How do trans-acting and cis-acting elements differ in gene regulation?

• A. Trans-acting factors are diffusible proteins that regulate genes on other DNA molecules.
• B. Cis-acting elements regulate genes on the same DNA molecule; trans-acting factors are diffusible and regulate genes elsewhere.
• C. Cis-acting elements regulate translation; trans-acting regulate splicing.
• D. Trans-acting elements are all RNA molecules; cis-acting elements are DNA sequences.

Answer: (B)

Cis-acting elements are DNA sequences that regulate genes on the same DNA molecule by serving as binding sites for regulatory proteins and RNA polymerase. Their effect is local because they are physically linked to the gene they control, such as promoters, enhancers, or silencers on the same chromosome or plasmid.

Trans-acting factors are diffusible products, usually proteins (or sometimes RNAs), that can move throughout the cell and act on target genes located on different DNA molecules. They recognize cis-acting sequences and modulate transcription at multiple sites across the genome.

Thus, cis-acting elements regulate genes on the same DNA molecule, while trans-acting factors are diffusible and can regulate genes elsewhere. The other statements mischaracterize either what cis-acting elements do, what trans-acting factors can be, or the types of molecules involved.