MBB 267 Week 4: Barnes 4 Flashcards
What are retrotransposons?
Retrotransposons or retroposons: Transposons that “jump” via an RNA intermediate
-“Copy and paste” mechanism
What are the three main families in retrotransposons?
LTR elements, LINES, and SINES.
What are LTR retrotransposons?
Long Terminal Repeat; and the sequence contains;
1-pol encodes three protein activities required for transposition: reverse transcriptase, RNAse H, integrase
2-Long terminal repeats (200-600 bp) are important for the reverse transcription mechanism
3-Direct repeats (5-10bp) are generated upon integration
What is the mechanism of LTR retrotransposon transposition?
Use reverse transcriptase mechanism involving the retrotransposon reverse transcriptase (creates cDNA from mRNA template) and RNAse H, which degrades the RNA template. This makes a double strandedDNA molecule. Integrase transports dsDNA into nucleus, dsDNA inserted into the genome, with creation of target-site direct repeats.
What are examples of LTR retrotransposons?
In;
- mammals; ERV = endogenous retrovirus
- -8% of the human genome
- –Only LTRs are left, rest of transposon are lost because of homologous recombination between two LTRs.
- Yeast: Ty elements
- -~35 copies of Ty1 in the haploid yeast genome
What is LINEs?
Non-LTR retrotransposons: Long INterspersed Elements (LINEs)
What does a LINE contain?
contains
- An A/T-rich region
- ORF1 = RNA-binding protein
- ORF2 = reverse transcrtiptase and DNA endonuclease
- Target-site desert repeat
What is the mechanism of LINE transposition?
The transposon is transcribed and translsated by host machinery, and then polyadenykated
- ORF1 protein binds LINE RNA, ORF2 protein binds to -LINE seq polyA, in the cytoplasm.
- RNA is transported into nucleus
- ORF2/polyA RNA binds to complementary polyA secion of the DNA
- Enconucelase activity from ORF2 nicks the DNA
- ORF2 uses its reverse transcritpase activity to be primed by host DNA sequence.
What is polyadenylation?
The addition of a poly tail of Adenines
What are the different activities that ORF2 can do?
reverse transcriptase and Endonuclease
What are SINEs?
Non-LTR retrotrabsposons, Short INterpersed Elements (SINEs)
How do SINEs work?
SINES are nonautonomous: require enzymes from LINEs to function.
-AT-rich sequences that bind to ORF1 and ORF2
What are Alu elements?
An Alu element is a short stretch of DNA originally characterized by the action of restriction endonuclease. Alu elements are the most abundant transposable elements, containing over one million copies dispersed throughout the human genome.
