4. Role transposons in evolution

Question 1

Transposons

Answer 1

Are able to multiply and insert themselves almost anywhere in the genome

Question 2

Tandem repeat

Answer 2

a sequence of two or more DNA base pairs that is repeated in such a way that the repeats lie adjacent to each other on the chromosome

Question 3

TEs Class I: Retro-elements: Amplify via RNA

Answer 3

• Long terminal repeat (LTR) retrotransposons

- Non-LTR retrotransposons

Question 4

TEs Class II (helitrons): DNA elements: ‘Cut and paste’ transposition mechanism

Answer 4

• Class II transposons are able to move about the genome using DNA only, whereas class I transposons require an RNA intermediate to do so.
• Class II or DNA transposons are typically unable to copy themselves.

Question 5

“Target Primed Reverse Transcription” (TPRT)

Answer 5

Transposon transcript acts as template for DNA synthesis.
Starts with a single stranded nick in the DNA double helix, followed by hybridization: DNA-RNA.
Free and base-paired 3’OH nt of the DNA strand acts as primer

Question 6

Consequences of active retrotransposons

Answer 6

• Cis retrotransposition (= amplifies itself)
• Trans retrotransposition (=amplified with the help of another element)
• Insertional mutagenesis > deleterious mutations but also novel combinations
• May trigger unequal homologous recombination (Cross-overs during meiosis and mitosis)
• May create ‘new’ genes by inducing “exon-shuffling”

Question 7

TE exaptation

Answer 7

a process whereby TEs, which usually persist by replicating in the genome, transform into novel host genes, which thereafter persist by conferring phenotypic benefits

Question 8

DNA methylation

Answer 8

DNA methylation is arguably the best characterised regulator of transposable element activity. DNA methylation is associated with transposable element repression, and acts to limit their genotoxic potential