RNA Structures, Types and Functions

RNA (Ribonucleic acid )
RNA is a polymer of
ribonucleotides linked together
by 3’-5’ phosphodiester linkage.
The major role of RNA is to
participate in protein synthesis,
which requires three classes of
RNA.
2Biochemistry For Medics

Differences between RNA and DNA
RNA DNA
1) Single stranded mainly except when self
complementary sequences are there it
forms a double stranded structure (Hair
pin structure)
Double stranded (Except for
certain viral DNA s which are
single stranded)
2) Ribose is the main sugar The sugar moiety is deoxy
ribose
3) Pyrimidine components differ. Thymine is
never found(Except
tRNA)
Thymine is always there but
uracil is never found
4) Being single stranded structure- It does
not follow Chargaff’s rule
It does follow Chargaff's rule.
The total purine content in a
double stranded DNA is always
equal to pyrimidine content.

RNA DNA
5) RNA can be easily destroyed by
alkalies to cyclic diesters of
mono nucleotides.
DNA resists alkali action due to
the absence of OH group at 2’
position
6) RNA is a relatively a labile
molecule, undergoes easy and
spontaneous degradation
DNA is a stable molecule. The
spontaneous degradation is
very 2 slow. The genetic
information can be stored for
years together without any
change.
7) Mainly cytoplasmic, but also
present in nucleus (primary
transcript and small nuclear
RNA)
Mainly found in nucleus, extra
nuclear DNA is found in
mitochondria, and plasmids
etc
8) The base content varies from
100- 5000. The size is variable.
Millions of base pairs are there
depending upon the organism

RNA DNA
9) There are various types of RNA –
mRNA, r RNA, t RNA, Sn RNA, Si
RNA, mi RNA and hn RNA. These
RNAs perform different and
specific functions.
DNA is always of one type and
performs the function of
storage and transfer of genetic
information.
10) No variable physiological forms
of RNA are found. The different
types of RNA do not change
their forms
There are variable forms of
DNA (A to E and Z)
11) RNA is synthesized from DNA, it
can not form DNA(except by the
action of reverse transcriptase).
It can not duplicate (except in
certain viruses where it is a
genomic material )
DNA can form DNA by
replication, it can also form
RNA by transcription.
12) Many copies of RNA are present
per cell
Single copy of DNA is present
per cell.

RNA V/S DNA

Types of RNA
In all prokaryotic and eukaryotic
organisms, three main classes of RNA
molecules exist-
1) Messenger RNA(m RNA)
2) Transfer RNA (t RNA)
3) Ribosomal RNA (r RNA)
The other are –
o small nuclear RNA (SnRNA),
o micro RNA(mi RNA) and
o small interfering RNA(Si RNA) and
o heterogeneous nuclear RNA (hnRNA).

1. Messenger RNA (m-RNA)
 Carries the genetic information for a protein
from the DNA in the nucleus to the
ribosomes.
 Exist long enough to carry the message and
have its corresponding protein synthesized
then the mRNA is degraded.

Biochemistry For Medics 9
Message: forms of the TRIPLETS OF BASES
specifying AMINO ACIDS OR PUNCTUATIONS
called CODONS.
Comprises only 5% of the RNA in the cell
All members of the class function as
messengers carrying the information in a gene
to the protein synthesizing machinery

2. Heterogeneous nuclear RNA
(hnRNA)
 In mammalian nuclei , hnRNA is the
immediate product of gene transcription
 The nuclear product is heterogeneous in
size (Variable) and is very large.
Molecular weight may be more than 107,
while the molecular weight of m RNA is
less than 2x 106
 75 % of hnRNA is degraded in the
nucleus, only 25% is processed to mature
m RNA

3. Transfer RNA (t- RNA)
smallest of three major species of RNA molecules
They have 74-95 nucleotide residues
Translate the genetic code of the mRNA into the
primary sequence of amino acids in the protein.
 They are synthesized by the nuclear processing of
a precursor molecule
They transfer the amino acids from cytoplasm to
the protein synthesizing machinery, hence the name
t RNA.
They are also called Adapter molecules.
There are at least 20 species of t RNA one
corresponding to each of the 20 amino acids
required for protein synthesis.

Structural characteristics of t- RNA
1) Primary structure- The nucleotide
sequence of all the t RNA molecules
allows extensive intrastand
complimentarity that generates a
secondary structure.
2) Secondary structure- Each single t-
RNA shows extensive internal base
pairing and acquires a clover leaf like
structure. The structure is stabilized by
hydrogen bonding between the bases
and is a consistent feature.

Secondary structure of t- RNA
The carboxyl group of amino acid is attached to 3’OH group of
Adenine nucleotide of the acceptor arm. The anticodon arm
base pairs with the codon present on the m- RNA 13Biochemistry For Medics

Tertiary structure of t- RNA
 The L shaped tertiary
structure is formed by
further folding of the clover
leaf due to hydrogen bonds
between T and D arms.
 The base paired double
helical stems get arranged
in to two double helical
columns, continuous and
perpendicular to one
another.

4. Ribosomal RNA (rRNA)
The mammalian ribosome contains two major
nucleoprotein subunits—a larger one with a
molecular weight of 2.8 x 106 (60S) and a smaller
subunit with a molecular weight of 1.4 x 106 (40S).
 Is a structural and functional component of
ribosomes w/ are “platforms” on which protein
synthesis occur.
 Consist of about 35% protein and 65%
ribosomal RNA.
 Complexed w/ proteins, the rRNA forms the
cellular structures called the ribosomes.

Ribosomal RNA (rRNA)

Ribosomal RNA (rRNA)
The functions of the ribosomal RNA
molecules in the ribosomal particle are not
fully understood, but they are necessary for
ribosomal assembly and seem to play key
roles in the binding of mRNA to ribosomes
and its translation
Recent studies suggest that an rRNA
component performs the peptidyl transferase
activity and thus is an enzyme (a ribozyme).

5. Small Nuclear RNA
Most of these molecules are
complexed with proteins to form small
nuclear ribonucleoproteins
particles{snurps} and are distributed in
the nucleus, in the cytoplasm, or in both.
They range in size from 20 to 300
nucleotides and are present in 100,000–
1,000,000 copies per cell.

Small Nuclear RNAs (snRNAs)
Function:
 Help w/ the processing of the initial
mRNA into mature form that is ready
for export out of the nucleus-{
splicing}.
Of the several snRNAs, U1, U2, U4,
U5, and U6 are involved in intron
removal and the processing of hnRNA
into mRNA
The U7 snRNA is involved in
production of the correct 3' ends of 19Biochemistry For Medics

Micro RNAs, miRNAs, and
Small Interfering RNAs,
siRNAs
These two classes of RNAs represent a
subset of small RNAs; both play
important roles in gene regulation.
miRNAs and siRNAs cause inhibition
of gene expression by decreasing specific
protein production albeit apparently via
distinct mechanisms

6.Micro RNAs (miRNAs)
Composed only of 22 nucleotides
long but are important in the timing
of an organism’s development.
Bind to the sections of mRNA and
prevent their translation.

Micro RNAs (miRNAs)
microRNAs, short non-coding RNAs present in all living organisms, have
been shown to regulate the expression of at least half of all human
genes. These single-stranded RNAs exert their regulatory action by
binding messenger RNAs and preventing their translation into
proteins.

7. Small Interfering RNAs
(siRNAs)
 short stetches of rna( 20-30 nucleotides long).
Found to have an enormous control over gene
expression.
This process serve as a protective mechanism in
many species , being used to eliminate expression
of undesirable gene.
 become an explosion of new biotechnology,
many companies have been created to produce
and market designer siRNA to knockout
hundreds of known genes.

Significance of mi RNAs and si
RNAs
Both miRNAs and siRNAs represent
exciting new potential targets for
therapeutic drug development in humans.
In addition, siRNAs are frequently used to
decrease or "knock-down" specific protein
levels in experimental procedures in the
laboratory, an extremely useful and
powerful alternative to gene-knockout
technology.

Summary
RNA exists in several different single-stranded
structures, most of which are directly or indirectly
involved in protein synthesis or its regulation.
The linear array of nucleotides in RNA consists of
A, G, C, and U, and the sugar moiety is ribose.
The major forms of RNA include messenger RNA
(mRNA), ribosomal RNA (rRNA), transfer RNA
(tRNA), and small nuclear RNAs (snRNAs;
miRNAs).
 Certain RNA molecules act as catalysts
(ribozymes).
miRNAs and siRNAs represent exciting new
potential targets for therapeutic drug
development in humans.

Biochemistry For Medics 26
RNA TYPE SIZE FUNCTION
Transfer RNA Small Transport amino acids to
site of protein synthesis
Ribosomal RNA Several kinds Combines w/ proteins to
form ribosomes.
Messenger RNA Variable Directs amino acids
sequence of proteins
Small Nuclear RNA Small Process initial mRNA to
its mature form in
eukaryotes.
Micro RNA Small Affects gene expression.
Small Interfering RNA small Affects gene expression;
used by the scientist to
knock out a gene being
studied.

RNA Structures, Types and Functions

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