GENE EXPRESSION THE FLOW OF GENETIC INFORMATION part 2

6TH WEEK, BIO-1053 GENE EXPRESSION THE FLOW OF GENETIC INFORMATION 6th week General Genetics-BIO1053 Translation: from mRNA to protein Translation is the process by which the sequence of nucleotides in a mRNA directs the assembly of the correct sequence of amino acids in the corresponding polypeptide 6th week General Genetics-BIO1053 Translation: from mRNA to protein Transfer RNAs (tRNAs) mediate translation of mRNA codons to amino acids Translation takes place on ribosomes that coordinate movement of tRNAs carrying specific amino acids with genetic instruction of an mRNA tRNAs are short single-stranded RNAs of 74 – 95 nt Each tRNA has an anticodon that is complementary to an mRNA codon A specific tRNA is covalently coupled to a specific amino acid (charged tRNA) Base pairing between an mRNA codon and an anticodon of a charged tRNA directs amino acid incorporation into a growing polypeptide 6th week General Genetics-BIO1053 Aminoacyl-tRNA synthetases catalyze attachment of amino acids to specific tRNAs Base pairing between an mRNA codon and a tRNA anticodon determines which amino acid is added to a polypeptide 6th week General Genetics-BIO1053 Wobble: Some tRNAs recognize more than one codon for the amino acid they carry 6th week General Genetics-BIO1053 Figure 8.3 Hartwell, Genetics: From Genes to Genomes, 3e Ribosomes are site of polypeptide synthesis Different parts of a ribosome have different functions Small subunit binds to mRNA Large subunit has peptidyl transferase activity Three distinct tRNA binding areas – E, P, and A sites Initiation of translation in eukaryotes Small ribosomal subunit binds to 5' cap, then scans the mRNA for the first AUG codon Initiator tRNA carries Met (not fMet) 6th week General Genetics-BIO1053 Elongation Addition of amino acids to C-terminus of polypeptide Charged tRNAs ushered into A site by elongation factors 6th week General Genetics-BIO1053 Polyribosomes consist of several ribosomes translating the same mRNA 6th week General Genetics-BIO1053 Termination of translation No normal tRNAs carry anticodons for the stop codons Release factors bind to the stop codons Release of ribosomal subunits, mRNA, and polypeptide 6th week General Genetics-BIO1053 Posttranslational processing can modify polypeptide structure (a) Cleavage may remove an amino acid (c) Chemical constituent addition may modify a protein (b) Cleavage may split a polyprotein 6th week General Genetics-BIO1053 Differences in translation between prokaryotes and eukaryotes 6th week General Genetics-BIO1053 The effect of mutations on gene expression and gene function Mutations in the coding sequence of a gene can alter the gene product Mutations outside the coding sequence - Mutations outside the coding sequence can also alter gene expression Promoter sequence Termination signals Splice-acceptor and splice-donor sites Ribosome binding sites 6th week General Genetics-BIO1053 Loss-of-function mutations result in reduced or abolished protein activity Loss-of-function mutations are usually recessive • Null (amorphic) mutations – completely block function of a gene product (e.g deletion of an entire gene) • Hypomorphic mutations – gene product has weak, but detectable, activity Xanthine dehydrogenase Incomplete dominance arises when phenotype varies in proportion to the amount of protein Rarely, loss-of-function mutations are dominant Heterozygotes for the null mutation of the T locus in mice have short tails because have an insufficient amount of protein to produce a wide-type tail Gian-of-function mutations are almost always dominant Mutant Wild-type Mutation in Antennapedia gene of Drosophila causes ectopic expression of a leg-determining gene in structures that normally produce antennae The cellular components of gene expression Mutations in genes encoding gene products for transcription, RNA processing, translation, and protein processing are often lethal Some mutations in tRNA genes can suppress mutations in proteincoding genes 6th week General Genetics-BIO1053 Some mutations in tRNA genes can suppress mutations in protein-coding genes Q&A 6th week General Genetics-BIO1053 ... Translation is the process by which the sequence of nucleotides in a mRNA directs the assembly of the correct sequence of amino acids in the corresponding polypeptide 6th week General Genetics-BIO1053... 6th week General Genetics-BIO1053 Wobble: Some tRNAs recognize more than one codon for the amino acid they carry 6th week General Genetics-BIO1053 Figure 8.3 Hartwell, Genetics: From Genes to... polyprotein 6th week General Genetics-BIO1053 Differences in translation between prokaryotes and eukaryotes 6th week General Genetics-BIO1053 The effect of mutations on gene expression and gene function