9.3 Genome Evolution and Mobile Elements Flashcards
What was the Nancy Kleckner experiment?
They infected bacteria with phages that have the tetracycline (Tc) resistant gene in their genome (the Tc resistant gene was a transposon called T10).
What was the argument in the Nancy Kleckner experiment?
If bacteria were able to grow on Tetracycline (i.e. they were resistant to Tc), then that phage had successfully integrated its genome into the host DNA
What were the results on NancyKleckner’s experiment?
(1) Several colonies grew on Tc.
(2) We expected to find phage DNA integrated into the host DNA, but no phage was observed in the DNA of these resistant colonies.
(3) These colonies had variated defects (some had defective flagella, other needed specific amino acids to grow, etc.)
What actually is the gene for Tc resistance?
A mobile element called transposon, that hoped off the phage and to the host DNA (via transposition) before the phage DNA was destroyed.
What may explain the defects observed in the Nancy Kleckner experiment?
Tc may have landed in protein-encoding genes
What are IS elements?
These are short sequences that act as the simplest transposable elements. They only code for proteins involved in the transposition activity. So they are unlike transposons, which can carry accessory genes such as antibiotic resistance gene
What are proteins encoded by the IS element?
(1) The transposase protein
2) Regulatory protein (which either stimulates or inhibits the transposition activity
What does the transposase protein do?
It will recognize terminal inverted repeats (TIR) of the IS elements and make a cut. The IS will get removed and then inserted in another DNA site.
What are Tn5 transposase and HIV-1 integrase in relation to one another?
Orthologs. They have evolved independently in different organisms but have similar structure and similar way of inserting DNA sequences into the genome
What are the 2 basic mechanisms for insertion of mobile elements?
(1) Cut and paste mechanism
(2) Replciative mechanism
What happens in the cut and paste mechanism?
Protein binds and forms complex with DNA. When it hits target DNA it transfers both strands.
What happens in the replicative mechanism?
This is used by some transposases. In this case, instead of transferring both DNA strands, only 1 strand is transferred. Replication at both the host and receipt strands will then reform the dsDNA.This results in an increase in genome size
What are the ways IS elements promote genome evolution?
(1) They promote integration and genetic exchange
(2) Gene duplication or deletion
(3) Creating mobile elements
Promoting integration and genetic exchange?
Some plasmids may have IS elements that are also present on the host DNA. These IS elements will recognize each other and will become the site of homologous recombination. Since the inserted plasmid has an OriT sequence (origin of transfer sequence), a large part of the DNA in which the plasmid is inserted can get mobilized with the plasmid during conjugation between bacteria. This leads to several gene exchanges between bacteria.
Gene duplication or deletion?
Homologous recombination naturally occurs between homologous chromosomes. When an IS element has several copies on a on a chromosome, uneven crossing over can occur during homologous recombination. This may lead to the duplication of some genes and the deletion of others. These duplicated genes may develop independently generating a set of paralogs.This duplication mechanism is the origin of the alpha and betta globulin genes that gave our hemoglobin protein
