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U OF M / Biology / BIOL 3301 / Once viral infection of a host cell occurs, what will happen?

Once viral infection of a host cell occurs, what will happen?

Once viral infection of a host cell occurs, what will happen?


School: University of Minnesota
Department: Biology
Course: Biology of Microorganisms
Professor: Christian mohr
Term: Fall 2016
Tags: Microbiology
Cost: 25
Name: Microbio Week 3 Lectures 4 (cont), 5, and 6
Description: These are the last set of lecture notes included in our upcoming exam, keep an eye out for my Midterm #1 Study Guide!
Uploaded: 09/22/2016
7 Pages 84 Views 3 Unlocks

Prof. Mohr  

Once viral infection of a host cell occurs, what will happen?


Reading: Chapter Sections 6.1, 6.2, 6.3

Lecture 4 (cont)


Viral Multiplication/Life Cycle

1. Attachment to host cell

2. Entry and uncoating  

3. Synthesis of viral proteins and nucleic acids

4. Assembly of capsids

5. Release of virions  

Once viral infection of a host cell occurs...

∙ Viral genome replicated

∙ Viral mRNA made (transcription), used to make viral proteins (translation) ∙ Consider the Central Dogma of Information Flow

∙ The primary factor determining the life cycle of an animal virus is the form of  its genome

What is the difference between viroids and prions?

∙ DNA viruses

 Typically replicate in nucleolus  

 Use host DNA polymerase DNA -> DNA

 Exception: Herpes Viruses, use their own DNA polymerase  

∙ RNA Viruses

 Typically replicate in cytoplasm  

 Use viral RNA Replicases RNA -> RNA

∙ Retroviruses (ex. HIV)

 Use reverse transcriptase

 Copies RNA genome into DNA We also discuss several other topics like How fast do images have to move to get rid of the black flickers?

 DNA copy becomes integrated into host chromosome  

Animal Virus Synthesis and Assembly:

∙ Synthesis

∙ All viruses make proteins using host ribosomes

∙ Translation in cytoplasm  

What sources can microbes use in nitrogen?

∙ Assembly

 Capsid + genome = Nucleoid capsid

 In cytoplasm or nucleus  

 Spike proteins insert into membrane

∙ Animal Virus release  

 Lysis or Budding (membrane lipids surround capsid to form envelope)

Prof. Mohr  


Reading: Chapter Sections 6.6, 6.7 Viroids and Prions, Chap 3: pgs 49 - 53 Lecture 5 We also discuss several other topics like What is the purpose of family?

Microbial Nutrition and Growth  

Even smaller than a virus:

∙ Viroids- RNA only

 Infectious agent but not a virus

 Do not encode proteins  

 Plant diseases: avocado, peach, coconut, tomato

 How? RNA may pair with plant RNA (RNA silencing)  

∙ Prions- Infectious Proteins  

 Neurodegenerative diseases: scrapie, mad cow, Creutzfeldt – Jakob  How? Abnormal protein form (PrPSc) causes misfolding, aggregation of  normal form (PrPc) Don't forget about the age old question of What is reactive nonmetals?

 Aggregates lead to disease symptoms – destruction of brain and other  nervous tissue  

Microbial Nutrition:

∙ To obtain energy and construct new cellular components, microbes must have a supply of raw materials and nutrients  

∙ Nutrients- substances used in biosynthesis and energy release (required for  growth)

∙ 95% of microbial cell dry weight is made up of a few major elements ∙ Macronutrients (or macroelements)

 Required in large amounts  

 C, O, H, N, S, P, Fe

∙ Micronutrients (or trace elements)

 Required in small amounts  

 Co, Cu, Zn, Mn

Growth factors- Some microbes are unable to synthesis certain organic  compounds and may require specific growth factors.  

3 major classes:

∙ Amino acids

∙ Purines and Pyrimidines

∙ Vitamins

Let’s look at nitrogen. What sources can microbes use? If you want to learn more check out How much photosynthate is kept in the source tissue and how much is exported?

∙ Microbes can use ammonia (NH3) or nitrate (NO3) Don't forget about the age old question of Why are obligations under civil code juridical necessity?

∙ A few use nitrogen gas N2-79% of earth’s atmosphere ∙ Nitrogen fixation– N2 reduced to ammonia

 Rhizobium- in symbiosis with plants

 Azotobacter- free living in soil  

Acquiring Nutrients:

∙ Rapid growth of microbes present challenges in acquiring nutrients  ∙ Food must enter:

 At high rates

 Across membranes

 Selective fashion

 Often against concentration gradient  

∙ Passive Transport

 No energy required

 Does require gradient from higher to lower

o Passive diffusion  

o Facilitated Diffusion- uses carrier molecules called permeases   Only a few small sugars can enter by diffusion  

 Carrier saturation effect

∙ Active Transport

 Energy dependent

 Moves nutrients against gradient

 ATP or proton motive force used  

Primary active transport: ABC transporters (ATP binding cassette)

∙ All domains of life  

∙ 2 types  

 Uptake ABC – to move nutrients in

 Efflux ABC- multidrug efflux pumps (out) (able to pump cancer  treatment out)

1. After binding solute the solute binding protein approaches ABC transporter  2. Solute binding protein attaches to transporter and releases solute Energy  released by hydrolysis of ATP drives movement of solute across membrane  ∙ Fig 3.13 We also discuss several other topics like What happens to a cell if the lac operon promoter is damaged?

Secondary Active Transport:

∙ Uses potential energy of ion gradients

∙ Ex. Electron transport across membrane generates proton (H+) gradient   Can use this gradient to do work

∙ Symport, Antiport

∙ Fig 3.12 

Active Transport: Group Translocation

∙ Phosphotransferase system (PTS) is an example in all bacteria ∙ Nutrient chemically altered  

∙ Energy from phosphoenolpyruvate (PEP) attaches P to sugars  ∙ Fig 3.14 

Iron Uptake:

∙ Problem: All microbes require iron (Fe), but there is little free Fe available,  often insoluble form (ferric iron, Fe3+)

∙ Solution: microbes release siderophores to acquire or bind up Fe ∙ Siderophore – Fe complex then transported into cell often using ABC  transporters

 (Ex. Enterobactin: An Escherichia coli siderophor)

Prof. Mohr


Reading: Chapter 7 Sections 7.1, 7.2, 7.3, 7.6, 7.7, and 7.8 Lecture 6

Microbial Growth  

∙ Acyclovir (drug) – inhibits Herpesvirus DNA Polymerase  

∙ Microbial growth is typically defined as an increase in the number of  cells in a population. So how do microbes increase in cell number?

∙ Reproductive Strategies:

 Eukaryotic microbes-

 Sexual and asexual

 Haploid and diploid

 Prokaryotic microbes-

 Asexual

 Binary fission

 Haploid cells  

∙ Binary Fission:

 DNA replicates

 Cell elongates, chromosomes segregate (FtsZ ring)

 Septum forms

 Cells divides, creating a daughter clone

∙ Population growth is often studied by analyzing the growth curve of  a microbial culture.

 Observed when microbes cultivated in a batch culture – closed vessel,  single batch of medium.

 Lag Phase  

 No growth

 Cells synthesizing new components (replenish, adapt)

 Length varies  

 Exponential or log phase

 Balanced, constant- double in number at regular intervals

 Rate of exponential growth expressed as generation (or  

doubling) time - Time required for cells (or population) to divide  Range: 7 min to 24 hours or more

 Plotting exponential growth starting with one cell with a generation  time of 20 min. Fig 7.30

 We can also determine the generation time (g) from the  

exponential phase of a growth curve.  

 Stationary Phase

 Growth of population is ceasing (plateau)

 Happens because: competition, waste products, crowding, lack  of air, ect.

 In stationary phase, some microbes undergo drastic changes!  Sporulation- nutrient limiting conditions, some microbes form stress resistant, dormant spores 

 Bacterial examples: Bacillus, Clostridium

 Death phase

 Cells dying, usually at an exponential rate

 Often cell lysis

 Death rate may slow or reverse – resistant cells

∙    Continuous Culture systems can maintain microbial populations in  exponential growth (add media, remove waste)

 Chemostat – Rate of new media in = rate of medium with microbes out

∙ Measuring microbial growth:

 Often measure changes in number of cells in a population 1. Direct cell counts - counting chambers (Petroff-Hausser)

2. Viable cell counts – Plating – Colony Forming Units (CFUs) 

3. Turbidity measurements  

 Microbial cells scatter light striking them

 More turbid – more cells – more light scattered

 Spectrophotometer  

∙ Extremophiles:

 Thermophile- can grow at extremely high temperatures

 Mesophiles- grow at (20-45 degrees C)

 Psychrophiles- can grow at extremely low temperatures  

 Osmophiles- can grow at extreme levels of concentrations of solutes  Halophiles- Osmophiles where the solute is salt  

 Acidophiles- grow at extremely low pH  

 Obligate anaerobes- grow with extremely low oxygen  

∙ Microbes can grow under a wide range of oxygen concentrations:  Obligate aerobe - Needs oxygen

 Facultaive anaerobe - Prefer oxygen

 Aerotolerant anaerobe - Ignore oxygen

 Obligate anaerobe - Oxygen is toxic

 Microaerophile - 2-10% oxygen  

 Fig 7.13

∙ What is the basis of different oxygen sensitivities?

 Oxygen can be reduced to toxic products called Reactive Oxygen  Species. Examples include superoxide radical (O2) and hydrogen  peroxide (H2O2). Microbes that live in the presence of oxygen need  enzymes t0 detoxify.


 Superoxide dismutase

 O2 + O2 +2H --> H2O2 +O2

 Catalase

 H2O2 +H2O2 --> 2H20 +O2

∙ Microbes can grow under a wide range of temperature:  Hyperthermophiles > 80 degrees C

 Current record holder: 122 degrees C

 Hermophiles 40 – 80 degrees C

 Mesophiles 20 – 45 degrees C

 Psychrophiles 0- 20 degrees C

∙ High temperature disrupts membranes, denatures proteins and DNA. How do thermophiles adapt?

 Proteins stabilized  

 Increased hydrogen and covalent bonds  

 Molecular chaperones – bind, refold damaged proteins

 DNA stabilized

 Synthesize proteins to coat DNA

 Membrane stabilized – How? See last paragraph pg. 146

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