L9: Transposition Flashcards
who is Barbara McClintock
- plant geneticist
- won Nobel Prize for identifying mechanism of jumping genes (transposons) in corn/maize
Barbara McClintock - how does transposons work in corn?
- kernel color gene is next to transposable element
- bc of this they are prone to being turned off/inactivated
- transposable element can ‘jump’ onto gene
- causes a mutation which changes the original color
Barbara McClintock - how does the kernel get spotted colors
- the transposable element hops in and then out again
- can get clones or different regions of cells where some of it has a functional kernel color gene and a functional transposable element
what are the two classes of genetic recombination
- conservative site-specific recombination (CSSR)
- transpositional recombination (transposition)
two classes of genetic recombination - conservative site-specific recombination
- recombination between two specific sequence elements
- not random, is specific
two classes of genetic recombination - transpositional recombination
- recombination between a specific sequence (jumping gene) and a nonspecific sequence (where the gene ends up)
- nonspecific sequence is random
homologous recombination vs site-specific recombination comparison - homologous recombination
- occurs between two homologous DNA segments
- little specificity about the site at which crossover occurs
- Holliday junction formation can move and increase the amount of DNA being exchanged
homologous recombination vs site-specific recombination comparison - site-specific recombination
- short, unique nucleotide sequences in two DNA molecules
- recognized by enzymes called recombinases which catalyze the joining of the two molecules
conservative site-specific recombination (CSSR) - importance from a molecular biology standpoint
- specific recombination sites allow certain phages integrate within the genome of their bacterial host to become a prophage
- but they need to move around to infect other cells (jumping genes)
conservative site-specific recombination (CSSR)
- the mobile DNA contains recombination sites (short specific sequence elements)
- this generates three types of DNA rearrangements depending on recombination site orientations
- requires recombinase
conservative site-specific recombination (CSSR) - three types of DNA rearrangements
- this is what prophages will do to excise itself from the genome:
1. direct repeats: deletion
2. inverted repeats: inversion
3. on two different DNA molecules: insertion - all are reversible
conservative site-specific recombination (CSSR) - direct repeats: deletion
- sites initially are similar in sequence and orientated in the same direction
- recombinase then deletes some genes
- deletion then results in a circular DNA molecule
- other genes deleted will join together
conservative site-specific recombination (CSSR) - inverted repeats: inversion
- sites initially are inverted (pointed away/towards from each other)
- recombinase causes DNA to be flipped in orientation
- nothing is lost/excised
conservative site-specific recombination (CSSR) - on two different DNA molecules: insertion
- sites initially have two molecules with contemporary recombination sites
- recombinase will integrate the DNA into another piece of DNA
- reverse of deletion
conservative site-specific recombination (CSSR) - what is recombinase
an enzyme that recognizes recombination and facilitates recombination
explain the biological role of CSSR via Hin Recombinase of Salmonella
- Hin recombinase: inverts a chromosomal segment to allow expression of two alternative forms of the protein flagellin (H1 or H2)
- its a common target for the immune system since its superficial
- some bacteria in a population switch forms randomly to escape recognition from the immune system
biological role of CSSR via Hin Recombinase of Salmonella - how does the gene switch forms
- an invertible segment contains genes for Hin recombinase and a promoter
- behind it are inverted hixL and hixR recombination sites
- Hin recombinase promotes inversion
- then the promoter drives expression of fljB (a H2-type flagellin) and fljA (a repressor protein of H1-type flagellin)
biological role of CSSR via Hin Recombinase of Salmonella - what results in the inversion of the locus
- hixR, fljB, and fljA is not expressed
- hixL is instead expressed
- H2 is no longer expressed (due to fljB not expressed)
- H1 is expressed (due to fjlA not expressed)
transpositional recombination
- the mobile genetic element is called a transposable element or transposon
- movement can occur with or without duplication of the element (copy and paste mechanism)
- transposition is a common source of new mutations since there is no selectivity in insertion sequence
transpositional recombination - human genome composition
- little protein-coding genes
- many transposable elements
transpositional recombination - DNA transposons
- often carry genes for recombinase enzyme called transposase which recognizes inverted repeat recombination sites
- transposase enzyme cuts inverted repeats to move to new location
- not like recombinase which inverts sequence flanked by repeats
- results in a “cut-and-paste” mechanism
transpositional recombination: DNA transposons - “cut-and-paste” mechanism
liberating itself from the genome and being incorporated somewhere else
transpositional recombination: “cut-and-paste” mechanism - what is the problem with this mechanism?
- how is the transposon maintained?
- it should be lost by genetic drift
transpositional recombination: “cut-and-paste” mechanism - what is the answer to the problem?
- transposition often occurs when DNA is hemi-methylated
- and after the replication fork passes by and therefore can be replicated
transpositional recombination: “cut-and-paste” mechanism - what happens during replication
- the transposition could jump to upstream the replication fork, leaving a double stranded break behind
- when replication fork continues, you will get an extra copy upstream (integrated in both strands of DNA)
transpositional recombination: “cut-and-paste” mechanism - what happens to the double stranded break left behind?
- repaired by NHEJ (not ideal)
- but since there is a homologous strand nearby and can be used as a template
- so can use homology directed repair (more precise)
- creates the transposition on that end
