Transcript Small RNA World

Small RNA World
microRNAs(miRNAs)
ECES458
FALL 2001
Student: Kai He
Instructor: Dr. Sahinalp
imRNAs: A New Class of
Small RNA Molecules

siRNAs: small interfering RNAs
 stRNAs: small temporal RNAs
siRNAs

Small interfering RNAs that have an integral role
in the phenomenon of RNA interference(RNAi), a
form of post-transcriptional gene silencing
 RNAi: 22 nt fragments, which bind to the
complementary portion of the target mRNA and
tag it for degradation
 siRNA: a role in conferring resistance and in
preventing transcription
stRNAs

Small RNAs refer to small temporal RNAs
 Including 22 nt, such as lin-4 and let-7 RNAs
 Role: the temporal regulation of Caenorhabditits
elegans development, preventing translation of
their target mRNAs by binding to the target’s
complementary 3’ untranslated regions(UTRs)
 From 70 nt ssRNA(single-stranded RNA)
Dicer

A RNaseIII, which cleaves double-stranded
RNA into discrete sizes
 Involving generate siRNA
 Dicer homologs exist in many organisms
including C.elegans, yeast and humans
 Roles: cause defeats in development and
gene silencing
Small RNA Molecules




Found in:
– Drosophila
– C.elegans
– Hela cells
Genes
– Lin-4
– Let-7
Structures
– Precursor RNA is folded into stem-loop
Roles
– Regulate translation
– Modify mRNA stability
Identifying miRNA

Using biochemical and bioinformatics
approaches
 Using Dicer to produce siRNA and stRNA,
and obtain ~22 nt with 5’ phosphate region
and 3’ hydroxy region
 mRNAs, tRNAs, or rRNAs: degradation
products - miRNAs
Charateristics of miRNAs

Found: More than 100 new miRNAs

From: Drosophila, C.elegans, Hela cells and so on

Conservation: 15% of these miRNAs were conserved with 1-2
mismatched across worm, fly, and mammalian genomes
Location: located at either the 3’ or 5’ region of a stem-loop within ~70
nt RNA precursor
Expression pattern: varied and some were expressed in all cells and at
all developmental stages and others had a more restricted spatial and
temporal expression pattern
Suggestions:



– miRNA might be involved in post-transcriptional regulation of
developmental genes
Problems

Now we are not able to identify the miRNA targets
by using an informatics approaches
 Why: maybe the entire miRNA does not have to
be complementary to the target in order for
binding to occur
 Questions:
– What do these miRNAs do?
– Are they involved in post-transcriptional gene
regulation, if so, what are their target genes?