18) Transposons

Question 1

What are transposons? How were they found?

Answer 1

Mobile genetic elements
Barbara McClintock: Saw that mutations were unstable and could revert back to original phenotype
Kernel colours
Dominant C allele= blue/purple kernel colour
Recessive c allele= Mutate= Give yellow allele
BUT sometimes yellow alleles will mutate back to give blue cells again
Another gene: To give unstable yellow ‘mutants’= Ds genes
Thought that: Ds genes could move into C allele, then daughter cells would switch back to being blue again

Question 2

What are the different classes of Transposons?

Answer 2

Class II: Move by using DNA intermediates
DNA is cut out of the genome and moved somewhere else= Use DNA intermediate
‘Cut and paste’ and is catalysed by transpose enzyme

Class I: Movement is mediated by RNA intermediate
Transposon is first transcribed usually by RNA pol II= RNA transcript= Transcript is then reverse transcribed to give DNA sequence that is then inserted back into the genome somewhere else
Can also be subdivided into:
1) LTR retrotransposons= long sequences that contain various signals for targeting and movement of transposons
2) Non-LTR:
Don’t need LTRs to identify where TEs are sitting= Many TEs make do with much shorter sequences called TSDs

Most are silenced

Question 3

What do transposons carry?

Answer 3

Carry bits of genome with them when they move= Exon shuffling and result in rapid creation of new proteins