Chapter 10 Molecular Biology of the Gene Sabotage Inside Our Cells • Viruses are biological saboteurs – Hijack the genetic material of host cells in order to reproduce themselves – May remain permanently dormant in the body • Viruses share some characteristics of living organisms but are not generally considered alive – Genetic material composed of nucleic acid PowerPoint Lectures for Biology: Concepts and Connections, Fifth Edition – Campbell, Reece, Taylor, and Simon – Not cellular – Cannot reproduce on their own Lectures by Chris Romero Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings THE STRUCTURE OF THE GENETIC MATERIAL 10.1 Experiments showed that DNA is the genetic material • "Transforming factor" postulated in 1928 by Frederick Griffith • Hershey-Chase experiments in 1952 determined that the heredity material was DNA not protein • First understanding of DNA based on viruses Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 10-1b Phage Bacterium DNA Batch 1 Radioactive protein Mix radioactively labeled phages with bacteria. The phages infect the bacterial cells. – Studied the simple bacteriophage T2 – Showed that the virus injects its DNA into host cells and reprograms them to produce more viruses Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Radioactive protein Batch 2 Radioactive DNA Empty protein shell Radioactivity in liquid Phage DNA Centrifuge Agitate in a blender to separate phages outside the bacteria from the cells and their contents. Pellet Centrifuge the mixture Measure the so bacteria form a radioactivity in pellet at the bottom of the pellet and the test tube. the liquid. Radioactive DNA Centrifuge Pellet Radioactivity in pellet LE 10-1c 10.2 DNA and RNA are polymers of nucleotides • Nucleic acids are polynucleotides made of long chains of nucleotide monomers – Nitrogenous bases Phage attaches to bacterial cell. Phage injects DNA. Phage DNA directs host cell to make more phage DNA and protein parts. New phages assemble. • Single-ring pyrimidines: thymine (T), cytosine ( C) Cell lyses and releases new phages. • Double-ring purines: adenine (A), guanine (G) – Sugar-phosphate backbone Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 10-2a • DNA and RNA are identical except for two things Sugar-phosphate backbone Phosphate group A – Nitrogenous bases • DNA: A, C, G, T • RNA: A, G, C, U C Nitrogenous base Sugar DNA nucleotide Animation: DNA and RNA Structure Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phosphate group Nitrogenous base (A, G, C, or T) Thymine (T) G • RNA: ribose C T T – Sugars • DNA: deoxyribose A G T T Sugar (deoxyribose) DNA nucleotide DNA polynucleotide LE 10-2b LE 10-2c Nitrogenous base (A, G, C, or U) Phosphate group Uracil (U) Cytosine (C) Thymine (T) Adenine (A) Guanine (G) Purines Pyrimidines Sugar (ribose) 10.3 DNA is a double-stranded helix • James Watson and Francis Crick worked out the three-dimensional structure of DNA, based on X-ray crystallography by Rosalind Franklin • DNA consists of two polynucleotide strands wrapped around each other in a double helix – Sugar-phosphate backbones are on the outside and nitrogenous bases on the inside – Each base pairs with a complementary partner • A with T, and G with C – Hydrogen bonds between the bases hold the strands together • The Watson-Crick model of DNA suggested a molecular explanation for genetic inheritance Animation: DNA Double Helix Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 10-3c LE 10-3d C G T A Base pair C Hydrogen bond T A T G C G A T A C G C G T C A G A T A T G A T A T A C T Ribbon model Computer model Partial chemical structure Twist DNA REPLICATION LE 10-4a 10.4 DNA replication depends on specific base pairing • The Watson-Crick model of DNA structure suggested a mechanism for its replication – DNA strands separate – Enzymes use each strand as a template to assemble new nucleotides into complementary strands • The mechanism of DNA replication is semiconservative – Each new double helix consists of one old and one new strand Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A T A C G C G C G A T A T A T Parental molecule of DNA T G C A C A Nucleotides Both parental strands serve as templates T A T A T G C G C G C G C G C T A T A T A T A T A Two identical daughter molecules of DNA LE 10-4b G C • DNA replication is a complex process A T A T C T G C A T T Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 10-5a Origin of replication Parental strand Daughter strand • DNA replication begins at specific sites (origins of replication) on the double helix – Proteins attach and separate the strands Bubble – Replication proceeds in both directions, creating replication bubbles • Parent strands open, daughter strands elongate – Replication occurs simultaneously at many sites Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings T C A T Animation: DNA Replication Overview 10.5 DNA replication: A closer look A T A A A A T T T T G A C T G C G G G C G C G G C C A C G A T A G C A – Some of the helical DNA molecule must untwist Two daughter DNA molecules LE 10-5b 3¢ end 5¢ end • DNA's sugar-phosphate backbones are oriented in opposite directions P 4¢ 3¢ – The enzyme DNA polymerase adds nucleotides at only the 3’ end HO 5¢ 2¢ 1¢ 2¢ A P P C P P T • The two strands are connected by the enzyme DNA ligase 4¢ G P G • The other strand is synthesized as a series of short pieces 3¢ 1¢ 5¢ P C • One daughter strand is synthesized as a continuous piece T OH 3¢ end A P 5¢ end Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 10-5c DNA polymerase molecule THE FLOW OF GENETIC INFORMATION FROM DNA TO RNA TO PROTEIN 3¢ 5¢ 5¢ Daughter strand synthesized continuously Parental DNA 3¢ 3¢ 5¢ Daughter strand synthesized In pieces 10.6 The DNA genotype is expressed as proteins, which provide the molecular basis for phenotypic traits • The information constituting an organism's genotype is carried in its sequence of DNA bases • A particular gene—a linear sequence of many nucleotides—specifies a particular polypeptide 5¢ 3¢ DNA ligase Overall direction of replication Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The flow of genetic information 1. Transcription of the genetic information in DNA into RNA 2. Translation of RNA into the polypeptide • Beadle-Tatum one gene-one enzyme hypothesis – Studies of inherited metabolic disorders in mold suggested that phenotype is expressed through proteins 10.7 Genetic information written in codons is translated into amino acid sequences • Genetic information flows from DNA to RNA to protein • Nucleotide monomers represent letters in an alphabet that can form words in a language – Triplet code – A gene dictates production of a specific enzyme • Three-letter words (codons) – The hypothesis has been restated to one geneone polypeptide • Each word codes for one amino acid in a polypeptide Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 10-7a DNA molecule Gene 1 • The genetic code specifies the correspondence between RNA codons and amino acids in proteins Gene 2 Gene 3 – Includes start and stop codons DNA strand A A A C C G G C A A A A U U U G G C C G U U U U Transcription RNA 10.8 The genetic code is the Rosetta stone of life – Redundant but not ambiguous • Nearly all organisms use exactly the same genetic code Codon Translation Polypeptide Amino acid Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 10-8a LE 10-8b Strand to be transcribed Second base UUU UUC U UUA First base UAC U UGU Tyr UGC Cys Ser C UAA Stop UGA Stop A UAG Stop UGG Trp G CUU CCU CAU CUC CCC CAC CUA Leu AUU AUC lle AUA AUG GUC GUA GUG CCA Val Pro CAA CCG CAG ACU AAU ACC AAC ACA Met or start GUU G UAU UCC UCA Leu CUG A UCU G UCG UUG C Phe A His Gln U CGU C CGA Arg U AGU Asn Lys AGC AGA Ser Arg A AAG AGG G GCU GAU GGU U GCC GAC GCA Ala GCG GAA GGA GGG Gly • One DNA strand serves as a template for the new RNA strand • RNA polymerase constructs the RNA strand in a multistep process – Initiation • RNA polymerase attaches to the promotor Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings T C A A A A T C A T G A A G T T T T A G U A G A U G A A G U U U RNA Stop codon Start codon Translation A G 10.9 Transcription produces genetic messages in the form of RNA • Synthesis starts T C GGC Glu GAG C Transcription C ACG Asp A A G CGG Thr AAA CGU T DNA Third base C U Polypeptide Met Lys Phe • Elongation: – RNA synthesis continues – RNA strand peels away from DNA template – DNA strands come back together in transcribed region • Termination – RNA polymerase reaches a terminator sequence at the end of the gene – Polymerase detaches Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 10-9a LE 10-9b RNA polymerase RNA nucleotides RNA polymerase DNA of gene Promoter DNA Terminator DNA Initiation A C T C A A T T T C U G A C G A T G G A G U C U T C A C T A A Termination Direction of transcription Area shown In Figure 10.9A Elongation C Template strand of DNA Growing RNA Completed RNA Newly made RNA 10.10 Eukaryotic RNA is processed before leaving the nucleus • The RNA that encodes an amino acid sequence is messenger RNA (mRNA) • In prokaryotes, transcription and translation both occur in the cytoplasm • RNA Splicing – Noncoding segments called introns are cut out – Remaining exons are joined to form a continuous coding sequence – A cap and a tail are added to the ends • In eukaryotes, RNA transcribed in the nucleus is processed before moving to the cytoplasm for translation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings RNA polymerase Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 10-10 Exon Intron Exon Intron Exon DNA 10.11 Transfer RNA molecules serve as interpreters during translation Transcription Addition of cap and tail Cap RNA transcript Introns removed with cap and tail Tail • Transfer RNA (tRNA) molecules match the right amino acid to the correct codon • tRNA is a twisted and folded single strand of RNA Exons spliced together – Anticodon loop at one end recognizes a particular mRNA codon by base pairing mRNA Coding sequence Nucleus – Amino acid attachment site is at the other end • Each amino acid is joined to the correct tRNA by a specific enzyme Cytoplasm Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 10-11a LE 10-11b Amino acid attachment site Amino acid attachment site Hydrogen bond RNA polynucleotide chain Anticodon Anticodon LE 10-12a Growing polypeptide tRNA molecules 10.12 Ribosomes build polypeptides • A ribosome consists of two subunits Large subunit – Each is made up of proteins and ribosomal RNA (rRNA) • The subunits of a ribosome – Hold the tRNA and mRNA close together in binding sites during translation – Allow amino acids to be connected into a polypeptide chain mRNA Small subunit Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 10-12b tRNA-binding sites LE 10-12c Next amino acid to be added to polypeptide Large subunit Growing polypeptide tRNA mRNA binding site mRNA Codons Small subunit 10.13 An initiation codon marks the start of an mRNA message • Initiation is a two-step process – Step 1 • The initiation phase of translation • mRNA binds to a small ribosomal subunit – Brings together mRNA, a specific tRNA, and the two subunits of a ribosome • Initiator tRNA, carrying the amino acid Met, binds to the start codon – Establishes exactly where translation will begin – Step 2 • A large ribosomal subunit binds to the small one, forming a functional ribosome • Ensures that mRNA codes are translated in the correct sequence Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Initiator tRNA fits into one binding site; the other is vacant for the next tRNA Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 10-13a LE 10-13b Start of genetic message Met Met Large Ribosomal subunit Initiator tRNA P site U A C AUG Start codon mRNA End A site U A C AUG Small ribosomal subunit LE 10-14 Amino acid Polypeptide 10.14 Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation • A site P site Anticodon mRNA Codons Condon recognition Once initiation is complete, amino acids are added one by one in a three-step elongation process mRNA movement Stop codon 1. Codon recognition Peptide bond formation 2. Peptide bond formation New peptide bond 3. Translocation • Elongation continues until a stop codon reaches the ribosome's A site, terminating translation Translocation Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 10-15 Transcription DNA 10.15 Review: The flow of genetic information in the cell is DNA ® RNA ® protein • The sequence of codons in DNA, via the sequence of codons in RNA, spells out the primary structure of a polypeptide mRNA RNA polymerase Translation Amino acid Enzyme ATP Anticodon Initiator tRNA Start Codon mRNA Large Initiation of ribosomal polypeptide synthesis subunit The mRNA, the first tRNA, and the ribosomal Sub units come together. Small ribosomal subunit New peptide bond forming Growing polypeptide 2. Attachment of amino acid to tRNA 3. Initiation of polypeptide synthesis Each amino acid attaches to its proper tRNA with the help of a specific enzyme and ATP. tRNA U AC AU G 1. Transcription of mRNA from a DNA template mRNA is transcribed from a DNA template. Codons mRNA Elongation 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. Polypeptide 4. Elongation 5. Termination Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Termination Stop codon The ribosome recognizes a stop codon. The polypeptide is terminated and released. LE 10-16a 10.16 Mutations can change the meaning of genes Normal hemoglobin DNA • Mutation: any change in the nucleotide sequence of DNA – Caused by errors in DNA replication or recombination, or by mutagens T C Mutant hemoglobin DNA T mRNA A T G U A mRNA A G – Can involve large regions of a chromosome or a single base pair C A Normal hemoglobin Sickle-cell hemoglobin Glu Val – Can cause many genetic diseases, such as sickle-cell disease Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 10-16b Normal gene • Two general categories of genetic mutations – Base substitutions replace one base with another • Most are harmful but may occasionally have no effect or be beneficial – Base insertions or deletions alter the reading frame • Result is most likely a nonfunctioning polypeptide • Mutagenesis caused by spontaneous error or a physical or chemical mutagen Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A U G A A G U U U G G C G C A mRNA Protein Met Lys Phe Gly Ala Base substitution A U G A Met A G U Lys G A Phe Base deletion A U U C G Ser C A Ala U Missing U Met G A A Lys G U U Leu G G C Ala G C A His U MICROBIAL GENETICS – Lytic cycle 10.17 Viral DNA may become part of the host chromosome • Host produces more viruses • Viruses are infectious particles consisting of nucleic acid enclosed in a protein capsid • Host cell lyses (breaks open) to release new viruses • Viruses depend on their host cells for the replication, transcription, and translation of their nucleic acid – DNA enters host bacterium, circularizes, and enters one of two pathways Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 10-17 – Lysogenic cycle Phage • Phage DNA inserted by recombination into the host chromosome; is now a prophage • Prophages replicated each time host cell divides; passed on to generations of daughter cells • Does not destroy host • Environmental signal may trigger switch from lysogenic to lytic cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Attaches to cell Phage DNA Cell lyses, releasing phages Bacterial chromosome Phage injects DNA Many cell divisions Lytic cycle Phages assemble Lysogenic cycle Phage DNA circularizes Prophage Lysogenic bacterium reproduces normally, replicating the prophage at each cell division OR New phage DNA and proteins are synthesized Phage DNA inserts into the bacterial chromosome by recombination LE 10-18a CONNECTION Membranous envelope 10.18 Many viruses cause disease in animals • Structure of a virus that invades animal cells – Genetic material may be RNA (examples: flu, HIV) or DNA (examples: hepatitis, herpes) RNA – Protein coat Protein coat – Sometimes a membranous envelope with glycoprotein spikes • The envelope helps the virus enter and leave the host cell during its reproductive cycle Glycoprotein spike Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 10-18b VIRUS Viral RNA (genome) Plasma membrane of host cell Glycoprotein spike Protein coat Envelope Entry CONNECTION 10.19 Plant viruses are serious agricultural pests • Most plant viruses Uncoating Viral RNA (genome) Protein synthesis RNA synthesis by viral enzyme RNA synthesis (other strand) mRNA New viral proteins Template Assembly New viral genome – Have RNA genomes – Enter their hosts via wounds in the plant's outer layers – May spread throughout the plant through plasmodesmata • There is no cure for most plant viruses Exit Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CONNECTION 10.20 Emerging viruses threaten human health • Emerging viruses have appeared suddenly or have recently come to the attention of scientists 10.21 The AIDS virus makes DNA on an RNA template • HIV, the AIDS virus, is a retrovirus – Flow of genetic information is RNA _ DNA – Examples: HIV, SARS, Ebola, West Nile – Inside a cell, HIV uses its RNA as a template for making DNA • Processes contributing to emergence – Mutation – The enzyme reverse transcriptase catalyzes reverse transcription – Contact between species – Spread from isolated populations Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 10-21a Animation: HIV Reproductive Cycle Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 10-21b Envelope Glycoprotein Protein coat RNA (two identical strands) Reverse transcriptase Viral RNA CYTOPLASM NUCLEUS DNA strand Chromosomal DNA Doublestranded DNA Viral RNA and proteins Provirus DNA RNA LE 10-22a DNA enters cell 10.22 Bacteria can transfer DNA in three ways • Bacteria can transfer genes from cell to cell by one of three processes – Transformation: the uptake of foreign DNA from the surrounding environment Fragment of DNA from another bacterial cell – Transduction: transfer of bacterial genes by a phage – Conjugation: union of two bacterial cells and the transfer of DNA between them Bacterial chromosome (DNA) Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 10-22b Phage LE 10-22c Mating bridge Sex pili Fragment of DNA from another bacterial cell (former phage host) Donor cell (“male”) Recipient cell (“female”) LE 10-22d • Once new DNA is in a bacterial cell, part of it may integrate into the recipient's chromosome Donated DNA Crossovers Degraded DNA – Occurs by crossing over between the two molecules – Leaves the recipient with a recombinant chromosome Recipient cell’s chromosome Recombinant chromosome Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 10-23a 10.23 Bacterial plasmids can serve as carriers for gene transfer • The F factor is a piece of bacterial DNA – Carries genes for things needed for conjugation F factor (integrated) Male (donor) cell Origin of F replication Bacterial chromosome F factor starts replication and transfer of chromosome Recipient cell – Contains an origin of replication – Can transfer chromosomal DNA by integrating into the donor bacterium's chromosome or entering the cell as a plasmid Only part of the chromosome transfers Recombination can occur Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 10-23b F factor (plasmid) Male (donor) cell Bacterial chromosome F factor starts replication and transfer • Plasmids – Small circular DNA molecules separate from the bacterial chromosome – Can serve as carriers for the transfer of genes Plasmid completes transfer and circularizes Cell now male Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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