Transposable Elements in Genetics

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Questions and Answers

What is one consequence of transposons inserting within a gene?

They exclusively alter the gene's protein coding sequence.

They permanently eliminate the entire gene.

They can only enhance gene expression.

They may cause a mutation. (correct)

How can transposable elements assist in gene purification?

By permanently attaching to each gene.

By fragmenting the genome and isolating mutated fragments. (correct)

By duplicating the entire genome.

By eliminating all non-coding sequences.

Which hypothesis suggests that transposable elements provide some important cellular function?

The cellular-function hypothesis. (correct)

The genetic-variation hypothesis.

The genome-duplication hypothesis.

The selfish-DNA hypothesis.

What role do transposons play regarding plasmids?

They participate in plasmid fusion and F plasmid insertion into the E. Coli chromosome. Signup and view all the answers

What does the genetic-variation hypothesis propose about transposable elements?

They induce mutations which provide evolutionary flexibility. Signup and view all the answers

What distinguishes Class I transposable elements from Class II transposable elements?

Class I transposable elements can be either replicative or non-replicative. Signup and view all the answers

Which of the following is a characteristic of insertion sequences in bacteria?

They range in length from 800 to 2000 bp. Signup and view all the answers

What does a composite transposon consist of?

A central region with genes flanked by two identical insertion sequences. Signup and view all the answers

Which transposable element is specifically associated with reverse transcription?

Class II element. Signup and view all the answers

What do terminal inverted repeats indicate in transposable elements?

The ability to generate flanking direct repeats. Signup and view all the answers

Why are non-composite transposons significant?

They lack insertion sequences entirely. Signup and view all the answers

Which of the following best describes the structure of Class I transposable elements?

Short terminal inverted repeats with flanking direct repeats. Signup and view all the answers

Which of the following examples illustrate Class II transposable elements?

Ty from Yeasts and Alu from Humans. Signup and view all the answers

What characterizes transposable elements in the genomes of organisms?

They are mobile DNA sequences that frequently cause mutations. Signup and view all the answers

Which of the following is a common feature of all types of transposition?

Staggered breaks are made in the target DNA. Signup and view all the answers

What are terminal inverted repeats?

They are inverted complements located at the ends of some transposable elements. Signup and view all the answers

What is the name for transposable elements that transpose through an RNA intermediate?

Retrotransposons Signup and view all the answers

Who first discovered transposable elements and in which organism?

Barbara McClintock in maize Signup and view all the answers

How long are the flanking direct repeats generated during the transposition process?

3 to 12 bp long Signup and view all the answers

Which statement about the regulation of transposition is correct?

They can regulate their own transposition by inhibiting itself. Signup and view all the answers

What is the significance of staggered breaks in transposition?

They allow the transposable element to be joined to single-stranded ends. Signup and view all the answers

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.