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Questions and Answers
What is one consequence of transposons inserting within a gene?
What is one consequence of transposons inserting within a gene?
How can transposable elements assist in gene purification?
How can transposable elements assist in gene purification?
Which hypothesis suggests that transposable elements provide some important cellular function?
Which hypothesis suggests that transposable elements provide some important cellular function?
What role do transposons play regarding plasmids?
What role do transposons play regarding plasmids?
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What does the genetic-variation hypothesis propose about transposable elements?
What does the genetic-variation hypothesis propose about transposable elements?
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What distinguishes Class I transposable elements from Class II transposable elements?
What distinguishes Class I transposable elements from Class II transposable elements?
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Which of the following is a characteristic of insertion sequences in bacteria?
Which of the following is a characteristic of insertion sequences in bacteria?
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What does a composite transposon consist of?
What does a composite transposon consist of?
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Which transposable element is specifically associated with reverse transcription?
Which transposable element is specifically associated with reverse transcription?
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What do terminal inverted repeats indicate in transposable elements?
What do terminal inverted repeats indicate in transposable elements?
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Why are non-composite transposons significant?
Why are non-composite transposons significant?
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Which of the following best describes the structure of Class I transposable elements?
Which of the following best describes the structure of Class I transposable elements?
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Which of the following examples illustrate Class II transposable elements?
Which of the following examples illustrate Class II transposable elements?
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What characterizes transposable elements in the genomes of organisms?
What characterizes transposable elements in the genomes of organisms?
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Which of the following is a common feature of all types of transposition?
Which of the following is a common feature of all types of transposition?
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What are terminal inverted repeats?
What are terminal inverted repeats?
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What is the name for transposable elements that transpose through an RNA intermediate?
What is the name for transposable elements that transpose through an RNA intermediate?
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Who first discovered transposable elements and in which organism?
Who first discovered transposable elements and in which organism?
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How long are the flanking direct repeats generated during the transposition process?
How long are the flanking direct repeats generated during the transposition process?
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Which statement about the regulation of transposition is correct?
Which statement about the regulation of transposition is correct?
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What is the significance of staggered breaks in transposition?
What is the significance of staggered breaks in transposition?
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Study Notes
What are Transposable Elements?
- Mobile DNA sequences present in all organisms
- Frequently cause mutations and DNA rearrangements
- Many transposable elements control their own transposition
- Barbara McClintock discovered transposable elements in the 1940s studying maize genetics
Characteristics
- Short flanking direct repeats (3 to 12 bp) are generated during insertion, not part of the element itself
- Many transposable elements have terminal inverted repeats (9 to 40 bp) that are inverted complements of each other
Transposition
- Movement of a transposable element from one location to another
- Several mechanisms are used for transposition in both prokaryotes and eukaryotes
- All transposition types share these features:
- 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 an RNA intermediate
- Transposable elements that transpose as DNA are called DNA transposons or Class I transposable elements
- Transposable elements that transpose through an RNA intermediate are called retrotransposons or Class II transposable elements
- Transposition can be:
- Replicative: a copy of the transposable element is moved to a new site
- Non-replicative: the transposable element is excised from the old site and moved to a new site
Characteristics of the Two Major Classes
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: IS1 (E.coli), Tn3 (E. coli), Ac, Ds (Maize), p elements (Drosophila)
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 eukaryotes possess a variety of transposable element types with different structures
Transposable Elements in Bacteria
- Two major groups of DNA transposons:
- Simple transposable elements (insertion sequences): carry only the information needed for movement
- More complex transposable elements (composite transposons): contain DNA sequences not directly related to transposition
I. Insertion Sequences
- Simplest type of transposable element in bacterial chromosomes and plasmids
- Carry only the genetic information necessary for movement
- Typically 800 to 2000 bp in length
- Possess terminal inverted repeats
- Generate flanking direct repeats at the site of insertion
- Most contain one or two genes that code for transposase
II. Composite Transposons
- Segment of DNA flanked by two copies of an insertion sequence
- Contain genes other than those required for transposition
- Transposable elements lacking insertion sequences are referred to as non-composite transposons
- Example: Tn3 (5000 bp - Possesses terminal inverted repeats of 38 bp and generates 5 bp flanking direct repeats)
Effects of Transposable Elements
- Can insert into a gene, causing mutations or stimulating DNA rearrangements that lead to deletions
- Can be used to purify genes and study their function (fragment the genome and isolate the mutated fragment)
- Can turn eukaryotic and prokaryotic genes on and off
- Located in plasmids and participate in plasmid fusion and insertion of F plasmids into the E. coli chromosome
- Play an important role in the generation and transfer of new gene combinations
Evolutionary Significance of Transposable Elements
- Cellular-function hypothesis: transposable elements provide an important cellular function
- Genetic-variation hypothesis: provide evolutionary flexibility by inducing mutations
- Selfish-DNA hypothesis: 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 play a crucial role in genetic mutations and rearrangements. Discover their characteristics, mechanisms of transposition, and their historical significance as uncovered by Barbara McClintock in maize genetics. This quiz will test your understanding of these essential genetic components.
