5.9.1 Transcription unit and gene

. . Transcription unit and gene A transcriptional unit in DNA is defined by three regions, a promoter , the structural gene and a terminator . The promoter is located towards the ' end of the coding strand. It is a DNA sequence that provides binding site for RNA polymerase. The presence of promoter in a transcription unit, defines the template and coding strands. The terminator region located towards the ' end of the coding strand contains a DNA sequence that causes the RNA polymerase to stop transcribing. In eukaryotes the promoter has AT rich regions called TATA box ( Goldberg-Hogness box ) and in As they move away in both directions, newly synthesized complementary nucleotides are paired with the existing nucleotides on the parent strand and covalently bonded together by DNA polymerase . Formation of new strand requires a primer (a short stretch of RNA)for initiation. The primer produces a '-OH end on the sequence of ribonucleotides, to which deoxy ribonucleotides are added. The RNA primer is ultimately removed leaving a gap in the newly synthesized DNA strand. It is removed from ' end one by one by the exonuclease activity of DNA polymerase. Finally, when all the nucleotides are in position, gaps are sealed by the enzyme DNA ligase . At the point of origin of replication, the helicases and topoisomerases (DNA gyrase) unwind and pull apart the strands, forming a Y-Shaped structure called the replication fork . There are two replication forks at each origin. The two strands of a DNA helix have an antiparallel orientation. The enzyme DNA polymerase can only catalyse the addition of a nucleotide to the new strands in the ' ' direction, as it can only add nucleotides to the ' carbon position.