The genetic code in non-overlapping - each nucleotide in mRNA belongs to just one codon except in rate cases where genes overlap and a nucleotide sequence is read in two different reading frames
MBG 2040 32 3. The genetic code is comma-free - there are no commas or other forms of punctuation within the coding regions (but it does have start and stops) 4. The genetic code is degenerate - all but two of the amino acids are specified by more than one codon 5. The genetic code is ordered - multiple codons for a given amino acid and codons for amino acids with similar chemical properties are closely related, usually differing by a single nucleotide 6. The genetic code contains start and stop codons - specific codons are used to initiate and to terminate polypeptide chains 7. The genetic code is nearly universal - with minor exceptions, the codons have the same meaning in all living organisms, from viruses to humans A Triplet Code - twenty different amino acids are incorporated into polypeptides during translation - at least 20 different codons must be formed with the four bases available in mRNA - two bases per codon would result in only 4 2 or 64 possible codons - an apparent excess - the reading frame of an mRNA is the series of nucleotide triplets that are read (positioned in the A site of the ribosome) during translation A Triplet Code Single Base Pair Insertion - a single base-pair addition or deletion will alter the reading frame of the gene and mRNA for that portion of the gene distal to the mutation Suppressor Mutant - second-site mutations that resort the wild-type phenotype in a mutant organism are called suppressor mutations - they cancel or suppress the effects of the original mutation 3 Base Pair Insertion - recombinants with three (+) mutations or three (-) mutations often exhibited the wild-type phenotype - the addition of 3 base pairs or the deletion of 3 base pairs left the distal portion of the gene with the wild- type reading frame - this result is expected only if each codon contained 3 nucleotides Evidence of a Triplet Code: In Vitro Translation Studies - trinucleotides were sufficient to stimulate specific binding of aminoacyl-tRNAs to ribosomes - chemically synthesized mRNAs containing repeated dinucleotide sequences directed the synthesis of copolymers with alternating amino acid sequences - mRNAs with repeating trinucleotide sequences AT G TA C AT G TA C T T T A A A A T T T A A C C C G G G T C C A G G A A A T T T C A A G T T G G G C C C A G G T C C T T T A A A G T T C A A T A A C C C C G G G C C C G G G TA G AT C A U G U U U C C C A A A G G G U U U C C C U A G C TA G AT G A G C T C A U G A U U U C C C A A A G G G U U U U G C C C C U A G A G DNA (Wild-type allele) DNA (Mutant allele) mRNA mRNA Protein Protein Single base-pair insertion Met Phe Pro Lys Gly Phe Pro (term) Met Ile Ser Gln Arg Val Leu A T Inserted base pair alters reading frame. Altered amino acid sequence Wild-type phenotype Mutant phenotype Transcription Transcription Translation Translation AT G TA C AT G TA C A T T T A A AT T TA A T C C A G G T C C A G G C A A G T T A A A T T T A G G T C C G G G C C C G T T C A A T T T A A A T A A C C C C G G G C C C G G G C TA G AT G A G C T C A U G A U U
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- DNA, RNA Chains