Final answer:
Actinomycin D inhibits transcription elongation by intercalating between G=C base pairs in DNA, disrupting RNA polymerase movement and halting the transcription process.
Step-by-step explanation:
Actinomycin D inhibits elongation during transcription by intercalating into the DNA double helix. It specifically inserts itself between successive G=C base pairs, which disrupts the movement of RNA polymerase along the DNA template. This binding prevents the RNA polymerase from progressing, thereby jamming and stopping the transcription process. Both DNA-directed RNA synthesis and DNA synthesis can be inhibited by Actinomycin D, but RNA synthesis is more susceptible, especially the synthesis of ribosomal RNA.