BIOL10221 Molecular Biology Lecture 1 PDF

BIOL10221 “Molecular Biology” Dr David Boam Module 7 – From RNA to Protein Lecture 1: The Genetic code  Aims of this lecture Degeneracy of the genetic code How it the genetic code was broken Reading frames What does tRNA do? Synthesis of aminoacyl tRNA...

BIOL10221 “Molecular Biology” Dr David Boam Module 7 – From RNA to Protein Lecture 1: The Genetic code  Aims of this lecture Degeneracy of the genetic code How it the genetic code was broken Reading frames What does tRNA do? Synthesis of aminoacyl tRNA Wobble – a mechanism for permitting degeneracy Lecture 7/1 The genetic code  Protein Synthesis (Translation) mRNA protein The Genetic Code Proteins made from 20 amino acids but only 4 nucleotides in messenger RNA. So, a group of nucleotides is required to code for 20 amino acids. 2 nucleotides 16 (42) 3 nucleotides 64 (43) A 3 nucleotide or triplet code used to specify the amino acids. = codon Lecture 7/1 The genetic code  Philip Leder Francis Crick George Gamow Marshall Nirenberg Har Gobind Khorana Lecture 7/1 The genetic code  How the genetic code was broken – The Nirenberg- Matthaei experiment UUUUUUUUUU Isolate polypeptides +19 amino acids + 1 labelled amino acid Only labelled phe bacterial gave a labelled extract polypeptide competent for translation Lecture 7/1 The genetic code  How the genetic code was broken How the genetic code was broken Polynucleotides UUU UUU UUU Phenylalanine AAA AAA AAA Lysine CCC CCC CCC Proline ACA CAC ACA CAC ACA CAC ACA Thr His Thr His Thr His Thr AAC AAC AAC AAC AAC AAC AAC Asn Asn Asn Asn Asn Asn Asn 61 of 64 codons coded A ACA Thr ACA Thr ACA Thr ACA Thr ACA Thr ACA Thr ACA Thr for amino acids, three codons didn’t code for AA CAA CAA CAA CAA CAA CAA CAA any amino acids - stop codons. Gln Gln Gln Gln Gln Gln Gln Trinucleotides and tRNA binding to ribosomes Lecture 7/1 The genetic code Of the 64 possible codons, 61 specify amino acids, 3 specify stop codons Many amino acids have more than one codon (e.g. Leucine, Serine and Arginine have 6 codons) The genetic code is degenerate - redundant Lecture 7/1 The genetic code  Reading Frames e sequence of codons that runs from a specific start on to a specific stop codon is an open reading frame Almost every protein chain initiates with AUG - codes for Methionine….. ……but not all met codons are equal! (next lecture….) 3 stop codons UAA, UGA, and UAG do not specify amino acids mRNA polypeptide Lecture 7/1 The genetic code  Transfer RNA (tRNA) Structure and Function Anticodon - interacts with codon Amino-acid acceptor arm - where amino acid binds Modified nucleotides especially in the anticodon loop Function – to link to a specific amino acid and to recognise a codon in mRNA - ensures amino acid-codon match Lecture 7/1 The genetic code Aminoacyl-tRNA synthetases Each tRNA is recognised by only one of 20 aminoacyl-tRNA synthetases Each synthetase is specific for amino acid and tRNA – proofreading! >1 tRNA for some amino acids Paradox : > 20 tRNAs but < 61. So some tRNAs recognise more than one codon! Lecture 7/1 The genetic code  How does one tRNA recognise more than one codon? (A) G-U Base pairing 3’ tRNAAla2 5’ 3’ tRNA Ala1 5’ CGG CGU Wobble 5’ GC A 3’ 5’ GCC 3’ 5’ GCG 3’ 5’ GCU 3’ Alanine codons Alanine codons (B) Inosine base-pairs with A, C and U 3’ tRNAIle 5’ Wobble - allows UA I unconventional 5’ AUC 3’ base pairing between third 5’ AUA 3’ base 5’ AUU 3’ in codon and first base in Isoleucine codons anticodon Lecture 7/1 The genetic code  Summary A degenerate triplet genetic code is used to translate nucleotide to amino acid sequences Specificity is primarily by amino acid- tRNA interactions Degeneracy - – 61 codons for 20 amino acids – >1 tRNA species for certain amino acids – wobble Lecture 7/1 The genetic code  Learning objectives for this lecture You should be able to understand – The meaning and significance of degeneracy in the genetic code – How the genetic code was broken – What a reading frame is – The role of tRNA in protein synthesis – What wobble base pairing is, its mechanism and role in degeneracy Lecture 7/1 The genetic code