SBI 4U: Metablic Processes

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SBI 4U: Metablic Processes TRANSCRIPTION Copying of the DNA code for a protein into RNA 4 Steps: Initiation Elongation Termination MODIFICATION Section 1.3

SBI 4U: Metablic Processes INITIATION RNA polymerase binds to the DNA molecules UPSTREAM from the gene to be transcribed. Known as a promoter region Usually high in A and T This is because there are only 2 H bonds b/w them, therefore it costs less energy for RNA polymerase to open up the DNA helix here RNA polymerase can only bind at this site to get started, it does not recognize any other sequence. Section 1.3

SBI 4U: Metablic Processes ELONGATION Once bound to the DNA, RNA polymerase opens the DNA and starts building mRNA in the direction of 5' to 3'. No primer is needed to start building. The promoter does not get transcribed. RNA polymerase uses one strand of DNA as its template for mRNA synthesis (TEMPLATE STRAND) CODING STRAND: the strand not used for the template The mRNA will be complementary to the template strand and identical (except for Us in place of Ts) to the coding strand. Section 1.3

SBI 4U: Metablic Processes TERMINATION RNA polymerase reaches a terminator sequence mRNA dissociates from the DNA, called primary transcript RNA polymerase is free to bind with another promoter In order to prevent the mRNA from being degraded as it exits the nucleus, a few modifications need to be made to the primary transcript. http://library.thinkquest.org/C0123260/basic%20knowledge/images/basic%20knowledge/RNA/transcription.jpg Section 1.3

POSTTRANSCRIPTIONAL MODIFICATION SBI 4U: Metablic Processes POSTTRANSCRIPTIONAL MODIFICATION CAPPING: A 5' cap is added to the start (a 7-methyl guanosine to form a modified guanine nucleoside triphosphate). The cap plays a role in protection and also in the initiation of translation. TAILING: a string of ~200 adenosine nucleotides are added to the 3' end by poly-A polymerase to create a poly-A tail. In eukaryotic DNA further modifications are made. In our DNA we have coding regions (EXONS) and non-coding regions (INTRONS). The introns sit between the exons, so a primary transcript will have parts that don't code for protein. Introns need to be removed before translation or the new protein will not fold properly. Spliceosomes (RNA and proteins) remove the introns and join the exons together. http://faculty.ircc.edu/faculty/tfischer/images/introns-exons.jpg Section 1.3

SBI 4U: Metablic Processes Section 1.3

SBI 4U: Metablic Processes QUALITY CONTROL Once the modifications have been made the primary transcript is known as the mRNA transcript. It can now leave the nucleus. *Unlike DNA replication, there is no proofreading enzyme, therefore more errors are made in transcription than replication. Not a big problem, since hundreds of transcripts are being made (vs. one copy of DNA) and therefore enough protein will be synthesized. Section 1.3