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Questions and Answers
What effect can transposable elements have on genes?
Which hypothesis suggests that transposable elements provide evolutionary flexibility?
How do transposons participate in plasmids?
Which statement accurately represents the selfish-DNA hypothesis?
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What role do transposable elements play in evolution?
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What is the primary function of transposable elements in genomes?
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What mechanism do many transposable elements use to regulate their own transposition?
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What feature is common to most transposable elements during insertion?
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What type of transposable element is referred to as DNA Transposons?
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Which of the following accurately describes retrotransposons?
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What is a key step in the process of transposition?
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Which of the following sequences is typically produced at the ends of transposable elements?
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What was a significant contribution made by Barbara McClintock in the field of genetics?
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What is the main characteristic of Class I transposable elements?
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Which gene is primarily associated with Class II transposable elements?
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Which example is a Class I transposable element?
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What structure do insertion sequences have?
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How do composite transposons differ from simple transposable elements?
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What type of transposition involves the element excising from one site and moving to another?
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What feature do retrotransposons primarily use for transposition?
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What distinguishes non-composite transposons from composite transposons?
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Study Notes
Transposable Elements: Mobile DNA Sequences
- Transposable elements are DNA sequences that move within a genome.
- Their movement can cause mutations and DNA rearrangements.
- They are found in all organisms.
- Many transposable elements regulate their own transposition.
- Barbara McClintock first discovered them in the 1940s while studying maize genetics.
General Characteristics
- Most transposable elements create flanking direct repeats at their insertion sites.
- These repeats are 3 to 12 base pairs long.
- They are not part of the transposable element itself but are generated during insertion.
- Many transposable elements also have terminal inverted repeats.
- These repeats are inverted complements of each other and range from 9 to 40 base pairs long.
Transposition
- Transposition refers to the movement of a transposable element from one location to another.
- There are various transposition mechanisms in prokaryotes and eukaryotes.
- All transposition events share these common features:
- Staggered breaks occur in the target DNA.
- The transposable element joins with the single-stranded ends of the target DNA.
- DNA replication fills the single-strand gaps.
Mechanisms of Transposition
- Transposition can occur through a DNA molecule or through an RNA intermediate.
- Transposable elements that move as DNA are called DNA transposons or class I transposable elements.
- Transposable elements that move through an RNA intermediate are called retrotransposons or class II transposable elements.
- Retrotransposons use reverse transcriptase to copy their RNA intermediate back into DNA.
- Transposition can be replicative, where the element is copied and the copy moves to a new location.
- Alternatively, it can be non-replicative, where the element is excised from its original site and moves to a new location.
Classes of Transposable Genetic Elements
Class I (DNA Transposons)
- Structure: Short terminal inverted repeats and 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 and 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
- DNA transposons in bacteria are divided into two major groups:
- Simple transposable elements (insertion sequences): Carry only the information required for movement.
- Complex transposable elements (composite transposons): Contain DNA sequences not directly related to transposition.
Insertion Sequences
- The 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 the characteristics of transposable elements: terminal inverted repeats and flanking direct repeats at their insertion site.
- Most insertion sequences contain one or two genes coding for transposase.
Composite Transposons
- Consist of a central region flanked by two copies of an insertion sequence.
- Contain genes other than those required for transposition.
- Examples: 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 causing a mutation or stimulate DNA rearrangements leading to deletions.
- Can be used to purify genes and study their function.
- Can turn on or off both prokaryotic and eukaryotic genes by moving.
- Found in plasmids and participate in processes like plasmid fusion and insertion of F plasmids into E. coli chromosomes.
- Play a critical role in the generation and transfer of new gene combinations.
Evolutionary Significance of Transposable Elements
- Cellular function hypothesis: Transposable elements might provide a vital function for the cell.
- Genetic variation hypothesis: Transposable elements induce mutations and provide evolutionary flexibility.
- Selfish DNA hypothesis: Transposable elements do not benefit the cell but can replicate and spread.
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Description
Explore the fascinating world of transposable elements, the mobile DNA sequences that can move within genomes. This quiz covers their characteristics, mechanisms of transposition, and their significance in genetics, including historical discoveries. Understand how these elements can impact mutations and gene regulation across different organisms.
