing region can redirect splice-site choice and produce aberrant transcripts.
Northern Blotting
/ NOR-thern BLOT-ing / · Named informally after Southern blotting
Northern Blotting is a laboratory technique for detecting specific RNA sequences in a sample by separating RNA molecules by size using gel electrophoresis, transferring them to a membrane, and hybridizing the membrane with a labeled probe complementary to the target sequence.
Total RNA or mRNA is denatured with formaldehyde to prevent secondary structure formation, separated by size on an agarose gel, then transferred to a nylon or nitrocellulose membrane by capillary or electroblotting. A labeled probe, historically radioactive but now often fluorescent or chemiluminescent, hybridizes to the target transcript, and the resulting signal is detected by autoradiography or imaging. Band size confirms transcript length, and band intensity reflects the relative abundance of the mRNA in each sample lane.
Northern blotting was the standard method for confirming gene expression and transcript size before quantitative RT-PCR and RNA sequencing became widely available in the 2000s.
The technique was named "Northern blotting" as a play on the Southern blotting method developed by Edwin Southern in 1975 for DNA detection. Neither name refers to a geographic direction; Western blotting for proteins followed the same naming convention.
Northern blotting detects proteins because it sounds similar to Western blotting. Northern blotting detects RNA; Western blotting detects protein; and Southern blotting, the original technique, detects DNA.
A Northern blot comparing RNA from mouse liver, brain, and skeletal muscle can reveal that a transcript is present in one tissue but absent in others. The same blot also reports transcript size, for example showing a band of approximately 2.4 kilobases in liver RNA.
Nuclear Receptor
/ NYOO-klee-er reh-SEP-ter / · Latin nucleus, kernel; Latin receptus, received
Nuclear Receptor is a ligand-activated transcription factor that binds small lipophilic molecules such as steroid hormones, thyroid hormone, or retinoids inside the cell and then directly regulates target gene transcription by binding specific DNA sequences.
Because their ligands are lipophilic, they cross the plasma membrane without requiring a cell-surface receptor, binding instead to nuclear receptors in the cytoplasm or nucleus. Ligand binding triggers a conformational change that exposes a DNA-binding domain, allowing the receptor to bind hormone response elements in the promoters or enhancers of target genes. Bound receptors then recruit coactivator complexes that remodel chromatin and stimulate transcription, or corepressor complexes that silence it, depending on the ligand and cellular context.
The human genome encodes 48 nuclear receptors, and their dysfunction underlies diseases ranging from breast and prostate cancer to metabolic syndrome.
The liver X receptors (LXRs) are nuclear receptors that sense excess cellular cholesterol and respond by activating genes that export cholesterol from cells. Synthetic LXR agonists are under investigation as treatments for atherosclerosis because they can reduce cholesterol accumulation in arterial macrophages.
Endocrine System Fun Facts →All cell signals work only at the cell surface. Lipophilic hormones such as estrogen and cortisol diffuse through the plasma membrane and activate nuclear receptors inside the cell, bypassing surface receptors entirely.
The glucocorticoid receptor, when bound to cortisol in human immune cells, translocates to the nucleus and suppresses transcription of genes encoding inflammatory cytokines such as interleukin-6 and tumor necrosis factor-alpha. This mechanism underlies the anti-inflammatory effect of synthetic glucocorticoids like dexamethasone, which are used clinically at doses as low as 0.1 milligrams per kilogram to suppress immune responses.
Nuclease
/ NOO-klee-ays / · Latin: nucleus + -ase
Nuclease is an enzyme that cleaves phosphodiester bonds in DNA or RNA, cutting nucleic acid strands internally or trimming nucleotides from their ends.
Nucleases are grouped by where they cut: endonucleases cleave internal bonds within a nucleic acid strand, while exonucleases remove nucleotides from a 5-prime or 3-prime end. Restriction endonucleases such as EcoRI recognize short DNA sequences and generate defined fragments, making them central to cloning and genome mapping. Cellular nucleases also perform essential housekeeping roles, including removal of RNA primers during replication, processing of recombination intermediates, and degradation of damaged or foreign nucleic acids.
Because uncontrolled nuclease activity would destroy genomes, cells regulate these enzymes by compartmentalization, protein inhibitors, and activation only at specific DNA or RNA substrates.
Cas9 is a nuclease whose cutting specificity is programmed by a guide RNA rather than by a fixed protein recognition sequence. This RNA-guided specificity is why CRISPR systems can be redirected to new genomic targets by changing only the guide sequence.
Building Blocks of Nucleic Acids →Nucleases randomly destroy all DNA in a cell. Cells control nuclease activity tightly, directing individual enzymes to specific substrates such as damaged DNA, viral genomes, RNA primers, or programmed genome-editing targets.
EcoRI is a restriction nuclease from Escherichia coli that recognizes the 6-base-pair sequence GAATTC and cuts both DNA strands at a defined position. The enzyme generates 4-nucleotide sticky ends, allowing compatible DNA fragments to be ligated together during molecular cloning.
Nucleosome
/ NOO-klee-oh-sohm / · Latin: nucleus + Greek: soma (body)
Nucleosome is the fundamental repeating unit of eukaryotic chromatin, consisting of approximately 147 base pairs of DNA wrapped around a histone octamer.
The histone octamer contains two copies each of H2A, H2B, H3, and H4, forming a protein core around which DNA wraps about 1.7 turns. Adjacent nucleosomes are separated by linker DNA that commonly ranges from 10 to 80 base pairs, and histone H1 can bind this linker region to promote higher-order compaction. Nucleosome positioning influences whether promoters, enhancers, splice sites, and repair sites are accessible to regulatory proteins.
Histone tail modifications and ATP-dependent remodeling complexes can shift, evict, or restructure nucleosomes, making chromatin packaging a dynamic regulator of gene expression rather than a passive storage system.
A human diploid nucleus contains enough DNA to stretch about 2 meters if fully extended, yet nucleosomes help package that DNA into a nucleus only about 6 to 10 micrometers wide. This compaction still leaves regulatory regions accessible when remodeling complexes reposition nucleosomes.
Difference Between Prokaryotic and Eukaryotic Cells →Nucleosomes are permanent blocks that freeze genes in place. Nucleosomes are dynamic particles that can slide, unwrap, or be modified within minutes when transcription, replication, or repair machinery needs access to DNA.
Human beta-globin promoters become accessible during erythroid differentiation as nucleosomes are repositioned around regulatory sequences. Genome-wide mapping shows that active promoters usually contain a nucleosome-depleted region of roughly 100 to 200 base pairs near the transcription start site.