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Lecture 20

Biology Lecture 20 Outcomes

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Western University
Biology 1002B
Tom Haffie

Lecture 20 Does telomerase work on both ends? Telomerase is active on both ends of DNA, there are 3’ and 5’ ends at both sides Which of the following can extend a 3” OH on RNA? Primase, DNA polymerase, polyApolymerase Silent Mutations: Changed codon specifies the same amino acid Missense mutations: Changed codon specifies changed amino acid Nonsense mutations: changed codon specifies “stop” (likely to have a dramatic affect) Common types of changes in DNA:  Base substitution  Insertion/deletion/reorganization In/Del Mutations  Tend to be sequence specific  There are sequences that lend themselves to in/del mutations  Insertion o A string of similar bases can shift over thru slippage=an extra base in NEW strand o Ie Huntington’s  Deletion o A string of similar bases on the TEMPLATE strand loop out= del of base  Changes reading frame for translation all the way down stream Reading frames  There are three possible reading frames in any given gene  Frameshift mutation: when inserting or deleting a pair causes a shift in the way codons are read= garbage  In the ribosome: the mRNA will read thru the stop codon and keep translating till the end of the mRNA, or if it sees a new stop codon before or after original How many reading frames are there?  6 frames in a double stranded DNA theoretically, but no DNA codes that many genes in one stretch… Splicing  snRPS bind with cutting signals, they don’t understand genetic code, so in/del mutations won’t affect the splicing process  mess up splice sequence is the only way to interfere with spliceosomes DNA repair  DNA polymerase is effective at proofreading  If 3’ OH is not in the right place, elongation stops  DNA polymerase chews out previous bases and refills it again  Its not perfect though  Form of proof reading  RNA polymerases don’t require 3’ Oh to start  they cannot proof read  reverse transcriptase ineffective at proofreading  The mismatch can be repaired  Damage is being repaired NOT mutations  However
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