Transposable element facts for kids

A transposable element (or TE), also called a transposon or jumping gene, is a special piece of DNA. It can move around within an organism's genome (all of its DNA). Sometimes, these movements can cause mutations, which are changes in the DNA. They can also change the size of the genome. When a jumping gene moves, it often makes a copy of itself. In humans, two common types are called L1 and Alu elements.

These jumping genes make up a big part of our DNA. Even though they are sometimes called "selfish" because they copy themselves, many are very important. They help the genome work and change over time. Scientists also use jumping genes as tools to change DNA in living things.

There are two main types of jumping genes:

• Class I TEs (or retrotransposons) work by making an RNA copy first.
• Class II TEs (or DNA transposons) move directly as DNA. They use a special protein called transposase.

Who Discovered Jumping Genes?

Barbara McClintock found the first jumping genes. She discovered them in maize (corn) plants. This happened at the Cold Spring Harbor Laboratory in New York. McClintock was studying corn plants with broken chromosomes.

In the winter of 1944–1945, McClintock grew corn from seeds that had been self-pollinated. These plants had broken ends on their ninth chromosomes. As the corn grew, McClintock saw strange color patterns on the leaves. For example, one leaf had two white patches of the same size.

She thought that during cell division, some cells lost genetic material. Others gained what they had lost. But when she compared the chromosomes, she found something else. Parts of the chromosomes had switched places! This was a big surprise. At the time, scientists thought genes stayed in fixed spots. McClintock showed that genes could move. She also found they could be turned on or off. This could happen because of the environment or during different stages of cell growth.

McClintock also proved that gene changes could be reversed. She shared her findings in 1951. She published an article about her discoveries in 1953.

When she first talked about her work in 1951, people didn't understand it. Her ideas were mostly ignored for many years. Then, in the late 1960s and 1970s, jumping genes were found in bacteria. People then realized how important McClintock's work was. She won a Nobel Prize in 1983 for her discovery. This was more than 30 years after her first research!

How Jumping Genes are Classified

Jumping genes are a type of mobile genetic elements. They are put into two classes. This depends on how they move. They either "copy and paste" (Class I TEs) or "cut and paste" (Class II TEs).

Copy and Paste Genes (Retrotransposons)

Class I TEs move in two steps. First, their DNA is copied into RNA. Then, this RNA is copied back into DNA. This new DNA copy is then put into a new spot in the genome. A special enzyme called reverse transcriptase helps with this step. The jumping gene itself often makes this enzyme. Retrotransposons are similar to retroviruses, like HIV.

Retrotransposons are usually grouped into three main types:

• LTR retrotransposons have long repeats at their ends. They make reverse transcriptase.
• LINEs (long interspersed nuclear elements) also make reverse transcriptase. But they don't have the long repeats.
• SINEs (short interspersed nuclear elements) do not make reverse transcriptase.

Retroviruses can also be seen as jumping genes. After a retrovirus's RNA becomes DNA inside a cell, this new DNA goes into the host cell's genome. These are called proviruses. A provirus is a special retrotransposon. It can make RNA that leaves the cell and infects other cells.

Cut and Paste Genes (DNA Transposons)

How a DNA transposon moves: It cuts itself out and pastes into a new spot.

Class II TEs move by a "cut and paste" method. They do not use an RNA step. Special enzymes called transposases help them move. Some transposases can attach to any DNA spot. Others attach to specific DNA sequences.

The transposase makes a jagged cut in the target DNA. It then cuts out the DNA transposon. Then, it pastes the transposon into the new spot. Other enzymes fill in the gaps. This creates small repeated sequences at the new insertion site.

Class II TEs make up less than 2% of the human genome. Most of the rest are Class I.

Independent vs. Dependent Jumpers

Jumping genes can also be "autonomous" or "non-autonomous." This applies to both Class I and Class II TEs. Autonomous TEs can move by themselves. Non-autonomous TEs need another jumping gene to help them move. This is often because dependent TEs are missing the transposase or reverse transcriptase enzyme.

For example, the Activator element (Ac) is autonomous. The dissociation element (Ds) is non-autonomous. Ds cannot move without Ac.

Where are Jumping Genes Found?

About 64% of the maize genome is made of jumping genes. They also make up 44% of the human genome. Almost half of mouse genomes are also TEs.

New studies show where jumping genes are found. They are often near the start of genes. Older jumping genes are usually not found in these spots. This might be because they could get in the way of how genes are copied.

Examples of Jumping Genes

• The first jumping genes were found in maize (corn) by Barbara McClintock in 1948. She saw that these elements caused parts of chromosomes to be added, removed, or moved. These changes could, for example, change the color of corn kernels.
• In the fruit fly Drosophila melanogaster, a family of TEs is called P elements. They appeared in the mid-1900s. In 50 years, they spread through all fruit fly populations. Scientists use artificial P elements to put genes into fruit flies.
• In bacteria, jumping genes often carry extra genes. These genes are for things other than moving. For example, they can carry genes for antibiotic resistance. This means bacteria can become resistant to medicines. Bacterial transposons can jump from the main DNA to small circles of DNA called plasmids. This helps spread genes like those for antibiotic resistance.
• In humans, the most common jumping gene is the Alu sequence. It is about 300 bases long. It can be found up to a million times in the human genome. Alu elements make up about 15–17% of our DNA.
• Mariner-like elements are another type found in many species, including humans. They are known for moving between different species.
• In human embryos, two types of jumping genes work together. They help make special RNA that helps stem cells develop. Stem cells are very important. They can turn into all the different cells in the body.
• In peppered moths, a jumping gene in a gene called cortex made the moths' wings turn completely black. This helped them hide on trees covered in soot during the Industrial Revolution.

How Jumping Genes Can Cause Problems

Jumping genes have lived with organisms for thousands of years. They have become part of many organisms' genomes. While they can have good effects, they can also cause problems. They can lead to diseases and harmful genetic changes.

How They Cause Changes

Jumping genes can harm the host cell's genome in different ways:

• If a jumping gene lands inside a working gene, it can stop that gene from working.
• When a DNA transposon leaves a gene, the gap it leaves might not be fixed correctly.
• Many copies of the same sequence, like Alu sequences, can make it hard for chromosomes to pair up correctly. This can lead to parts of chromosomes being duplicated.

Jumping genes can also cause problems by making harmful proteins. Many TEs have promoters. These are signals that tell genes to start working. These promoters can cause other genes nearby to work incorrectly, leading to disease.

Diseases Linked to Jumping Genes

Some diseases often linked to jumping genes include:

• Hemophilia A and B (a blood clotting disorder)
• Severe combined immunodeficiency (a problem with the immune system)
• Porphyria (a group of liver disorders)
• A higher chance of getting cancer
• Duchenne muscular dystrophy (a muscle weakening disease)
• Alzheimer's Disease and other brain disorders.

How Cells Control Jumping Genes

Cells have ways to stop jumping genes from spreading too much. They use small RNA molecules called piRNAs and siRNAs. These molecules can "silence" jumping genes after they have been copied.

Even though organisms have many jumping genes, diseases caused by them are not super common. This is because most jumping genes are silenced. This happens through special processes like DNA methylation. This stops them from moving or causing problems.

Scientists think that only about 100 LINE1 sequences are active in humans. This is even though they make up 17% of our genome. Human cells silence LINE1 sequences using an RNA interference (RNAi) process.

Jumping Genes and Evolution

Jumping genes are found in almost all living things. Scientists are still learning about how they evolved and how they affect the evolution of genomes. It's not clear if they came from one common ancestor or appeared many times.

While some jumping genes help their hosts, most are seen as "selfish DNA" or parasites. They are similar to viruses in this way. Viruses and jumping genes also share similar structures and abilities. This makes some scientists think they might have a common ancestor.

Because too much jumping gene activity can be harmful, many organisms have ways to stop them. Bacteria can remove TEs from their genomes. Eukaryotic organisms (like plants and animals) often use RNA interference to stop TE activity.

However, large amounts of jumping genes can still be helpful for evolution. They can create repeated sequences in the genome. These repeats can protect new gene sequences from being changed. This helps new genes develop. Jumping genes might also have been used by the immune system to create different types of antibodies.

Jumping genes can carry many types of genes. These include genes for antibiotic resistance. They can also carry integrons. Integrons are genetic parts that can pick up and use genes from other sources.

Sometimes, jumping genes don't cut themselves out perfectly. They might take some nearby DNA with them. This process is called "exon shuffling." Shuffling different parts of genes can create new gene products.

Some jumping genes in plants can copy parts of genes and move them around the genome. This can duplicate genes. It can also help create new genes through exon shuffling.

Using Jumping Genes in Science

Jumping genes can be used in laboratories and research. Scientists use them to study organisms' genomes. They can even use them to create new genetic sequences. Using jumping genes can be split into two main areas: for genetic engineering and as a genetic tool.

Genetic Engineering

• Scientists can use jumping genes to insert new DNA sequences. This is often done to remove a DNA sequence or change a gene.
• Sometimes, putting a jumping gene into a gene can stop it from working. But this can be reversed. If the transposon is removed, the gene can work again.
• This allows scientists to study how genes work in different cells.

As a Genetic Tool

Besides genetic engineering, jumping genes are also used as a genetic tool:

• They help scientists study how genes work and what proteins do.
• This tool helps researchers find out what effects genes have. They can compare the original gene with a changed one.

Specific Uses

• Jumping genes are used to cause changes in many organisms studied in labs. The Sleeping Beauty transposon system is one example. It has been used to find cancer genes.
• The Sleeping Beauty transposon system can work in mammal cells. Scientists are looking into using it for gene therapy in humans.
• Jumping genes are used to figure out how different species are related. They can act as a natural way to cause changes in bacteria.
• Common organisms where jumping genes are used include:
  • Drosophila (fruit flies)
  • Arabidopsis thaliana (a small flowering plant)
  • Escherichia coli (a type of bacteria)

Images for kids

• 

  A bacterial DNA transposon, showing how it's put together.

See also

In Spanish: Transposón para niños