Molecular Biology Terms Starting With F
Molecular Biology Glossary: F
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FISH
/ FISH / · Acronym from Fluorescence In Situ Hybridization, combining Latin fluorescere, to flow, and in situ, in place.
FISH is a cytogenetic technique that uses fluorescently labeled nucleic acid probes to hybridize to specific DNA or RNA sequences inside fixed cells or tissues.
In DNA FISH, chromosomes or nuclei are fixed, denatured to expose single-stranded targets, and incubated with fluorescent probes that bind complementary sequences. The technique can detect aneuploidy, gene amplification, deletions, translocations, and spatial genome organization, with practical resolution depending on probe size, sample preparation, and whether metaphase spreads, interphase nuclei, fiber-FISH, or super-resolution imaging is used. Routine clinical FISH often detects changes in the kilobase-to-megabase range rather than single-base variants, making it complementary to sequencing.
RNA FISH adapts the same hybridization principle to visualize individual transcripts or expression patterns in cells and tissues.
Spectral karyotyping is a multicolor FISH method that paints all 24 human chromosome types in different colors. It can reveal complex cancer rearrangements that look ambiguous under conventional chromosome banding.
FISH can detect every mutation at nucleotide resolution. Standard FISH detects larger sequence gains, losses, rearrangements, or locations, while single-nucleotide variants usually require sequencing or allele-specific molecular assays.
Clinical laboratories use FISH to diagnose chronic myeloid leukemia by detecting fusion of the BCR locus on chromosome 22 with the ABL1 locus on chromosome 9. Dual-color probes show abnormal signal fusion in more than 95 percent of typical chronic myeloid leukemia cases carrying the Philadelphia chromosome.
Folding Chaperone
/ FOHL-ding SHAP-er-ohn / · From Old English fealdan, to fold, and French chaperon, protector or guardian.
Folding Chaperone is a protein that assists other proteins in achieving their correct three-dimensional structure without becoming part of the final folded complex.
Folding chaperones prevent aggregation and misfolding by binding to exposed hydrophobic regions of nascent or denatured polypeptides; up to 30% of newly synthesized proteins in Escherichia coli require chaperone assistance to reach their native state. The GroEL-GroES system forms a barrel-shaped cavity where a single polypeptide folds in isolation, shielded from competing interactions with other unfolded chains. Heat shock proteins such as HSP70 and HSP90 are ATP-dependent, cycling between high-affinity and low-affinity binding states to release substrates in a controlled manner that promotes correct folding.
When chaperone capacity is overwhelmed, proteins misfold into pathological conformations that contribute to Alzheimer’s disease, Parkinson’s disease, and cystic fibrosis. Many chaperones are transcriptionally upregulated within minutes of heat shock, oxidative stress, or pH shifts, protecting the proteome from widespread denaturation.
AAA+ disaggregase chaperones such as Hsp104 in yeast (Saccharomyces cerevisiae) can thread aggregated, misfolded proteins through a narrow central pore and unfold them completely, giving the polypeptide a second opportunity to fold correctly, a capability that mammalian cytosol largely lacks.
Folding chaperones provide the template or instructions that determine a protein's final shape. Chaperones only prevent misfolding and aggregation; the amino acid sequence itself encodes all the information needed to specify the correct three-dimensional structure.
In the bacterium Escherichia coli, the GroEL-GroES chaperone system assists approximately 10 to 15% of all cytosolic proteins in reaching their native state. Among its obligate substrates is the metabolic enzyme DapA, which aggregates almost completely in the absence of GroEL even at normal growth temperatures of 37 degrees Celsius.
Footprinting
/ FOOT-prin-ting / · English: footprint + -ing
Footprinting is a technique that identifies the DNA sequences bound by a protein by revealing which regions are protected from nuclease or chemical cleavage when the protein is present.
In DNase I footprinting, a DNA fragment is end-labeled and then partially digested with DNase I in the presence and absence of a bound protein. Where the protein contacts DNA, it physically blocks the enzyme, and a gap, the footprint, appears in the otherwise continuous ladder of bands on a sequencing gel. Comparing the protected and unprotected lanes reveals the exact nucleotides contacted by the protein, defining the binding site with single-base resolution.
Hydroxyl radical footprinting offers even finer detail, probing individual nucleotides on both strands because the small radical cleaves DNA with minimal steric bias. Beyond DNase I, chemical footprinting agents such as dimethyl sulfate methylate exposed guanine residues, and protection from methylation marks positions directly contacted by the protein.
Genomic footprinting, developed in the 1980s, extended the technique to intact chromatin inside living cells by using ligation-mediated PCR to amplify the cleavage pattern from a specific locus, revealing protein-binding sites on endogenous chromosomes without purifying the protein first.
Are Enzymes Proteins? →Footprinting reveals the complete three-dimensional shape of a DNA-binding protein. The technique maps only the nucleotide positions on the DNA that are physically shielded or chemically protected by the protein, not the protein's structure itself.
Building Blocks of Nucleic Acids →When Carl Wu and colleagues used DNase I footprinting on the Drosophila melanogaster heat shock gene hsp26 in the early 1980s, they identified a protected region spanning roughly 20 base pairs at the promoter. Subsequent work confirmed this footprint corresponded to the binding site of heat shock factor, one of the first transcription factor binding sites characterized at nucleotide resolution in a eukaryote.
Forward Primer
/ FOR-werd PRY-mer / · English: forward + primer
Forward Primer is a short, single-stranded oligonucleotide that anneals to the antisense strand of a DNA template in the same orientation as the coding strand, initiating DNA synthesis toward the reverse primer during PCR.
Together with a reverse primer that anneals to the opposite strand, the forward primer defines both the boundaries and the orientation of the amplified product. Typical forward primers are 18 to 25 nucleotides long and are designed to have a melting temperature between 55 and 65 degrees Celsius to ensure specific annealing during the PCR annealing step. Primer design must avoid self-complementarity, primer-dimer formation, and stable secondary structures, all of which reduce amplification efficiency.
Mismatches at the 3-prime end of a forward primer are particularly disruptive because DNA polymerase extends from that terminus; even a single mismatch at the last position can abolish amplification. In allele-specific PCR, researchers deliberately introduce a mismatch at the 3-prime end of the forward primer so that it extends only from one allele, distinguishing single-nucleotide variants without sequencing.
Padlock probes, used in rolling-circle amplification, incorporate a sequence equivalent to a forward primer at one arm of a circular template; when the probe ligates and the circle is amplified, a single target molecule can generate more than 1,000 copies in under an hour, far exceeding conventional PCR sensitivity for certain in situ applications.
A single primer is sufficient for standard PCR amplification. PCR requires both a forward and a reverse primer flanking the target region; without both, the polymerase cannot generate the exponentially accumulating double-stranded product that defines the technique.
In quantitative PCR assays targeting the SARS-CoV-2 nucleocapsid gene, the forward primer anneals to the antisense strand at a position chosen to produce an amplicon of roughly 100 to 150 base pairs. Each thermal cycle approximately doubles the number of copies, so a 3.3-cycle difference corresponds to about a tenfold difference in starting template amount.