BIOL 112 Lecture Notes - Lecture 22: Reading Frame, Citrulline, Tata Box

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21 Jul 2016
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!Fred Sanger - sequenced the first protein (insulin)
o!Showed that a sequence of amino acids is characteristic of a protein
!64 possible sequences, only 20 amino acids, so yeah some redundancy
!Experiment to figure out which sequences encode for which amino acids:
o!RNA with only one of: UUU, AAA, or CCC (repeated)
o!Conclusion: each triplet is mRNA for a different amino acid
!Gobind Khorana: AAGAAGAAG
o!Some proteins were made of LysLysLys (amino acids)
o!Others, ArgArgArg ... others, GluGluGlu
o!Due to the three possible reading frames for any sequence of DNA
o!If you do enough experiments like that, you can figure out which amino acid
each codon corresponds to
o!Methionine: AUG/ATG, start codon
o!Stop codon: TAG/TAA/TGA (or with U)
o!Code is degenerate: while DNA uniquely determines the amino acid
sequence, the reverse is not true
!How does the ribosome know how to interpret the RNA sequence?
o!Unlike in DNA, where one strand has natural hydrogen bond affinity to the
other strand, this is not true for ribosomes
o!No affinity of amino acids to specific codons in RNA
o!So there is another type of RNA - transfer RNA, has a 3' and 5' end, and has
an anticodon
!!The reverse complement of the codon that encodes for the amino
acid
!!There is 1+ tRNA for each amino acid
!!So for CAT the tRNA would be GTA I guess
!!Each tRNA has a unique shape
!!So the tRNA binds to the mRNA, and the correct amino acid is on the
tRNA (other side)
!!Which is how you get the amino acid, the amino acids join up etc
o!How does the cell know which amino acid to attach to a specific tRNA?
!!Answer lies in the structure of the tRNA itself - shape, also tRNA
synthase
!!Enzymes recognise specific tRNA and amino acids, attach them to
each other
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