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BIOLOGY 2C03 (150)
Joe Kim (16)

Module 10(1) - March 22 - BIO 2C03.docx

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McMaster University
Joe Kim

BIO 2C03 2013 Module 10 Lecture 1 – Transposable Elements (TE); control by siRNAs Transposable Elements (TE) – Definition and Significance  Transposable Elements (TE) – DNA sequences that are able to move from one location to another in the genome  Found in prokaryotes and eukaryotes  TE represents o 10% of fish genomes o 13% of nematode genomes o 45% of human genome o >80% of some plant genomes (eg/ maize)  TE can o Inactivate gene upon insertion o Regulate gene expression o Induce illegitimate recombination o Be considered as “selfish DNA” or “junk DNA”  TE have also been “domesticate” to perform some functions in the cell (eg/ in recombination or promoter function) Structure  TE are classified according to the mechanism of transposition 1. Class I (Retrotransposons) – replicate through a RNA intermediate and function by retrotransposition 2. Class II (DNA Transposons) – use a “cut and paste” mechanism  ORF = open reading frame  UPR = untranslated region  RT = revese transcriptase  EN = endonuclease  TIR = terminated inverted repeats Replication 1. Class I – retrotransposons  Retrotransposons require the action of reverse transcriptase and integrase (or endonuclease) for transposition  Both activities are encoded in ORF2  Retrotransposons function like retroviruses 2. Life Cycle of Retroviruses  HIV 1 BIO 2C03 2013  Oncogenic Rous Sarcoma Virus (RSV  ORF2 in Class I Retrotransposons is analogous to the pol gene of retrovirus o RT = reverse transcriptase o EN = endonuclease (integrase) 3. Class II – DNA Transposons  Most class II transposons function through a non-replicative mechanism in which the transposon is excised from one location and re- integrated in the DNA at a second location  Transposase, encoded by the Class II TE, recognizes the Terminal Inverted Repeats (TIR) to promote excision and insertion  DNA transposition can be replicative or nonreplicative  Both mechanisms depend on the activity of a transposase  Non-replicative transposition 2 BIO 2C03 2013  Replicative Transposition – several cellular enzymes are also required during transposition, including DNA polymerases, DNA ligases and resolvase Direct and Indirect Repeats in ET  TE insertion generates flanking direct repeats  Many transposable elements generate inverted repeats o Inverted repeats are present in retrotransposons o Expression of these inverted repeats triggers RNA interference, transcript degradation and chromatin silencing o Expression of transposable elements is therefore restricted in the cell 3 BIO 2C03 20
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