Molecular Biology of Prokaryotes VIRUSES AND BACTERIA

Look at the relative size of these cells.

Viruses Have a genome, but can only reproduce in a host cell. Structure: infectious particles consisting of nucleic acid enclosed in a protein coat Genome can consist of double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA (depends on virus type)

Capsids and Envelopes Capsids are the protein shell enclosing the viral genome. Most complex in viruses affecting bacteria (phages). Depending on the virus type, the capsid may be rod-shaped, polyhedral, or a more complex shape. Capsids are built from protein subunits called capsomeres. Viral envelopes surrounding viruses come from host cell phospholipids and membrane proteins.

Viral Reproductive Cycles Viruses are obligate intracellular parasites- they cannot reproduce without a host cell. Viruses lack enzymes for metabolism. No ribosomes for protein synthesis Viruses can affect only a limited number of host cells called the host range. Viruses identify host cells by a “lock and key” mechanism. Virus binds to receptor molecules on the host cell surface.

Lytic vs. Lysogenic Cycles Lytic cycles result in the death of the host. The bacterium lyses (breaks open) and releases phages produced by the cell. Virulent phages reproduce this way. In lysogenic cycles, the phage genome is replicated without destroying the host cell. Viral genome becomes incorporated into the bacterial chromosome as a prophage.

LYTIC CYCLE

Animal Viruses Viruses are classified by the form of genetic material they contain. (see chart in textbook) Nearly all animal viruses with RNA genomes have an envelope, as do some with DNA genomes. Some enveloped viruses enter the host cell by fusion of the envelope with the cell’s plasma membrane, similar to endocytosis.

Human Immunodeficiency Virus (HIV) Retrovirus that causes AIDS Reverse transcriptase transcribes an RNA template into DNA, providing a RNADNA information flow (opposite the usual direction)

This is hepatitis

Viral Diseases in Animals The damage done by a virus depends on the ability of the affected tissue to regenerate by cell division. The immune system is the basis for preventing viral infections by the use of vaccines. Vaccines are harmless variants or derivatives of pathogenic microbes that stimulate the immune system to defend against the pathogen.

Influenza virus

Flu epidemic of 1915

Child with small pox

Child with measles

Measles virus

Herpes simplex virus

Child with polio

Emerging Viruses Major source is the mutation of existing viruses Spread from one host species to another Spread of disease from a small, isolated population can lead to widespread epidemics Examples: HIV, Ebola, SARS

EBOLA

Why are emerging viruses so dangerous? Viruses can evolve very quickly, mutating rapidly. Viruses can sometimes jump between species, especially if exposed to populations without defenses against them Medications that treat viral diseases are difficult to develop, and often have to target host cells.

Plant Viruses More than 2,000 types of viral diseases affecting plants are known. Symptoms include: bleached or brown spots on leaves or fruits, stunted growth, damaged roots and flowers Decrease crop and food production Most have RNA genomes

Tobacco Mosaic Virus in squash

Horizontal vs. Vertical transmission of plant viruses Horizontal transmission: plant is infected by an external source of the virus. Plant is more vulnerable if it has been damaged in some way. Vertical transmission: plant inherits a viral infection from a parent. Occurs through asexual reproduction (cuttings) or sexual reproduction by infected seeds.

Electron Micrograph Of TMV

TMV isolated TTm

Viroids Viroids are plant pathogens composed of molecules of naked circular DNA several hundred nucleotides long. A single molecule can be an infectious agent that spreads disease.

Prions Prions are infectious proteins which appear to cause a number of degenerative brain diseases in animal species. Ex) Mad cow disease and Creutzfeld-Jakob Prions may be transmitted in food. Prions have two disturbing characteristics: 1) they have long incubation periods, and 2) heating fails to deactivate them

PROKARYOTES The short generation span of bacteria facilitates their adaptation to changing environments. Composed of one double-stranded circular molecule of DNA, found in a nucleoid region (no nucleus) Transcription and translation occur simultaneously. Some bacteria contain extrachromosomal DNA in plasmids.

Binary Fission After replication of the bacterial chromosome, bacteria divide by binary fission. Bacteria divide rapidly. For example, E.coli reproduce every 20 minutes. Binary fission is an asexual process, so most bacteria in a colony are identical to the parent cell.

E. coli dividing

Mutation and Genetic Variation Because bacteria can reproduce rapidly, new mutations can quickly increase a population’s genetic variation. Genetic recombination of DNA from two different bacterial cells increases diversity in populations as well.

Gene Transfer and Genetic Recombination New bacterial strains can arise by the transfer of DNA from one cell to another cell. In transformation, naked DNA enters the cell from the surroundings.

E. coli DNA

Gene transfer, cont. In transduction, bacterial DNA is carried from one cell to another by phages. In conjugation, an F+ donor cell, which contains F plasmid, transfers plasmid DNA to an F- recipient cell. R plasmids transfer resistance to antibiotics.

Plasmids

Conjugation: pili and plasmids