The trunk is made up of DNA strand . The branches are the nascent RNAs in increasing length. The RNA polymerase ,
The RNAP scans the duplex DNA to find the sites for transcription initiation, known as promoters, and bind to them, exposing the DNA template.
Once the active site of RNAP is in the correct position, the Transcriptional Elongation Complex (TEC) starts the elongation phase. During this phase, the RNAP polymerizes RNA chains, incorporating a ribonucleoside complementary to the nucleotide present in its active site. After incorporation, the RNAP moves along one nucleotide in the template strand and then restarts the process.
Different approaches have been proposed to model the elongation kinetics.
Although some models consider the presence of only one transcribing RNAP on the DNA strand, experimental evidence has been gathered to suggest that multiple RNAPs can simultaneously participate in the process.
Ribosomal genes, for instance, are highly transcribed and electron micrographs of the process are remarkable for their Christmas-tree-like molecular organization, where the “trunk” of the tree are the DNA strand, and the “branches” are the nascent RNAs in increasing length.