Pathway of protein synthesis is called translation because the “language” of the nucleotide sequence on the mRNA is translated into the “language” of an amino acid sequence Mutations Codons 4 nucleotide bases (A,T,G,C) or (A,U,G,C) use to produce 3 base codon hence 64 combinations produced

61 among 64 codes for 20 amino acid & remaining 3 are termination codons UAG, UGA, UAA CHARACTERISTICS of genetic code Specificity Universality Degeneracy (may have more than one triplet coding) i.e. Arg specified by 6 codons only met & Trp have just one coding triplet Nonoverlapping and commaless

Effects of changing single nucleotide base

Trinucleotide repeat expansion: result in Huntigton’s & X-linked diseases respectively Frame shift mutations: result in addition of different aa or loss of whole aa resulting in serious pathology i.e. cystic fibrosis deletion of phenylalanine results in lung and digestive deficiencies

Components required for translation 1) Aminoacids: if 1 aa is missing, translation stops at codon specifying that aa so 2) tRNA: one specific type of tRNA is required for each amino acid species In human & bacteria some amino acids have more than one specific tRNA molecule. This is particularly true of those amino acids that are coded for by several codons

 Amino acid attachment site: Each tRNA molecule has an attachment site for a specific amino acid at its 3'-end The carboxyl group of the amino acid is in an ester linkage with the 3'-hydroxyl at tRNA  Anticodon: Each tRNA molecule also contains a three-base nucleotide sequence—the anticodon—that pairs with a specific codon on the mRNA

3) Aminoacyl-tRNA synthetases: enzymes required for the transfer of aa to 3’ end of tRNA through covalent modifications & requires ATP These synthetases have proof reading activity 4) mRNA : The specific mRNA required as a template for the synthesis of the desired polypeptide chain must be present 5) Competent ribosomes: Values of ribosomal subunits are not additive

The small ribosomal subunit binds mRNA and correct base-pairing between the codon in the mRNA and the anticodon of the tRNA. The large ribosomal subunit catalyzes formation of the peptide bonds that link amino acid residues in a protein Ribosomal RNA (3 & 4 subunits) Ribosomal proteins

A, P, and E sites on the ribosome: binds to different sites of tRNA Cellular location of ribosomes: in eukaryotic cells, the ribosomes may be “free” in the cytosol or are in close association with the ER. Some proteins after formation destined to different sites as golgi, lysosomes

6) ATP and GTP are required as sources of energy Cleavage of four high-energy bonds is required for the addition of one amino acid to the growing polypeptide chain: two from ATP in the aminoacyl-tRNA synthetase reaction & two from GTP

CODON RECOGNITION BY tRNA Antiparallel binding between codon and anticodon : Binding of the tRNA anticodon to the mRNA codon follows the rules of complementary and antiparallel binding, that is, the mRNA codon is “read” 5'→3' by an anticodon pairing in the “flipped” (3'→5') orientation

Wobble hypothesis The mechanism by which tRNAs can recognize more than one codon for a specific amino acid is described by the “wobble” hypothesis The result of wobble is that there need not be 61 tRNA species to read the 61 codons that code for amino acids i.e. GGU, GGC, GGA all codes for gycine all can form basepair with one anti codon 3’-CCI-5’

So pairing is as 1 st base from anticodon (5’) pairs with last base of codon (3’) 3’ CCI 5’ (anticodon on tRNA ) 5’ GGU 3’ 5’ GGC 3’ 5’ GGA3 ’ (codons)

Steps in protein synthesis The process of protein synthesis translates the three-letter alphabet of nucleotide sequences on mRNA into the 20-letter alphabet of aa mRNA is translated from its 5'-end to its 3'- end, producing a protein synthesized from its amino-terminal end to its carboxyl-terminal Prokaryotes have several coding regions polycistronic

Difference in translation of pro & eukaryotes Translation steps: 1) initiation: involves 2 ribosomal subunits, mRNA, aminoacyl tRNA, GTP & initiation factors

In prokaryotes IF1 to IF3 & in eukaryotes IF1- IF10 ribosome recognizes the nucleotide sequence (AUG) that initiates translation: Shine-Dalgarno sequence: located six to ten bases upstream of the initiating AUG codon on the mRNA molecule— that is, near its 5'-end

The 16S rRNA component of the 30S ribosomal subunit has a nucleotide sequence near its 3'-end that is complementary to all or part of the SD sequence 5' end of the mRNA and the 3'-end of the 16S rRNA can form complementary base pairs

Eukaryotic messages do not have SD sequences so ribosomal subunit binds close to the cap structure at the 5-end of the mRNA and moves down the mRNA until it encounters the initiator AUG This “scanning” process requires ATP Initiation codon: The initiating AUG is recognized by a special initiator tRNA Recognition is facilitated by IF2-GTP in prokaryotes and eIF2-GTP in eukaryotes

In bacteria and in mitochondria : the initiator tRNA carries an N-formylated methionine In eukaryotes, the initiator tRNA carries a methionine that is not formylated In both prokaryotic and eukaryotic cells, this N-terminal methionine is usually removed before translation is completed