Chapter 18

The Genetics of Viruses and Bacteria

Introduction

• Viral/bacterial studies led to understanding

mechanisms of heredity due to simplicity

(viruses – nucleic acid & protein coat only)

Studies led to:

• better understanding of disease

• emergence of biotechnology

Discovery of viruses• 1883 A. Mayer – studying tobacco mosaic

disease (thought to be carried by unusually small bacteria unseen through microscope)

• 1897 Beijerenck – discovered infectious agent could reproduce, therefore, not just a bacterial toxin

• 1935 Stanley – crystallized infectious particle; then others could be identified with the aid of the electron microscope

Structure of viruses• very small (20 nm smallest)

• virion: nucleic acid & protein coat called a capsid

• genome can be ds DNA, ssDNA, dsRNA,

ssRNA

• linear or circular NA (4 - 700 genes)

• acellular

Identified by shape

• capsid – protein coat may be:

1) rod (helical)

ex: TMV

2) polyhedral

ex: adenovirus

3) complex (combination)

ex: T4 bacteriophage

• Some have viral envelopes, membranes cloaking their capsids derived from membrane of host cell

ex: HIV

Viral Reproduction

Lytic cycle (virulent viruses) p.3321) adsorption (attachment) – virus

attaches to host cell2) entry – viral NA enters host cell3) replication – virus NA takes over

host NA & makes new viral NA & viral protein

4) assembly – viral NA & proteins are joined to make new viruses

5) release – viruses break out & destroy host cell (cell is lysed)

Lysogenic cycle (temperate viruses) p. 333

1) adsorption2) entry – viral NA is integrated into

host NA called a prophage or provirus-may remain latent in host cell or may be triggered to complete lytic cycle

Possible triggers may include:

• Stress

• Increased temperature

ex: herpes simplex I virus

(oral)

Animal viruses

• Enveloped viruses may have easier access into cells

• Herpes viruses derive the envelope from nuclear membrane

Retroviruses have the most complicated life cycles

• Contain reverse transcriptase

RNA DNA ex: HIV

• In some cases, viral damage is easily repaired (respiratory epithelium after a cold), but in others, infection causes permanent damage (nerve cells after polio)

Emergent viruses• HIV

• New influenza viruses

• Ebola

due to :

1) mutation of existing viruses (flu strains)

2) spread of existing viruses from 1 species to another (bird flu, hantavirus)

3) dissemination of a viral disease from a small isolated population (AIDS)

• Viruses appear to cause certain human cancers

-hepatitis B – liver cancer

-Epstein-Barr virus – several cancers

including Burkitt’s lymphoma

-Papilloma viruses – cervical cancer

-HTLV -1 – adult leukemia

Plant viruses

• serious agricultural pests (stunt growth,

diminish yield)

• spread easily

• most are RNA viruses

Origin• taxonomic puzzle;

they do evolve, but they are not cells

Bacterial genome• circular chromosome with a few

associated proteins

• accessory genes found on smaller rings of DNA called plasmids

• replication is bidirectional from a single origin

Reproduction in bacteria

• Asexually:

binary fission – splitting in two

• Sexually:

1) transformation – gene transfer where bacterial cell assimilates foreign DNA from its environment

Reproduction in bacteria

2) transduction – gene transfer from one bacterium to another by a phage

a) generalized – random pieces of host’s DNA are packaged within phage capsid

b) specialized – prophage takes piece of bacterial chromosome with it

Reproduction in Bacteria

3) conjugation – direct transfer of genes between 2 temporarily joined bacteria

Antibiotic resistance

• bacterial genes may code for enzymes that destroy certain antibiotics

• these genes are carried by plasmids known as R plasmids (R = resistance)

• resistant bacteria survive to pass on these genes (causes increase in bacterial strains that are antibiotic resistant)

• R plasmids can be transferred by conjugation

episomes – plasmids that can integrate into bacterial chromosome

Transposons

transposons – described by Barbara McClintock; mobile segments of DNA that may move within a chromosome & to & from plasmids

a) conservative – changes location without replicating first

b) replicative - replicates, remaining in its original position, also inserting in a new location (can move genes to totally new areas)

Insertion sequences• simplest transposons

• consist only of DNA necessary for transposition (transposase)

• IS flanked by inverted repeats (noncoding

segments 20-40 nucleotides long)

• enzyme (transposase) recognizes these

inverted repeats; enzyme binds to catalyze

cutting and resealing

• While insertion sequences may not benefit bacteria in any specific way, composite transposons may help bacteria adapt to new environments.• For example, repeated movements of

resistance genes by composite transposition may concentrate several genes for antibiotic resistance onto a single R plasmid.

• In an antibiotic-rich environment, natural selection factors bacterial clones that have built up composite R plasmids through a series of transpositions.

• Transposable genetic elements are important components of eukaryotic genomes as well.

• In the 1940s and 1950s Barbara McClintock investigated changes in the color of corn kernels. – She postulated that the changes in kernel color only

made sense if mobile genetic element moved from other locations in the genome to the genes for kernel color.

– When these “controlling elements” inserted next to the genes responsible for kernel color, they would activate or inactivate those genes.

– In 1983, more than 30 years after her initial break-through, Dr. McClintock received a Nobel Prize for her discovery.

• The control of gene expression enables individual bacteria to adjust their metabolism to environmental change

• An individual bacterium, locked into the genome that it has inherited, can cope with environmental fluctuations by exerting metabolic control.– First, cells vary the number of specific enzyme

molecules by regulating gene expression.– Second, cells adjust the activity of enzymes

already present (for example, by feedback inhibition).

• For example, the tryptophan biosynthesis pathway demonstrates both levels of control.– If tryptophan levels are high, some of the

tryptophan molecules can inhibit the first enzyme in the pathway.

– If the abundance of tryptophan continues, the cell can stop synthesizing additional enzymes in this pathway by blocking transcription of the genes for these enzymes.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 18.19

Operator

• On – binding of activator protein stimulates transcription (usually the case)

• Off – binding of specific repressor protein

shuts off transcription

constitutive genes – unregulated genes

(always needed by cell)

Repressible operon• Inhibited by an anabolic end product (such

as trp - tryptophan)

• trp operon is an example

• tryptophan is a corepressor that binds to repressor protein, changing it to active state which switches off trp operon

• When trp decreases, repressor protein is no longer bound; operator not repressed, RNA polymerase attaches to promoter & trp synthesis continues

Inducible operon• function in catabolic pathways• inducer (substrate for pathway) derepresses

operon by inhibiting the otherwise active repressor protein

• example is lac operon (lactose)• has 3 structural genes, repressor innately

active, binding to lac operator & switching off the operon

• allolactose (lactose isomer) acts as inducer, binds to and inactivates repressor protein so operon can be transcribed

Repressible vs. Inducible

Repressible• inhibited by anabolic end product• inherently “on”• innately inactive repressor• under negative control• ex: trp operon

Repressible vs. inducible

Inducible• function in catabolic pathway• inherently “off”• innately active repressor• under negative & positive control• ex: lac operon

• Both of these are examples of negative control – operons are switched off by the active form of the repressor protein

• Positive control – when an activator molecule interacts directly with the genome to switch transcription on

• Positive gene control occurs when an activator molecule interacts directly with the genome to switch transcription on.

• Even if the lac operon is turned on by the presence of allolactose, the degree of transcription depends on the concentrations of other substrates.

– If glucose levels are low (along with overall energy levels), then cyclic AMP (cAMP) binds to cAMP receptor protein (CRP) which activates transcription.

• The cellular metabolism is biased toward the utilization of glucose.

• If glucose levels are sufficient and cAMP levels are low (lots of ATP), then the CRP protein has an inactive shape and cannot bind upstream of the lac promotor.– The lac operon will

be transcribed but at a low level.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 18.22b

• For the lac operon, the presence / absence of lactose (allolactose) determines if the operon is on or off.

• Overall energy levels in the cell determine the level of transcription, a “volume” control, through CRP.

• CRP works on several operons that encode enzymes used in catabolic pathways.– If glucose is present and CRP is inactive, then the

synthesis of enzymes that catabolize other compounds is slowed.

– If glucose levels are low and CRP is active, then the genes which produce enzymes that catabolize whichever other fuel is present will be transcribed at high levels.