• GENETIC CODE
genetic code links groups of nucleotides in mRNA to amino acids in a
protein
• It allows DNA & RNA sequences to be decoded into aminoacids
Codons
• Cells decode mRNAs by reading their nucleotides in groups of three,
called codons
• Most codons specify an amino acid
• Three "stop" codons mark the end of a protein (UAA, UAG,UGA)
• One "start" codon, AUG, marks the beginning of a protein and also
encodes the amino acid methionine
Dr.Sadia Ghousia Baig
Genetic Code Table
Dr.Sadia Ghousia Baig
Characteristics of Genetic code
• Degeneracy : every amino acid except MET is represented by several codons
• Wobbling phenomenon: Reduced specificity at the last position. Also called 3rd
base degeneracy
• Unambiguity: a given codon designates only 1 specific amino acid
• Universality: in all living organisms the genetic code is same ( few exceptions )
• Commaless : codons are arranged as continuous structure.
Dr.Sadia Ghousia Baig
Anticodon : Sequences of nucleotides that are complementary to codons. They are
found in tRNA
• TRANSLATION (Protein Synthesis) Overview
• The ribosomal translation is initiated when the ribosomes recognize the starting
point of mRNA, where it binds a molecule of tRNA that bears a single amino acid.
• In prokaryotes, the initial amino acid in methionine. during elongation, the second
amino acid is linked to the first one.
• The ribosome then shifts its position on the mRNA and repeats the elongation
cycle.
• When the elongation process reaches the stop codon, the amino acid chain folds
spontaneously to form a protein.
• The ribosomes then split into two subunits, but later rejoin before another mRNA
is translated.
• Protein synthesis is facilitated by several catalytic proteins which include initiation,
elongation, termination factors, and guanosine triphosphates (GTP)
Dr.Sadia Ghousia Baig
Requirements of Translation :
• Amino acids
• mRNA to be translated
• tRNA for each amino acid
• Ribsomes (Prokaryotic 70S (50s & 30 s) Eukaryotic 80 S (60 & 40 s)
• Energy source (GTP)
• Initiation factors
• Elongation factors
• Termination factors
• Aminoacyl tRNA synthetase ( attaches aa with tRNA utilizing ATP,
synthesizes amino acyl tRNA or charged tRNA)
Dr.Sadia Ghousia Baig
tRNA
attachment side for aa is its 3 prime end
Charged tRNA = attached with amino acid
• Anticodon: 3 base nucleotide sequence that pairs with codons on mRNA
• Translation steps (initiation, elongation, translocation, termination)
• 1st tRNA goes to P site . Then ribosome moves on mRNA ( from 5′ to 3′)
• Next charged tRNA comes on A site
• Peptide formation is catalyzed by peptidyl transferase
• Uncharged tRNA moves from P to E site
• Peptidyl tRNA moves from A to P site
• Termination occurs when termination codon move into A site recognized by
releasing factors which hydrolyze bonds linking peptide to tRNA at P site
Dr.Sadia Ghousia Baig
Dr.Sadia Ghousia Baig
