Lesson 20 21 The Genetic Code Central dogma is the concept that cells are governed by a cellular chain of command DNA RNA protein The flow of information from gene to protein is based on a triplet code a series of non overlapping three nucleotide words o The words of a gene are transcribed into complementary non overlapping three nucleotide words of mRNA o These words are then translated into a chain of amino acids forming polypeptides Ex The triplet code ACC on DNA codes for UGG on mRNA which codes for the amino acid tryptophan on a polypeptide o During transcription the template strand provides a template for ordering the sequence of complementary nucleotides in an RNA transcript o During translation the mRNA base triplets called codons are read in the 5 to 3 direction Each codon specifies the amino acid one of 20 to be placed at the corresponding position along a polypeptide Of the 64 triplets 61 code for amino acids other 3 are stop signals to end translation The genetic code is redundant more than one codon may specify a particular amino acid but not ambiguous This is a good thing because if there is a small mistake during transcription there is a chance that through the redundancy similar codons could both code for the same amino acid Codons must be read in the correct reading frame in order for the specified polypeptide to be produced o Start codon AUG Met o Stop codons UAA UAG and UGA Genetic information flows from mRNA to protein through the process of translation o During this process the sequence of bases in mRNA codes for the assembly of specific sequence of amino acids in a protein primary structure In prokaryotes mRNA binds to ribosomes in the cytoplasm In eukaryotes mRNA undergoes processing before leaving the nucleus and binding to ribosomes in the cytoplasm o A cell translates an mRNA message into protein with help of transfer RNA tRNA tRNAs transfer amino acids to the growing polypeptide in a ribosome tRNA molecules are not identical each carries a specific amino acid on one end of it s structure each has an anticodon on the other end tRNA molecule consists of a single RNA strand that is able 80 nucleotides long Straightened out it is shaped like a clover remember RNA can form hydrogen bond with nucleotides within its own molecule into a three dimensional molecule that is L shaped o Accurate translation requires two steps First a correct match between a tRNA and an amino acid done by the enzyme aminoacyl tRNA synthetase this enzyme attaches the amino acid onto the 3 end of the tRNAs that can carry that amino acid Second a correct match between the tRNA anticodon and an mRNA codon o Flexible pairing at the third base of a codon is called wobble and allows some tRNAs to bind to more than one codon Ribosomes Ex For proline the mRNA codon is CCU CCC CCA CCG o Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons in protein synthesis o The two ribosomal subunits large and small are made of proteins and ribosomal RNA rRNA 2 3 of ribosome mass is composed of rRNA o A ribosome has 3 binding sites for tRNA The P site holds the tRNA that carries the growing polypeptide chain The A site holds the tRNA that carries the next amino acid to be added to the chain The E site is the exit site where discharged tRNAs leave the ribosome Building a polypeptide the 3 stages of translation all 3 stages require protein factors that aid in the process o 1 Initiation stage of translation that brings together mRNA a tRNA with the first amino acid and the two ribosomal subunits First a small ribosomal subunit binds with mRNA and a special initiator tRNA In prokaryotes the small subunit can bind to either the initiator tRNA or the mRNA at a specific sequence upstream of the start codon AUG In eukaryotes the small subunit attaches to the initiator tRNA first The complex bind on the 5 cap of the mRNA and scans down the stand until it reached AUG Once the small subunit reaches the start codon AUG translation is started Proteins called initiation factors bring in the large subunit that completes the translation initiation complex o Energy in the form of GTP is used to form the translation initiation complex o 2 Elongation amino acids are added one by one to the preceding amino acid at the C terminus of the growing chain Each addition involves proteins called elongation factors and occurs in 3 steps codon recognition peptide bond formation and translocation 1 Codon recognition GTP is used to increase accuracy and efficiency of the proper charged tRNA entry into A site 2 Peptide bond formation rRNA of large ribosomal subunit catalyzes peptide bond formation and transfer of polypeptide from P site to A site tRNA 3 Translocation 1 tRNA moves to E site 2 tRNA moves to the P site and mRNA moves towards the 3 end exposing the next codon to the A site o 3 Termination occurs when a stop codon in the mRNA reaches the A site of ribosome The A site accepts a protein called a release factor it is not charged with an amino acid Release factor is a protein not a tRNA The release factor causes the addition of a water molecule instead of an amino acid This reaction releases the polypeptide and the translation assembly then comes apart The hydrolysis of the polypeptide from the tRNA occurs when the release factor binds onto the exposed stop codon at the A site GTP not used o Polyribosomes enable a cell to make many copies of a polypeptide very quickly A number of ribosomes can translate a single mRNA simultaneously forming a polyribosome or polysome o Completing and Targeting the Functional Protein Often translation is not sufficient to make a functional protein During and after synthesis a polypeptide chain spontaneously coils and folds into its three Polypeptide chains are modified after translation or targeted to specific sites in the cell dimensional shape o Protein Folding and Post Translational Modification Proteins may also require post translational modifications before doing their job Some polypeptides are activated by enzymes that cleave them Other polypeptides come together to form the subunits of a protein o Targeting Polypeptides to Specific Location Ex Hemoglobin has 4 subunits Two populations of ribosomes are evident in cells free ribsomes in the cytosol and bound ribosomes attached to the ER Free ribosomes mostly synthesize proteins that function in the cytosol Bound ribosome make proteins of the endomembrane system and proteins that are secreted from the cell Remember
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