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DNA and the Genetic Code June 14, 2011 DNA and the Genetic Code Protein synthesis Proteins are biochemical compounds involved in virtually every process within cells. Protein synthesis requires two steps: transcription and translation. DNA and the Genetic Code Protein synthesis (cont.) DNA and the Genetic Code DNA (Deoxyribonucleic acid) DNA is a two-stranded, helical polymer that contains genetic instructions used during development and functioning. DNA is comprised of 4 nucleotide bases: guanine (G), adenine (A), cytosine (C), and thymine (T). The bases are paired: G on one strand pairs with C on the other strand; A pairs with T. DNA and the Genetic Code Transcription Transcription is the synthesis of messenger RNA (mRNA) from DNA. The two DNA strands separate from one another. One strand is used for replication, the other for transcription. mRNA is the complement strand of the original DNA except with T replaced by U (uracil). mRNA carries the genetic information out of the nucleus, into the cytoplasm where it is translated to produce protein. DNA and the Genetic Code Translation Translation is the process in which ribosomes decode mRNA to produce amino acids. mRNA is decoded in three-base sections called codons. A codon codes for a single amino acid, or for “stop!” There are 64 (43 ) different codons but only 20 amino acids. ⇒ several different codons can specify the same amino acid. " &&1 /. DNA and the Genetic Code Decoding mRNA Scientists knew that amino acid sequences were specified by sequences of bases, but they did not know how the mRNA sequence was “read” to make proteins. In the early 1960s, Marshall Nirenberg conducted experiments using E. coli. He inserted an RNA strand made up only of uracil (UUUUU...), which produced an amino acid chain made up only of phenylalanine (Phe-Phe-Phe...). DNA and the Genetic Code Decoding mRNA (continued) This was the first step in breaking the genetic code. Nirenberg’s mRNA sequence: UUUUUUUUUUUUUUU Sample mRNA sequence: AUGGCCUAGGUACUAAAU Questions Nirenberg couldn’t answer with this experiment: How long are codons (“words”)? Are they the same length? Do the codons overlap? Are codons consecutive bases? Is there “punctuation” between codons? DNA and the Genetic Code Decoding mRNA (continued) H. Gobind Khorana performed a similar experiment to finish cracking the genetic code. of the genetic code. He synthesized artificial message analyzed the resulting polypeptides. His data are show the resulting amino-acid couplet also repeats indefinit use these data (plus Nierenberg’s result) to figure out code as you can. In particular, show that the code mu as consecutive, non-overlapping triplets without punc third position is often degenerate, and that some codo He used more complex mRNA sequences to obtain the data at right: DNA and the Genetic Code Your task...should you choose to accept DNA and the Genetic Code Your task...should you choose to accept Just kidding! You don’t have a choice - you have to accept. :) DNA and the Genetic Code Your task...which you will accept Use the previous data from the two experiments to figure out the genetic code. In particular, show that the code: is consecutive, non-overlapping triplets. has no punctuation. often has a degenerate third position. contains some codons meaning “stop”. Also, explain why coding with doublets is impossible the only possible reading restart interval less than 9 is 3. DNA and the Genetic Code Sample mRNA sequence AUGGCCUAGGUACUAAAU DNA and the Genetic Code