Molecular Biology Terms Starting With P

Polyadenylation is the cleavage and addition of a poly-adenosine tail to the 3-prime end of most eukaryotic mRNAs, increasing stability, export efficiency, and translation.

The polyadenylation machinery recognizes an AAUAAA signal usually located 10 to 30 nucleotides upstream of the cleavage site, along with downstream sequence elements that help position the cut. After cleavage, poly(A) polymerase adds adenosine residues without using a template, and poly(A)-binding proteins coat the tail to protect the transcript from rapid decay. In mammals, newly made poly(A) tails are commonly about 200 to 250 nucleotides long in the nucleus, though they shorten during cytoplasmic mRNA aging.

Alternative polyadenylation can change the length of the 3-prime UTR, removing or adding microRNA binding sites and thereby altering protein output without changing the coding sequence.

Post-Translational Modification is a covalent chemical change added to a protein after translation that alters the protein's activity, stability, localization, or interactions.

Common post-translational modifications include phosphorylation, acetylation, methylation, ubiquitination, SUMOylation, glycosylation, lipidation, and proteolytic cleavage. These modifications can be reversible, as with phosphorylation by kinases and removal by phosphatases, or permanent, as with cleavage of proinsulin into mature insulin. The human proteome contains hundreds of thousands of experimentally detected modification sites, allowing a limited number of genes to produce many functionally distinct protein states.

Crosstalk among modifications is especially important on histones, where combinations of acetylation, methylation, and ubiquitination help define active, repressed, or poised chromatin.

Pre-mRNA is the initial RNA polymerase II transcript of a protein-coding gene, containing exons, introns, and end-processing signals before maturation into mRNA.

Pre-mRNA processing begins while transcription is still underway, so capping, splicing, and 3-prime-end formation are physically coupled to RNA polymerase II. A 7-methylguanosine cap is added near the 5-prime end, introns are removed by the spliceosome, and cleavage followed by polyadenylation creates a mature 3-prime end. Processing factors bind the phosphorylated carboxy-terminal domain of RNA polymerase II, which coordinates the order and quality control of these reactions.

Defective pre-mRNA processing can cause disease even when the protein-coding sequence is intact, because exon skipping, intron retention, or faulty polyadenylation can eliminate functional protein production.

Primer is a short nucleic acid strand with a free 3-prime hydroxyl group that DNA polymerase extends to begin synthesis on a template strand.

DNA polymerases cannot usually start synthesis de novo because the first phosphodiester bond requires an existing 3-prime hydroxyl acceptor. In cellular DNA replication, primase makes short RNA primers that are later removed and replaced with DNA, while PCR uses synthetic DNA primers designed to flank a target sequence. Primer length, melting temperature, GC content, and 3-prime-end specificity determine whether amplification is efficient and specific.

Poorly designed primers can anneal to unintended genomic sites, form primer dimers, or fail to extend, producing false-negative or misleading results in diagnostic PCR.

Protein Misfolding is the failure of a polypeptide to adopt or maintain its correct three-dimensional conformation, often causing loss of function, aggregation, or cellular toxicity.

Protein folding is guided by the amino acid sequence but is influenced by temperature, pH, molecular crowding, mutation, oxidative stress, and the availability of folding chaperones. Misfolded proteins often expose hydrophobic surfaces that are normally buried, causing them to aggregate with other partially folded chains. Cells counter this threat through chaperones, the ubiquitin-proteasome system, autophagy, and the unfolded protein response in the endoplasmic reticulum.

Persistent misfolding contributes to cystic fibrosis, alpha-1 antitrypsin deficiency, prion diseases, and neurodegenerative disorders such as Alzheimer’s and Parkinson’s disease.

Protein Trafficking is the directed movement of newly synthesized or mature proteins to their correct cellular destinations, including organelles, membranes, lysosomes, the nucleus, or the extracellular space.

Protein sorting depends on sequence-encoded address labels such as signal peptides, nuclear localization signals, mitochondrial targeting sequences, peroxisomal targeting motifs, and lysosomal sorting signals. Proteins entering the secretory pathway are directed to the endoplasmic reticulum, modified in the Golgi apparatus, and packaged into vesicles coated by COPII, COPI, or clathrin. Rab GTPases, SNARE proteins, tethering factors, and cytoskeletal motors ensure that vesicles bud from the correct compartment, move along defined tracks, and fuse with the correct target membrane.

Roughly one-third of eukaryotic proteins enter the secretory pathway or membrane-trafficking system, so trafficking errors can affect hormone secretion, receptor signaling, lysosomal enzyme delivery, and immune recognition.