Silent alleles are not expressed in a given context because gene expression is controlled by epigenetic regulation or random variation among cells. In mammals, this can happen for several reasons: imprinting, where only one parental copy is expressed while the other is silenced; X-chromosome inactivation in females, which silences one entire X chromosome; and random or “noisy” expression where stochastic differences cause a cell to skip expression of one allele. Gene silencing can occur through several mechanisms. DNA methylation, especially at promoter CpG islands, tends to repress transcription by promoting a closed chromatin state. Histone modifications, such as deacetylation or repressive methylation marks, also compact chromatin and reduce transcription. RNA interference can target the RNA itself for degradation or block translation, and it can even guide chromatin changes that promote silencing. All of these routes show that silencing isn’t limited to a single level of control; it can act at transcriptional or post-transcriptional stages depending on the context. So a silent allele isn’t expressed due to mutation alone; it can be silenced by imprinting, X-inactivation, or random regulation, and through DNA methylation, histone changes, or RNA interference.

tRNA acts as the adaptor that delivers the correct amino acid to the ribosome as the genetic code is read. Each tRNA carries a specific amino acid and has an anticodon that base-pairs with a complementary codon on the mRNA. This pairing ensures the ribosome adds the proper amino acid to the growing polypeptide chain. The amino acid is attached to the tRNA by an aminoacyl-tRNA synthetase, ensuring fidelity. As translation proceeds, tRNA molecules enter at the ribosome’s active site, donate their amino acids, and exit, allowing the next tRNA to bring in its amino acid. tRNA does not synthesize mRNA, replicate DNA, or transport ribosomes.

tRNA carries amino acids to the ribosome during translation. Each tRNA is matched to a specific amino acid by a charging enzyme, attaching the amino acid to the tRNA’s end. It also carries an anticodon that pairs with the codon on mRNA, ensuring the correct amino acid is brought in at the right step of protein synthesis. The ribosome uses these charged tRNAs to form peptide bonds and extend the growing polypeptide chain. In contrast, mRNA provides the codon sequence that specifies which amino acids to add, rRNA forms and helps execute the ribosome’s catalytic and structural roles, and DNA stores the genetic information that is transcribed into mRNA. So the carrier of amino acids to the ribosome is the tRNA, specifically the charged form that delivers SAMPLEthe amino acid during translation.

Histone acetylation is a modification where acetyl groups are added to lysine residues on histone tails by histone acetyltransferases. This neutralizes part of the histone’s positive charge, reducing its attraction to the negatively charged DNA. The result is a more open chromatin structure (euchromatin), which makes DNA more accessible to transcription factors and RNA polymerase, leading to increased transcription. Deacetylation reverses this, tightening the chromatin and generally reducing transcription. The idea that histone acetylation degrades histones or tightens chromatin, or that it recruits ribosomes for translation, doesn’t fit the mechanism or the process—acetylation modulates accessibility for transcription in the nucleus, not translation.

The bonds that hold the RNA chain together are phosphodiester bonds. As transcription proceeds, each new ribonucleotide is added to the growing RNA strand by a condensation reaction that links the 3' hydroxyl of the existing sugar to the 5' phosphate of the incoming nucleotide, forming a covalent phosphodiester linkage and releasing pyrophosphate. This creates the sugar–phosphate backbone of the RNA. Hydrogen bonds, on the other hand, stabilize base pairing (A–U and G–C) between strands or within RNA structures, but they do not join nucleotides along the backbone. Peptide bonds connect amino acids in proteins, and glycosidic bonds connect sugars in carbohydrates, so they aren’t the bonds forming during RNA transcription.