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molecular genetics > Transposition 2 > Flashcards

Transposition 2 Flashcards

1
Q

who was the scientist who first discovered transposition?

A

Dr. Barabara McClintock

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2
Q

what can maize transposons cause?

A

maize transposons can cause chromosome breakage adn rearrangements
- controlling elements often insert next to genes with visible phenotypes in a heterozygote

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3
Q

variegation

A

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

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4
Q

what is a sign that a Ds element jumped sooner? later?

A
  • 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
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5
Q

what does Ds transposition result in?

A

accentric fragments, which are lost
- breakage of the chromosome

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6
Q

what is the Ds breakage-fusion-bridge cycle?

A
  • 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
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7
Q

autonomous transposition

A
  • transpose independently
  • encode their own mobility enzymes (transposase)
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8
Q

nonautonomous transposons

A
  • need an autonomous transposon to provide the transposase through trans-activation
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9
Q

how much of the maize genome is comprised of transposons?

A

70%
- the genome of maize has roughly doubled in the last 6 million years due to transposase activity

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10
Q

how are Ds elements derived?

A

Ds elements arise by deletions of Ac

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11
Q

Ds elements

A
  • are shorter than Ac elements (due to deletions) and contain the same 11 bp inverted repeats at each strand
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12
Q

double Ds element

A

has one inverted repeat inserted into another
- lands in the middle of a copy of itself
- these are especially prone to cause chromosome breakage

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13
Q

Ds1 elements

A

represent an extreme deleted form
- contain only 300-500 bo of sequences between the inverted repeats
- called MITES
- common in eukaryotes

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14
Q

MITES

A
  • Ds1 element
  • miniature inverted repeat transposable element
  • common in many eukaryotes
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15
Q

Ac elements

A
  • 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
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16
Q

autonomous maize transposon families

A
  • Ac (activator)
  • Mp (modulator)
  • Spm (suppressor-mutator)
  • En (enhancer)
  • Dt (dotted)
  • MuDR (mutator) - most active
17
Q

nonautonomous maize transposon families

A
  • Ds (dissociation)
  • dSpm (defective Spm)
  • I (inhibitor)
  • rD (receptor of dotter)
  • Mu
18
Q

What is epigenetic silencing of MuDR?

A
  • 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
19
Q

When a P element inactivates, what occurs in drosophila?

A

the white locus (w) eye color goes from Red (wt) to white

20
Q

what are the characteristics of P elements?

A
  • 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
21
Q

when are P elements active?

A

only active when male P strains are crossed with M females

22
Q

hybrid dysgenesis

A

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

23
Q

How can the copy number of Ac increase even though it uses non-replicative transposition?

A
  • 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
24
Q

what cells are P elements active in?

A

only germline

25
Q

what controls the expression of P elements?

A

differential splicing between somatic and germline tissues

26
Q

What keeps P elements from jumping in somatic cells?

A
  • 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
27
Q

Why is the P element only active in germline tissues?

A
  • 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
28
Q

How do P cytotype females suppress P element activity in their eggs and germline cells? Does it also operate in somatic tissues?

A

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

29
Q

M cytotype female x M cytotype male

A

no P element, normal offspring

30
Q

M cytotype female x P cytotype male

A

active P element, hybrid dysgenesis

31
Q

P cytotype female x M cytotype male

A

inactive P element, normal offspring
- protected by iRNA in egg

32
Q

P cytotype female x P cytotype male

A

inactive P element, normal offspring
- protected by iRNA in egg

33
Q

Dicer

A

chops up the duplex RNA

34
Q

agronaute complex

A

RNase that uses the 21 nt RNA as a guide to target mRNAs

molecular genetics (28 decks)

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