Transposition 2 Flashcards
who was the scientist who first discovered transposition?
Dr. Barabara McClintock
what can maize transposons cause?
maize transposons can cause chromosome breakage adn rearrangements
- controlling elements often insert next to genes with visible phenotypes in a heterozygote
variegation
the change in phenotypes that occurs during developments
- can result from insertions, deletions, or chromosome breakage and arrangement
- the Ds element is particularly prone to causing chromosome breakage
what is a sign that a Ds element jumped sooner? later?
- the sooner a Ds element jumped, the less color there will be, perhaps it will be spotted
- the later a Ds element jumps, the more color, for example the whole kernel being purple indicates the Ds element jumped very soon
what does Ds transposition result in?
accentric fragments, which are lost
- breakage of the chromosome
what is the Ds breakage-fusion-bridge cycle?
- fusion-bridge forms after a break at Ds
- replication occurs
- in the first fusion bridge cycle, sister chromatids form and acentric fragment is lost
- centromeres separate at mitosis
- resulting in random breakage and refusion causing a loss of alleles and duplication of others
autonomous transposition
- transpose independently
- encode their own mobility enzymes (transposase)
nonautonomous transposons
- need an autonomous transposon to provide the transposase through trans-activation
how much of the maize genome is comprised of transposons?
70%
- the genome of maize has roughly doubled in the last 6 million years due to transposase activity
how are Ds elements derived?
Ds elements arise by deletions of Ac
Ds elements
- are shorter than Ac elements (due to deletions) and contain the same 11 bp inverted repeats at each strand
double Ds element
has one inverted repeat inserted into another
- lands in the middle of a copy of itself
- these are especially prone to cause chromosome breakage
Ds1 elements
represent an extreme deleted form
- contain only 300-500 bo of sequences between the inverted repeats
- called MITES
- common in eukaryotes
MITES
- Ds1 element
- miniature inverted repeat transposable element
- common in many eukaryotes
Ac elements
- transposition by nonreplicative mechanism (cut and paste)
- transposition associated with genome replication, but does not occur if DNA is fully methylated
- target site is often near the donor site on the same chromosome, if it jumps to a site that has not yet replicated, it will result in an increase in number
autonomous maize transposon families
- Ac (activator)
- Mp (modulator)
- Spm (suppressor-mutator)
- En (enhancer)
- Dt (dotted)
- MuDR (mutator) - most active
nonautonomous maize transposon families
- Ds (dissociation)
- dSpm (defective Spm)
- I (inhibitor)
- rD (receptor of dotter)
- Mu
What is epigenetic silencing of MuDR?
- Robinson’s mutator
- Have a copy nearby that is a flip copy
- If read by RNA polymerase and have an inverted repeat to form a hairpin structure
- This will silence the duplex part and the mutator
- The ds RNA will then be chopped into small fragments (21) which will be attached to a complex that direct methylation of the Mu element. This triggers the formation of heterochromatin methylation of the DNA will inactivate the Mu element.
- or pre-RNA produced by run-on of cellular transcript synthesized from the opposite strand
- This type of silencing occurs in plants and animals and is a major way transposons are prevented from becoming active
When a P element inactivates, what occurs in drosophila?
the white locus (w) eye color goes from Red (wt) to white
what are the characteristics of P elements?
- length: 2.9 kb
- terminal inverted repeats: 31 bp
- target site direct repeats: 8 bp
- carries 30-40 copies in the genome, present on all chromosomes, but only 1/3 are full length
when are P elements active?
only active when male P strains are crossed with M females
hybrid dysgenesis
seen when cytogenic P male is crossed with an M female
- a series of defects arise including mutations, chromosomal aberrations and reduced fertility in the offspring
- no hybrid dysgenesis when a P female is crossed with an M male
How can the copy number of Ac increase even though it uses non-replicative transposition?
- It will catch the genome in the middle of replication and the host will reciprocate
- Host made a copy of Ac
- Jumps and goes to an area not yet replicated and the host genome will replicate the Ac
- Started with one, now you have 3
- Slowly increases Ac levels
what cells are P elements active in?
only germline
what controls the expression of P elements?
differential splicing between somatic and germline tissues
What keeps P elements from jumping in somatic cells?
- There is a protein that has an RNA binding domain that matches the 3rd intron splice of the transposase
- When the cellular proteins binds there, you cannot properly process mRNA
- Third intron has a stop codon and blocks splicing
- Truncated copy of the transposase gene
- Almost perfect - recognize inverted repeats, sit and chew but cannot release
- Acting as a repressor
- Preventing WT transposase from cutting
Why is the P element only active in germline tissues?
- Functional transposase produced in germline
- Transposition occurs when a chromosome carrying P elements is introduced into an egg having an M cytotype (no P element in cell previously)
- no blocking protein in germlines
How do P cytotype females suppress P element activity in their eggs and germline cells? Does it also operate in somatic tissues?
P cytotype female is immune to P element due to iRNA production
- certain strains of P infections gives rise to P cytotype
- When the RNA polymerase reads will read in the gene
If there is an inverted element it will run into the antisense strand of the P element, which will hybridize with the transposase and the hairpin forms as a silencing messaging
Interferes with translation
- epigenetic gene silencing
M cytotype female x M cytotype male
no P element, normal offspring
M cytotype female x P cytotype male
active P element, hybrid dysgenesis
P cytotype female x M cytotype male
inactive P element, normal offspring
- protected by iRNA in egg
P cytotype female x P cytotype male
inactive P element, normal offspring
- protected by iRNA in egg
Dicer
chops up the duplex RNA
agronaute complex
RNase that uses the 21 nt RNA as a guide to target mRNAs
