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Ch. 19 - Viruses

Overview: A Borrowed Life Viruses called bacteriophages can infect and

set in motion a genetic takeover of bacteria, such as Escherichia coli

Viruses lead “a kind of borrowed life” between life-forms and chemicals

The origins of molecular biology lie in early studies of viruses that infect bacteria

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Figure 19.1

0.5 mm

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Concept 19.1: A virus consists of a nucleic acid surrounded by a protein coat

Structure of Viruses Viruses are not cells A virus is a very small infectious particle

consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope

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Viral Genomes

Viral genomes may consist of either Double- or single-stranded DNA, or Double- or single-stranded RNA

Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus

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Capsids and Envelopes

A capsid is the protein shell that encloses the viral genome

Capsids are built from protein subunits called capsomeres

A capsid can have various structures

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Figure 19.3

Capsomereof capsid

RNA CapsomereDNA

Glycoprotein Glycoproteins

Membranousenvelope RNA

CapsidHead

DNA

Tailsheath

Tailfiber

18 250 nm 80 225 nm70–90 nm (diameter) 80–200 nm (diameter)

20 nm 50 nm 50 nm 50 nm(a) Tobacco

mosaic virus(b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4

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Some viruses have membranous envelopes that help them infect hosts Viral envelopes surround the capsids of

influenza viruses and many other viruses found in animals

Derived from the host cell’s membrane Contain a combination of viral and host cell

molecules

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Bacteriophages, also called phages, are viruses that infect bacteria Have the most complex capsids

found among viruses Have an elongated capsid head

that encloses their DNA A protein tail piece attaches the

phage to the host and injects the phage DNA inside

http://scitechdaily.com/video-animation-on-how-a-flu-virus-works/

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Overview: Masters of Adaptation Utah’s Great Salt Lake can reach a salt

concentration of 32% Its pink color comes from living prokaryotes

Ch. 27 - Prokaryotes

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Thrive almost everywhere, including places that are too: Acidic Salty Cold/Hot

Most are microscopic, but what they lack in size they make up for in #s

There are more in a handful of fertile soil than the number of people who have ever lived

Prokaryotes are divided into two domains: Bacteria Archaea

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Prokaryotes

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Concept 27.1: Structural and functional adaptations contribute to prokaryotic success Most likely 1st organisms on Earth Most are unicellular, although some species

form colonies Sizes are usually 0.5–5 µm

Eykaryotic cells are usually 10–100 µm Come in a variety of shapes, the 3 most

common shapes are: spheres (cocci) rods (bacilli) spirals

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Cell-Surface Structures

Cell wall: Maintains cell shape Protects the cell Prevents cell from bursting in a hypotonic

environment A eukaryotic cell wall is made of cellulose or

chitin Bacterial cell walls contain peptidoglycan

A network of sugar polymers cross-linked by polypeptides

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Archaea contain polysaccharides and proteins but lack peptidoglycan

Scientists use the Gram stain to classify bacteria by cell wall composition Gram-positive bacteria have simpler walls with

a large amount of peptidoglycan Gram-negative bacteria have less

peptidoglycan and an outer membrane that can be toxic

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Figure 27.3

(a) Gram-positive bacteria: peptidoglycan traps crystal violet.

Gram-positivebacteria

Peptido-glycanlayer

Cellwall

Plasmamembrane

10 m

Gram-negativebacteria

Outermembrane

Peptido-glycanlayer

Plasma membrane

Cellwall

Carbohydrate portionof lipopolysaccharide

(b) Gram-negative bacteria: crystal violet is easily rinsed away, revealing red dye.

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Many antibiotics target peptidoglycan and damage bacterial cell walls

Gram-negative bacteria are more likely to be antibiotic resistant

A polysaccharide or protein layer called a capsule covers many prokaryotes

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Figure 27.4

Bacterialcell wall

Bacterialcapsule

Tonsilcell

200 nm

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Some prokaryotes have fimbriae, which allow them to stick to their substrate or other individuals in a colony

Pili (or sex pili) are longer than fimbriae and allow prokaryotes to exchange DNA

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Figure 27.5

Fimbriae

1 m

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Motility

In a heterogeneous environment, many bacteria exhibit taxis: the ability to move toward or away from a

stimulus Chemotaxis is the movement toward or away

from a chemical stimulus Towards nutrients Away from toxins

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Most motile bacteria propel themselves by flagella scattered about the surface or concentrated at one or both ends

Flagella of bacteria, archaea, and eukaryotes are composed of different proteins and likely evolved independently

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Figure 27.6

Flagellum

Hook

Motor

Filament

RodPeptidoglycan

layerPlasma

membrane

Cell wall

20 nm

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Evolutionary Origins of Bacterial Flagella

Bacterial flagella are composed of a motor, hook, and filament

Many of the flagella’s proteins are modified versions of proteins that perform other tasks in bacteria

Flagella likely evolved as existing proteins were added to an ancestral secretory system

This is an example of exaptation, where existing structures take on new functions through descent with modification

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Internal Organization and DNA

Prokaryotic cells usually lack complex compartmentalization

Some prokaryotes do have specialized membranes that perform metabolic functions

These are usually infoldings of the plasma membrane

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The prokaryotic genome has less DNA than the eukaryotic genome

Most of the genome consists of a circular chromosome

The chromosome is not surrounded by a membrane; it is located in the nucleoid region

Some species of bacteria also have smaller rings of DNA called plasmids

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Figure 27.8

Chromosome Plasmids

1 m

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There are some differences between prokaryotes and eukaryotes in DNA replication, transcription, and translation

These allow people to use some antibiotics to inhibit bacterial growth without harming themselves

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Reproduction and Adaptation

Prokaryotes reproduce quickly by binary fission and can divide every 1–3 hours

Key features of prokaryotic reproduction: They are small They reproduce by binary fission They have short generation times

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Many prokaryotes form metabolically inactive endospores, which can remain viable in harsh conditions for centuries

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Figure 27.9

Coat

Endospore

0.3 m

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Their short generation time allows prokaryotes to evolve quickly

For example, adaptive evolution in a bacterial colony was documented in a lab over 8 years

Prokaryotes are not “primitive” but are highly evolved

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