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STRUCTURE OF NUCLEIC ACID :
RIBONUCLEIC ACID (RNA )
rRNA,mRNA,tRNA and Micro RNA
BY Dr. Ichha Purak
University ProfessorRanchi Women’s College,Ranchi
http://www.dripurak.com/http://drichhapurak.webnode.com/
Friedrich Miescher in 1869 discovered substance
nuclein from the nuclei of pus cells which was
later termed nucleic acid because of having
phosphate group and was of acidic nature.
Nucleic acids are present in all living organisms
(plants,animal,bacteria and even Virus and viriods)
and therefore origin of life is suspected with the
formation of nucleic acids. All living cells contain
both DNA and RNA, while viruses contain either
DNA or RNA, usually not both.
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The nucleic acids are most important
biological macromolecules responsible for transmission and
storage of heredity or genetic information from one
generation to next by means of Replication, Transcription
and Translation.
Nucleic acids are fibre like molecules having much greater
length in comparision to diameter
Nucleic acids are the found in abundance in all living things,
where they function in encoding, transmitting and
expressing genetic information
The nucleic acids are generally associated with proteins to
form nucleoproteins
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There are two types of nucleic acids :
RNA (Ribonucleic acid ) = (Ribonucleotide )n –A U C
G
DNA ( Deoxyribonucleic
acid)=(Deoxyribonucleotide)n– A T C G
These are polymers consisting of long chains of monomers called nucleotides linked in a chain through phosphodiester bond
A nucleotide consists of a nitrogenous base, a
pentose sugar and a phosphate group
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There are two related pentose sugars:
- RNA contains ribose
- DNA contains deoxyribose
The sugars have their carbon atoms numbered with
primes to distinguish them from the nitrogen bases
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NITROGEN BASES The nitrogen bases in nucleotides consist of two general types: - pyrimidines: cytosine (C), thymine (T) and Uracil (U) - purines: adenine (A) and guanine (G)
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The primary structure of a nucleic acid
is the nucleotide sequence joined by phosphodiester bonds
The 3’-OH group of the sugar in one nucleotide forms an ester
bond to the phosphate group on the 5’-carbon of the sugar of the
next nucleotide
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READING PRIMARY STRUCTURE
A nucleic acid polymer has a free 5’-phosphate group at one end and a free 3’-OH group at the other end
The sequence is read from the free 5’-end using the letters of the bases
This example reads
5’—A—C—G—T—3’
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EXAMPLE OF RNA PRIMARY STRUCTURE
In RNA, A, C, G, and U are linked by 3’-5’ ester bonds between ribose and phosphate
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RNA is polymer of
Ribonucleotides
RNA is a polymeric
constituent of all living
cells and many viruses. it
is long single stranded
chain of alternating
phosphate and ribose
sugar units with the
nitrogenous bases
Adenine & guanine
(purines) and cytosine and
uracil (pyrimidines)
bonded to ribose sugar.
RIBONUCLEIC ACID (RNA )0
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Chemical structure of RNA
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RBONUCLEOTIDES
(AMP )adenosine monophosphate
(GMP) gwanosine monophosphate
PURINES
(CMP) cytidine monophosphate
(UMP) uridine monophosphatePYRIMIDINES
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STRUCTURE OF RIBO NUCLIC ACIDS
Polymers of four nucleotides Linked by alternating sugar-phosphate
bonds RNA: ribose and A, C, G, U
P sugar
A
P sugar
G
P sugar
C
P sugar
U
AMPCMP
GMP
UMP
RNAs are usually single
stranded, but many RNA
molecules have secondary
structure in which
intramolecular loops are
formed by complementary
base pairing (tRNA and
rRNA ).
Base pairing in RNA follows
exactly the same principles
as with DNA: the two
regions involved in duplex
formation are antiparallel
to one another, and the
base pairs that form are A-
U and C-G
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RNA is much more abundant than DNADFFERENCES BETWEEN RNA AND DNAS N RNA DNA
1 RNA is polymer of Ribonucleotides DNA is polymer of Deoxyribonucleotides
2 RNA has Ribose sugar DNA has Deoxyribose sugar3 Ribose sugar (C5H10O5) has OH-
at C2,C3 &C5Deoxyribose Sugar has OH- only at C3 & C5
4 RNA is generally single stranded Exception tRNA & rRNA at some places are double stranded
DNA is Generally Double Stranded Exception In Bacteriophage Ф- 174 and S-13
5 RNA has Pyrimidines Cytosine and Uracil
DNA has Pyrimidines Cytosine and Thymine
6 RNA generally Non Genetic but in some Viruses it is Genetic Material
DNA is Genetic Material
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7 RNA is not stable DNA is stable in alkaline condition
8 RNA is synthesized by transcription using one strand of DNA as Template
DNA is Synthesized by Replication using both strands of DNA as templates during Interphase
9 In helical regions A-U and C-G are the Base Pairs
A- T and C-G are the Base Pairs
10 The helix geometry of RNA is of A-FormIs destroyed by enzymes and produced again
The helix geometry of DNA is of B-FormProtects itself against enzymes
11 RNA performs the function of a messenger between DNA and the proteins.
DNA performs long term storage and transfer of Genetic Information
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.
RIBOSE
1
OHOCH2
H
H
OH
H
OH
H
OH23
4
5OHOCH2
H
H
OH
H
OH
H
H
1
23
4
5
2-DEOXY-RIBOSE
O N
H N
O
H
CH3
THYMINE
O N
H N
O
H
URACIL
RNA DNA
Molecular Differences between Ribonucleic Acid (RNA)& 2-deoxy-ribonucleic acid (DNA).
Ribose replaces deoxyribose; uracil replaces thymine
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S N Coding RNA Non-coding RNAs
1 mRNA (Messenger RNA)
2 tRNA ( Transfer RNA)
3 rRNA (Ribosomal RNA
4 snRNA (Small nuclear RNA )
5 snoRNA (Small nucleolar RNA )
6 scRNA (Small cytoplasmic RNA)
7 aRNA (Antisense RNA )
8 miRNA (microRNA )
9 siRNA (small interference RNA )
10 Ribozyme (catalytic RNA )
11 cRNA (Complementary RNA )
12 gRNA ( guide RNA )
TYPES OF RNAs0
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MAJOR TYPES OF RNAS0
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SYNTHESIS AND MAIN FUNCTIONS OF DIFFERENT RNAs
BRIEF DESCRIPTION OF DIFFERENT RNAS
Messenger RNA(mRNA )-is the only coding RNA as it bears a complementary copy of deoxyribonucleotide sequence present on Gene (DNA segment ) as ribonucleotide sequence. It encodes chemical blueprint for protein synthesis in 5’-3’ direction. It is single stranded
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Transfer RNA (tRNA) is a
small RNA chain(73-95 nucleotides )
that transfers a specific amino acid
to a growing polypeptide chain at
ribosome during translation.
It acts as a adapter molecule
because it can recognise both a
specific Amino Acid as well as its
codon on mRNA. For loading 20
different protein Amino Acids there
are different tRNAs.(1-20) Each
amino acid is recognized by one or
more specific tRNA
tRNA has a tertiary structure
that is L-shaped one end
attaches to the amino acid
and the other binds to the
mRNA by a 3-base
complimentary sequence
Anticodon
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Ribosomal RNA(rRNA )is the most stable RNA .It is
synthesized by Nucleolar genes
by RNA polymerase I and make
40-60% by weight of total RNA
and is 80% of total RNA of the
cell. It may measure upto 7000
Aᵒ in extended form . It is main
component of ribosomes along
with proteinsRibosomes are the sites of
protein synthesis ,consisting of
ribosomal RNA (65%) and
proteins (35%), have a small
and a large subunit
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Small nuclear RNA
(snRNA) a class of small
eukaryotic RNA molecules
found in the nucleus usually
as ribonucleoproteins which
are involved in processing
heterogenous nuclear
messenger RNA (Splicing of
Exons )U1, U2 U4, U5 and
U6 and also used in RNA
interference
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Small nucleolar RNA (snoRNAs ) are a class of
small RNA molecules that guide chemical modification
(methylation or puedouridylation ) of ribosomal RNAs
Small cytoplasmic RNA (scRNA) small (7S,129
nucleotides) RNA found in cytosol and rough endoplasmic
reticulum associated with proteins ,component of SRP (Signal
Recognition Particle ) that are involved in specific selection and
transport of secretory proteins by recognizing signal sequence at
amino terminus of growing polypeptide chain.
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Signal Sequence directing Protein Targetting
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Antisense RNA (aRNA) is a single stranded RNA
that is complementary to a mRNA strand . Antisence mRNA
(artificially synthesized ) may be introduced into a cell to
inhibit translation of a complementary mRNA by base pairing
to it and physically obstructing (blocking) the translation
machinery. It is thought as promising technique for disease
therapy.
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microRNAs (miRNA )
MicroRNAs were first described in 1993 by Rosalind Lee, Victor
Ambros, and Rhonda Feinbaum in the nematode Caenorhabditis
elegans
MicroRNAs constitute a recently discovered class of non-coding RNAs
that play key roles in the regulation of gene expression by gene
silencing
Mature microRNAs are short, single-stranded RNA molecules
having about 21- 23 nucleotides in length
The genes encoding miRNAs are much longer than the
processed mature miRNA molecule
MicroRNA genes are transcribed by RNA polymerase II as large
primary transcripts (pri-microRNA)
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Pri-microRNA has a cap and Poly-A tail before undergoing
processing like mRNA Pri-microRNAs are processed by a protein
complex Drosha containing RNase III enzyme to form an
approximately 70 nucleotide precursor microRNA (pre-microRNA).
This molecule has stem-loop structure
This precursor is subsequently transported to the cytoplasm where it
is processed by a second RNase III enzyme, DICER, to form a mature
microRNA of approximately 22 nucleotides
This mature miRNA has some complementary sequences to one or
more mRNAs
The mature microRNA is then incorporated into a ribonuclear particle
to form the RNA-induced silencing complex, RISC, which mediates
gene silencing through RNA interference.
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MicroRNAs usually induce gene silencing by binding to target sites
found within the 3’UTR of the targeted mRNA.
This interaction prevents protein production by suppressing protein
synthesis by initiating mRNA degradation
Since most target sites on the mRNA have only partial base
complementarity with their corresponding microRNA, individual
microRNAs may target as many as 100 different mRNAs.
MicroRNAs play significant role in cell cycle control,apoptosis,stem
cell differentiation etc.MicroRNAs are also involved in tissue-specific
expression
MicroRNAs are a class of post-transcriptional regulators
They are short ~22 nucleotide RNA sequences that bind to
complementary sequences in the 3’ UTR of multiple target mRNAs,
usually resulting in their silencing.
.
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Processing of miRNA
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Function of Micro RNA ( MiRNA) Small but mighty
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Small interfering RNA ( siRNA ) is considered
exogenous double-stranded RNA that is taken up by cells, or
enters via vectors like viruses, while miRNA is single stranded
and comes from endogenous (made inside the cell) non-coding
RNA, found within the introns of larger RNA molecules.
siRNA are double stranded RNA fragments,which trigger
catalytically mediated gene silencing by targetting RNA
Induced Silencing Complex ( RISC) to bind and degrade the
mRNA
Both siRNA and miRNA can play a role in epigenetics through a
process called RNA-induced transcriptional silencing
(RITS). Likewise, both are important targets for therapeutic
use, because of the roles they play in the controlling gene
expression.
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Ribozyme ( Catalytic RNA ) is ribonucleic acid molecules having catalytic activity as protein enzymes
Till 1989 it was believed that all enzymes are proteinaceous in
nature. During observing spilicing mechanism of Group I rRNA
intron from protozoa Tetrahymena thermophila by Thomas Cech
and Sydney Altman, it was realized that no proteins were
involved in this process. It was revealed that RNAs could also
have catalytic function. In this case the rRNA was processed for
removal of intron and spilicing exons without any intervention of
any protein.
This post transcriptional processing of rRNA led to discovery of
RNA enzyme or RIbozyme or Non-protein enzymes.
ExamplesRNase PGroup I and Group II intronsPeptidyl transferase 23S rRNA
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Splicing mechanism of Group I intron .The nucleophile in the first step may be gwanosine,GMP,GDP or GTP
Guide RNA (gRNA) are RNA genes that function in RNA
editing, found in mitochondria by inserting or deleting stretches
of uridylates (Us) . The gRNA forms part of editosome and contain
sequences to hybridize to matching sequences in the mRNA to
guide the mRNA modifications.
Complementary RNA( cRNA ) viral RNA that is transcribed
from negative sense RNA and serves as a template for protein
synthesis
Negative sense RNA viral RNA with a base sequence complementary
to that of mRNA during replication it serves as a template to the
transcription of viral complementary RNA
Positive sense RNA viral RNA with same base sequence as mRNA
during replication function as mRNA ,serving as template for protein
synthesis
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RNA interference Experimental use by introducing
small ds RNAs for functional deactivation of specific genes ,which
induces degradation of complementary single stranded mRNA
encoded by the gene , gene silencing.
RNA interference is a related process in which double stranded
RNA fragnments called small interfering RNAs trigger catalytically
mediated gene silencing most typically by targeting the RNA
induced Silencing Complex (RISC) to bind to and degrade the
mRNA
Attempts to genetically engineer transgenic plants to express
antisense RNA instead activate the RNA pathway, although the
processess result in differing magnitude of gene silencing.
Well known examples include Flavr Savr tomato and two
cultivars of ringspot papaya
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FUNCTIONS OF DIFFERENT RNA
mRNA – It carries genetic formation of DNA ( Gene ) for
protein synthesis from nucleus to ribosome in the form of
genetic code
tRNA – Acts as adapter molecule ,carries Amino Acid and
drops it to particular location by recognising codon on mRNA
by virtue of having anticodon
rRNA – It makes complex with proteins and form ribosomal
subunits which provide space for protein synthesis ,single
ribosomal RNA of smaller subunit helps correct orientation of
mRNA during attachment with respect to P and A sites
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snRNA – play significat role in eukayotic mRNA processing
By splicing of exons as snRNPs or snurps U1,U2, U4,U5 &U6
scRNA – being component of Signal Recognition Particle (SRP )
helps in targetting of seceretary proteins
snoRNA – Plays role in gene silencing
miRNA – play important role in gene silencing by blocking mRNA
and preventing translation
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Si RNA – Plays important role in gene silencing by interfering
transcription
gRNA- help in RNA editing in mitochondria , forms part of
editosome and hybridize with matching sequence of mRNA
Catalytic RNA –Ribozymes act as protein enzymes in
catalyzing removal of intron, peptide bond formation etc
aRNA- antisense RNA - is artificially used to block translation
of perticular messenger RNAs so as to prevent formation of
some harmful proteins
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THANK YOU