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BIOLOGY CONCEPTS & CONNECTIONS Fourth Edition Neil A. Campbell • Jane B. Reece • Lawrence G. Mitchell • Martha R. Taylor CHAPTER 10 Molecular Biology of the Gene Modules 10.17 – 10.22 From PowerPoint® Lectures for Biology: Concepts & Connections Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings VIRUSES: GENES IN PACKAGES The Molecular Genetics of Viruses Viral DNA may become part of the host chromosome Phage Attaches to cell Bacterial chromosome Phage DNA Cell lyses, releasing phages Phage injects DNA Many cell divisions Occasionally a prophage may leave the bacterial chromosome 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 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Phage DNA inserts into the bacterial chromosome by recombination Translation of viral genes in Prokaryotes • Prophage genes inserted in the DNA of essentially harmless bacteria cause botulism, diptheria, and scarlet fever. This occurs when prophage genes become active in the host genome. Also, an environmental signal such as radiation or a certain chemical triggers a switchover from the lysogenic cycle to the lytic cycle. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 10.18 Connection: Many viruses cause disease in animals • Many viruses have RNA, rather than DNA, as their genetic material Membranous envelope • RNA viruses include: flu, cold, measles, mumps as well as AIDS and polio. RNA • DNA viruses include: hepatitis, chicken pox, herpes • Not all viruses reproduce in the cytoplasm, some like herpes reproduce in the nucleus. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Protein coat Glycoprotein spike Figure 10.18A Viral Classification 20 - 300 nm, origin not clear because ther is no fossil record. The smallest genomes code for only four proteins and weigh about 106 daltons, while the largest weigh about 108 daltons and code for over one hundred proteins. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Baltimore Classification • IdsDNA viruses • IIssDNA viruses • IIIdsRNA viruses • IV(+)ssRNA viruses • V(-)ssRNA viruses • VIssRNA-RT viruses • VIIdsDNA-RT viruses • ss: single-stranded • ds: double stranded RT: reverse transcribing In taxonomy, the classification of viruses is rather difficult due to the lack of a fossil record and the dispute over whether they are living or non-living. They do not fit easily into any of the domains of biological classification and therefore classification begins at the family rank. However, the domain name of Acytota (without cells) has been suggested. This would place viruses on a par with the other domains of Eubacteria, Archaea, and Eukarya. Not all families are currently classified into orders, nor all genera classified into families. . Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings As an example of viral classification, the chicken pox virus belongs to family Herpesviridae, subfamily Alphaherpesvirinae and genus Varicellovirus. It remains unranked in terms of order. The general structure is as follows: Order (-virales) Family (-viridae) Subfamily (-virinae) Genus (-virus) Species (-virus) The International Committee on Taxonomy of Viruses (ICTV) developed the current classification system. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Some animal viruses steal a bit of the host cell’s membrane (Herpes virus) • Some remain latent unless cell is stressed • How quickly we fight them off with our immune system and how quickly our cells go through mitosis is a factor in how and IF we recover 100% from a viral infection. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Glycoprotein spike VIRUS Protein coat Viral RNA (genome) Plasma membrane of host cell Envelope 1 Entry 2 Uncoating 3 RNA synthesis by viral enzyme Viral RNA (genome) 4 Protein synthesis 5 RNA synthesis (other strand) Template mRNA New viral proteins protein 6 New viral genome Assembly Exit 7 Figure 10.18B 10.19 Connection: Plant viruses are serious agricultural pests • Most plant viruses have RNA – Example: tobacco mosaic disease – These viruses enter damaged plants and spread through the plasmodesmata Protein RNA Figure 10.19 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 10.20 Connection: Emerging viruses threaten human health • The deadly Ebola virus causes hemorrhagic fever (1976, Africa) – Each virus is an enveloped thread of protein-coated RNA • Hantavirus is another enveloped RNA virus caused by a virus infecting rodents (1993) • Viral Encephalitis-inflammation of the brain • West Nile (1999-48 states by 2004) • SARS (severe acute respiratory syndrome) (China 2003-3months 8,450 infected) Figure 10.20A, B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 10.21 The AIDS virus makes DNA on an RNA template • HIV is a retrovirus using reverse transcriptase to make a DNA copy of its genome. Envelope Glycoprotein Protein coat RNA (two identical strands) Reverse transcriptase Figure 10.21A Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • Inside a cell, HIV uses its RNA as a template for making DNA to insert into the host chromosome Viral RNA CYTOPLASM 1 NUCLEUS DNA strand Chromosomal DNA 2 3 Doublestranded DNA Provirus DNA 4 5 RNA Viral RNA and proteins 6 Figure 10.21B Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Spread of Lethal viruses • Viral disease can go from a small population to an epidemic or even pandemic • Lethal viruses are more threatening due to biological and technological factors: international travel, blood transfusions, sexual promiscuity, IV drug abuse, jumping the “species barrier” Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The Molecular Genetics of the Simplest Living Organisms • Bacteria replicate through binary fission • Bacteria can transfer DNA in three ways – Bacteria can transfer genes from cell to cell by one of three processes • Transformation, transduction, or conjugation DNA enters cell Fragment of DNA from another bacterial cell Bacterial chromosome (DNA) Mating bridge Phage Phage Fragment of DNA from another bacterial cell (former phage host) Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Sex pili Donor cell (“male”) Recipient cell (“female”) – Once new DNA gets into a bacterial cell • Part of it may then integrate into the recipient’s chromosome Donated DNA Figure 10.22D Recipient cell’s chromosome Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Crossovers Degraded DNA Recombinant chromosome The ability of E.coli to carry out conjugation • 10.23 Bacterial F factors and plasmids can serve as carriers for gene transfer • F Factor (fertility) is integrated into the host genome • Plasmids are not integrated • Are small circular DNA molecules separate from the bacterial chromosome • Confers antibiotic resistence Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings – Plasmids can serve as carriers • For the transfer of genes F factor (plasmid) F factor (integrated) Male (donor) cell Origin of F replication Bacterial chromosome F factor starts replication and transfer of chromosome Male (donor) cell Bacterial chromosome F factor starts replication and transfer Only part of the chromosome transfers Plasmid completes transfer and circularizes Plasmids Recombination can occur. Cell still female Figure 10.23A–C Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Cell now male Colorized TEM 2,000 Recipient cell 10.22 Virus research and molecular genetics are intertwined • Virus studies help establish molecular genetics • Molecular genetics helps us understand viruses – such as HIV, seen here attacking a white blood cell Figure 10.22 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings