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Download Lecture 17 Protein synthesis pp101-110
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Lecture 20 Protein synthesis 1 3.12 Proteins are made from amino acids linked by peptide bonds • Amino acids, the building blocks of proteins, have an amino group and a carboxyl group – Both of these are covalently bonded to a central carbon atom – Also bonded to the central carbon is a hydrogen atom and some other chemical group symbolized by R Copyright © 2009 Pearson Education, Inc. Amino group Carboxyl group Leucine (Leu) Hydrophobic Serine (Ser) Aspartic acid (Asp) Hydrophilic 3.12 Proteins are made from amino acids linked by peptide bonds • Amino acid monomers are linked together to form polymeric proteins – This is accomplished by an enzyme-mediated dehydration reaction – This links the carboxyl group of one amino acid to the amino group of the next amino acid – The covalent linkage resulting is called a peptide bond Copyright © 2009 Pearson Education, Inc. 3.13 A protein’s specific shape determines its function • A polypeptide chain contains hundreds or thousands of amino acids linked by peptide bonds – The amino acid sequence causes the polypeptide to assume a particular shape – The shape of a protein determines its specific function Copyright © 2009 Pearson Education, Inc. Protein Synthesis • DNA serves as master blueprint for protein synthesis • Genes are segments of DNA carrying instructions for a polypeptide chain • Triplets of nucleotide bases form the genetic library • Each triplet specifies coding for an amino acid Genetic information flows: DNA RNA polypeptide Remember for further information Roles of the Three Types of RNA • Messenger RNA (mRNA) – carries the genetic information from DNA in the nucleus to the ribosomes in the cytoplasm • Transfer RNAs (tRNAs) – bound to amino acids base pair with the codons of mRNA at the ribosome to begin the process of protein synthesis • Ribosomal RNA (rRNA) – a structural component of ribosomes From DNA to Protein Nuclear envelope DNA Transcription Pre-mRNA RNA Processing mRNA Ribosome Translation Polypeptide Figure 3.33 From DNA to Protein DNA Figure 3.33 From DNA to Protein Transcription DNA Figure 3.33 From DNA to Protein DNA Transcription Pre-mRNA RNA Processing mRNA Figure 3.33 From DNA to Protein Nuclear envelope DNA Transcription Pre-mRNA RNA Processing mRNA Figure 3.33 From DNA to Protein Nuclear envelope DNA Transcription Pre-mRNA RNA Processing mRNA Ribosome Translation Polypeptide Figure 3.33 DNA molecule Gene 1 Gene 2 Gene 3 DNA strand Transcription RNA Codon Translation Polypeptide Amino acid DNA strand Transcription RNA Codon Translation Polypeptide Amino acid Transcription: • RNA Polymerase, An enzyme that oversees the synthesis of RNA Unwinds the DNA template (17 base pair at a time) • Adds complementary ribonucleoside triphosphates on the DNA template • Joins these RNA nucleotides together • Encodes a termination signal to stop transcription RNA polymerase Transcribes the DNA code into RNA code DNA = G A T C A T T A G RNA = C U A G U A A U C 10.9 Transcription produces genetic messages in the form of RNA – Overview of transcription – The two DNA strands separate – One strand is used as a pattern to produce an RNA chain, using specific base pairing – For A in DNA, U is placed in RNA – RNA polymerase catalyzes the reaction Copyright © 2009 Pearson Education, Inc. 10.9 Transcription produces genetic messages in the form of RNA Stages of transcription – Initiation: RNA polymerase binds to a promoter, where the helix unwinds and transcription starts – Elongation: RNA nucleotides are added to the chain – Termination: RNA polymerase reaches a terminator sequence and detaches from the template Copyright © 2009 Pearson Education, Inc. 5 RNA nucleotides RNA polymerase Direction of transcription Newly made RNA Template strand of DNA RNA polymerase DNA of gene Promoter DNA Terminator DNA 1 Initiation 2 Elongation 3 Termination Completed RNA Area shown in Figure 10.9A Growing RNA RNA polymerase Transcription makes a mRNA copy of DNA RNA polymerase DNA of gene Promoter DNA 1. RNA polymerase enzyme binds to DNA (Initiation) 2. RNA polymerase catalyzes the production of mRNA, using DNA as a template (Elongation) 3. RNA polymerase unbinds from DNA and RNA production stops (Termination) Figure 10.9B Terminator DNA 1 Initiation Area shown In Figure 10.9A 2 Elongation 3 Termination Completed RNA Growing RNA RNA polymerase Initiation of Translation • A leader sequence on mRNA attaches to the small subunit of the ribosome • Methionine-charged initiator tRNA binds to the small subunit • The large ribosomal unit now binds to this complex forming a functional ribosome 10.8 The genetic code is the Rosetta stone of life – Characteristics of the genetic code – Triplet: Three nucleotides specify one amino acid – 61 codons correspond to amino acids – AUG codes for methionine and signals the start of transcription – 3 “stop” codons signal the end of translation Copyright © 2009 Pearson Education, Inc. 10.8 The genetic code is the Rosetta stone of life – Redundant: More than one codon for some amino acids – Unambiguous: Any codon for one amino acid does not code for any other amino acid – Does not contain spacers or punctuation: Codons are adjacent to each other with no gaps in between – Nearly universal Copyright © 2009 Pearson Education, Inc. Genetic Code • RNA codons code for amino acids according to a genetic code Figure 3.35 Strand to be transcribed DNA Strand to be transcribed DNA Transcription RNA Start codon Stop codon Strand to be transcribed DNA Transcription RNA Start codon Polypeptide Met Translation Lys Phe Stop codon 10.10 Eukaryotic RNA is processed before leaving the nucleus – Messenger RNA (mRNA) contains codons for protein sequences – Eukaryotic mRNA has interrupting sequences called introns, separating the coding regions called exons – Eukaryotic mRNA undergoes processing before leaving the nucleus – Cap added to 5’ end: single guanine nucleotide – Tail added to 3’ end: Poly-A tail of 50–250 adenines – RNA splicing: removal of introns and joining of exons to produce a continuous coding sequence Copyright © 2009 Pearson Education, Inc. Exon Intron Exon Intron Exon DNA Cap RNA transcript with cap and tail Transcription Addition of cap and tail Introns removed Tail Exons spliced together mRNA Coding sequence Nucleus Cytoplasm Comparison • • • • • • • • DNA Deoxyribonucleic Acid Double stranded Deoxyribose GATC Long in length Nucleus – always Coding 1 type • • • • • • • • RNA Ribonucleic Acid Single stranded Ribose GAUC Shorter in length Nucleus cytosol Messenger 3 Types Thank you