Genetics of Viruses and Bacteria - Introduction to Biology - Lecture Slides, Slides for Biology. Aliah University


Description: These are the lecture slides of Introduction to Biology. Key important points are: Genetics of Viruses and Bacteria, Collection of Techniques, Importance of Microbes, Fermentation Products, Mycorrhizal Fungi, Types of Viruses, Protein Capsids, Virion Particle
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The Chromosomal Basis of Inheritance

Chapter 18: The Genetics of

Viruses and Bacteria

W ha

t i s

M ic

ro bi

ol og

y?  Microbiology is the science that studies

microorganisms  Microorganisms, roughly, are those living things

that are too small to be seen with the naked eye  Microorganisms cannot be distinguished

phylogenetically from “Macroorganisms”, e.g., includes fungi as well as bacteria, etc. (that is, they are not, as a whole, a closely related group of organisms)

 Microbiology is more a collection of techniques: Aseptic technique, Pure culture technique, Microscopic observation of whole organisms, etc.

 A microbiologist usually first isolates a specific microorganism from a population and then cultures it

Im po

rta nc

e of

M ic

ro be

s  Microbes are produces—they provide energy to ecosystems  Microbes are fixers—they make nutrients available

from inorganic sources, e.g., nitrogen  Microbes are decomposers—they free up nutrients

from no longer living sources  Microbes form symbioses (such as mycorrhizal fungi

associated with plant roots—though these are somewhat macroscopic; also the bacteria found in legume root nodules, etc.)

 Microbes serve as emdosymbionts (e.g., chloroplasts and mitochondria)

 Microbes make fermentation products (ethanol!), food (beer! Cheese! Yogurt! Half-sour pickles!), Biotech products (e.g., recombinant insulin), etc.

 Germ theory of disease; Normal flora

Examples of Types of Viruses

W ha

t i s

a Vi

ru s?

 Viruses consist of protein capsids and nucleic

acid (DNA or RNA) plus some viruses (virions) have a lipid envelope (enveloped viruses)

 Viruses are… “...infectious agents of small size and simple composition that can multiply only in living cells of animals, plants and bacteria [plus fungi & protozoa].

 Viruses are obligate parasites that are metabolically inert when they are outside their hosts. They all rely, to varying extents, on the metabolic processes of their hosts to reproduce themselves.

 The viral diseases we see are due to the effects of this interaction between the virus and its host cell (and/or the host’s response to this interaction).” Encyclopedia Britannica

Vi ru

s (V

iri on

P ar

tic le

) The Virion is what defines a virus as a virus A Virion is the extracellular state of a virus

The job of Virions is to find new cells to infect

As such, Virions are a durable state that is “designed” to attach to susceptible cells

The Virion is then responsible for translocation of the virus genome into the cell

The Virion consists of a DNA (or RNA) genome surrounded by Protein that, in turn, may be surrounded by a Lipid Bilayer

The Protein layer is called a Capsid

The Lipid Bilayer is called an Envelope

S te

ps o

f V iru

s R

ep lic

at io

n 1. Adsorption (attachment) 2. Penetration (nucleic-acid release) 3. Synthesis (of RNA and proteins, as

well as DNA if DNA genome)

4. Maturation (assembly of virion) 5. Release (lysis or chronic release, e.g.,

budding, with the latter coinciding with release for various enveloped viruses)

Caveat: It is important to realize that variation among viruses is between virus strains/species; any one kind of virus cannot replicate in multiple ways, have more than one virion morphology, or vary in genome type, etc.

Bacteriophage Lytic Cycle


Lysogeny (Temperate Phage)

Only temperate phage are

able to display



E nv

el op

ed R


Vi ru


An example of an animal


Acquisition of plasma membrane

as envelope


B ac

te ria

S ex

 Viruses move genetic material from cell to

cell  Mostly this material is their own genomes,

i.e., genes that collectively code for the production of new viruses

 Bacteria DNA also can move from cell to cell  Once received by a cell, this DNA may be

incorporated into the bacterial genome via recombination

 This idea of DNA sourced from different parents recombining into a single chromosome is equivalent to eukaryotic sex (i.e., fertilization followed by recombination)

 Transformation, Transduction, Conjugation

Transformation Transformation: DNA picked up directly from the medium and recombined into the genome

Competent cell: capable of picking up DNA


Moves plasmid more so than chromosomal


C on

tro l o

f G en

e E

xp re

ss io

n  It makes energetic sense to make or use proteins responsible for certain metabolic processes only when those processes are needed

 Trp operon • response to presence of tryptophan • Repressible (Corepressed) operon • Example of negative control of transcription  Lac operon • Response to presence of lactose • Inducible operon • Example of negative control of transcription  Catabolite repressor protein • Response to absence of glucose • Example of positive control of transcription  Protein binding to the operator controls RNA

polymerase activity

Trp Operon (higher trp densities)

Equilibrium: Likelihood of being in bound state

depends on trp density

Negative regulation Corepression

Lac Operon (higher Lac densities)

Equilibrium: Likelihood of being in bound state

depends on Lac density

Don’t worry about the names of these genes and products except for β-Galactosidase

An inducible operon

C at

ab ol

ite R

ep re

ss or

P ro

te in

(C A

P )

Positive control

(binding increases


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