Introduction to Transposons

Summary

This video explores transposons, or 'jumping genes,' which are mobile genetic elements found in nearly all organisms. Transposons can move within genomes through two mechanisms: Class 1 transposons use RNA intermediates for reverse transcription, while Class 2 transposons follow a 'cut and paste' DNA process. The video details their structure, including terminal inverted repeats and transposase enzymes, and explains the differences between autonomous and non-autonomous transposons. It also highlights their role in genetic variation, their impact on genome stability, and their evolutionary significance, with a focus on lab applications in creating transgenic animals.

Takeaways

• 😀 Transposons are mobile genetic elements found in almost all organisms, often called 'jumping genes' because they can move around a genome.
• 😀 Barbara McClintock inferred the existence of transposons, for which she won the 1983 Nobel Prize in Physiology or Medicine.
• 😀 There are two classes of transposons: Class 1, which move via an RNA intermediate, and Class 2, which move directly in DNA form.
• 😀 Class 1 transposons are copied into RNA, which is then reverse transcribed into DNA and reinserted into the genome.
• 😀 Class 2 transposons move via a 'cut and paste' mechanism, where the transposon is excised from one site and inserted into another.
• 😀 Class 2 transposons contain terminal inverted repeats at both ends, which are essential for transposes (the enzyme) to recognize and bind.
• 😀 Transposons also have direct repeats at their insertion sites, which are left behind when the transposon moves to a new location.
• 😀 Transposition of Class 2 transposons involves three steps: excision, drift, and integration, facilitated by the transposes enzyme.
• 😀 A transposon is considered autonomous if it has a functional transposes gene, while a non-autonomous transposon relies on an external transposes source to move.
• 😀 Transposons are widespread in nature, with remnants making up around 50% of the human genome, but their movement is highly mutagenic and controlled to avoid damage.
• 😀 Despite their mutagenic risks, transposons provide genetic variation, which can offer evolutionary advantages to organisms.

Q & A

What are transposons, and why are they referred to as 'jumping genes'?

Transposons are mobile genetic elements that can move around a genome. They are often referred to as 'jumping genes' because they move seemingly at random from one location to another within the DNA.

Who first inferred the existence of transposons, and for which achievement did they receive the Nobel Prize?

Barbara McClintock inferred the existence of transposons, and she was awarded the 1983 Nobel Prize in Physiology or Medicine for this discovery.

What are the two main classes of transposons?

The two main classes of transposons are Class 1 and Class 2. Class 1 transposons move through an RNA intermediate, while Class 2 transposons move directly in DNA form.

How do Class 1 transposons move within the genome?

Class 1 transposons move by being copied into RNA through transcription, which is then reverse-transcribed back into DNA using the enzyme reverse transcriptase. This DNA is then inserted back into the genome.

What distinguishes Class 2 transposons from Class 1 transposons?

Class 2 transposons do not use an RNA intermediate; they remain in DNA form throughout the process of transposition. They typically encode a multifunctional enzyme called transposase, which facilitates their movement.

What is the role of the enzyme transposase in Class 2 transposons?

Transposase recognizes the terminal inverted repeats at the ends of the transposon, cuts it out of its original location, and inserts it into a new location in the genome. This is called a cut-and-paste mechanism.

What are terminal inverted repeats and how do they contribute to the movement of transposons?

Terminal inverted repeats are DNA sequences at the three-prime and five-prime ends of a transposon. These sequences are reversed complements of each other and serve as the binding site for the transposase enzyme, which is necessary for cutting and moving the transposon.

What are direct repeats, and how are they formed during transposition?

Direct repeats are sequences in the genome that flank the transposon after it has been inserted. These repeats are formed at the recipient site during the insertion process and are not part of the transposon itself.

What are the three main steps in the transposition process of Class 2 transposons?

The three main steps in transposition are excision, drift, and integration. First, transposase excises the transposon from the donor site, then the transposon drifts through the genome, and finally, it integrates into a new location.

What happens if the transposase gene or the inverted repeat sequence is mutated?

If the transposase gene or the inverted repeat sequence is mutated, transposase cannot bind to the transposon, leading to a failure in its excision and reinsertion. This results in the transposon being 'stuck' in the genome.

What is the difference between autonomous and non-autonomous transposons?

Autonomous transposons carry both the transposase gene and the necessary inverted repeat sequences, allowing them to move independently. Non-autonomous transposons lack an intact transposase gene and require an external source of transposase to move.

How are transposons related to the human genome, and what evolutionary role might they play?

Transposons make up about 50% of the human genome. Despite their potential for causing mutations, they may provide an evolutionary advantage by enabling genetic variation when mobilized.

What are some applications of transposons in scientific research?

Transposons are used in genetic engineering, such as creating transgenic animals, by controlling their movement with the help of transposase enzymes. They can also be used to study gene function and genome evolution.