BIOL 202- Transposable Elements

Maize endosperm

Double fertilization leads to 2 sperm and 1 egg

Triploid but rest of plant is diploid

Dissociation locus (Ds)

Chromosome 9 breakage that leads to only phenotype on the non broken chromosome

Can unmask recessive allele

Nonautonomous transposable element (needs Ac)

Activator locus (Ac)

Element in another chromosome that activates Ds breakage

Ac and Ds lead to excises and breakage

Encodes transposase that allows Ds to excise, autonomous transposable element

Ac heterozygote and Dc homozygote cross

50% no breakage (normal blue kernels)

50% mixed kernels (with Ac, uncovers c so blue with white spots)

Rare kernel where Ds jumped into C (c-m1) is white with blue spots

C vs c allele

C is blue (dominant)

c is white (recessive)

Dc can break off either allele is Ac present, leading to monoallelic

c-m1 allele

Mutable unstable allele

Ds jumps into C, creating a new allele

c-M1 and c give white kernel with blue spots

Ty elements in yeast

Family of transposons with LTR's on each side

Retrotransposon that is transcribed to RNA, converted into dsDNA, inserted back into yeast genome

Copy/paste

long terminal repeat (LTR)

Hundreds of bp, flank retroviruses and transposons

LTR retrotransposons

Class 1 transposable elements (copy/paste)

Retrotransposons with long terminal repeats that resemble retroviruses

Moves using a RNA intermediate that is inserted into DNA by reverse transcriptase

CANNOT LEAVE CELL (unlike retrovirus)

Do retrotransposons have introns

NO

copia-like element

Ty-like retrotransposons in drosophila that has LTRs

Solo LTR

Single LTR instead of 2

Drives His transcription in His revertant mutants

Galactose effect on Ty

Increased transcription of Ty DNA, suggesting it has galactose-sensitive promoter

DNA transposons

Class 2 transposable elements (cut/paste)

Moves to new location in genome without RNA intermediate

P elements are DNA transposons

P element

DNA transposon in Drosophila with inverted repeats and codes transposase for mobilization

hybrid dysgenesis

In drosophila, crossing a lab female with wild male leads to atrophic F1's

But lab male and wild female is normal F1

This is due to P elements in wild strain that is only silenced by female piRNA

M vs P cytotype

M is lab strain (no P elements)

P is wild strain (has P elements)

long interspersed elements (LINEs)

Long, repetitive sequences found interspersed in the genomes of higher organisms

Move like retrotransposons with reverse transcriptase but no LTR

short interspersed elements (SINEs)

Short DNA sequence repeated many times and interspersed throughout the genome, nonautonomous and mobilized by transcriptase by LINES

Transposable elements in humans

1. LINES (autonomous)

2. SINES (nonautonomous)

3. DNA transposons (both)

Alu

Most abundant SINE in humans, over 10% of genome

Target site for Alu restriction enzyme

Mostly introns but also in exons

Safe havens

Regions between genes where transposons insert into because there are few genes and lots of repeats

Usually in rRNA and tRNA (few genes)

Ty3 safe havens

Usually in tRNA because it doesn't disrupt essential genes

R1 and R2 in drosophila

LINES that only insert into rRNA genes because there are many tandem repeats, tolerable

Targets rRNA

unc-22 gene

Gene in C. Elegans that is target of Tc1, causing twitching by blocking its function

When Tc1 was excised, wt is expressed (normal smooth movement)

genome surveillance

Host can suppress transposons such as Tc1 to protect genome stability

Tc1

DNA transposon in C. Elegans that can insert into Unc-22 gene, disrupting it to cause twitching phenotype

Insertion causes twitching and allowed identification of unc-22 by mutant phenotypes

Usually REPRESSED

RNAi mechanism

1. Transposon (Tc1) forms dsRNA by inverted ends

2. Cut into siRNA by Dicer

3. siRNA and RISC complex can silence all Tc1's by degradation

piRNA

Silence transposons in germ cells to protect genome from damage, in egg so wild strain mother carries it

Binds Piwi and finds transposon DNA to destroy it

Pi clusters

Region in genomes that codes for clusters of piRNAs and trap transposons

Transposons are transcribed and processed into piRNA

Bind Piwi-argonaute

Complementary mRNA is immediately degraded and not translated