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BIOB11H3 (13)
Dan Riggs (8)
Chapter 11

Chapter 11 notes

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Biological Sciences
Dan Riggs

Chapter 11 Garrod discovered the relationship between a genetic defect, a specific enzyme, and a specific metabolic condition - inborn errors of metabolism. Beadle and Tatum first proposed the one gene-one enzyme hypothesis. It was modified to one gene-one poplypeptide. Then it was discovered that a single gene often generates more than one polypeptide. Ingram demonstrated that a mutation in a single gene had caused a single substitution in the amino acid sequence of a single polypeptide (valine in place of glutamic acid caused sickle cell anemia). The use of mRNA allows the cell to separate information storage from information utilized. It also allows the cell to greatly amplify its synthetic output. DNA is always double-helical while RNAs may fold into different structures and thus perform many functions. With the help of transcription factors, RNA polymerase binds to the promoter site on DNA, which contains information that determines which strand is going to be transcribed and where to begin. Polymerase moves 3-5 on DNA, so assembles RNA 5-3. www.notesolution.comBacteria like E.Coli contain a single type of RNA polymerase composed of 5 subunits that are tightly associated to from a core enzyme. An accessory polypeptide called sigma factor attached to core enzyme increases its affinity for promoter sites in DNA and so, transcription only begins at selected locations. After 10-12 nucleotides have been transcribed, the enzyme undergoes a conformation change -> transcriptional elongation complex that can move processively along the DNA. This is when sigma factor is released. Two stretches of DNA are similar in bacteria genes: TTGACA (-35 element; consensus sequence) and TATAAT (-10 element; Pribnow box) which is responsible for identifying the precise nucleotide at which transcription begins. Bacterial cells possess a variety of different sigma factors that recognize different versions of the promoter sequence, e.g. sigma 70 the housekeeping sigma factor initiates transcription of most genes. For termination of transcription, a ring-shaped protein called rho is required - separates the RNA transcript from the DNA. Sometimes, transcription is terminated when polymerase reaches a terminator sequence and releases the RNA chain (no additional factors). Eukaryotic cells have 3 distinct transcribing enzymes in the nucleus (Table 11.1 P.437). www.notesolution.com
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