MCB 2610 Lecture Notes - Lecture 3: Viral Envelope, Nitrogen Fixation, Nitrogen
14.3 How do microbes influence the cycling of nitrogen:
●The atmosphere, made up of almost 80 percent N as dinitrogen gas, is by far the largest
reservoir of nitrogen
●Nitrogen fixation: carried out by certain bacteria and archaea, is essential for converting
atmospheric dinitrogen to forms in which it can be incorporated into biomass.
●Nitrogen fixation process is O2 sensitive due to irreversible activation of the Nitrogenase
complex by O2
●Nitrification: conversion of ammonia to nitrite and then nitrate, is carried out by
sequential activities of nitrifiers, include ammonia and nitrite oxidizers.
Both nitrification and denitrification is chemolithoautrophic process
Nitrifiers: aerobic ammonia and nitrite oxidizers : convert NH3 to NO2.
●Denitrification: conversion of nitrate to dinitrogen gas through a series of reduction
steps carried out by reductase enzymes during anaerobic respiration
-complete the nitrogen cycle causing nitrogen to be lost from a habitat
-denitrifiers are chemoorganoheterotrophs
-denitrification is harmful to the soil because it reduces the effectiveness of nitrogen fertilizer. ->
nitrapyrin inhibit ammonia oxidation.
-denitrification is harmful to the environment because it depletes of marine nitrogen.
●Haber- Bosch process: synthetic nitrogen fertilizer.
5.1 Basic Overview of Viruses:
Viruses - small subcellular particles that can replicate only within living host cells
+obligate intracellular parasites-can’t replicate independently
+capsid : protective protein shell that consists of RNA and dna genome - constructed from
symmetrically arranged capsomeres
+nucleocapsid: icosahedral or helical morphology
+viral envelope surrounds the capsids of some viruses.
+bacteriophages: complex morphology, with virions containing an icosahedral head and a helical
tail
Viral envelopes: without an intact envelope, virus would be non infectious
●In eveloped viruses, a lipid bilayer surrounds the capsid.
●Viral replication involves an interaction between the viral attachment protein and the
host cell receptor, penetration of the host cell membrane, disassembly of the capsid (for
viruses of eukaryal cells), replication of the viral genome and production of viral
proteins, assembly of new virus particles and exit from the cell of these new particles.
Entry mechanism of animal viruses (draw it out)
●Non-enveloped viruses like rhinovirus often enter through endocytosis:
1) The virus attaches to the cell receptor
2) Endocytosis is initiated
3) An endosome forms with the virus inside
4) The nucleocapsid escapes to the cytoplasm and uncoats to release the genome
●Enveloped viruses like HIV undergo a membrane fusion event at the cell surface
1) the virus attaches to the cell receptor
2) A conformational change in the attachment protein and bound receptor initiates
membrane fusion
3) The viral envelope fuses with the plasma membrane
4) The nucleocapsid enters the cytoplasm and uncoats to release the genome
●Enveloped viruses like influenza first enter the cell via endocytosis. A fusion event
between the viral envelope and vehicle membrane then occurs.
1) The virus attaches to the cell receptor
2) Endocytosis is initiated
3) An endosome forms with the virus inside
4) The low pH of the endosome initiates fusion of the viral envelope with the endosome
membrane. The nucleocapsids are released
●Entry mechanisms of bacteriophages:
1) Tail fibers attach to receptors
2) Conformational change in tail fibers bring base of the tail in contact with host cell surface
3) Rearrangement of tail protein allows inner core tube proteins to extend down into cell
wall
4) Contact with the plasma membrane initiates transfer of DNA through a pore formed in
the lipid bilayer
●General steps :
+Attachment to host cell
+Entry and unocating of the viral genome
+Gene expression and protein production
Document Summary
14. 3 how do microbes influence the cycling of nitrogen: The atmosphere, made up of almost 80 percent n as dinitrogen gas, is by far the largest reservoir of nitrogen atmospheric dinitrogen to forms in which it can be incorporated into biomass. Nitrogen fixation: carried out by certain bacteria and archaea, is essential for converting. Nitrogen fixation process is o2 sensitive due to irreversible activation of the nitrogenase. Nitrification: conversion of ammonia to nitrite and then nitrate, is carried out by complex by o2 sequential activities of nitrifiers, include ammonia and nitrite oxidizers. Nitrifiers: aerobic ammonia and nitrite oxidizers : convert nh3 to no2. steps carried out by reductase enzymes during anaerobic respiration. Denitrification: conversion of nitrate to dinitrogen gas through a series of reduction. Complete the nitrogen cycle causing nitrogen to be lost from a habitat. Denitrification is harmful to the soil because it reduces the effectiveness of nitrogen fertilizer. nitrapyrin inhibit ammonia oxidation.