MBG 2040 Lecture Notes - Lecture 5: Lysogenic Cycle, Genetic Variability, Chromosome
2 May 2018
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Bacterial Genetics
Outline:
• Viruses and Bacteria in Genetics
• Mechanisms of Genetic Exchange in Bacteria
o Conjugation
o Transformation
o Transduction
• Evolutionary Significance of Genetic Exchange in Bacteria
Outcomes:
• Understand the differences in the genetics of prokaryotes and eukaryotes
• Differentiate between a lytic pathways and a lysogenic pathways, during which a virus'
chromosome is inserted into the chromosome of a bacterium
• Define the tree parasexual processes (transformation, conjugation, and transduction) that occur in
bacterial gene transfer
Bacteria and Viruses in Genetics:
• Bacteria and viruses have made most important contributions to the science of genetics
o Evidence for the nature of genetic material (DNA)
o First characteristics and definitions of a gene
o First evidence for gene interactions at the molecular level
o Basic principles of inheritance
• What makes bacteria valuable research subjects:
o Small size
o Rapid reproduction
o Selective media (e.g. antibiotics) that can easily identify the presence of an active allele
o Simple structures and physiology
o Genetic variability
• Bacterial Viruses (bacteriophages):
o Reproduce by infecting bacterial cells
o Several important genetic concepts have been discovered through studies of bacteriophages
o Bacteria grow in liquid or solid media
o Bacteriophages produces clearances (plaques) on plates with dense bacterial cultures within
hours of infection
• The clearances are areas of dead and lyzed cells
• Each clearance contains millions of phages
o Example 1: Bacteriophage T4
• *see slide for structure
• Protein head
• Genome contains 168,800 base pairs and 150 characterized genes
▪ This is a fairly large and complex genome
• The phage goes through a lytic phase, meaning it lyses the cell to infect other cells
▪ This feature provides for quick and simple genetic experiments
• Life Cycle:
▪ T4 bacteriophage attaches to an E. coli cell and injects its DNA
▪ Synthesis of phage-specific mRNAs begin
▪ Replication of phage DNA begins
• Host DNA has been degraded by phage-encoded nucleases
▪ DNA filled heads, tails lacking tail fibers, and assembled tail fibers first appear
▪ The first intact phage particles are assembled
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▪ The host bacterium is lysed, released about 300 progeny phage
o Example 2: Bacteriophage Lambda
• *see slide for structure
• Genome contains 48,502 base pairs and ~50 genes
• May be lytic (like T4) or lysogenic (inserts DNA into host cell genome and goes into
latency)
• Life Cycle:
▪ Infection:
• Tail binds to E. coli cell and transmits the lamda DNA in the head
• There is then a circular form of lambda chromosome (in addition to the E.
coli chromosome) within the cell
• The viral chromosome can then enter a lytic or lysogenic pathway
▪ Lytic Pathway:
• Viral chromosome duplicates and assembles into the head and tail
• Cell lysis then releases the phages
▪ Lysogenic Pathway:
• The viral chromosome enters the cell's chromosome via site-specific
recombination
• Linear packed form of the lambda chromosome is converted into a
circular intracellular form
• Site-specific recombination is mediated by lambda integrase
• The lambda prophage is then incorporated into the bacterial cell's DNA
• After recombination, a circular chromosome is evident in the
lysogenic E. coli cell
• The bacterial cell replicates with the prophage still within the cell's DNA
• Key Points:
▪ Bacteriophage lambda can enter a lytic pathway or a lysogenic pathways (during
which its chromosome is inserted into the chromosome of the bacterium)
▪ In its integrated state, the lambda chromosome is called a prophage, and its lytic
genes are kept turned off
▪ The study on the repression of lytic genes provided one of the first molecular
insights on the interaction of genes
• Genetics of Bacteria:
o Bacterial genomes are large, circular molecules of several million base pairs = bacterial
chromosomes
o Additional genetic material resides in plasmids (small circular DNA) and episomes (large
circular DNAs)
o Bacteria do not have meiosis, hence they cannot have chiasmata
o Nevertheless, bacterial recombination is possible
o Gene transfer in bacteria is unidirectional: from donor cells to recipient cells
• Phenotypes in Bacteria:
o Gene mutations can be readily observed via:
• Colony colour and morphology
• Nutritional mutants (can not metabolic certain sugars like lactose or galactose)
• Prototrophs and auxotrophs (cannot make certain amino acids, these need to be added
to the growth medium)
• Antibiotic resistance
• Unidirectional Gene Transfer in Bacteria:
o *see slide
o Gene transfer in bacteria produces a partially diploid recipient cell containing a fragment of
the donor cell's chromosome
find more resources at oneclass.com
find more resources at oneclass.com
