microRNAs and small RNAs
Types and functions of small RNAs
siRNA – small non-coding RNA that regulates coding genes and is part of antiviral defence.
miRNA- (microRNA) regulates protein coding genes
piRNA- (piwi interacting RNA) transposon silencing RNA.
RNAi- a non-coding antisense RNA that functions in experimental studies, viral resistance, genome stability through keeping mobile elements silent, keeping chromatin condensed preventing transcription and repressing protein synthesis.
miRNA processing pathway
Genes encoding miRNA are found in the DNA. dsRNA is recognised by the protein DGCR8 leading to the enzyme Drosha to associate and catalyse the cleavage of the double stranded hairpin loop structure, cutting the RNA into smaller precursor miRNA. This allows the miRNA to be transported into the cytosol. Once in the cytosol, dicer recognises and attaches to the RNA. Dicer is a RNase protein that cleaves the terminal stem loop to form an ever-shorter molecule. TAR RNA binding protein is the cofactor. Then Argonaut protein interacts and forms the RISC complex (RNA induced silencing complex.) miRNA is unwound and one strand is released. Remaining miRNA is guided to the target sequence (with the help of RISC), this is energy dependent and chaperone mediated. Once miRNA reaches its target, it binds by complementarity and prevents the mRNA from being translated as binding causes degradation of mRNA.
Similarities and Differences between small RNA processing pathways
Both use dicer
Both use Argonaut
Both form RISC complex
Only one strand of dsRNA is actually used to silence
Both begin as dsRNA
Both are synthesised by RNA polymerase II
End result: miRNA bind to target mRNA, prevent its translation and cause its degradation. siRNA bind to target and cleave it.
miRNA are endogenous, so the processing pathway begins in the nucleus. siRNA are exogenous and so processing begins in the cytoplasm
miRNA are short hair pin loop structures, siRNA are not
Physiological functions of small RNAs
stem cell development
cell cycle control
some are suppressors of cancer, some support cancer
miRNA resemble small interfering RNAs of the RNA interference pathway.
The human genome may encode over 1900 miRNAs.
In the early 1990s the first miRNA was discovered
Gene regulation is essential as it increases adaptability of organisms by allowing the cells to expression proteins when needed.
Genes can be regulated at:
Modifications to DNA can be structural and chemical. Histone modification can be phosphorylation, ubiquitination, acetylation or methylation. Depending on whether chromatin is opened or closed will determine whether transcription takes place. Open chromatin allows for transcription. The histone code hypothesis suggests that chemical modifications made to histones can influence the conformation of chromatin and thus influence transcription.
mRNA processing is highly important. mRNA must be capped, be cleaved, spliced and polyadenylated. Capping is important for protection of mRNA from degradation, allowing for the mRNA to leave the nucleus, for recognition of mRNA being different to other RNAs and to allow for ribosome binding during translation.