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Chapter 10: The genetic code Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-1 Genes • A single gene provides the genetic instructions for one polypeptide • There is a specific relationship between the DNA sequence of the gene and the amino acid sequence of the protein • The process of converting DNA information into protein molecules is gene expression • The information is deciphered using the genetic code Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-2 RNA • • RNA has the same primary structure as DNA. It consists of a sugar-phosphate backbone, with nucleotides attached to the 1' carbon of the sugar The differences between DNA and RNA are that – RNA has a hydroxyl group on the 2' carbon of the sugar (the difference between deoxyribonucleic acid and ribonucleic acid) – RNA uses the pyrimidine base uracil (U) to pair with adenine (A) – RNA exists as a single-stranded molecule Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-3 Transcription • DNA acts as a template for the synthesis of RNA in a process called transcription • Only one strand of DNA is used as the template • Like DNA replication, transcription proceeds 5’ 3’ on the strand being produced • Nucleotides are added according to the complementary base pairing rules Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-4 Fig. 10.2a: Transcription of DNA by RNA polymerase Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-5 Fig. 10.2b: The structure of RNA polymerase, shown in the act of transcribing DNA to produce RNA Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-6 Initiation • RNA polymerases do not require priming • The polymerase binds just downstream of the promoter • Elongation of the RNA continues by addition of complementary nucleotides until a termination signal is reached • The transcribed region is called a transcription unit with the RNA being a primary transcript Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-7 Fig. 10.4: Formation of the first phosphodiester bond to initiate transcription Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-8 Transcription • Transcription generates three major RNAs – messenger RNA (mRNA) determines the amino acid sequence of the protein during translation – ribosomal RNA (rRNA) is one of the components of the ribosome involved in translation – transfer RNA (tRNA) is a small RNA that can bind an amino acid at one end, and mRNA at the other end. It acts as an adaptor to carry the amino acid elements of a protein to the appropriate place as coded for by the mRNA (cont.) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-9 Transcription (cont.) • In prokaryotes transcription occurs in the cytoplasm • Since many bacterial genes are arranged in operons (Chapter 11) the RNA transcripts are polycistronic • Transcription and translation are often coincident Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-10 Fig. 10.5: Coincident transcription and translation in prokaryotes Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-11 Eukaryotic transcription • Eukaryotic transcription occurs in the nucleus • Eukaryotic genes are usually monocistronic— coding for a single polypeptide • The eukaryotic nucleus has an organelle called the nucleolus which is the site for ribosomal RNA synthesis • The three major classes of RNA are transcribed by different RNA polymerases Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-12 mRNA processing • Both ends of the primary RNA transcript are modified before transport to the cytoplasm – A methyl guanidine cap is added to the 5’ end – A poly adenine or poly A tail is added to the 3’ end of the transcript • One of the most important stages in RNA processing is RNA splicing. In many genes, the DNA sequence coding for proteins, or ‘exons’, may be interrupted by stretches of non-coding DNA, called ‘introns’ (cont.) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-13 mRNA processing (cont.) • • In the cell nucleus, the DNA that includes all the exons and introns of the gene is first transcribed into a complementary RNA copy called 'nuclear RNA’ or nRNA Introns are then removed from nRNA by a process called RNA splicing – splicing occurs at specific sequences on the intron–exon boundaries – exons are joined together – the edited sequence is the final mRNA Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-14 Fig. 10.6: Post-transcriptional processing of eukaryotic mRNA transcripts Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-15 The genetic code • mRNA transfers information to protein in the form of a code defined by a sequence of nucleotide bases • Each amino acid is specified by three nucleotides called a codon • Since RNA is constructed from four types of nucleotides, there are 64 possible codons (4x4x4). • Three of these codons called stop codons specify the termination of the polypeptide chain (cont.) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-16 The genetic code (cont.) • That leaves 61 codons to specify only 20 different amino acids • Therefore, most of the amino acids are represented by more than one codon • Thus, the genetic code is to be degenerate • Particularly in the third nucleotide position, a base change often does not change the amino acid specified Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-17 Fig. 10.7: The genetic code Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-18 Reading frames • • In order for the protein to be synthesised, translation must start and stop correctly The region of the mRNA used to encode the amino acid sequence is called the open reading frame Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-19 Fig. 10.8: The concept of reading frames Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-20 Frame shift mutations • Each mRNA potentially has three reading frames but only one gives the correct sequence for the protein • The reading frame is set by the position of the start codon (AUG) • Within a gene, small deletions or insertions of a number of bases not divisible by 3 will result in a frame shift in the mRNA during translation (cont.) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-21 Fig. 10.9: Frameshift mutations Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-22 Reading frame mutations (cont.) • A nonsense mutation creates a stop codon where none previously existed – this shortens the resulting protein, possibly removing essential regions • A missense mutation changes the code of the mRNA – for example if an AGU is changed to an AGA, the protein will have an arginine instead of serine – the shape or function of the protein may be altered Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-23 Translation • • • During protein synthesis, ribosomes move along the mRNA molecule and read its sequence one codon at a time from the 5' end to the 3' end Each amino acid is specified by the mRNA's codon Codons pair with a sequence of three complementary nucleotides (anticodon) carried by a particular tRNA Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-24 Transfer RNA (tRNA) • • tRNAs have an anticodon at one end and an amino acid at the other They act as adaptor molecules to bring the correct amino acid to the mRNA codon (cont.) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-25 Fig. 10.10: Transfer RNA structure Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-26 Transfer RNA (tRNA) (cont.) • Each tRNA only binds the appropriate amino acid for its anticodon and is recharged after depositing its amino acid into the growing chain Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-27 Fig. 10.11: Activation of RNA by aminoacylation Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-28 Ribosomes • • Consist of two protein subunits, large and small, and associated rRNAs Provide a precise method of aligning codons and tRNAs to ensure amino acids are synthesised in the correct order Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-29 Fig. 10.12: Ribosomes Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-30 Polypeptide synthesis • Initiation – the ribosomes form from subunits – tRNA methionine binds to start codon – Large subunit of ribosome binds to form A and P sites (cont.) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-31 Fig. 10.14: The three phases of protein synthesis Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-32 Polypeptide synthesis (cont.) • Elongation – begins with the formation of a peptide bond between the methionine and the second amino acid – the process involves the addition of a sequence of amino acids, specified by the codons – as each new amino acid is brought into position, a peptide bond is formed with the preceding amino acid – translation proceeds 5’ to 3’ along the mRNA – the peptide is synthesised from the amino (NH2) end to the carboxyl (COOH) end (cont.) Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-33 Polypeptide synthesis (cont.) • Termination – when the ribosome arrives at a stop codon the elongation process stalls because there is no tRNA for stop codons – termination factors remove the last amino acid from its tRNA – the ribosome separates into its two subunits and leaves the mRNA Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-34 Fig. 10.15a: Initiation Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-35 Fig. 10.15b: Elongation Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-36 Fig 10.15c: Termination Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-37 Protein processing • • • • Polypeptide synthesis is only the first step in the production of a mature protein The protein may be further modified by the addition of chemical residues Proteins are targeted to particular organelles by the addition of signal sequences which bind to receptors at the correct location Proteins for secretion are sorted and packaged in the Golgi apparatus Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-38 Fig. 10.16: Pathways of targeting in eukaryotic cells Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint 10-39