Transcription

• Transcription is the process in which a gene’s code, stored in DNA, is transcribed (or “copied”) into messenger RNA (mRNA)
• RNA is a “disposable copy” of the gene, thus it can

  • be modified/ edited
  • Leave the nucleus to go where ribosomes are
• In transcription, DNA is used as a template to make a complementary strand of RNA
• The double stranded DNA is unwound near the gene to be transcribed (gene of interest)
• DNA strand antiparallel to gene of interest becomes template for building acomplementary RNA transcript
• DNA strand that actually contains the gene of interest is the nontemplate or coding strand
• RNA nucleotides complementary to the template are linked together (polymerized) to form the RNA transcript by a protein enzyme called RNA polymerase
• RNA polymerase reads DNA in 3’ to 5’ direction, new RNA strand is built in 5’ to 3’ direction
• Genes can be coded on either strand of DNA
• Transcription is very similar in prokaryotes and eukaryotes, with a few key differences

  • One key difference is where transcription takes place
• In both prokaryotes and eukaryotes transcription takes place in 3 steps

  1. Initiation
  2. Elongation
  3. Termination
• Transcription begins at a promoter

  • A sequence of nucleotides along DNA strand just before (or “upstream of”) the gene of interest
  • where RNA polymerase and transcription factors bind in order to start the transcription process
• Transcription factor – a protein involved in transcription
• In eukaryotes, the promoter often contains a short “recognition” sequence called the TATA box that RNA polymerase will recognize, then bind to

  • Typically, RNA polymerase begins transcribing template strand ~ 25 bps after this sequence
• In addition to promotors, eukaryotic DNA also has enhancer sequences that play a role in transcription initiation as well

  • Like the promotor, enhancers are regions of DNA where transcription factors (proteins) can attach and help to start the process of transcription
• Before RNA polymerase can bind to the promoter and start transcribing template DNA, transcription factors must first bind there

  • acts like scaffolding to stabilize the RNA polymerase when it arrive
• Likewise, another set of transcription factors must bind to the enhancer region

  • When these bind, they will will attract a complex of proteins called the “mediator complex”
• The mediator complex, in turn, recruits the RNA polymerase to the scene
• The mediator complex has binding sites for• RNA polymerase• Enhancer transcription factors• Promotor transcription factors
• Mediator complex brings all the things together at the promoter region, at which point transcription can finally begin!
• RNA polymerase....

  1. Facilitates local unwinding of the DNA
  2. Stabilizes pairing of DNA – RNA at active transcription site to enable base pairing
• RNA polymerase....

  1. Has a channel to funnel in free RNA nucleotides (rNTPs) towards transcription site
  2. On top of all that, it’s polite, too! Restores original DNA double helix as it passes
• RNA polymerase....

  1. If incoming rNTP is proper complement to DNA, catalyzes formation of phosphodiester bonds to 3’ end of previous rNTP
• Transcription ends at the terminator

  • When RNA polymerase reaches signal, transcript is cleaved, forming the 3’ end and RNA polymerase isreleased from DNA template
• Transcription ends at the terminator

  • If more RNA is required (like if you want to make a lot of this protein) then RNA polymerase can bind to promoter again and start whole process over (and over and over)
• If you know the DNA sequence you can predict thetranscribed RNA

  • The DNA is read in the 3’ to 5’ direction.
  • RNA is transcribed in the direction of 5’ to 3’.