Molecular Superman : The RNAi protein Dicer
A new role of RNAi protein Dicer in preventing collision during DNA replication has been identified.
In the world of fantasy a Superman dive down from the heavens hundreds of times and rescue a victim in critical condition.
This dramatic scene happens in real life during each and every cell division. For the cell division occur successfully, our genetic material must be faithfully replicated by extremely complex machinery, whose parts are minute enough to navigate among the strands of the double helix.
Now our DNA is in continuous use. The transcription machinery is constantly plucking at its strands to gain access to crucial genes and the letters of DNA are being copied to form a template, which will guide to form proteins. But these two copying machines cannot occupy the same bit of genetic track together. Inevitably they will collide – unless some molecular Superman plays the role of removing the transcription machinery and allow the replication to happen in an uninterrupted way.
In a recent study, Scientists from Cold Spring Harbor Laboratory (CSHL) have found that this molecular Superman is none other than the Dicer protein. Dicer protein is known for its role in selectively silencing genes via a process called RNA interference (RNAi). It is now believed that Dicer proteins help to free transcription machinery from DNA so that replication can occur without any disturbance across the entire genome. It controls the release at hundreds of extremely active genes, which are in constant use by the cell – we call many of them ‘housekeeping’ genes because they are required for basic survival. At any given time, transcription machinery can be found near these genes. The team, led by Robert Martienssen, a CSHL Professor and Howard Hughes Medical Institute Investigator, concludes that that collision between the replication and transcription machinery lead to massive changes across the genome which are associated with aging and diseases like cancer. Previously the researchers found that RNAi resolves the conflict between transcription and DNA replication in isolated areas of the genome where genes are being silenced. When Dicer is mutated, replication stalls and DNA in the region becomes damaged explains.
Are these collisions really so disastrous for the cell? The team found that these accidents lead to loss of massive segments of DNA with each cell division. These chromosome rearrangements, known as genomic instability, are involved in aging and cancer, experts say. Other groups have shown that mutations in Dicer are similarly associated with an increased risk of tumour formation, strengthening the view. Thus Dicer may protect the genome by preventing collisions between transcription and replication.
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