CHAPTER 10 Molecular Biology of the Gene

1. CHAPTER 10Molecular Biology of the Gene Modules 10.17 – 10.22

2. VIRUSES: GENES IN PACKAGES The Molecular Genetics of Viruses Viral DNA may become part of the host chromosome Phage Attachesto cell Bacterialchromosome Phage DNA Cell lyses,releasing phages Phage injects DNA Many celldivisions Occasionally a prophagemay leave the bacterialchromosome LYTIC CYCLE LYSOGENIC CYCLE Phagesassemble Phage DNAcircularizes Lysogenic bacteriumreproduces normally,replicating the prophageat each cell division Prophage OR New phage DNA andproteins are synthesized Phage DNA inserts into the bacterialchromosome by recombination

3. 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.

4. 10.18 Connection: Many viruses cause disease in animals Membranousenvelope • Many viruses have RNA, rather than DNA, as their genetic material • RNA viruses include: flu, cold, measles, mumps as well as AIDS and polio. • DNA viruses include: hepatitis, chicken pox, herpes • Not all viruses reproduce in the cytoplasm, some like herpes reproduce in the nucleus. RNA Proteincoat Glycoprotein spike Figure 10.18A

5. 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. • 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 strandedRT: reverse transcribing

6. 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. .

7. 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.

8. Glycoprotein spike VIRUS Protein coat Viral RNA(genome) Envelope Plasmamembraneof hostcell Entry 1 Uncoating 2 • 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. Viral RNA(genome) RNA synthesisby viral enzyme 3 Proteinsynthesis RNA synthesis(other strand) 5 4 mRNA Template New viral genome Newviral protein Newviral proteins Assembly 6 Exit 7 Figure 10.18B

9. 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

10. 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

11. 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 Proteincoat RNA(two identicalstrands) Reversetranscriptase Figure 10.21A

12. Viral RNA CYTOPLASM 1 NUCLEUS • Inside a cell, HIV uses its RNA as a template for making DNA to insert into the host chromosome DNAstrand ChromosomalDNA 2 3 ProvirusDNA Double-strandedDNA 4 5 RNA ViralRNAandproteins 6 Figure 10.21B

13. 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”

14. The Molecular Genetics of the Simplest Living Organisms Mating bridge DNA enters cell Phage Phage Fragment of DNAfrom anotherbacterial cell Fragment of DNA fromanotherbacterial cell(former phagehost) Sex pili Bacterial chromosome (DNA) Recipient cell(“female”) Donor cell(“male”) • 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

15. Degraded DNA Crossovers Donated DNA Recombinantchromosome Recipient cell’schromosome Figure 10.22D • Once new DNA gets into a bacterial cell • Part of it may then integrate into the recipient’s chromosome

16. 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

17. F factor (plasmid) F factor (integrated) Male (donor) cell Male (donor) cell Origin of F replication Bacterial chromosome Bacterial chromosome F factor starts replication and transfer F factor starts replication and transfer of chromosome Recipient cell Plasmid completes transferand circularizes Only part of the chromosome transfers Plasmids Colorized TEM 2,000 Recombination can occur. Cell still female Cell now male Figure 10.23A–C • Plasmids can serve as carriers • For the transfer of genes

18. 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