Viruses and Prokaryotes

Chapter 21 Part 1

Impacts, Issues

The Effects of AIDS

Some viruses and bacteria help us; others, such

as the HIV virus that causes AIDS, can kill

21.1 Viral Characteristics and Diversity

A virus consists of nucleic acid and protein

A virus is smaller than any cell and has no

metabolic machinery of its own

Viruses

Viruses

• Noncellular infectious particles that multiply only

inside living cells

• Consist of genetic material and a protein coat;

some also have a lipid envelope

• Some viruses cause disease (pathogens); others

control disease-causing organisms

Characteristics of a Virus

Examples of Viruses

Viruses that infect plants (tobacco mosaic virus)

Viruses that infect bacteria or archaeans

(bacteriophages)

Naked viruses (adenoviruses)

Enveloped viruses (herpesviruses)

Examples of Viruses

Fig. 21-2a, p. 334

RNA

protein

subunits

of coat

Fig. 21-2b, p. 334

DNA

inside

protein

coat

sheath

tail

fiber

Fig. 21-2c, p. 334

Fig. 21-2d, p. 334

Fig. 21-2e (1), p. 334

viral DNA and enzymes

lipid envelope with

protein components

protein coat

inside envelope

Fig. 21-2e (2), p. 334

lipid envelope with

protein components

protein coat

inside envelope

Effects of Plant Viruses

Viral Origins and Evolution

Viruses may have descended from cells that

were parasites of other cells

Viruses may be genetic elements that escaped

from cells

Viruses may represent a separate evolutionary

branch

21.2 Viral Replication

All viruses replicate only inside host cells, but

the details of the process vary among viral

groups

Steps in Viral Replication

Table 21-2, p. 336

Stepped Art

Bacteriophage Replication

Lytic pathway

• Under direction of viral genes, the host makes an

enzyme that lyses and kills the cell

Lysogenic pathway

• Virus enters a latent state

• Host replicates viral genes and passes them on

to descendents before entering lytic pathway

Bacteriophage Replication

Fig. 21-4, p. 337

A1 Viral DNA is

inserted into host

chromosome by

viral enzyme

action.

A Virus particle binds,

injects genetic material.

A2 Chromosome and integrated viral DNA are replicated.E Lysis of host cell

lets new virus particles

escape.

Lytic

Pathway

Lysogenic

Pathway

D Accessory parts are

attached to viral coat.B Host replicates

viral genetic material,

builds viral proteins.

A3 Cell

divides;

recombinant

DNA in each

daughter cell.

C Viral proteins self-

assemble into a coat

around viral DNA.

A4 Viral

enzyme excises

viral DNA from

chromosome.

Fig. 21-4, p. 337

Stepped Art

A2 Chromosome and integrated viral DNA are replicated.E Lysis of host cell

lets new virus particles

escape.

A Virus particle binds,

injects genetic material.

Lytic

Pathway

A1 Viral DNA is

inserted into host

chromosome by

viral enzyme

action.

Lysogenic

Pathway

D Accessory parts are

attached to viral coat.B Host replicates

viral genetic material,

builds viral proteins.

A3 Cell

divides;

recombinant

DNA in each

daughter cell.

C Viral proteins self-

assemble into a coat

around viral DNA.

A4 Viral

enzyme excises

viral DNA from

chromosome.

Animation: Bacteriophage multiplication

cycles

Animation: Lytic pathway

Animation: Lysogenic pathway

Replication of an Enveloped DNA Virus

Example: Herpes

• Viral envelope fuses with host membrane; viral

DNA enters host cytoplasm

• Viral DNA enters nucleus, directs synthesis of

new viral DNA and proteins

• New viral particles are assembled and enveloped

in host nuclear membrane

• New viral particles exit cell by exocytosis

Replication of a Retrovirus

Example: HIV

• Virus binds to receptors on certain white blood

cells; viral envelope fuses with host membrane;

viral RNA enters host cytoplasm

• Enzyme (reverse transcriptase) converts viral

RNA to DNA, which integrates with host DNA

• Host cell produces viral RNA and proteins which

assemble into new viral particles

• New viruses are enveloped in host plasma

membrane and exit by exocytosis

HIV Replication

Fig. 21-5, p. 337

viral enzyme

(reverse transcriptase)

C Viral DNA gets

integrated into the

host’s chromosome.

D Viral DNA gets

transcribed along

with the host’s genes.

A Viral RNA

and protein

enter the

host cell.

E Some RNA

transcripts are

new viral RNA.

Others are

translated into

viral proteins.

RNA and

proteins

assemble as

new virus

particles.

viral coat

proteins

nucleus

B Viral reverse

transcriptase

uses viral RNA

to make double-

stranded viral

DNA.

viral proteins

viral RNA

viral DNAone of two

strands of

viral RNA

lipid envelope

with proteinsF Viral particles

bud from the

infected cell.

Animation: HIV replication cycle

21.3 Viroids and Prions

Viroids and prions are infectious particles that

are even simpler than viruses

Viroid

• Infectious RNA, not surrounded by a protective

protein coat

Prion

• Proteins in the nervous system that can misfold,

and cause other prions to misfold

Viroid Disease in Plants

Prion Diseases

Scrapie: A prion disease that affects sheep

Bovine spongiform encephalopathy (BSE or mad

cow disease): Affects cattle that have eaten feed

made with infected sheep

Variant Creutzfeldt-Jacob disease (vCJD):

Affects humans who have eaten infected beef

Prion Diseases

21.1-21.3 Key Concepts: Viruses and

Other Noncellular Infectious Particles

Viruses are noncellular particles made of protein

and nucleic acid; they replicate by taking over

the metabolic machinery of a host cell

Viroids are short sequences of infectious RNA

Prions are infectious misfolded versions of

normal proteins

21.4 Prokaryotes—Enduring,

Abundant, and Diverse

Prokaryotes

• Structurally simple cells that lack a nucleus

• Evolved before eukaryotes

Prokaryotes still persist in enormous numbers

and show great metabolic diversity

Evolutionary History and Classification

Automated gene sequencing and comparative

biochemistry helps classify species and

subgroups (strains) of prokaryotes

p. 339

to ancestors of eukaryotic cells

DOMAIN BACTERIA DOMAIN ARCHAEA

biochemical and molecular origin of life

Abundance and Metabolic Diversity

Prokaryotes are Earth’s most abundant organisms

Metabolic diversity contributes to their success

• Example: Saprobes that break down wastes or

remains are important decomposers

All four forms of nutrition are used by prokaryotes

Prokaryotic Nutritional Modes

21.5 Prokaryotic Structure and Function

Prokaryotic cells have many structural features

that adapt them to their environment

The typical prokaryote is a walled cell with

ribosomes but no nucleus

Prokaryotic Cell Characteristics

Prokaryotic structure

• Nucleoid region contains a single, circular

chromosome

• Cell wall surrounds the plasma membrane, with

a slime layer (capsule) outside the cell wall

• Flagella rotate like propellers

• Pili extend from the cell surface for adhesion or

motion

Prokaryotic Cell Characteristics

Prokaryotic Body Plan

Prokaryotic Cell Size and Shape

Prokaryotic cells are

much smaller than

eukaryotic cells (about

the size of

mitochondria)

Prokaryotes have

three typical shapes:

Fig. 21-8a, p. 340

coccus

bacillus

spirillum

Fig. 21-8b, p. 340

cytoplasm, with ribosomes

DNA, in

nucleoid region

pilus

bacterial

flagellum

outer capsule

cell wall

plasma

membrane

Prokaryotic Reproduction

Prokaryotic chromosome

• A circular, double-stranded DNA molecule

Prokaryotic fission

• DNA replicates; parent cell divides in two

Prokaryotic Fission

Fig. 21-10, p. 341

A The bacterial

chromosome is

attached to the

plasma membrane

prior to DNA

replication.

B Replication starts

and proceeds in two

directions from a

certain site in the

bacterial chromosome.

C The DNA copy

becomes attached at

a membrane site

near the attachment

site of the parent

DNA molecule.

D Then the two DNA

molecules are moved

apart by membrane

growth between the

two attachment sites.

E Lipids, proteins, and

carbohydrates are built

for new membrane and

new wall material. Both

get inserted across the

cell’s midsection.

F The ongoing,

orderly deposition

of membrane and

wall material at the

midsection cuts

the cell in two.

D Then the two DNA

molecules are moved

apart by membrane

growth between the

two attachment sites.

F The ongoing,

orderly deposition

of membrane and

wall material at the

midsection cuts

the cell in two.

Fig. 21-10, p. 341

A The bacterial

chromosome is

attached to the

plasma membrane

prior to DNA

replication.

B Replication starts

and proceeds in two

directions from a

certain site in the

bacterial chromosome.

C The DNA copy

becomes attached at

a membrane site

near the attachment

site of the parent

DNA molecule.

E Lipids, proteins, and

carbohydrates are built

for new membrane and

new wall material. Both

get inserted across the

cell’s midsection.

Stepped Art

Animation: Prokaryotic fission

Horizontal Gene Transfers

Conjugation

• Transfer of a plasmid (non-chromosomal DNA)

between prokaryotic cells via a sex pilus

Transduction

• Transfer of prokaryotic genes via bacteriophages

Transformation

• Prokaryotic genes acquired from the environment

Conjugation

Fig. 21-11a, p. 341

A Conjugation in E.

coli begins when a

cell with a specific

type of plasmid

extends a sex pilus

to another E. coli

cell that lacks this

plasmid. The pilus

attaches the cells to

one another. When

it shortens, the cells

are drawn together.

sex pilus

Fig. 21-11 (b-d), p. 341

nicked plasmid conjugation tube

B A conjugation tube forms,

connecting the cytoplasm of

the cells. An enzyme nicks

the plasmid in the donor cell.

C As a single strand of

plasmid DNA moves into the

recipient, each cell makes a

complimentary DNA strand.

D The cells separate and the

plasmid resumes its circular

shape.

Fig. 21-11, p. 341

Stepped Art

A Conjugation in E. coli begins when a

cell with a specific type of plasmid

extends a sex pilus to another E. coli

cell that lacks this plasmid. The pilus

attaches the cells to one another. When

it shortens, the cells are drawn

together.sex pilus

C As a single strand of plasmid DNA

moves into the recipient, each cell

makes a complimentary DNA strand.

D The cells separate and the

plasmid resumes its circular

shape.

nicked plasmid conjugation tube

B A conjugation tube forms,

connecting the cytoplasm of

the cells. An enzyme nicks the

plasmid in the donor cell.

Animation: Prokaryotic conjugation

21.4-21.5 Key Concepts

Features of Prokaryotic Cells

Prokaryotes are single-celled organisms that do

not have a nucleus or the diverse cytoplasmic

organelles found in most eukaryotic cells

Collectively, prokaryotes show great metabolic

diversity; they divide rapidly and exchange DNA

by a variety of mechanisms