Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Unit 4 Proteins Transcription (DNA to mRNA) Translation (mRNA to tRNA to proteins) Gene expression/regulation (turning genes on and off) Viruses 1

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Yesterday’s Exit Ticket Eukaryotic genes all have enhancers, made up of control elements. Specific combinations of specialized transcription factors (called activators) bind to the control elements and help RNA polymerase initiate transcription. Without that specific combination of activators, a gene won’t be transcribed. 2

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Today’s Plan Viruses – What are viruses? – General viral reproduction – Host specificity – Variety in viral reproduction Exam 4 Review 3

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Today’s Plan Viruses – What are viruses? – General viral reproduction – Host specificity – Variety in viral reproduction Exam 4 Review 4

Fig µm Viruses are not cells and are not alive Dependent on host cells to reproduce

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Structure of Viruses Consist of nucleic acid enclosed in a protein coat (capsid) Some viruses are surrounded by a membranous envelope DNA Capsid RNA DNA Membranous envelope RNA Capsid 6

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Viral Genomes Viral genomes can be: – Double-stranded DNA – Single-stranded DNA – Double-stranded RNA – Single-stranded RNA 7

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Bacteriophages, also called phages, are viruses that infect bacteria Phages have an elongated capsid head that encloses their DNA A protein tail piece attaches the phage to the host and injects the phage DNA inside 8

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Today’s Plan Viruses – What are viruses? – General viral reproduction – Host specificity – Variety in viral reproduction Exam 4 Review 9

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Viruses reproduce only in host cells Each virus has a limited number of host cell types that it can infect 10

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings General Features of Viral Reproductive Cycles Viruses insert their genomes into a host cell The cell manufactures viral proteins The virus uses host enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules Viral nucleic acids and proteins spontaneously self- assemble into new viruses Basic Virus Reproduction Animation 11

Fig Attachment 1

Fig Entry of phage DNA and degradation of host DNA Attachment 1 2

Fig Synthesis of viral genomes and proteins Entry of phage DNA and degradation of host DNA Attachment 1 2 3

Fig Phage assembly Assembly Synthesis of viral genomes and proteins Entry of phage DNA and degradation of host DNA Attachment HeadTailTail fibers 3

Fig Phage assembly HeadTailTail fibers Assembly Release Synthesis of viral genomes and proteins Entry of phage DNA and degradation of host DNA Attachment

Transcription and manufacture of capsid proteins Self-assembly of new virus particles and their exit from the cell Entry and uncoating Fig VIRUS DNA Capsid 4 Replication HOST CELL Viral DNA mRNA Capsid proteins Viral DNA

Today’s Plan Viruses –What are viruses? –General viral reproduction –Host specificity –Variety in viral reproduction Exam 4 Review

Viral Envelopes Many animal viruses have a membranous envelope Membranous envelope RNA Capsid (c) Influenza viruses

Viral Envelopes Viruses use cellular communication mechanisms to their advantage galaxywire.net Cell-cell recognition

Viral Envelopes Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cell Membranous envelope RNA Capsid Glycoproteins Cell-cell recognition Viruses imitate identifiers on the outside of host cells to “trick” cells into letting them attach

Viral Envelopes Many viral envelopes are formed from the host cell’s plasma membrane as the viral capsids exit

Fig Capsid RNA Envelope (with glycoproteins) Capsid and viral genome enter the cell HOST CELL Viral genome (RNA) Template mRNA ER Glyco- proteins Capsid proteins Copy of genome (RNA) New virus

Today’s Plan Viruses –What are viruses? –General viral reproduction –Host specificity –Variety in viral reproduction Exam 4 Review

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Nucleic Acids and Viral Reproduction The type of nucleic acid in a virus determines how it reproduces DNA transcribed translated RNAtranslated directly as mRNA copied to produce mRNA translated reverse transcribed into DNA 25

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings RNA as Viral Genetic Material Retroviruses use reverse transcriptase to copy their RNA genome into DNA HIV (human immunodeficiency virus) is a retrovirus that causes AIDS (acquired immunodeficiency syndrome) – HIV infects helper t-cells, and others, compromising the immune system Ebay.com 26

Fig. 19-8a Glycoprotein Reverse transcriptase HIV RNA Capsid Viral envelope HOST CELL Reverse transcriptase Viral RNA RNA-DNA hybrid DNA NUCLEUS Provirus Chromosomal DNA RNA genome for the next viral generation mRNA New virus Because reverse transcriptase doesn’t exist naturally in animal cells, the virus brings it to the party

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The viral DNA that is integrated into the host genome is called a provirus The provirus remains a permanent resident of the host cell HIV Reproduction Animation 28

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings So what did doctors really do to save Emma? 29

Today’s Exit Ticket What are the key parts of a virus? What extra component do retroviruses contain?

Today’s Plan Viruses –What are viruses? –General viral reproduction –Host specificity –Variety in viral reproduction Exam 4 Review