Transposition 2

Question 1

who was the scientist who first discovered transposition?

Answer 1

Dr. Barabara McClintock

Question 2

what can maize transposons cause?

Answer 2

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

Question 3

variegation

Answer 3

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

Question 4

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

Answer 4

• 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

Question 5

what does Ds transposition result in?

Answer 5

accentric fragments, which are lost
- breakage of the chromosome

Question 6

what is the Ds breakage-fusion-bridge cycle?

Answer 6

• 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

Question 7

autonomous transposition

Answer 7

• transpose independently
• encode their own mobility enzymes (transposase)

Question 8

nonautonomous transposons

Answer 8

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

Question 9

how much of the maize genome is comprised of transposons?

Answer 9

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

Question 10

how are Ds elements derived?

Answer 10

Ds elements arise by deletions of Ac

Question 11

Ds elements

Answer 11

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

Question 12

double Ds element

Answer 12

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

Question 13

Ds1 elements

Answer 13

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

Question 14

MITES

Answer 14

• Ds1 element
• miniature inverted repeat transposable element
• common in many eukaryotes

Question 15

Ac elements

Answer 15

• 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

Question 16

autonomous maize transposon families

Answer 16

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

Question 17

nonautonomous maize transposon families

Answer 17

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

Question 18

What is epigenetic silencing of MuDR?

Answer 18

• 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

Question 19

When a P element inactivates, what occurs in drosophila?

Answer 19

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

Question 20

what are the characteristics of P elements?

Answer 20

• 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

Question 21

when are P elements active?

Answer 21

only active when male P strains are crossed with M females

Question 22

hybrid dysgenesis

Answer 22

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

Question 23

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

Answer 23

• 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

Question 24

what cells are P elements active in?

Answer 24

only germline

Question 25

what controls the expression of P elements?

Answer 25

differential splicing between somatic and germline tissues

Question 26

What keeps P elements from jumping in somatic cells?

Answer 26

- 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

Question 27

Why is the P element only active in germline tissues?

Answer 27

- 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

Question 28

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

Answer 28

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

Question 29

M cytotype female x M cytotype male

Answer 29

no P element, normal offspring

Question 30

M cytotype female x P cytotype male

Answer 30

active P element, hybrid dysgenesis

Question 31

P cytotype female x M cytotype male

Answer 31

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

Question 32

P cytotype female x P cytotype male

Answer 32

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

Question 33

Dicer

Answer 33

chops up the duplex RNA

Question 34

agronaute complex

Answer 34

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