chapter 8 – bacterial and viral genetic systems
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Chapter 8 – Bacterial and Viral Genetic Systems. Bacteria. Prototrophic Wild-type Can grow on minimal media Contains minimal nutrients – carbon, nitrogen, phosphorous, vitamins, ions Auxotrophic Can not produce an essential enzyme or manufacture essential molecules - PowerPoint PPT PresentationTRANSCRIPT
Chapter 8 – Bacterial and Viral Genetic Systems
Bacteria• Prototrophic
– Wild-type– Can grow on minimal media
• Contains minimal nutrients – carbon, nitrogen, phosphorous, vitamins, ions
• Auxotrophic– Can not produce an essential enzyme or manufacture
essential molecules– Will only grow on media that contains the “missing”
substance • Complete media
Culturing bacteria• Suspension culture
– Liquid media
– Bacteria dies off when nutrients are used up or waste buildup becomes toxic
– Bacteria grow singularly – no colonies
Culturing bacteria
• Petri dishes
– Growth media in agar
– Isolate individual colonies
• Each colony originates from a single bacterium
Replica plating• Gives “carbon
copies” of petri dish colonies
• Use sterilized velvet to make a stamp– Some bacteria from
each colony is transferred to velvet, and then transferred to new dishes
Bacterial genome• Most consist of a single,
circular chromosome– Some have several
chromosomes, and a few have linear chromosomes
• Very little “extra” DNA between genes
• Plasmids – Small, circular, extra-
chromosomal DNA• Usually non-essential
– Replicate independent of chromosomal DNA
• Have their own origin of replication
F factor episome
• Episome– A plasmid that
can replicate independently AND also has the ability to incorporate into chromosomes
Gene transfer in bacteria - Conjugation• One bacteria directly transfers DNA to another bacterium
• Cytoplasmic connection forms, and either entire plasmid or part of the chromosome is transferred from donor to recipient
• Crossing over may occur between homolgous regions– Creates recombinant DNA– Extra DNA is degraded
Gene transfer in bacteria - Transformation• Bacteria takes up DNA from surrounding
environment
• Recombination may occur
Gene transfer in bacteria - Transduction
• Viral particle introduced DNA from a bacterium into a new bacterium
Conjugation• Fertility factor/F factor contains ori and genes needed for
conjugation
Conjugation• Hfr bacteria
– F+ cell that has F factor incorporated into chromosome
• As F factor enters recipient, some chromsome enters – amount depends on time length of contact
• Donor DNA made into double-stranded– Crossing over can occur
between homologous regions
– Any DNA not incorporated is degraded
• Recipient is not usually converted to F+ since the F factor is nicked in the middle
Conjugation• F′ bacteria
– F factor excises out of a chromosome in a Hfr cell• May remove part of chromosome as well
– F′ plasmid now contains F factor and some genes from chromosome• Enters F- bacteria
– Produces merozygotes – partially diploid
Transformation• Uptake of DNA and incorporation into chromosome or plasmid
• Competent – cells able to take up DNA– CaCl2, heat shock, electrical fields
• Makes membrane more permeable to DNA– DNA does not have to have bacterial origin
• Transformants – bacteria that have incorporated foreign DNA
E. Coli has model organism• Many strains are avirulent
• Small and rapid reproduction
• Easy to culture
• Genome is single chromosome - haploid– Small genome– 4.5 million bp/4,000 genes
• Wild-type are prototrophic
Viral genetics• DNA or RNA
(single or double stranded) as genetic material
• Can not reproduce on their own
• Bacteriophages – viral particles that infect bacteria
Bacteriophage – lytic cycle• Virulent phages
• Viral DNA is injected into host cell where it replicated, transcribed, and translated into more phages
• Host cell bursts open to release viral particles
Bacteriophages – lysogenic cycle• Temperate phages
• Phage DNA is incorporated into host genome – prophage
• Passed onto all progeny cells
• Can be transcribed and translated
• Can exit from host genome to enter lytic cycle
Transduction• Generalized
– Any gene is transferred– During lytic cycle, bacterial DNA is degraded
• Some may enter viral protein coat instead of viral genetic material – Transducing phages
• Can become incorporated into new host’s genome
Transduction• Specialized
– Few genes are transferred/genes near certain sites of chromosome
– During lysogenic cycle, prophage enters at specific sites of host’s genome
– When prophage excises, it may do so imperfectly and bring some hot DNA with it
• Then introduced to new host
RNA viruses• Positive strand RNA viruses
– Single strand directly codes for viral proteins
• Negative strand RNA viruses– Must make complementary RNA strand, which then codes for proteins
• Retroviruses– Incorporate into host genome
• Must make DNA from RNA• Reverse transcriptase
– Makes cDNA from DNA or RNA template
– Enters host genome as a provirus• Can be transcribed and translated
– Some retroviruses contain oncogenes• Cause tumors