16(ii). Translation: mRNA  Protein Key ConceptsAn Introduction to TranslationSlide 3Slide 4Prokaryotic Cells – Structural OverviewTranscription and Translation in BacteriaTranscription and Translation in EukaryotesSlide 8The Genetic CodeSlide 10The Characteristics of Transfer RNAWhat Happens to the Amino Acids Attached to tRNA?Slide 13What Do tRNAs Look Like?Slide 15How Many tRNAs Are There?How many tRNAs are there?The Structure and Function of RibosomesRibosomes and the Mechanism of TranslationSlide 20Requirements for Translation:Translation: Involves more than 100 macromolecules.InitiationINITIATION:Initiation in BacteriaElongationMBSlide 28Is the Ribosome an Enzyme or a Ribozyme?Moving Down the mRNASlide 31TerminationSlide 33Antibiotics blocks prokaryotic translation:Post-Translational ModificationsSlide 36Slide 37Cytosolic vs secreted proteins:Slide 39Slide 4016(ii). Translation: mRNA  Protein Key ConceptsRibosomes translate mRNAs into proteins with the help of intermediary molecules called transfer RNAs (tRNAs).Each transfer RNA carries an amino acid corresponding to the tRNA’s three-base-long anticodon.In the ribosome, the tRNA anticodon binds to (complementary base pairing, H-bonding) a three-base-long mRNA codon, causing the amino acid carried by the transfer RNA to be added to the growing protein.An Introduction to Translation•In translation, the sequence of bases in the mRNA is converted to an amino acid sequence in a protein.•Ribosomes involved in translation of the mRNA sequence into protein.A cell builds the proteins it needs from instructions encoded in its genome according to the central dogma of molecular biology.rRNA (ribosomal)t RNA (transfer)mRNA (messenger)All 3 involved in protein synthesis4.3RNAmay beprocessed bymay functiondirectly incell as16.2• Splicing• Addition of 5 cap• Addition of poly(A) tailto form16.416.5• tRNA (transfer RNA)• rRNA (ribosomal RNA)mRNA(messeger of RNA)16.2is thenTRANSLATED byaffectRibosomes16.5to formProteins3.216.5changed byproduce13.1Phenotype• Folding• Glycosylation• Phosphorylation• Degradation3.45.39.118.4Prokaryotic Cells – Structural Overview•All prokaryotes lack a membrane-bound nucleus.•Bacterial cells vary greatly in size and shape, but most bacteria contain several structural similarities:–Plasma membrane–A single chromosome–Ribosomes, which synthesize proteins–Stiff cell wallTranscription and Translation in Bacteria•In bacteria, transcription and translation can occur simultaneously. Bacterial ribosomes begin translating an mRNA before RNA polymerase has finished transcribing it. –Multiple ribosomes attached to an mRNA form a polyribosome.  increase rate of protein synthesis from a single mRNATranscription and Translation in Eukaryotes•In eukaryotes, transcription and translation are separated. mRNAs are synthesized and processed in the nucleus and then transported to the cytoplasm for translation by ribosomes.The Genetic CodeIt is redundant All amino acids except two are encoded by more than one codon.It is unambiguous  One codon never codes for more than one amino acid.It is nearly universal  With a few minor exceptions, all codons specify the same amino acids in all organisms.It is conservative  The first two bases are usually identical when multiple codons specify the same amino acid.The Genetic Code•Salient features of the genetic code–triplet:triplet: a sequence of three nucleotides (a codon) is needed to specify one amino acid–nonoverlapping:nonoverlapping: no bases are shared between consecutive codons–commaless:commaless: no intervening bases between codons–redundant (degenerate):degenerate): more than one triplet codon can code for the same amino acid; Leu, Ser, and Arg, for example, are each coded for by six triplets–Nearly universal:Nearly universal: the same in viruses, prokaryotes, and eukaryotes; the only exceptions are some codons in mitochondria, chloroplast…..The adapter molecule was later found to be a small RNA called transfer RNA (tRNA).Francis Crick proposed that an adapter molecule holds amino acids in place while interacting directly and specifically with a codon in mRNA.How Does an mRNA Triplet Specify an Amino Acid?The Characteristics of Transfer RNA•ATP is required to attach tRNA to an amino acid.•Enzymes called aminoacyl tRNA synthetases “charge” the tRNA by catalyzing the addition of amino acids to tRNAs. For each of the 20 amino acids, there is a different aminoacyl tRNA synthetase and one or more tRNAs.A tRNA covalently linked to its corresponding amino acid is called an aminoacyl tRNA.What Happens to the Amino Acids Attached to tRNA?•Experiments with radioactive amino acids revealed that they are lost from tRNAs and incorporated into polypeptides synthesized in ribosomes.These results inspired the use of “transfer” in tRNA’s name, because amino acids are transferred from the RNA to the growing end of a new polypeptide. The experiment also confirmed that aminoacyl tRNAs act as the interpreter in the translation process: tRNAs are Crick’s adapter molecules.What Do tRNAs Look Like?•The CCA sequence at the 3' end of each tRNA is the binding site for amino acids.•The triplet on the anticodon loop base pairs with the mRNA codon.•The secondary structure of tRNA (clover-leaf) folds over to produce an L-shaped tertiary structure.All of the tRNAs in a cell have the same structure, shaped like an upside-down L. They vary at the anticodon and attached amino acid.tRNAmRNA-tRNA interaction:[Codon-anticodon interaction]How Many tRNAs Are There?•There are 61 different codons but only about 40 tRNAs in most cells. •To resolve this deficit, Francis Crick proposed the wobble hypothesis. This hypothesis proposes that the anticodon of tRNAs can still bind successfully to a codon whose third position requires a nonstandard base pairing. •Thus, one tRNA is able to base pair with more than one type of codon.How many tRNAs are there?Wobble: specificity for the base at the 3′ end of the codon is not always observed.Example: codons for alanine—GCA, GCC, and GCU—are recognized by the same tRNA.Allows cells to produce fewer tRNA species; but in all cases—the genetic code remains unambiguous.The wobble hypothesis provides insight into some aspects of the degeneracy of the codeThe Structure and Function of Ribosomes•Ribosome , the “work bench”  site of protein