Chromatin architecture controls access to DNA for transcription. The way DNA is packaged—how nucleosomes are positioned and which histone marks decorate their tails—determines whether transcriptional machinery can reach promoter and enhancer regions. When nucleosomes cover a promoter, or when repressive histone marks are present, the DNA is less accessible and transcription is repressed. In contrast, remodeling that shifts or removes nucleosomes near a gene, along with histone acetylation and other activating marks, loosens the chromatin and makes the DNA accessible to RNA polymerase II and transcription factors, enabling transcription. Euchromatin is the open, accessible form associated with active transcription, while heterochromatin is the condensed, less accessible form associated with repression. DNA methylation can further reinforce repression by promoting a closed chromatin state. This combination of nucleosome positioning and histone modifications, rather than DNA sequence alone, sets how readily a gene is transcribed.

Base pairing across the two DNA strands is held together by hydrogen bonds between complementary bases. These non-covalent bonds are individually weak, but many of them together provide enough stability to keep the double helix intact while still allowing the strands to separate when needed for replication or transcription. Adenine pairs with thymine via two hydrogen bonds, and guanine pairs with cytosine via three hydrogen bonds, which explains why GC-rich regions are more stable. The covalent bonds in DNA form the backbone of each strand rather than the cross-strand links, and while van der Waals interactions help with base stacking inside the same strand, they are not the links that bind the two strands together.

Differential RNA processing through alternative splicing lets a single gene produce multiple protein isoforms by including or excluding certain exons during maturation of the mRNA. After transcription, the initial transcript is processed to remove introns and join exons, but sometimes specific exons are skipped in some transcripts. Skipping an exon means that part of the coding sequence is not included in the mature mRNA, so the produced protein has a different amino acid sequence and often a different function. This mechanism expands the variety of proteins a single gene can generate without changing the DNA sequence. Other options don’t fit this description because gene duplication creates extra copies of the gene rather than changing how a single transcript is processed; RNA editing tweaks individual nucleotides within the RNA without removing exons; and protein folding occurs after translation to shape the final three-dimensional structure, not to alter which exons are present in the protein-coding sequence.

Both terms describe how foreign DNA gets into cells to drive gene expression, but they apply to different kinds of cells. In bacteria, transformation means the uptake of naked DNA from the surrounding environment by competent cells. Once inside, the DNA—often a plasmid—can be replicated and expressed using the bacterial transcription and translation machinery, and selective markers help identify cells that carry the new DNA. In contrast, introducing DNA into eukaryotic cells is called transfection. This delivery is usually achieved with methods like liposomes, electrical pulses, or viral vectors, and the DNA must reach the nucleus to be transcribed and translated by the cell’s machinery. Expression can be transient or, if the DNA integrates or a replicating vector is used, more stable. So, the difference hinges on the host organism and the delivery approach: transformation for bacteria, transfection for eukaryotic cells, with both ultimately aiming to get gene expression in the host.