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Chapter 10 Molecular Biology of the Gene PowerPoint Lectures for Biology: Concepts & Connections, Sixth Edition Campbell, Reece, Taylor, Simon, and Dickey Copyright © 2009 Pearson Education, Inc. What do you know? 1. What are the two types of nucleic acids? 2. What is the role of each nucleic acid in the cell? 3. What is the structure of a nucleic acid – be specific. 4. What are 3 examples of proteins found in the body? What are their functions? 5. How are proteins made in the body? THE STRUCTURE OF THE GENETIC MATERIAL DNA DNA vs. RNA Copyright © 2009 Pearson Education, Inc. DNA and RNA are polymers of nucleotides The monomer unit of DNA and RNA is the nucleotide, containing: 1. Nitrogenous base 2. 5-carbon sugar 3. Phosphate group Called polynucleotides – many nucleotides joined together Sugar-phosphate backbone – covalent bonds Copyright © 2009 Pearson Education, Inc. Sugar-phosphate backbone Phosphate group Nitrogenous base Sugar Nitrogenous base (A, G, C, or T) DNA nucleotide Phosphate group Thymine (T) Sugar (deoxyribose) DNA nucleotide DNA polynucleotide In DNA, there are four different nitrogen bases: Thymine (T) Cytosine (C) Pyrimidines Adenine (A) Guanine (G) Purines Nitrogenous base (A, G, C, or U) Phosphate group In RNA, Uracil (U) Instead of thymine, they have the base uracil Sugar (ribose) DNA is a double-stranded helix Watson and Crick deduced the secondary structure of DNA, with data from Rosalind Franklin and Maurice Wilkins Copyright © 2009 Pearson Education, Inc. DNA Structure DNA is 2 polynucleotide strands bonded together through hydrogen bonds between nitrogen bases Twisted into a helical shape Sugar-phosphate on outside Nitrogen bases on inside Base pair rules - adenine always pairs with thymine 2 H+ bonds - Guanine always pairs with cytosine 3 H+ bonds Twist Hydrogen bond Base pair Ribbon model Partial chemical structure Computer model DNA REPLICATION Copyright © 2009 Pearson Education, Inc. DNA replication depends on specific base pairing DNA replication follows a semiconservative model – The two DNA strands separate – Each strand is used as a pattern to produce a complementary strand, using specific base pairing – Each new DNA helix has one old strand with one new strand Copyright © 2009 Pearson Education, Inc. Parental molecule of DNA Nucleotides Parental molecule of DNA Both parental strands serve as templates Nucleotides Parental molecule of DNA Both parental strands serve as templates Two identical daughter molecules of DNA DNA replication proceeds in two directions at many sites simultaneously DNA replication begins at the origins of replication 1. Helicase (enzyme) unwinds DNA at the origin to produce a “bubble” by breaking hydrogen bonds between nitrogen bases 2. DNA polymerase brings in nucleotides to the existing DNA strands (using complimentary base pairing) and replication proceeds in both directions from the origin - one chain is continuous - one chain is brought in pieces 3. DNA ligase joins pieces on non-continuous strand 4. Replication ends when products from the bubbles merge with each other Copyright © 2009 Pearson Education, Inc. Origin of replication Parental strand Daughter strand Bubble Two daughter DNA molecules 5 end P 5 4 3 2 1 P 3 end 2 3 1 4 5 P P P P P P 3 end 5 end DNA polymerase molecule 5 3 3 5 Daughter strand synthesized continuously Parental DNA 3 5 5 3 DNA ligase Overall direction of replication Daughter strand synthesized in pieces THE FLOW OF GENETIC INFORMATION FROM DNA TO RNA TO PROTEIN Copyright © 2009 Pearson Education, Inc. The DNA genotype is expressed as proteins, which provide the molecular basis for phenotypic traits A gene - sequence of DNA that directs the synthesis of a specific protein – DNA is transcribed into RNA – RNA is translated into protein The presence and action of proteins determine the phenotype of an organism Copyright © 2009 Pearson Education, Inc. DNA Transcription RNA Nucleus Cytoplasm Translation Protein Genetic information written in codons is translated into amino acid sequences The sequence of nucleotides in DNA provides a code for constructing a protein – Protein construction requires a conversion of a nucleotide sequence amino acid sequence – Transcription rewrites DNA code into RNA – Three nucleotides on RNA is a codon – Translation takes codons and makes them into amino acids – Each amino acid is specified by a codon – 64 codons are possible – Some amino acids have more than one possible codon Copyright © 2009 Pearson Education, Inc. 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 The genetic code is the Rosetta stone of life Characteristics of the genetic code – Triplet (codon): 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. Third base First base Second base Strand to be transcribed DNA Transcription RNA Start codon Polypeptide Translation Stop codon Transcription produces genetic messages in the form of RNA Transcription – making DNA into RNA; occurs in nucleus 1. Initiation – RNA polymerase binds to region called promoter 2. Elongation – RNA polymerase brings in RNA nucleotides complimentary to DNA strand - 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 3. Termination – RNA polymerase reaches terminator (bases on DNA say “stop”) 4. Polymerase detaches, DNA is joined back together 5. RNA leaves the nucleus (after processing) Copyright © 2009 Pearson Education, Inc. 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 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 one end: single guanine nucleotide – Tail added to other end – 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 Transfer RNA molecules serve as interpreters during translation Amino acid attachment site Transfer RNA (tRNA) match an amino acid to its corresponding mRNA codon – structure allows it to convert one language to the other – An amino acid attachment site allows each tRNA to carry a specific amino acid – An anticodon allows the tRNA to bind to a specific mRNA codon, complementary in sequence – A pairs with U, G pairs with C Copyright © 2009 Pearson Education, Inc. Anticodon Translation - Ribosomes build polypeptides Translation - occurs on the surface of the ribosome - mRNA used to make amino acid chains/polypeptides – Ribosomes have two subunits: small and large – Each subunit is composed of ribosomal RNAs and proteins – subunits come together during translation – have binding sites for mRNA and tRNAs Copyright © 2009 Pearson Education, Inc. tRNA molecules Growing polypeptide Large subunit mRNA Small subunit tRNA-binding sites Large subunit mRNA binding site Small subunit Next amino acid to be added to polypeptide Growing polypeptide tRNA mRNA Codons Translation occurs in steps 1. Initiation occurs in two steps 1. mRNA binds to a small ribosomal subunit, and the first tRNA binds to mRNA at the start codon – The start codon reads AUG – The first tRNA has the anticodon UAC, brings in methionine 2. A large ribosomal subunit joins the small subunit, allowing the ribosome to function Start End Copyright © 2009 Pearson Education, Inc. Large ribosomal subunit Initiator tRNA 1 mRNA Start codon Small ribosomal subunit 2 Translation occurs in steps 2. Elongation - addition of amino acids to the polypeptide chain Each cycle of elongation has three steps 1. Codon recognition: next tRNA binds to the mRNA 2. Peptide bond formation: joining of the new amino acid to the chain 3. Translocation: tRNA is released from the ribosome and moves down the length of the mRNA molecule - continues until ribosomes reacher STOP codon on mRNA Copyright © 2009 Pearson Education, Inc. Translation occurs in steps 3. Termination – The completed polypeptide is released – The ribosomal subunits separate – mRNA is released and can be translated again Copyright © 2009 Pearson Education, Inc. Amino acid Polypeptide A site P site Anticodon mRNA Codons 1 Codon recognition mRNA movement Stop codon 2 Peptide bond formation New peptide bond 3 Translocation Review: The flow of genetic information in the cell is DNA RNA protein Does translation represent: – DNA RNA or RNA protein? Where does the information for producing a protein originate: – DNA or RNA? Which one has a linear sequence of codons: – rRNA, mRNA, or tRNA? Which one directly influences the phenotype: – DNA, RNA, or protein? Copyright © 2009 Pearson Education, Inc. Transcription DNA mRNA RNA polymerase Amino acid 1 mRNA is transcribed from a DNA template. Translation 2 Each amino acid attaches to its proper tRNA with the help of a specific enzyme and ATP. Enzyme ATP tRNA Anticodon Large ribosomal subunit Initiator tRNA 3 Initiation of polypeptide synthesis The mRNA, the first tRNA, and the ribosomal sub-units come together. Start Codon mRNA Small ribosomal subunit New peptide bond forming Growing polypeptide 4 Elongation Codons A succession of tRNAs add their amino acids to the polypeptide chain as the mRNA is moved through the ribosome, one codon at a time. mRNA Polypeptide 5 Termination Stop codon The ribosome recognizes a stop codon. The polypeptide is terminated and released. Mutations can change the meaning of genes A mutation is a change in the nucleotide sequence of DNA – Base substitutions: replacement of one nucleotide with another – Effect depends on whether there is an amino acid change that alters the function of the protein – Deletions or insertions – Alter the reading frame of the mRNA, so that nucleotides are grouped into different codons – Lead to significant changes in amino acid sequence downstream of mutation – Cause a nonfunctional polypeptide to be produced Copyright © 2009 Pearson Education, Inc. 10.16 Mutations can change the meaning of genes Mutations can be – Spontaneous: due to errors in DNA replication or recombination – Induced by mutagens – High-energy radiation – Chemicals Copyright © 2009 Pearson Education, Inc. Normal hemoglobin DNA Mutant hemoglobin DNA mRNA mRNA Normal hemoglobin Sickle-cell hemoglobin Glu Val Normal gene mRNA Protein Met Lys Phe Gly Ala Lys Phe Ser Ala Base substitution Met Base deletion Met Missing Lys Leu Ala His