BIOL239 Lecture Notes - Lecture 13: Sigma Factor, Start Codon, Enzyme
14 May 2018
School
Department
Course
Professor
Set 13: Transcription (making mRNA)
●The process by which the polymerization of ribonucleotides guided by complementary
base pairing produces an RNA transcript of a gene
●Nucleotides are added in the 5’ to 3’ direction
●Uracil is incorporated in place of thymine in RNA
●In prokaryotes:
oRNA polymerase (DNA-dependent RNA polymerase) – enzyme that catalyzes
transcription
o(In bacteria) made of two components: sigma factor and RNA polymerase core
enzyme; together, called the RNA polymerase holoenzyme
▪Sigma factor recognizes promoter region, binds to promoter region
▪Once bound, sigma factor is released; core enzyme carries out synthesis
Promoters – DNA sequences near the beginnings of genes that signal RNA polymerase where to
begin transcription (“start codon” is only for translation)
●AT rich
Terminators – sequences in the RNA products that tell RNA polymerase where to stop
(encoded by DNA that polymerase can recognize and bind to)
●Terminator signal is encoded in the gene
●In bacteria (not eukaryotes!), at least two methods for transcription termination:
oRho-dependent – some terminators; recognized by a protein; disengages RNA
polymerase
oRho-independent – no protein involved; sequence-mediated; two complementary
regions
▪Sequences that can bind/base-pair with itself → form secondary hair-pin
structure, which destabilizes the polymerase
▪Second complementary region is flanked by strings of As or Ts – weak
hydrogen bonding, releases transcript
Document Summary
The process by which the polymerization of ribonucleotides guided by complementary base pairing produces an rna transcript of a gene. Nucleotides are added in the 5" to 3" direction. Uracil is incorporated in place of thymine in rna. Promoters dna sequences near the beginnings of genes that signal rna polymerase where to begin transcription ( start codon is only for translation) Terminators sequences in the rna products that tell rna polymerase where to stop (encoded by dna that polymerase can recognize and bind to) Terminator signal is encoded in the gene. During transcription, no single stranded binding proteins makes a single-stranded molecule of rna that is released, dna pairing snaps back to keep the molecule intact, forcing out messenger rna molecule (nascent strand) Prokaryotes genes are what you see is what you get ; mrna and proteins can be directly deduced from the dna sequence. Introns non-amino acid coding regions; sequences found in a gene"s dna but.
