DNA transposition (ex 3 lec 5)

what are transposable elements

they move from one DNA molecule to another

exist in all life forms

can move within/between chromosomes 

cant exist outside of larger DNA molecule

what is insertion sequence (IS)

simple jumping genes that can move to new locations within a genome (using transposition)

containing a transposase gene flanked by a short inverted repeat sequences

what is another way you can think of a transposable element

a piece of DNA that has become able to transfer itself from one location to another

heart of transposable element

it has a gene encoding a transposase

DNA sequences on either side of the element that can be recognized/acted on

what are the sequences in the heart of the transposable elements called 

inverted repeats 

they are identical to each other on opposite strands 

how is inverted repeat layed on

inverted repeat-transposase gene-inverted repeat

5’-3’

3’-5’

what happens after a trnasposase has been translated from a gene within the IS

it then binds to the inverted repeat sequences

at the same time it interacts with another piece of DNA

what does the transposase make after interacting with another DNA and binding to transposons

it makes a pair of staggered cuts in the target DNA 

cuts in different but nearby places on the two strands 

what happens to the IS (insertion sequence) after staggered cuts are made

it is either moved or copied into spaces between the two pieces of target DNA

the short single stranded regions are filled in

what are the two mechanisms transposable elements do

replicative transposition and non replicative transposition

nonreplicative transposition 

the elements jump from one site to another 

a cut and a paste T.S is completely from OG spot and inserted into a new one leaving no copy behind

replicative transposition

the element is copied and one copy remains in the original site

what are transposon

they are transposable elements (jumping genes) that can jump from one location to another within a genome

same kind of genes that are in plasmids, including genes for transfer

what are conjugative transposons (also called integrative and conjugative elements or ICE)

they transfer from one cell to another via conjugation

same we saw for conjugative plasmids

they contribute to the spread of antibiotic resistance genes

what is SXT 

a conjugative transposon carried by Vibrio cholerae

spreading of these genes from one member of the species to another can therefore have medical consequences 

what happens in a cell with a copy of the SXT transposon

the transposon is integrated into a specific site in the chromosome (between a pair of direct repeats called att sites) under normal conditions the transposon will be along for the ride

the encoding gene (integrase) is not expressed when SXT is integrated

what will happen if the cell that SXT is in it healthy

SXT will be replicated every time the chromosome is replicated and the cell divides

what happens to the SXT transposon under stressful conditions 

the SXT has ability to transfer to a new cell by conjugation 

gene duplication 

is the most important mechanism for generating new genes and biochemical processes

it frees a gene from its previous functional constraints and allows evolution through mutation 

paralogous genes (paralogs)

genes that are related to eachother because they arose from duplication

orthologous genes (orthologs)

genes that are related to each other because they arose from speciation

what is genome reduction

a large scale loss of genes through evolution 

nonfunctional pseudogenes

appear to encode an enzyme but are nonfunctional because a portion is missing as a result of mutation

what are pseudogenes formed from

genes that it no longer needs after becoming an obligate intracellular pathogen

genomic islands 

they provide evidence for horizontal gene transfer

pathogenicity islands, symbiosis islands and fitness islands 

characteristics that reveal identity of genomic islands

direct repeats or insertions of elements on either sides

remnant of transposition event