BIOLOGYCONCEPTS & CONNECTIONS

Fourth Edition

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Neil A. Campbell • Jane B. Reece • Lawrence G. Mitchell • Martha R. Taylor

From PowerPoint® Lectures for Biology: Concepts & Connections

CHAPTER 10Molecular Biology of the Gene

Modules 10.17 – 10.22

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

VIRUSES: GENES IN PACKAGES The Molecular Genetics of Viruses

Viral DNA may become part of the host chromosomePhage

New phage DNA andproteins are synthesized

Phage DNA inserts into the bacterialchromosome by recombination

Attachesto cell

Phage DNA

Bacterialchromosome

Phage injects DNA

Occasionally a prophagemay leave the bacterialchromosome

Many celldivisions

Lysogenic bacteriumreproduces normally,replicating the prophageat each cell division

Prophage

Phage DNAcircularizes

LYSOGENIC CYCLE

Cell lyses,releasing phages

Phagesassemble

LYTIC CYCLE

OR

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Translation of viral genes in Prokaryotes

• Prophage genes inserted in the DNA of essentially harmless bacteria cause botulism, diptheria, and scarlet fever. This occurs when prophage genes become active in the host genome. Also, an environmental signal such as radiation or a certain chemical triggers a switchover from the lysogenic cycle to the lytic cycle.

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• Many viruses have RNA, rather than DNA, as their genetic material

• RNA viruses include: flu, cold, measles, mumps as well as AIDS and polio.

• DNA viruses include: hepatitis, chicken pox, herpes

• Not all viruses reproduce in the cytoplasm, some like herpes reproduce in the nucleus.

10.18 Connection: Many viruses cause disease in animals

Figure 10.18A

Membranousenvelope

RNA

Proteincoat

Glycoprotein spike

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Viral Classification 20 - 300 nm, origin not clear because ther is no fossil record. The smallest genomes code for only four proteins and weigh about 106 daltons, while the largest weigh about 108 daltons and code for over one hundred proteins.

• Baltimore Classification

• IdsDNA viruses

• IIssDNA viruses

• IIIdsRNA viruses

• IV(+)ssRNA viruses

• V(-)ssRNA viruses

• VIssRNA-RT viruses

• VIIdsDNA-RT viruses

• ss: single-stranded

• ds: double strandedRT: reverse transcribing

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In taxonomy, the classification of viruses is rather difficult due to the lack of a fossil record and the dispute over whether they are living or

non-living. They do not fit easily into any of the domains of biological classification and therefore classification begins at the

family rank. However, the domain name of Acytota (without cells) has been suggested. This would place viruses on a par with the other domains of Eubacteria, Archaea, and Eukarya. Not all families are

currently classified into orders, nor all genera classified into families..

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As an example of viral classification, the chicken pox virus belongs to family Herpesviridae, subfamily Alphaherpesvirinae and genus Varicellovirus. It remains unranked in terms of order. The general structure is as follows:

Order (-virales) Family (-viridae)

Subfamily (-virinae) Genus (-

virus)

Species (-virus)

The International Committee on Taxonomy of Viruses (ICTV) developed the current classification system.

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• Some animal viruses steal a bit of the host cell’s membrane (Herpes virus)

• Some remain latent unless cell is stressed

• How quickly we fight them off with our immune system and how quickly our cells go through mitosis is a factor in how and IF we recover 100% from a viral infection.

Figure 10.18B

VIRUS Glycoprotein spikeProtein coat

EnvelopeViral RNA(genome)

1Plasmamembraneof hostcell

Entry

2 UncoatingViral RNA(genome)

3 RNA synthesisby viral enzyme

4 Proteinsynthesis

5 RNA synthesis(other strand)

mRNA

Newviral proteinNewviral proteins

6 Assembly

7

Exit

Template

New viralgenome

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• Most plant viruses have RNA

– Example: tobacco mosaic disease

– These viruses enter damaged plants and spread through the plasmodesmata

10.19 Connection: Plant viruses are serious agricultural pests

Figure 10.19

Protein RNA

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• The deadly Ebola virus causes hemorrhagic fever (1976, Africa)

– Each virus is an enveloped thread of protein-coated RNA

• Hantavirus is another enveloped RNA virus caused by a virus infecting rodents (1993)

• Viral Encephalitis-inflammation of the brain

• West Nile (1999-48 states by 2004)

• SARS (severe acute respiratory syndrome) (China 2003-3months 8,450 infected)

10.20 Connection: Emerging viruses threaten human health

Figure 10.20A, B

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• HIV is a retrovirus using reverse transcriptase to make a DNA copy of its genome.

10.21 The AIDS virus makes DNA on an RNA template

Figure 10.21A

EnvelopeGlycoprotein

Proteincoat

RNA(two identicalstrands)

Reversetranscriptase

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• Inside a cell, HIV uses its RNA as a template for making DNA to insert into the host chromosome

Figure 10.21B

Viral RNA

1

2 3

5

4

6

DNAstrand

Double-strandedDNA

ViralRNAandproteins

CYTOPLASM

NUCLEUSChromosomalDNA

ProvirusDNA

RNA

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Spread of Lethal viruses

• Viral disease can go from a small population to an epidemic or even pandemic

• Lethal viruses are more threatening due to biological and technological factors: international travel, blood transfusions, sexual promiscuity, IV drug abuse, jumping the “species barrier”

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• Bacteria replicate through binary fission

• Bacteria can transfer DNA in three ways

– Bacteria can transfer genes from cell to cell by one of three processes

• Transformation, transduction, or conjugationDNA enterscell

Fragment of DNAfrom anotherbacterial cell

Bacterial chromosome

(DNA)

Phage

Fragment of DNA fromanotherbacterial cell(former phagehost)

Phage

Sex pili

Mating bridge

Donor cell(“male”)

Recipient cell(“female”)

The Molecular Genetics of the Simplest Living Organisms

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– Once new DNA gets into a bacterial cell• Part of it may then integrate into the

recipient’s chromosome

Recipient cell’schromosome

Recombinantchromosome

Donated DNACrossovers Degraded DNA

Figure 10.22D

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• 10.23 Bacterial F factors and plasmids can serve as carriers for gene transfer

• F Factor (fertility) is integrated into the host genome

• Plasmids are not integrated• Are small circular DNA molecules separate

from the bacterial chromosome

• Confers antibiotic resistence

The ability of E.coli to carry out conjugation

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– Plasmids can serve as carriers• For the transfer of genes

Plasmids

Col

oriz

ed T

EM

2,0

00

Cell now male

Plasmid completes transferand circularizes

F factor starts replication and transfer

Male (donor) cell

Bacterial chromosome

F factor (plasmid)

Recombination can occur. Cell stillfemale

Only part of the chromosome transfers

F factor starts replication and transfer of chromosome

Origin of F replicationBacterial chromosome

Male (donor) cellF factor (integrated)

Recipient cell

Figure 10.23A–C

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• Virus studies help establish molecular genetics

• Molecular genetics helps us understand viruses

– such as HIV, seen here attacking a white blood cell

10.22 Virus research and molecular genetics are intertwined

Figure 10.22