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THE GENETIC CODE © 2016 Paul Billiet ODWS The tRNA molecule • tRNA molecules do the final translating • A specific amino acid is attached at one end by a tRNA activating enzyme = first part of translating • An anticodon is at the other end, complementary to the mRNA codons = second part of translation. www.cs.stedwards.edu/.../CHEM43/tRNA/trna1.gif © 2016 Paul Billiet ODWS The 3-D structure of a tRNA library.thinkquest.org/.../content/_thb_tRNA.jpg © 2016 Paul Billiet ODWS The genetic code Made of 64 triplets of bases (codons). © 2016 Paul Billiet ODWS 1st position U C A G 2nd position 3rd position ↓ U C A G Phe Ser Tyr Cys U Phe Ser Tyr Cys C Leu Ser STOP STOP A Leu Ser STOP Trp G Leu Pro His Arg U Leu Pro His Arg C Leu Pro Gln Arg A Leu Pro Gln Arg G Ile Thr Asn Ser U Ile Thr Asn Ser C Ile Thr Lys Arg A Met Thr Lys Arg G Val Ala Asp Gly U Val Ala Asp Gly C Val Ala Glu Gly A Val Ala Glu Gly G Genetic code: characteristics • Only 61 triplets or codons code for amino acids • 3 stop codons (aka nonsense codons or terminator codons) UUA UAG UGA. © 2016 Paul Billiet ODWS 1st position U C A G 2nd position 3rd position U C A G Phe Ser Tyr Cys U Phe Ser Tyr Cys C Leu Ser STOP STOP A Leu Ser STOP Trp G Leu Pro His Arg U Leu Pro His Arg C Leu Pro Gln Arg A Leu Pro Gln Arg G Ile Thr Asn Ser U Ile Thr Asn Ser C Ile Thr Lys Arg A Met Thr Lys Arg G Val Ala Asp Gly U Val Ala Asp Gly C Val Ala Glu Gly A Val Ala Glu Gly G The degenerate genetic code • A few amino acids are coded for by a single codon • Most are coded for by more than one codon • Some are coded for by up to six codons • This is degeneracy in the code. © 2016 Paul Billiet ODWS 3rd position ↓ 2nd position 1st position U C A G U Phe Ser Tyr Cys U Phe Ser Tyr Cys C Leu Ser STOP STOP A Leu Ser STOP Trp G Leu Pro His Arg U Leu Pro His Arg C Leu Pro Gln Arg A Leu Pro Gln Arg G Ile Thr Asn Ser U Ile Thr Asn Ser C Ile Thr Lys Arg A Met Thr Lys Arg G Val Ala Asp Gly U Val Ala Asp Gly C Val Ala Glu Gly A Val Ala Glu Gly G C A G © 2016 Paul Billiet ODWS The degenerative code • Several codons code for the same amino acid • The first two letters seem to be the most important • The third one tends to be interchangeable Codon Amino acid UUU Codon UUA Phenylalanine UUC U & C are both pyrimidines © 2016 Paul Billiet ODWS Amino acid Leucine UUG A & G are both purines 3rd position ↓ 2nd position 1st position U C A G U Phe Ser Tyr Cys U Phe Ser Tyr Cys C Leu Ser STOP STOP A Leu Ser STOP Trp G Leu Pro His Arg U Leu Pro His Arg C Leu Pro Gln Arg A Leu Pro Gln Arg G Ile Thr Asn Ser U Ile Thr Asn Ser C Ile Thr Lys Arg A Met Thr Lys Arg G Val Ala Asp Gly U Val Ala Asp Gly C Val Ala Glu Gly A Val Ala Glu Gly G C A G Acidic Basic Uncharged Polar Non-polar Similar amino acids have similar codons Example Aspartic acid codons GAU and GAC Glutamic acid codons GAA and GAG Both are acidic amino acids. © 2016 Paul Billiet ODWS 1st position U C A G 2nd position 3rd position U C A G Phe Ser Tyr Cys U Phe Ser Tyr Cys C Leu Ser STOP STOP A Leu Ser STOP Trp G Leu Pro His Arg U Leu Pro His Arg C Leu Pro Gln Arg A Leu Pro Gln Arg G Ile Thr Asn Ser U Ile Thr Asn Ser C Ile Thr Lys Arg A Met Thr Lys Arg G Val Ala Asp Gly U Val Ala Asp Gly C Val Ala Glu Gly A Val Ala Glu Gly G Grammar in the code? • Three codons are nonsense codons; they represent the end of the information = STOP • The codon for methionine, found at the beginning of the information to be transcribed, means START • The methionine amino acid is usually removed from the finished protein. © 2016 Paul Billiet ODWS Punctuation? • There is no punctuation between each codon • The reading frame is set at the beginning of the gene • Frame shift mutations can be caused by the ADDITION or DELETION of only one or two bases Everything downstream is misread. © 2016 Paul Billiet ODWS Reading the code • mRNA is always read in the same direction 5’ to 3’ (same as transcription and replication) • The polypeptide chain is constructed from the amino end to the carboxyl end. © 2016 Paul Billiet ODWS A universal code • The code is used by all organisms • So it is very ancient • Permits investigations into common ancestry • Permits genetically transformed organisms. © 2016 Paul Billiet ODWS Well almost universal • Mitochondrial gene expression is a bit different • Mitochondria use UGA to code for tryptophan rather than as a stop codon • Animal mitochondria use AUA for methionine instead of isoleucine • All vertebrate mitochondria use AGA and AGG as stop codons • Yeast mitochondria use all codons beginning with CU for threonine instead of leucine. © 2016 Paul Billiet ODWS 20 is the limit • Some amino acids are chemically altered AFTER translation e.g. In collogen proline is converted to hydroxyproline • Therefore the total number of amino acids found in proteins is greater than 20 but the total used in translation is only 20. © 2016 Paul Billiet ODWS Well almost 20 • There are two codons that operate under exceptional circumstances • The stop codon UGA codes for selenocysteine • The stop codon UAG codes for pyrrolysine in species of archaea and bacteria. © 2016 Paul Billiet ODWS