BIOL 112 Chapter Notes - Chapter 16: Adenine, Ribozyme, Aminoacyl-Trna

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6 Apr 2012
Chapter 16 Notes
DNA transcription--> mRNA translation--> Proteins
16.1-Transcription in Bacteria
- RNA polymerase (preform template-directed synthesis from 5’3’ and no primer needed to begin
transcription) synthesize mRNA
- Transcription occurs when:
1. RNA polymerase matches the base of ribonuculeotide triphosphate with a complementary base in
gene (gene: section of DNA that codes for a protein or RNA)
2. RNA polymerase catalyzes formation of phosphodiester bond between 3’ end of growing mRNA and
the new ribonucleotide
3. RNA that is complementary to the gene is synthesized
- RNA Polymerase Structure and Function
* structure found by using: crystallography which allows biologists to obtain 3-D images of molecules
* shape: large, globular, many channels running through interior
* active site: located where several channels intersect
* DNA fits into one of the enzyme’s channels two strands of DNA in double-helix separate inside the
enzyme to produce single stranded template at active site
- Polymerase can’t do it on their own = Sigma (detachable protein subunit) must bind to polymerase
- Polymerase + Sigma = holoenzyme (“whole enzyme”) which is made of core enzyme (polymerase)
which contains active site and other needed proteins
- Sigma: guides RNA polymerase to specific location where transcription should begin = promoter
- promoters: 40-50 nucleotide base sequence on non-template strand of DNA, binds to RNA polymerase,
lets transcription start (prokaryotic- a single promoter for several genes, eukaryotic- each gene has own
- TATAAT: -10 box (centered 10 bases upstream from where RNA polymerase starts transcription)
- Downstream: direction RNA polymerase moves during transcription, Upstream: opposite direction
- +1 site: place where transcription beings
- TTGACA: -35 box, located 35 bases upstream from +1 site
- Sigma binds to -10 and -35 boxes sigma makes first contact with DNA to start transcription in
bacteria, = sigma is regulatory protein that tells RNA polymerase where and when to start synthesis
- After sigma binds, DNA helix opens up and single strand is threaded through active site, ribonucleoside
triphosphates (NTP’s) enter channel and diffuse into active site, NTP pairs with complementary base on
template strand of DNA, polymerization beings!
- entire process is spontaneous and exergonic (NTP’s have a lt of PE)
- RNA polymerase move along DNA template from 3’5’, synthesizing RNA in 5’3’ direction
- Inside the enzyme, group of projecting amino acids = enzyme’s zipper help open double helix upstream
- Rudders (amino acids) steer template/non-template through correct channels inside the enzyme
- At active site, addition of nucleotides to 3’ end of the growing RNA molecule at 50 nucleotide/sec
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