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Questions and Answers
What is one potential effect of transposable elements on genes?
What is one potential effect of transposable elements on genes?
Which hypothesis suggests that transposable elements provide important functions for the cell?
Which hypothesis suggests that transposable elements provide important functions for the cell?
What role do transposons play in plasmids?
What role do transposons play in plasmids?
Which hypothesis proposes that transposable elements create evolutionary flexibility by inducing mutations?
Which hypothesis proposes that transposable elements create evolutionary flexibility by inducing mutations?
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What does the selfish-DNA hypothesis state about transposable elements?
What does the selfish-DNA hypothesis state about transposable elements?
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What are transposable elements primarily known for?
What are transposable elements primarily known for?
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Who discovered transposable elements and in which organism?
Who discovered transposable elements and in which organism?
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What are flanking direct repeats in relation to transposable elements?
What are flanking direct repeats in relation to transposable elements?
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What is the role of transposase in transposable elements?
What is the role of transposase in transposable elements?
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How do retrotransposons move within the genome?
How do retrotransposons move within the genome?
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What are terminal inverted repeats associated with transposable elements?
What are terminal inverted repeats associated with transposable elements?
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What common feature do all types of transposition share?
What common feature do all types of transposition share?
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Which of the following statements about DNA transposons is true?
Which of the following statements about DNA transposons is true?
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What defines a replicative transposition process?
What defines a replicative transposition process?
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Which of the following is a characteristic of Class II transposable elements?
Which of the following is a characteristic of Class II transposable elements?
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What is a primary feature of insertion sequences found in bacterial transposable elements?
What is a primary feature of insertion sequences found in bacterial transposable elements?
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What distinguishes composite transposons from insertion sequences?
What distinguishes composite transposons from insertion sequences?
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Which of the following is not a function of the transposase gene?
Which of the following is not a function of the transposase gene?
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Which example is classified as a Class I transposable element?
Which example is classified as a Class I transposable element?
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What is true about transposable elements lacking insertion sequences?
What is true about transposable elements lacking insertion sequences?
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Which of the following correctly describes the structure of Class I transposable elements?
Which of the following correctly describes the structure of Class I transposable elements?
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Study Notes
Transposable Elements
- Mobile DNA sequences found in all organisms
- Frequently cause mutations and DNA rearrangements
- Discovered in the 1940s by Barbara McClintock while studying maize genetics
General Characters
- Most generate short, flanking direct repeats at the target site when inserted
- Many possess short terminal inverted repeats
- Flanking direct repeats are 3 to 12 bp long and present on both sides of most transposable elements
- Not part of the transposable element, they are generated during insertion
- Terminal inverted repeats are 9 to 40 bp long and are inverted complements of one another
- Found at the ends of many, but not all, transposable elements
Transposition
- Movement of transposable elements from one location to another
- Several different mechanisms are used for transposition in prokaryotic and eukaryotic cells
- All types of transposition have several features in common:
- Staggered breaks are made in the target DNA
- The transposable element is joined to single-stranded ends of the target DNA
- DNA is replicated at the single-strand gaps
Mechanisms of Transposition
- Can occur through a DNA molecule or through the production of an RNA molecule that is then reverse transcribed into DNA
- DNA Transposons (class I transposable elements): transpose as DNA
- Retrotransposons (class II transposable elements): transpose through an RNA intermediate
- RNA is transcribed from the transposable element (DNA) and then copied back into DNA using reverse transcriptase
- Transposition can be replicative (the element is copied and the copy moves) or non-replicative (the element excises and moves)
Characteristics of Two Major Classes of Transposable Genetic Elements
Class I
- Structure: Short, terminal inverted repeats; short flanking direct repeats at the target site
- Genes encoded: Transposase gene (and sometimes others)
- Transposition: Through DNA (replicative or non-replicative)
- Examples: IS1 (E. coli), Tn3 (E. coli), Ac, Ds (Maize), p elements (Drosophila)
Class II
- Structure: Long, terminal direct repeats; short flanking direct repeats at the target site
- Genes encoded: Reverse transcriptase gene (and sometimes others)
- Transposition: By RNA intermediate
- Examples: Ty (Yeasts), copia (Drosophila), Alu (Human)
Structure of Transposable Elements
Bacteria
- Two major groups:
- Simple transposable elements (insertion sequences): Carry only the information required for movement
- More complex transposable elements (composite transposons): Contain DNA sequences not directly related to transposition
- Insertion Sequences:
- Simplest type of transposable element in bacterial chromosomes and plasmids
- Carry only the genetic information necessary for their movement
- Typically 800 to 2000 bp in length and possess terminal inverted repeats and flanking direct repeats at the site of insertion
- Contain one or two genes that code for transposase
- Composite Transposons
- Segment of DNA flanked by two copies of an insertion sequence
- Consists of a central region with extra genes, flanked by identical or very similar IS elements
- Contain genes other than those required for transposition
- Lacking insertion sequences are referred to as non-composite transposons
- Example: Tn3 (5000 bp - Possesses terminal inverted repeats of 38 bp and generates flanking direct repeats that are 5 bp in length)
Effects of Transposable Elements
- Can insert within a gene to cause a mutation or stimulate DNA rearrangement leading to deletions of genetic material
- Used to purify genes and study their function
- Can fragment the genome and isolate the mutated fragment, purifying the gene
- Can turn on and off eukaryotic and prokaryotic genes
- Located within plasmids and participate in processes like plasmid fusion and the insertion of F plasmids into the E. coli chromosome
- Play an extremely important role in the generation and transfer of new gene combinations
Evolutionary Significance of Transposable Elements
- Cellular-function hypothesis: Suggest that transposable elements provide some important function for the cell
- Genetic-variation hypothesis: Proposes that transposable elements provide evolutionary flexibility by inducing mutations
- Selfish-DNA hypothesis: Suggests that transposable elements do not benefit the cell but are widespread because they can replicate and spread
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Description
Explore the fascinating world of transposable elements, the mobile DNA sequences that play a crucial role in genetics. This quiz covers their characteristics, mechanisms of transposition, and historical significance, particularly in relation to Barbara McClintock's discoveries. Test your knowledge on how these elements cause mutations and DNA rearrangements.
