STRUCTURE AND BIOLOGICAL ROLES OF RNAs

1. STRUCTURE AND BIOLOGICAL
ROLES OF RNAs
SUBMITTED TO,
DR. ARYA P MOHAN
ASSITANT PROFESSOR
DEP OF BOTANY
ST. TERESA’S COLLEGE ERNAKULAM
SUBMITTED BY,
SHWETHA U
ROLL NO:12
I M.SC. BOTANY
ST. TERESA’S COLLEGE ERNAKULAM
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2. RNA (Ribonucleic acid )
• RNA is a type of nucleic acid which synthesized in the
nucleus it is found mainly in the cytoplasm and nucleus.
• RNA is a single stranded structure consisting of long ,
unbranched polynucleotide chain in some viruses RNA is
double stranded e.g., in Reo viruses.
• RNA molecule has a backbone made of alternating
phosphate groups and the sugar ribose, rather than the
deoxyribose .
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3. TYPES OF RNA
• Mainly three types of non genetic RNA - mRNA, tRNA, rRNA (Non-genetic RNA is that
one that is transcribed from DNA but is not translated to amino acids during the
synthesis of protein):
1. rRNA (ribosomal RNA)
2. tRNA (transfer RNA)
3. tmRNA (Transfer-messenger RNA)
4. siRNA (small interfering RNA)
5. miRNA (Micro RNA)
6. piRNA (Piwi-interacting RNA)
7. lncRNA (Long noncoding RNA)
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4. RIBOSOMAL RNA (rRNA)
• The RNA found in ribosomes, the molecules responsible for catalysing protein synthesis,
is known as ribonucleic acid (rRNA).
• Over 60-80% of the weight of the ribosome is composed of ribosomal RNA, essential for
all of the ribosome’s activities, including binding to mRNA, attracting tRNA, and
catalysing the formation of peptide bonds between amino acids.
• Types of rRNA :
i. In eukaryotic cell have four kinds of rRNA molecule - 28s rRNA, 18s rRNA, 5s rRNA,
5.8s rRNA.
ii. In prokaryotic cells contain three kinds of rRNA molecules – 23s rRNA, 16s rRNA, 5s
rRNA.
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5. Structure of rRNA
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6. FUNCTION OF rRNA :
• Protein synthesis is the primary function of rRNA.
• The A(anchors an entering tRNA), P(for binding a developing polypeptide), and E
(creation of a peptide bond) sites are created within the ribosome by the unusual three-
dimensional structure of rRNA, which has internal helices and loops.
• By attaching to messenger RNA and transfer RNA, these molecules assure that the codon
sequence of the mRNA is appropriately translated into the amino acid sequence of
proteins.
• Gives structural integrity to ribosome.
• Serves as the site for mRNA.
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7. TRANSFER RNA (tRNA)
• Transfer RNA (tRNA) is a small RNA molecule that plays a key role in protein
synthesis.
• The tRNA which possesses the capacity to combine specially with only one
amino acid in a reaction mediated by a set of amino acid specific enzyme called
aminoacyl-tRNA synthetase.
• It transfers the amino acid from the ‘amino acid pool’ to the site of protein
synthesis and recognizes the codons of the mRNA, is know as the tRNA or
soluble RNA (sRNA).
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8. STRUCTURE OF tRNA
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9.  STRUCTURE OF tRNA
• The acceptor stem is a 7- to 9-base pair (bp) stem made by the base pairing of the 5′-terminal
nucleotide with the 3′-terminal nucleotide (which contains the CCA 3′-terminal group used to
attach the amino acid).
• In general, such 3′-terminal tRNA-like structures are referred to as 'genomic tags’.
• The acceptor stem may contain non-Watson-Crick base pairs.
• The CCA tail is a cytosine-cytosine-adenine sequence at the 3′ end of the tRNA molecule. The
amino acid loaded onto the tRNA by aminoacyl tRNA synthetases, to form aminoacyl-tRNA, is
covalently bonded to the 3′-hydroxyl group on the CCA tail. This sequence is important for the
recognition of tRNA by enzymes and critical in translation. In prokaryotes, the CCA sequence is
transcribed in some tRNA sequences. In most prokaryotic tRNAs and eukaryotic tRNAs, the CCA
sequence is added during processing and therefore does not appear in the tRNA gene.
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10. • The D loop is a 4- to 6-bp stem ending in a loop that often contains
dihydrouridine.
• The anticodon loop is a 5-bp stem whose loop contains the anticodon. The tRNA
5′-to-3′ primary structure contains the anticodon but in reverse order, since 3′-to-
5′ directionality is required to read the mRNA from 5′-to-3′.
• The ΨU loop is named so because of the characteristic presence of the unusual
base ΨU in the loop, where Ψ is pseudo uridine, a modified uridine. The modified
base is often found within the sequence 5' -TΨUCG-3’.
• The variable loop sits between the anticodon loop and the ΨU loop and, as its
name implies, varies in size from 3 to 21 bases.
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11. • The function of Transfer RNA (tRNA):
1. It is required for protein synthesis.
2. tRNA is an adapter molecule.
3. tRNA reads the code and binds to
specific amino acids.
4. Each amino acid has its tRNA.
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12. TRANSFER-MESSENGER RNA (tmRNA)
• Transfer-messenger RNA (tmRNA), also called 10Sa or SsrA RNA, is unique
among bifunctional RNAs in that it has properties of a tRNA and an mRNA.
• tmRNA is significantly larger than a tRNA, and in place of the anticodon loop
there are multiple pseudoknots and a specialized open reading frame.
• This unusual structure allows tmRNA to interact with specific ribosomes in a
reaction known as trans-translation.
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13. STRUCTURE OF tmRNA
• tmRNA is a remarkable chimeric molecule with both
transfer and messenger RNA activities. It ranges
from 230 to 400 nucleotides in length. Its modular
and highly-structured architecture includes a tRNA-
like domain (TLD), a huge ring made of pseudoknots
(PKs), a long and disrupted helix H2 connecting the
TLD to the PKs, and a short mRNA-like domain
(MLD) made of a single strand portion as well as a
conserved helix H5 carrying a termination codon
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14.  Function of tmRNA
• Acting both as a tRNA and an mRNA, in a process known as trans-translation,
tmRNA adds a short peptide tag to undesirable proteins.
• Trans-translation plays at least two physiological roles:
1. removing ribosomes stalled upon mRNA.
2. Targeting the resulting truncated proteins for degradation by proteases.
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15. SMALL INTERFERING RNA (siRNA)
• One of the most important advances
in biology has been the discovery
that siRNA (small interfering RNA) is
able to regulate the expression of
genes, by a phenomenon known as
RNAi (RNA interference).
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16.  FUNCTIONS OF siRNA
• It is involved in cellular defense. It controls the damage by transposons and viral
infections.
• siRNAs silence genes at the post-transcriptional level. They cleave mRNA
molecules with a sequence complementary to the siRNA molecule and thereby
stop the translation process or gene expression.
• siRNA can be used to treat various diseases such as cancer.
• Expression of any gene can be interfered with using siRNA having complementary
sequences. This can be utilized in drug development to regulate gene expression
by introducing siRNA into the cell.
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17. Micro RNA (miRNA)
• MicroRNAs are small, highly conserved non-coding RNA molecules involved in the
regulation of gene expression. MicroRNAs are transcribed by RNA polymerases II
and III, generating precursors that undergo a series of cleavage events to form
mature microRNA.
• Function : The miRNA functions as a guide by base-pairing with target mRNA to
negatively regulate its expression. The level of complementarity between the
guide and mRNA target determines which silencing mechanism will be employed;
cleavage of target messenger RNA (mRNA) with subsequent degradation or
translation inhibition Fig.
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18. Structure of miRNA
• miRNA is a single-stranded RNA molecule.
It is around 21-25 nucleotides long. It is
transcribed as long pre-miRNA, which
undergoes cleavage and processing to form
mature miRNA. miRNA is transcribed by
RNA polymerase II and III.
RNA
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19. • Piwi-interacting RNA (piRNA) is the largest class of small non-coding RNA
molecules expressed in animal cells.
• piRNAs form RNA-protein complexes through interactions with piwi-
subfamily Argonaute proteins.
• These piRNA complexes are mostly involved in the epigenetic and post-
transcriptional silencing of transposable elements and other spurious or
repeat-derived transcripts, but can also be involved in the regulation of
other genetic elements in germ line cells.
Piwi-interacting RNA (piRNA)
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20. STRUCTURE OF piRNA
• (piRNAs) are single-stranded, 23–36 nucleotide
RNAs that act as guides for an animal-specific
class of Argonaute proteins, the PIWI proteins.
The first piRNAs — derived from the Suppressor
of Stellate locus in Drosophila melanogaster
testes — were discovered in 2001.
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21.  FUNCTION OF piRNA
• piRNAs (piwi-interacting RNA) are a novel class of non-coding small single-
stranded RNAs with the length of 23-36 nt. The piRNAs play important biological
role through the specific interaction with the piwi proteins of the Argonaute
family.
• piRNA function in embryonic development, maintenance of germline DNA
integrity, silencing of transposon transcription, suppression of translation,
formation of heterochromatin, and epigenetic regulation of sex determination.
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22. LONG NONCODING RNA (lncRNA)
• Long noncoding RNAs (lncRNAs) are RNA molecules larger than 200 nucleotides.
They regulate gene expression at transcriptional, RNA processing, translational, and
post‐translational levels through interaction with nucleic acids and proteins.
• FUNCTION: lncRNAs are a new class of epigenetic regulators that play important
roles in epigenetic regulation. lncRNAs regulate epigenetic modification primarily in
the nucleus, regulating gene transcription at the transcriptional level by modulating
histone or DNA modification, primarily methylation and acetylation.
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23.  STRUCTURE
• The lncRNA is organized into a modular structure
comprising three domains, consisting of 12
helices, eight terminal loops, five sizeable internal
loops, and a five-way junction. This 5′ asymmetric
G-rich internal loop (RHT/AGIL motif) in vivo is
necessary for the interaction with CNBP.
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24. HOTAIR
• HOTAIR (for HOX transcript antisense RNA) is a human gene located between
HOXC11 and HOXC12 on chromosome 12.
• It is the first example of an RNA expressed on one chromosome that has been
found to influence transcription of HOXD cluster posterior genes located on
chromosome 2.
• The sequence and function of HOTAIR is different in human and mouse.
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25. XIST (X-inactive specific transcript)
• Xist, one of the most well-studied lncRNAs, is a 17-kb transcript responsible for
dosage compensation in placental mammals.
• Xist (X-inactive specific transcript) is a non-coding RNA on the X chromosome of
the placental mammals that acts as a major effector of the X-inactivation process.
It is a component of the Xic – X-chromosome inactivation center – along with two
other RNA genes (Jpx and Ftx) and two protein genes (Tsx and Cnbp2).
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26. Models of the localization and spreading of Xist.
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27. CIRCULAR RNA (circRNA)
• Circular RNA (circRNA) is a novel endogenous non-coding RNA (ncRNA) that, like
microRNA (miRNA), is a rapidly emerging RNA research topic.
• CircRNA, unlike traditional linear RNAs (which have 5' and 3' ends), has a closed-
loop structure that is unaffected by RNA exonucleases.
• Circular RNA is a type of single-stranded RNA which, unlike linear RNA, forms a
covalently closed continuous loop. In circular RNA, the 3' and 5' ends normally
present in an RNA molecule have been joined together.
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28. CIRCULAR RNA
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29. REFERENCE
• https://biologydictionary.net/ribosomal-rna/
• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3175250/
• Long Noncoding RNA - an overview | ScienceDirect Topics
• Cooper, G.,& Hausman, R. (2013). The cell: A molecular approach (6th ed.).
Sunderland, MA: Sinauer Associates.
• Karp, G. (2013). Cell and molecular biology: Concepts and Experiments. NJ: John
Wiley & Sons Inc.
• Lodish, H., Berk, A., Zipursky, S., Matsudaira, P., Baltimore, D., & Darnell, J. (2008).
Molecular cell biology. New York: W.H. Freeman and company.
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