Class Notes (838,404)
Canada (510,881)
BIOB10Y3 (39)

biob10 lec 15 notes

3 Pages
Unlock Document

Biological Sciences
Aarti Ashok

How is the gene expressed as traits? [DNA  RNA  Protein]  The relationship b/w genes and proteins was demonstrated by experiments done by Beadle & Tatum in 1940s  Looked at the growth of bread mould, Neurospora Crassa  They hypothesized that if the DNA of this organism were damaged in some way, certain genes may be mutated and hence would not be able to express specific proteins o This would then make it difficult for the mould to grow under conditions where the function of that protein is critical o Example: enzyme working in a specific metabolic pathway -One Gene – One Enzyme Hypothesis  We now know that we can’t just go simply from a gene to make a protein  We require an intermediate known as messenger RNA (mRNA)  The process of making this mRNA from DNA (genes) is called transcription o Carried out by DNA-dependent RNA polymerases o Carried out in the nucleus  mRNAs then exported to cytosol Transcription: from DNA to RNA 1. Go from info storage (DNA in nucleus) to info use (mRNA in cytosol) 2. Amplify synthetic output: several mRNAs can be made from each molecule of DNA; each mRNA can be made into large numbers of proteins/polypeptides Transcription: an overview Transcription (or the synthesis of RNA from DNA) is a cyclic process and can be broken down into 3 stages: 1. Initiation RNA polymerase binds a region of DNA close to the beginning of a gene known as a promoter sequence. -polymerase binds to promoter sequence in duplex DNA [CLOSED complex] -polymerase melts duplex DNA near transcription start site, forming a transcription bubble [OPEN complex] -polymerase catalyzes phosphodiester linkage of two initial rNTPs 2. Elongation Upon promoter binding, a conformational change occurs in the polymerase and upon unwinding of the DNA strands, a complementary RNA strand (which involves linking ribonucleotides by phosphodiester bonds) is synthesized. -polymerase advances 3’5’ down template strand, melting duplex DNA and adding rNTPs to growing RNA -complementary RNA strand moves from 5’3’ 3. Termination RNA polymerase reaches a signal on the DNA that causes an extended pause, the transcript is released from the enzyme, the DNA strands come back together and polymerase floats off to find another promoter. -at transcription stop site, polymerase releases completed RNA and dissociates from DNA  You can see that the mRNA made is complementary to one strand of DNA – called the template strand o It is a copy of info  hence, transcript  Ribonucleoside triphosphates are the building blocks of the mRNA o added on in the order dictated by the template strand o linked by phosphodiester bonds o added on 5’3’ direction  RNA polymerase itself covers about 35bp of DNA o The RNA:DNA hybrid is about 9bp long  Overwinding of the DNA ahead of the polymerase o Positive supercoiling  A more accurate depiction of RNA polymerase action: o DNA makes a sharp turn within the enzyme site o RNA exits out of a separate channel Studying RNA Polymerase  The rate of movement of RNA polymerase can be observed in a fluorescence microscope Transcription in Prokaryotes  One type of RNA polymerase: o 5 subunits = core enzyme  Loose association b/w DNA and core enzyme; RNA chains that are begun are not initiated at proper sites o 5 subunits + Sigma Factor (σ) = haloenzyme  Association of complete enzyme w/ DNA at proper site and opening of double helix  Sigma Factor increases the enzyme’s affinity for promoters o transcription initiates at correct sites  Initiation of transcription is difficult – RNA polymerase may make several attempts Promoters  +1 = site (nucleotide position) at which transcription begins  -35 element = consensus sequence Regulatory sequences required for initiating transcription at the correct site  -10 element = Pribnow box -Upstream = towards 3’ end of template (away from transcription start site) -Downstream = towards 5’ end of template  Once 10-12 nucleotides have been incorporated, the polymerase goes through a conformational change that is known as the ‘transcriptional elongation complex’  Sigma factor is lost following for of this compl
More Less

Related notes for BIOB10Y3

Log In


Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.