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Questions and Answers
What role do transposable elements play in genetic material?
What role do transposable elements play in genetic material?
Which hypothesis suggests that transposable elements provide an important function for the cell?
Which hypothesis suggests that transposable elements provide an important function for the cell?
What is a significant characteristic of transposons included in plasmids?
What is a significant characteristic of transposons included in plasmids?
Which statement best aligns with the selfish-DNA hypothesis regarding transposable elements?
Which statement best aligns with the selfish-DNA hypothesis regarding transposable elements?
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What are transposable elements primarily known for?
What are transposable elements primarily known for?
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Which of the following best describes terminal inverted repeats?
Which of the following best describes terminal inverted repeats?
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What mechanism is commonly used during the process of transposition?
What mechanism is commonly used during the process of transposition?
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What distinguishes retrotransposons from DNA transposons?
What distinguishes retrotransposons from DNA transposons?
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Flanking direct repeats generated during transposition are how long?
Flanking direct repeats generated during transposition are how long?
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Who first discovered transposable elements and in what organism?
Who first discovered transposable elements and in what organism?
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Which of the following statements about transposition is incorrect?
Which of the following statements about transposition is incorrect?
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What role does transposase play in transposable elements?
What role does transposase play in transposable elements?
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What is the key difference between replicative and non-replicative transposition?
What is the key difference between replicative and non-replicative transposition?
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Which of the following best describes Class I transposable elements?
Which of the following best describes Class I transposable elements?
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What type of transposable element contains only the necessary information for its movement?
What type of transposable element contains only the necessary information for its movement?
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Which gene is typically encoded by Class II transposable elements?
Which gene is typically encoded by Class II transposable elements?
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What defines a composite transposon?
What defines a composite transposon?
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How do transposons in bacteria differ from those in eukaryotic organisms?
How do transposons in bacteria differ from those in eukaryotic organisms?
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What characteristic is shared by both insertion sequences and composite transposons?
What characteristic is shared by both insertion sequences and composite transposons?
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Which of the following examples represents a Class I transposable element?
Which of the following examples represents a Class I transposable element?
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Study Notes
Transposable Elements
- Mobile DNA sequences found in all organisms.
- Frequently cause mutations and DNA rearrangements.
- Self-regulate transposition by controlling transposase or inhibiting the transposition event.
- Discovered in the 1940s by Barbara McClintock while studying maize genetics.
General Characteristics
- Most generate short flanking direct repeats at the target site when they insert.
- Many possess short terminal inverted repeats.
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Flanking direct repeats
- 3 to 12 bp long and are present on both sides of most transposable elements.
- Not part of the transposable element and don’t travel with it.
- Generated during transposition at the point of insertion.
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Terminal inverted repeats
- At the ends of many transposable elements, but not all.
- 9 to 40 bp in length and are inverted complements of one another.
Transposition
- Movement of a transposable element from one location to another.
- Several mechanisms used in prokaryotic and eukaryotic cells.
- Common features of all transposition events:
- 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
- Transposition can occur through a DNA molecule or through the production of an RNA molecule that is then reverse transcribed into DNA.
- DNA transposons: Transpose as DNA. Also called class I transposable elements.
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Retrotransposons: Transpose through an RNA intermediate. Also called class II transposable elements.
- RNA is transcribed from the transposable element (DNA).
- RNA is copied back into DNA using reverse transcriptase.
- Transposition can be replicative (the transposable element is copied and the copy moves to a new site) or non-replicative (the transposable element excises from the old site and moves to a new site).
Characteristics of Two Major Classes of Transposable Genetic Elements
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Class I (DNA Transposons)
- 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: ISI (E. coli); Tn3 (E.coli); Ac, Ds (Maize); p elements (Drosophila).
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Class II (Retrotransposons)
- 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 and eukaryotic organisms have a number of different types of transposable elements with varying structures.
Transposable Elements in Bacteria
- Two major groups of DNA transposons:
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Simple transposable elements (Insertion sequences): Carry only the information required for movement.
- Typically 800 to 2000 bp in length.
- Possess terminal inverted repeats and generate flanking direct repeats at the site of insertion.
- Contain one or two genes that code for transposase.
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More complex transposable elements (Composite transposons): Contain DNA sequences not directly related to transposition.
- Segment of DNA flanked by two copies of insertion sequences.
- Consists of a central region containing extra genes, flanked on both sides by IS elements that are identical or very similar in sequence.
- Contain genes other than those required for transposition.
- Non-composite transposons: Lack insertion sequences.
- Example: Tn3 (5000 bp - Possesses terminal inverted repeats of 38 bp and generates flanking direct repeats that are 5 bp in length).
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Simple transposable elements (Insertion sequences): Carry only the information required for movement.
Effects of Transposable Elements
- 1. Mutations and DNA rearrangements: Can insert within a gene to cause a mutation or stimulate DNA rearrangement leading to deletions of genetic material.
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2. Gene purification and function studies: Transposons can be used to purify genes and study their function.
- Transposable elements can fragment the genome and isolate the mutated fragment, thereby purifying the gene.
- 3. Gene regulation: Eukaryotic and prokaryotic genes can be turned on and off by transposon movement.
- 4. Plasmid processes: Transposons are located within plasmids and participate in processes like plasmid fusion and the insertion of F plasmids into the E. coli chromosome.
- 5. Genetic diversity: Transposable elements play a crucial role in the generation and transfer of new gene combinations.
Evolutionary Significance of Transposable Elements
- 1. Cellular function hypothesis: Transposable elements provide some important function for the cell.
- 2. Genetic variation hypothesis: Transposable elements provide evolutionary flexibility by inducing mutations.
- 3. Selfish-DNA hypothesis: 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, mobile DNA sequences that cause mutations and DNA rearrangements. This quiz covers their general characteristics, mechanisms of transposition, and their discovery in maize genetics. Test your knowledge of these critical genetic elements and their role in evolution.
