Chapter 18

Microbial Models: The Genetics of Viruses and Bacteria

In 20 years, 22 million have died from AIDS.

The 1918 flu epidemic killed 675,000 Americans and
more than 20 million people around the world

Outline

• Viruses

• Bacteria

• Control of Gene Expression in Bacteria

Figure 18.2 Comparing the size of a virus, a bacterium, and a eukaryotic cell

Viruses

• A virus is an infectious particle consisting of nucleic acid enclosed in a protein coat.

• The protein shell is called a capsid.

• Some viruses that infect animals are also surrounded by a membrane called a viral envelope.

Figure 18.4 Viral structure

Viral Reproduction

• Viruses can reproduce only within a host cell.

– They are ‘obligate intracellular parasites”.

• Viruses lack the enzymes for metabolism, production of proteins, etc, and rely on the host cell for these functions.

• When a virus infects a host cell, it usually takes over the machinery of the cell and directs the cell to make copies of it’s genome and proteins.

• Isolated viruses are packaged sets of genes in transit from one host cell to another.

Figure 18.5 A simplified viral reproductive cycle

Viral Reproduction

• Each type of virus can infect and parasitize only a limited range of host cells, called its host range.

• Virus recognize host cells by a lock and key fit between proteins on the outside of the virus, and receptors on the outside of the cell.

• Some viruses have broad host ranges, infecting several species.

• Other viruses have narrow host ranges, infecting only a single species.

• In eukaryotes, viruses are often specific for particular tissues or cell types.

– HIV infects T4 lymphocytes.

– Adenovirus infects cells lining the upper respiratory tract.

Cells in all kingdoms are vulnerable to viral infection.

HIV

• HIV, the virus that causes AIDS is a retrovirus.

• Retroviruses have an RNA genome, however a polymerase called “reverse transcriptase” makes a DNA copy of the genome.

• This copy can be inserted into the host cells genome, where it can lie dormant, passively replicating with the host cell for years.

• HIV is an enveloped virus, the specifically recognizes T4 lymphocytes and macrophages, disabling the host’s immune system.

Figure 18.10 HIV, a retrovirus

Emerging Viruses

• Many outbreaks of viruses occur suddenly.

– HIV, Hanta virus, Ebola.

Emerging Viruses

• Three factors contribute to the emergence of viral diseases:

1. Mutation of existing viruses.

• Most emergent viruses have RNA genomes.

– Influenza, ebola and hanta virus.

• RNA viruses have very high mutation rates.

– No proofreading during replication.

2. Spread of existing viruses from one host to another.

- Initially virus is extremely virulent in the new host.

- Due to natural selection, virulence decreases over time.

3. Spread of a viral disease from a small, isolated population.

• Environmental change, development of isolated areas.

Viruses and Cancer

• Some viruses can cause cancer in humans and other animals.

• These viruses often carry mutated forms of host genes called “oncogenes”.

• Oncogenes are mutated forms of normal host cell genes that cause cell proliferation.

Cell Division in Prokaryotes

• Bacterial genome is a single circular, DNA molecule.

– Attached to the plasma membrane

– Replication origin controls replication of the DNA

Cell Division in Prokaryotes

• Mechanism of cell division is called binary fission.

– Cell grows to about 2X normal size

– DNA is replicated

– Cell membrane and cell wall are assembled between the attachment sites of replicated DNA molecules, dividing the cell into two cells.

Genetic Variety in Humans and Bacteria

• Bacteria reproduce asexually, so there is less genetic variation.

• Asexual reproduction, and a small simple cell type allow for a short generation time.

• 20 minutes in E. coli.

• Genetic variation only occurs through new mutations, and exchange of genetic information between bacterial cells.

How new genetic information can enter a bacterial cell.

• Transformation – uptake of plasmid DNA molecules from the environment that confer new traits.

• Transduction – infection by a phage – genetic material is injected into the bacterial cell.

• Conjugation – Plasmid DNA from one cells is passed to another cell.

Plasmids

• Small circular DNA molecules (about 3-4,000 bp).

• Self replicating.

– Carry a replication origin

• F plasmids can transfer genes from one cell to another (conjugation).

• R plasmids carry genes for antibiotic resistance.

– Can enter an E. Coli cell by transformation

Figure 18.16 Transduction

Bacterial mating

Transcriptional Control in Prokaryotes

• Repressors are OFF switches

• Activators are ON switches

• Genes used for the same process are often found in clusters called operons.

Example of repression: the trp operon

• When tryptophan is present in the bacteria medium, the trp repressor shuts off transcription of the trp genes.

• When tryptophan is not present, the trp represssor is inactive, and transcription of the trp genes occurs.

Figure 18.20 Regulation of a metabolic pathway

Figure 18.21 The trp operon: regulated synthesis of repressible enzymes

The End.