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 2nd position U C A G 3rd position ↓ Phe Ser Tyr Cys Leu STOP Trp Pro His Arg Gln Ile Thr Asn Lys Met Val Ala Asp Gly Glu

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 Phe Ser Tyr Cys Leu STOP Trp Pro His Arg Gln Ile Thr Asn Lys Met Val Ala Asp Gly Glu

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

1st position 2nd position U C A G 3rd position ↓ Phe Ser Tyr Cys Leu STOP Trp Pro His Arg Gln Ile Thr Asn Lys Met Val Ala Asp Gly Glu © 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 UUA Phenylalanine Leucine UUC UUG U & C are both pyrimidines A & G are both purines © 2016 Paul Billiet ODWS

1st position 2nd position U C A G 3rd position ↓ Phe Ser Tyr Cys Leu STOP Trp Pro His Arg Gln Ile Thr Asn Lys Met Val Ala Asp Gly Glu 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 Phe Ser Tyr Cys Leu STOP Trp Pro His Arg Gln Ile Thr Asn Lys Met Val Ala Asp Gly Glu

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