Viral Replication EK 3C3: Viral replication results in genetic variation and viral infection can introduce genetic variation into the hosts.

Ways to describe viruses Genes packaged in protein coats Not alive but exist in a shady area between life-forms and chemicals Viruses lead “a kind of borrowed life” Packaged sets of genes in transit from one host cell to another Obligate intracellular parasites

What types of cells can a virus infect? Viruses can infect all types of cells: – Plant, animal, fungal, protista, bacteria… Although viruses can infect different types of cells they have a host range – a limited number of host it can infect Some viruses have a wide host range while others have a very narrow host range Bacteriophages: viruses that infect bacteria; aka phages

Structure of a Virus Every virus has: – Genome – Protein coat Some viruses have: – Viral envelope – Viral enzymes

Viral Genome Can be in the form of DNA or RNA – DNA Double stranded DNA Single stranded DNA – RNA Double stranded RNA Single stranded RNA Surrounded by a protein coat

Protein Coat The protein coat that surrounds the virus is called a capsid – protein shell that encloses the viral genome

Viral Envelopes membranous structure found in some viruses surrounds the capsid of a virus helps virus infect host by disguising itself as host cell contains glycoproteins for recognition – Protein with carbohydrates attached; found in membranes; serves as identification tags that are specifically recognized by membrane proteins of other cells derived from host cell membrane

Viral Enzyme Proteins needed by the virus to reproduce that are not made by the host cell Typically found in viruses that contain RNA – Ex: Reverse transcriptase Reverse transcriptase: enzyme in retroviruses that transcribe RNA into DNA; reverses the process

Viruses / Structures RNA Capsomere of capsid DNA Glycoprotein 18  250 nm70–90 nm (diameter) Glycoproteins 80–200 nm (diameter) 80  225 nm Membranous envelope RNA Capsid Head DNA Tail sheath Tail fiber 50 nm 20 nm (a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4

Basics of Viral Replication Attachment of virus to host cell Entry of genome Synthesis of viral genome and proteins – Machinery inside host is used to make more virus Assembly of virus Release of virus to infect more cells lifecycle lifecycle

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

Bacteriophage Infection Two types of reproduction cycles: – Lytic Cycle: results in the death of the host cell – Lysogenic Cycle: replicates phage genome without destroying the host cell Phage DNA Phage The phage injects its DNA. Bacterial chromosome Phage DNA circularizes. Daughter cell with prophage Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Binary Fission Cell divisions produce population of bacteria infected with the prophage. The cell lyses, releasing phages. Lytic cycle is induced or Lysogenic cycle is entered Lysogenic cycle Prophage The bacterium reproduces, copying the dormant prophage & transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a dormant prophage. New phage DNA and proteins are synthesized and assembled into phages.

Lytic Cycle Ends with death of host Produces new phages (viruses that infect bacteria) and digests the host’s bacterial wall releasing new progeny viruses Virulent Phage: viruses that only reproduce via the lytic cycle

Lysogenic Cycle Does not kill host Viral genome is incorporated into host’s chromosome Prophage: integrated viral DNA into host cell; dormant Every host divides the viral DNA is copied as well Remains dormant until an environmental trigger causes the viral genome to leave the host genome Temperate phages: viruses that use both the lytic and lysogenic cycles

Viral Reproductive Cycles: Lytic and Lysogenic Phage DNA Phage The phage injects its DNA. Bacterial chromosome Phage DNA circularizes. Daughter cell with prophage Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Binary Fission Cell divisions produce population of bacteria infected with the prophage. The cell lyses, releasing phages. Lytic cycle is induced or Lysogenic cycle is entered Lysogenic cycle Prophage The bacterium reproduces, copying the dormant prophage & transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a dormant prophage. New phage DNA and proteins are synthesized and assembled into phages.

HIV 95OI 95OI

Flu GShQ GShQ

Virus and Host Cell 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

The viral DNA that is integrated into the host genome is called a provirus. Unlike a prophage, a provirus remains a permanent resident of the host cell. The host’s RNA polymerase transcribes the proviral DNA into RNA molecules. The RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new virus particles released from the cell.