Some Codons Are biased: Because more than one codon exists for most amino acids, the possibility forvariation in codon usage arises.. Even in organisms of average base composition, codon usage may be biased·For example the Leu codons in of human genes,: CUG was used in excess of 48,000 times: CUC more than 23,000 times· UUA just 6000 times.?Preferred codons are represented by the most abundant isoacceptor tRNAs.Furthermore, mRNAs for proteins that are synthesized in abundance tend toemploy preferred codons.: Rare tRNAs correspond to rarely used codons, and messages containing suchcodons might experience delays in translation
Some Codons Are biased • Because more than one codon exists for most amino acids, the possibility for variation in codon usage arises. • Even in organisms of average base composition, codon usage may be biased. • For example the Leu codons in of human genes, • CUG was used in excess of 48,000 times • CUC more than 23,000 times • UUA just 6000 times. • Preferred codons are represented by the most abundant isoacceptor tRNAs. Furthermore, mRNAs for proteins that are synthesized in abundance tend to employ preferred codons. • Rare tRNAs correspond to rarely used codons, and messages containing such codons might experience delays in translation
TranslationThree types of RNA participate in translationmRNA: contains the genetic code to specify the amino acid sequence of theproteintRNA:carriestheaminoacidsand readsthecodeAnticodonpairswithcodonrRNA: part of the machinery (ribosome) that makes the protein productPsiteLargenbosomasubunittRNArRNA(+proteins)mRNAAOGMIMMMIOsmallribosomerRNAsubunit(+proteins)
Translation Three types of RNA participate in translation mRNA: contains the genetic code to specify the amino acid sequence of the protein tRNA: carries the amino acids and reads the code Anticodon pairs with codon rRNA: part of the machinery (ribosome) that makes the protein product
3tRNA-OHPAcceptorstemSets of tRNAs bind individual amino acids2D structuretRNAs have specific 2D and 3D shapetRNAshave3-baseanticodonGACACTYCCUGUGloopUCGDloop Base pairs to codons in mRNAVariableloopDecodesmessageAnticodonloop*NotresponsibleAnticodonforstructuresUracil=pseudouracilD=dihydrouracil
tRNA Sets of tRNAs bind individual amino acids tRNAs have specific 2D and 3D shape tRNAs have 3-base anticodon Base pairs to codons in mRNA Decodes message
tRNAoSets of tRNAs bind individual amino acidsUU30-40 different tRNAs in bacterial cells?AcceptorstemUpto150inmammaliancellsIsoacceptortRNAsACACTYCCUCGloopCUGUGDloopGAG different anticodons - same amino acidVariableloop2DstructureceptorendAnticodonloopAnticodon(Otherloopsarefolded)3D structureAnticodonloop
tRNA Sets of tRNAs bind individual amino acids 30-40 different tRNAs in bacterial cells Up to 150 in mammalian cells Isoacceptor tRNAs different anticodons – same amino acid
Charging the tRNARNAEnzymes add amino acid to tRNAAminoacyl-tRNA synthetase (AARS)aminoacidrequires energy of ATPNHAminoacyl-tRNAAMP added to amino acidAminoacidAcceptorAmino acid attached to tRNA at acceptor armcCAttachment Connected at alpha-COO- groupsitetRNAAlpha-NH, group freecorTVCloopDHU IoOPAnticodonSLLLLLLLLLLLLLLAUG
Charging the tRNA Enzymes add amino acid to tRNA Aminoacyl-tRNA synthetase (AARS) requires energy of ATP AMP added to amino acid Amino acid attached to tRNA at acceptor arm Connected at alpha-COO- group Alpha-NH2 group free