CH 21 & 22 Prokaryotes and Viruses

What must all cells have?

1. DNA- in nucleoid (prok.) or nucleus (euk.)- as genetic material

2. RNA- mRNA, tRNA, rRNA- involved in protein synthesis; also, other regulatory RNA

3. Cytoplasm- mostly water with solutes

   • Semifluid matrix of organelles and cytosol

4. Ribosomes

   • Synthesize proteins

5. Cell membrane- (unit membrane)- Phospholipid bilayer and proteins

6. Enzymes

7. Some internal structures and membranes

Prokaryotic cells

• Simplest organisms (aka bacteria)

  • Simplest cells

• Lack a membrane-bound nucleus

  • DNA is present in the nucleoid (region of cell)

  • No internal unit membranes

• Cell wall outside of cell membrane

• Contains ribosomes

• No membrane-bound organelles

• Two domains of prokaryotes

  • Archaea

  • Bacteria

Prokaryotic cell structure- 3 basic shapes

• Bacillus- Rod-shapes

• Coccus- Spherical

• Spirillum- Helical-shaped

Prokaryotic cell walls

• Cell walls in all domains have the following characteristics:

  1. Location- Outside the cell membrane

  2. Composition- Structural polysaccharides and amino acids or proteins

  3. Overall structure- rigid, strengthening component of cells

• Cell wall

  • Nearly all prokaryotes have cell walls

  • Domain bacteria have peptidoglycan in cell walls which adds strength

  • Domain archaea lack peptidoglycan; have different structural molecules for strength

  • Peptidoglycan forms a rigid network around cell

    • Maintains shape

    • Withstands osmotic pressure of water entering cell

    • Archaea have a similar molecule

  • Gram stain

    • Gram-positive bacteria have a thicker peptidoglycan wall and stain a purple color

    • Gram-negative bacteria contain less peptidoglycan and do not retain the purple-colored dye- retain counterstain and look pink

Prokaryotic Metabolism

• Acquisition of Carbon

  • Autotrophs- get carbon from inorganic CO2 and use it to synthesize organic molecules

    • Photolithoautotrophs- use energy from sun for synthesis (photosynthetic)

    • Chemolithoautotrophs- use energy found in inorganic substances for synthesis (ex: NH3, H2SO4, H2)

  • Heterotrophs- use organic molecules as source of carbon

    • Photoorganohetertrophs- light as energy source but obtain organic carbon made by other organisms

    • Chemoorganohetertroph- use both carbon atoms and energy from organic molecules

    • All protozoa, fungi, animals and many bacteria

Antibiotic Resistance

• A growing problem

• Arises from mutations in a cell(s) that can then be passed on to other cells

• R (resistance) plasmids

  • Encode antibiotic resistance genes

  • Can be transferred from one cell to another

  • Important factor in appearance of antibiotic resistant strains of Staphylococcus aureus

• Mutations (an plasmids) can spread rapidly through a population

  • Methicillin-resistant Staphylococcus aureus (MRSA)

  • Vancomycin-resistant Staphylococcus aureus (VRSA)

Benefits of prokaryotes

• Major decomposers- break down organic matter into inorganic matter; recycle elements

  • Ex: C6H12O6 to CO2 and H2O

• Carbon fixation

  • Photosynthesizers convert CO2 carbon into sugars

  • Nitrogen fixers reduce N2 to NH3

    • Rhizobium in soil

    • Important for soil fertility and agriculture

  • Symbiosis with other organisms (ex: humans)

Eukaryotic Cells

• More complex than prokaryotic cells

• Has much compartmentalization

  • Achieved through use of internal unit membrane-bound organelles and endomembrane system

  • Unit membrane- membrane consisting of a phospholipid bilayer ± proteins

• Possess a membrane-bound nucleus

• Possess a cytoskeleton for support and to maintain cellular structure

• Organelle- Unit membrane structure or compartment having 1 or more functions

Viruses

• Viruses are not cells; they lack cell membranes, cytoplasm, organelles

• Viruses are acellular infectious entities that must enter a specific cell, take control of it to one degree or other, and replicate itself

  • Viruses are thus obligate intracellular parasites

• Viruses are at the border line of living things and nonliving things

Basic structure of viruses

• All viruses have same basic structure

  1. Nucleic acid core (DNA or RNA, not both)

  2. Protein capsid surrounding nucleic acid (if just a nucleic acid and coat called naked virus)

  3. Some also have additional components

     a. Envelope- derived from host cell membrane (phospholipid membrane) with viral proteins; called enveloped virus

     • Corona of SARS-CoV-2 is an envelope of lipid and protein

     b. Some viruses store specialized enzymes

     • HIV virus has reverse transcriptase not found in host

Viral classification

• One method is by type of genome

  • RNA Viruses: genetic material RNA (stays RNA)

  • DNA Viruses: genetic material is DNA

  • Retroviruses: genetic material is RNA (starts as RNA, makes DNA, finishes as RNA)

• Virion is an inactive virus particle outside of a cell

  • Not alive or dead but inactive or active

Viral shapes

• Most viruses come in two simple shapes

  • Helical: TMV, rodlike or threadlike (less common)

  • Icosahedral: Soccer ball shaped

    • Structure with 20 equilateral triangular facets

    • Most animal viruses

    • Most efficient symmetrical arrangement that subunits can take to form a shell with maximum internal capacity

• Some viruses are complex

  • T-even bacteriophages: binal symmetry

  • Poxviruses: multilayered capsid

• Enveloped viruses are polymorphic

Viral genomes

• Vary greatly in both type of nucleic acid (DNA or RNA, but not both) and number of strands; also whether single stranded or double stranded

• Most DNA viruses are single-stranded

  • Replicated in nucleus of eukaryotic host cell

Viral hosts

• Viruses are obligate intracellular parasites in every kind of organism investigates

• Host range- types of organisms infected

  • Smallest host range is 1 host cell

  • Each type of virus has a limited host range

  • Tissue tropism: Inside a host the virus may only infect certain tissues

• Viruses can remain dormant or latent for years

  • Chicken pox can reemerge as shingles

Viral replication

• Infecting virus can be thought of as a set of instructions

• Viral nucleic acid tricks host cell into making viruses

• Viruses can only reproduce inside cells

  • Outside, they are metabolically inert virions

• Viruses lack their own ribosomes and enzymes for protein and nucleic acid synthesis

• Virus hijacks the cell’s transcription and translation machineries to make viral proteins

• End result is assembly and release of viruses

Bacteriophage

• Viruses that infect bacteria

• Diverse and united only by bacterial hosts

• E. coli-infecting viruses are the best studied

• Viruses have also been found in archaea

  • Different from bacterial viruses

  • Characterization in early stages

Reproductive cycles

• Attachment or absorption- specific and receptor-mediated

  • Target part of bacterial outer surface

• Penetration or injection

  • T4 pierces cell wall to inject outer viral genome

• Virus take control of cell to one degree or another

• Synthesis- phage may immediately take over the cell’s replication and protein synthesis enzymes to synthesize viral components

• Assembly- Assembly of components

• Release- Maure virus particles are released through enzyme lysing host or budding through host cell wall

• Lytic cycle

  • Virus lyses the infected host cell

  • Virulent or lytic phages

• Lysogenic cycle

  • Virus does not immediately kill infected cell

  • Inserts virus nucleic acid (ds DNA) into host cell DNA-prophage

  • Integration allows a virus to be replicated along with the host cell’s DNA as the host divides

  • Temperate or lysogenic phase

Emerging viruses

• Are viruses that extend their host range

• Often deadly to new host

• Considerable threat in the aviation age

• Ebola virus

  • Causes severe hemorrhagic fever

  • Among most lethal infectious diseases

  • Host is unknown

• SARS-CoV-2

  • Severe acute respiratory syndrome

  • Caused by a coronavirus

  • Host is a civet

  • Mutation rate low compared to HIV

  • SARS vaccines currently being developed

Viruses and cancer

• Viruses may contribute to about 15% of all human cancers (ex: HPV)

• Viruses can cause cancer by altering the growth properties of human cells

  • Triggering expression of oncogenes

  • Disrupt cell cycle control genes

Viroids

Tiny naked molecules of circular RNA- cause diseases in plants

Prions

• Cause transmissible spongiform encephalopathies (TSEs) (ex: mad cow disease)

• Host has normal prion proteins (PrPc)

  • Misfolded proteins (PrPc) cause disease