3) Chain termination: When RNA polymerase stops adding nucleotide at RNA chain, it releases a completed product and RNA chain get free from the terminator sequence. During termination all the hydrogen bonds break down which hold RNA-DNA hybrid together and when RNA chain is separated DNA again reform duplex. Site at which site enzyme stops adding nucleotides is known as chain termination site (Figure 28.7).

Figure 28.7 : Termination of Transcription

Two types of termination mechanism are present in bacteria: 1) Intrinsic termination, 2) rho factor dependent termination.

Intrinsic Termination: In this mechanism of termination rho factor is not required and termination depends on RNA product. It requires G-C rich hairpin; hairpin structure is followed by 7 U residues. RNA DNA hybrid requires forces for holding the elongation complex together thus when the hybrid gets detached; it collapses the elongation complex which causes the termination. In this type of termination dissociation of polymerase occurs by destabilizing the attachment of growing chain to the template. During this process hairpin structure is formed by the transcript via complementary base pairing. It includes palindromic sequences. This stem loop structure includes GC rich region followed by U rich region (Figure 28.8).