PCB3063 Exam 4 Chapter 13 Translation and Proteins 13 1 Translation of mRNA Translation of mRNA is the biological polymerization of amino acids into polypeptode chains The process requires amino acids messenger RNA ribosomes and transfer RNA tRNAs read the triplet codon into amino acids o They also have anticodons that compliment the RNAs Hydrogen bonding between tRNAs to mRNA holds the amino acid in proximity so that a peptide bond can be formed Ribosomes consist of ribosomal proteins and ribosomal RNAs The rRNAs perform important catalytic functions associated with translation They have a large and small subunit Both subunits consist of one or more molecules of rRNA and an array of ribosomal proteins When the two subunits are associated with each other in a single ribosome the structure is sometimes called a monosome o In class he called ribosomes a protein polymerase They synthesize a polypeptide Ribosomal Structure Prokaryotes monosome is 70 consists of 50S and 30S subunit The larger subunit in prokaryotes consists of a 23S rRNA molecule a 5S rRNA molecule and 31 ribosomal proteins 33 molecules in this subunit Eukaryotes monosome is 80S consists of 60S and 40S subunit The larger subunit in prokaryotes consists of a 28S rRNA molecule a 5 8S and 5S rRNA molecule and 46 ribosomal proteins The rRNA genes AKA rDNA are present as clusters at various chromosomal sites nucleous organizing region NOR Each cluster consists of tandem repeats with each unit separated by a noncoding spacer DNA sequence Essential point Translation is the synthesis of polypeptide chains under the direction of mRNA in association with ribosomes tRNA Structure Very small and stable They are transcribed and then posttranscriptionally modified It is not clear why such modified bases are created it is believed that their presence enhances hydrogen bonding efficiency during translation 2 examples of modified bases inosinic acid and pseudouridylic acid A tRNA has an anticodon that complementarily base pairs with the codon in the mRNA At the other end is the 3 accepter region where the amino acid is bound The two dimensional structure of tRNAs is a cloverleaf Charging tRNA tRNA molecules must be chemically linked to their respective amino acids This activation process called charging occurs under the direction of aminoacyl tRNA synthetases There are 20 different amino acids so there are 20 different synthetases ATP and a single amino acid are required In theory there could be 61 AA codons resulting in 61 specific tRNAs Because of third base wobble it is now thought that there are only 31 different tRNAs and 20 synthetases Essential point Translation depends on tRNA molecules that serve as adapters betwenn triplet codons in mRNA and that the corresponding amino acids 13 2 Initiation Translation is divided into three phases initiation elongation and termination Initiation requires the small and large ribosomal subunits mRNA GTP charged tRNA Mg2 and initiation factors that enhance binding affinity In prokaryotes the initiation codon of mrNA AUG calls for the modified amino acid formylmethionine fmet Shine Dalgarno sequence prok only purine rich sequence that precedes the start codon and binds to the small subunit of ribosome facilitating initiation Kozak sequence in eukaryotes ACCAUGG similar function but ribosome scans from 5 to 3 on mRNA Elongation Requires both ribosomal subunits assembles with the mRNA to form the P peptidyl site and A aminoacyl site The lengthening and growing polypeptide chain by one amino acid is called elongation Simple way to distinguish two sites following the shift the P site peptidyl contains tRNA attached to peptide chain wheras the A site aminoacyl contains tRNA with an amino acid attached The small subunit decodes the mRNA The large subunit catalyzes the peptide bond synthesis Termination http www hhmi org biointeractive translation advanced detail Stop codons UAG UAA UGA NO amino acids for these codons but they are annotated in GenBank as part of the ORF the last part Essential point Translation like transcription is subdivided into stages of initiation elongation and termination and relies on base pairing affinities between complementary nucleotides 13 3 Polyribosomes Polysomes are mRNAs with several ribosomes translating at once 13 4 Translation is more Complex in Eukaryotes Eukaryotes ribosomes are larger with RNA and protein components are more complex and longer lived than in bacteria One main difference is that translation occurs on larger ribosomes whose rRNA and protein components are more complex Transcription occurs in the nucleus The 5 end of mRNA is capped with a 7 methylguanosine residue at maturation which is essential for translation A poly A tail is added at the 3 end of mRNA Translation occurs in the cytoplasm Many eukaryotic mRNAs contain a purine A or G three bases upstream from the AUG initiator codon which is followed by a G This Kozak sequence is considered to increase the efficacy of translation by interacting with the initiator tRNA analogous to Shine Dalgarno sequence in prokes Translation is more Complex in Eukaryotes overview Eukaryotic ribosomes are larger No coupled translation transcription in Euk Eukaryotic mRNAs persist longer Start codon identified by 5 3 scanning No fMet in Euk iMet instead Membrane extracellular proteins synthesized on the ER first not plasma membrane 3 5 The initial insight that proteins are important in hereditary was provided by the study of inborn errors of metabolism Alkaptonuria results from a mutation that leads to a metabolic block which prevents them from metabolizing homogentisic acid o It accumulates in cells and tissues and is excreted in the urine o Oxidation products are black and detected in the diapers of newborns and also accumulate in cartiginous areas causing the ears and nose to darken o Homogentisic acid accumulates in joins leading to benign arthritis o Pedigree analysis indicates that this disease has a genetic basis and afflicted individuals had parents who were first cousins consanguine 3 6 Studies of Neurospora Led to One Gene One Enzyme Hypothesis Beadle and Tatum showed that nutritional mutations in the bread mold Neurospora caused the loss of enzymatic activity that catalyzes an essential reaction in wild type organisms Pink bread mold much known about its biochemistry easy to mutate and isolate single cell derivatives produced strains with mutations critical to the growth of the organism Basically if
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