Gene Transcription (DNA → RNA) – Easy Notes for NEET, Nursing & Biology Students
TRANSCRIPTION BY: SWARAJ BARIK
Introduction: Central dogma describes the flow of genetic information in cells. DNA ->RNA ->Protein From this it is clear that transcription is the first step of gene expression. Transcription is the process in which RNA is synthesized from DNA.
◦Comparison of Replication and Transcription Replication Transcription • Template Both strands Single Strand • Primer . Yes No • Enzyme Topoisomerase, Only RNA Polymerase Helicase,ssB Protein,Primase, DNA Polymerase,DNA ligase Base pair A-T&G-C A-U&G-C
◦Transcription Unit ◦ Transcription Unit has following regions 1-Promotor region – The beginning region on the DNA where initiation of transcription occurs 2- Elongation region – where the RNA transcript is elongated 3-Termination region – where transcription is terminated or stopped
◦RNA Polymerase ◦ RNA Polymerase in prokaryotes is catalyzed by a large enzyme called ‘DNA dependent RNA Polymerase ‘or ‘RNA Polymerase ‘ ◦ Sub-Units. Functions ◦ Alpha - determine the DNA to be transcribed ◦ Beta - catalyze polymerization ◦ Beta prime –Unwinds DNA strands ◦ Omega - Function is not known
◦Mechanism of Transcription ◦ Activation of Ribonucleotides ◦ Initiation ◦ Elongation ◦ Termination ◦ First of all sigma factor of RNAP recognises the Promoter region of DNA and binds to it. ◦ The first nucleotide corresponding to the position at which transcription begins is represented as +1. ◦ Upstream from the start point the region,TATAAT , “Called Pribnow box” is located; it is 10 and 35 base pairs upstream. ◦ They are called -10 and -35 elements. ◦ Transcription bubble extends about 14 base pairs. ◦ Transcription initiated at the free single stranded template strand, also called as Antisense strand. ◦ The other strand is called as the Coding Strand also called as Sense Strand; it is not used for transcription. ◦ The base sequence of RNA transcript is similar to that of coding strand besides presence of U instead of T.
◦Elongation ◦ After adding of about 10 nucleotides ,sigma factor is released from the Polymerase. ◦ The RNAP moves in 5’ to 3’ direction unwinding the template DNA. ◦ New ribonucleotides are added in that direction growing the RNA chain ◦ After the release of sigma factor ‘Nus A Protein ‘enters the enzyme and helps in elongation. ◦T
◦Termination ◦ Prokaryotes have two kinds of termination
◦ 1-Rho dependent termination
◦ The termination site is about 40 bp in length is inverted repeat.
◦ This site is recognised by rho protein ◦ It binds to the signal and does not allow RNAP to move forward.
◦ This leads to disruption of RNA-DNA hybrid and release RNA
◦ 2-Rho independent ◦ Hairpin loop is formed having a particular base sequence that causes RNA transcript to pair with itself to form a loop which displace the RNA transcript and Polymerase from DNA.
◦ 4 to 8 adenine bases along the DNA strand which follows hairpin loop and produces a sequence of poly U at the tail end of RNA transcript.
