CHARACTERS AND TYPES OF RNA

CHARACTERISTICS OF RNA
TYPES OF RNA

1. CHARACTERS AND TYPES OF RNA
Introduction:
RNA, abbreviation of ribonucleic acid, complex compound of high molecular weight that
functions in cellular protein synthesis and replaces DNA (deoxyribonucleic acid) as a carrier
of genetic codes in some viruses. RNA consists of ribose nucleotides (nitrogenous bases
appended to a ribose sugar) attached by phosphodiester bonds, forming strands of varying
lengths. The nitrogenous bases in RNA are adenine, guanine, cytosine, and uracil, which
replaces thymine in DNA.
The ribose sugar of RNA is a cyclical structure consisting of five carbons and one oxygen.
The presence of a chemically reactive hydroxyl (−OH) group attached to the second carbon
group in the ribose sugar molecule makes RNA prone to hydrolysis. This chemical ability of
RNA, compared with DNA, which does not have a reactive −OH group in the same position
on the sugar moiety (deoxyribose), is thought to be one reason why DNA evolved to be the
preferred carrier of genetic information in most organisms. The structure of the RNA
molecule was described by R.W. Holley in 1965.

Figure: 01 Ribonucleic acid (RNA)

2. CHARACTERS OF RNA
The characteristic features of ribonucleic acid (RNA) are as follows:
 Ribonucleic acid (RNA) is another nucleic acid type. Like deoxyribose nucleic acid
 (DNA), this is a polynucleotide but several differences are found in their structure.
 In RNA, the pentose sugar is ribose and not deoxyribose
 RNA contains uracil in place of thymine.
 Most RNAs are generally single stranded with partial double-stranded regions due to
folding back of its single chain.
 RNA serves as a genetic material in many viruses.
 There are three major classes of cellular RNAs which function during gene
expression.
 These three major classes of cellular RNAs are – (i) ribosomal RNA (rRNA), (ii)
messenger RNA (mRNA) and (iii) transfer RNA (tRNA).
 All these three types of molecules originate as complementary copies of one of the
two strands of a DNA segment that constitutes a gene, during the process of
transcription.
 This shows that RNA will have the same sequence as the other strand of DNA, except
that uracil will replace thymine against adenine.
 The abovementioned RNA types can be differentiated on the basis of their size,
sedimentation behaviour and genetic functions.
 Ribosomal RNA (rRNA) is generally the largest and most prevalent of the cellular
RNA species. This is an important structural component of ribosomes, the sites where
translation occurs during protein synthesis.
 Messenger RNA (mRNA) is responsible to carry the genetic message from DNA to
the ribosome. Their length and sequence vary depending upon the gene which is being
transcribed into m RNA.
 Transfer RNA (tRNA) is the smallest of the three types. It carries amino acids to the
ribosomes during translation. This species of RNA contain number of modified bases.

Types of RNA
RNA Could be divided into two categories in accordance with their coding potential,
that is, coding RNAs and non-coding RNAs.

3. Coding RNAs generally refer to mRNA that encodes protein to act as various
components including enzymes, cell structures, and signal transductors.
Human genome analysis contributed to the first discovery of long Sequences of
ncRNA such as tRNA and rRNA. In addition to this, other lncRNA sequences were
also identified whose function is not found in protein translation machinery. When
these long sequences of non-coding RNA were compared to ENCODE consortium
data, it was found that several classes of ncRNA molecules are generated through
pathways similar to that of protein-coding genes .These findings in relation to
previous studies sparked on the diversity of non-coding RNA, ncRNAs are classified
into two major categories: structural non-coding RNAs and regulatory non-coding
RNAs. Structural non-coding RNAs comprise of rRNAs and tRNAs. Regulatory
noncoding RNAs are further divided into three classes, small,medium, and long
noncoding RNAs.
Further, miRNA, siRNA, piRNA, cisRNA, telsRNA were considered as short non-
coding RNA with a size between 20–50 nucleotides, and snoRNA, prompts, tiRNA,
snRNA, and many more are classified as medium non-coding RNA with a size
between 50-200 nucleotides and a large class of RNA with maximum regulatory
potency containing greater than 200 nucleotides are classified as long non-coding
RNA. With increasing studies on highly abundant and functionally important
categories of lncRNAs (such as intronic, antisense, lincRNA, cisRNA, ceRNA, etc)
CODING RNA
Messenger RNA
mRNA - Jacob and Monod (1961) proposed the name messenger RNA for the RNA
carrying information for protein synthesis from the DNA (genes) to the sites of
protein formation (ribosomes). It consists of only 3 to 5% of the total cellular RNA.
Size of Messenger RNA - mRNA
The molecular weight of an average sized mRNA molecule is about 500,000, and its
sedimentation coefficient is 8S. It should be noted however, that mRNA varies greatly
in length and molecular weight. Since most proteins contain at least a hundred amino
acid residues, mRNA must have at least 100 X 3= 300 nucleotides on the basis of the

4. triplet code. Stability of Messenger RNA - mRNA - The cell does not contain large
quantities of mRNA. This is because mRNA, unlike other RNAs is constantly
undergoing breakdown. It is broken down to its constituent ribonucleotides by
ribonucleases.
Structure of Messenger RNA - mRNA
Messenger RNA is always single stranded. It contains mostly the basesadenine,
guanine, cytosine and uracil. There are few unusual substituted bases. Although there
is a certain amount of random coiling in extracted mRNA, there is no base pairing. In
fact base pairing in the mRNA strand destroys its biological activity. Since mRNA is
transcribed on DNA (genes), its base sequence is complementary to that of the
segment of DNA on which it is transcribed. This has been demonstrated by
hybridization experiments in which artificial RNA, DNA double strands are produced.
Hybridization takes place only if the DNA and RNA strands are complementary.
Usually each gene transcribes its own mRNA. Therefore, there are approximately as
many types of mRNA molecules as there are genes. There may be 1,000 to 10.000
different species of mRNA in a cell. These mRNA types differ only in the sequence of
their bases and in length. When one gene (cistron) codes for a single mRNA strand
the mRNA is said to be monocistronic. In many cases, however, several adjacent
cistrons may transcribe an mRNA molecule, which is then said to be polycistronic or
polygenic.
The mRNA molecule has the following structural features:
1. Cap. At the 5' end of the mRNA molecule in most eukaryote cells and animal virus
molecules is found a 'cap'. This is blocked methylated structure, m7Gpp Nmp Np or
m7Gpp Nmp Nmp Np. where: N = any of the four nucleotides and Nmp = 20 methyl
ribose. The rate of protein synthesis depends upon the presence of the cap. Without
the cap mRNA molecules bind very poorly to the ribosomes.
2. Noncoding region 1 (NC1). The cap is followed by a region of 10 to 100
nucleotides. This region is rich in A and U residues, and does not translate protein.
3. The initiation codon is AUG in both prokaryotes and eukaryotes

5. 4. The coding region consists of about 1,500 nucleotides on the average and translates
protein It is made up of 73-93 nucleotides.

Figure:02 Structure Of mRNA
NON CODING RNA
TRANSFER RNA
Transfer RNA (abbreviated tRNA) is a small RNA molecule that plays a key role in protein
synthesis. Transfer RNA serves as a link (or adaptor) between the messenger RNA (mRNA)
molecule and the growing chain of amino acids that make up a protein. Each time an amino
acid is added to the chain, a specific tRNA pairs with its complementary sequence on the
mRNA molecule, ensuring that the appropriate amino acid is inserted into the protein being
synthesized.

6. Figure: 03 Structure Of tRNA
Transfer RNA or tRNA. So tRNAs are in a way one of the molecules that I find really
fascinating. You know, imagine a chef, a master chef cooking in a kitchen. But, of course,
they're so busy and so involved that they need someone to bring them the appropriate
ingredients so that they can cook up their fabulous meal. So really, when we think about the
process in which an amino acid is constructed, the DNA, of course, is the code. So in a chef
analogy, it would be the recipe. The chef, themselves, is the ribosomal machinery that's
actually creating the meal. But really, the tRNA is that very, very important person in the
kitchen that goes and fetches the specific amino acids that are needed as a protein gets
constructed according to the code of DNA. So this is a very particular kind of RNA that has a
unique job of making sure that just the right amino acid is transferred, or that's why it's tRNA,
into the growing chain of the protein as it gets constructed.

rRNA forms ribosomes, which are essential in protein synthesis. A ribosome contains a large
and small ribosomal subunit. In prokaryotes, a small 30S and large 50S ribosomal subunit
make up a 70S ribosome. In eukaryotes, the 40S and 60S subunit form an 80S ribosome. The
ribosomes contain an exit (E), peptidyl (P), and acceptor (A) site to bind aminoacyl-tRNAs
and link amino acids together to create polypeptides.

7. Figure:04 Structure Of rRNA
MicroRNA
MicroRNAs (miRNA) are non-coding RNAs mainly involved in gene regulation. They are
mostly processed from introns and are transcribed into primary miRNA from the host gene by
RNA polymerase II. They are then modified by endonucleases, such as Drosha and Dicer into
a mature miRNA. Studies have shown that miRNAs that bind to an untranslated region
(3’UTR) on mRNAs suppress translation, while miRNA binding to promoter regions can
upregulate transcription.[13] miRNAs can also function similarly to hormones. They are
released into the extracellular fluid and taken up by target cells for regulation of cellular
activity. Additionally, researchers are studying these extracellular miRNAs as ideal
biomarkers for various diseases. Research has already shown circulating miRNAs to be
involved in cancer through its role in controlling oncogenes and tumor suppressors.

Figure: 05 Structure Of MicroRNA

8. Small Interfering RNA
SiRNA:
Small Interfering RNAs (siRNA) are double-stranded, non-coding RNAs that inhibit gene
expression through RNA interference. They interfere with gene expression by degrading
mRNA and preventing the translation of proteins. siRNAs form from long double-stranded
RNAs with the assistance of Dicer. Once fully formed, siRNA binds to an RNA induced
silencing complex (RISC) and cleaves mRNA through a catalytic RISC protein,
Argonaute. Small interfering RNAs have the potential to be therapeutic agents for diseases
due to their potency and ability to knock down genes. Unlike miRNAs, siRNAs can
specifically target a gene of choice, and a single siRNA guide strand can function multiple
times.


Figure: 06 Structure Of siRNA
CONCLUSION
The central dogma of molecular biology suggests that DNA maintains the information to
encode all of our proteins and that three different types of RNA rather passively convert this
code into polypeptides. Specifically, messenger RNA (mRNA) carries the protein blueprint

9. from a cell's DNA to its ribosomes, which are the "machines" that drive protein synthesis.
Transfer RNA (tRNA) then carries the appropriate amino acids into the ribosome for
inclusion in the new protein. Meanwhile, the ribosomes themselves consist largely of
ribosomal RNA (rRNA) molecules.
RNA molecules play numerous roles in both normal cellular processes and disease states.
Generally, those RNA molecules that do not take the form of mrna are referred to as
noncoding, because they do not encode proteins. The involvement of noncoding mRNAs in
many regulatory processes, their abundance, and their diversity of functions has led to the
hypothesis that an "RNA world" may have preceded the evolution of DNA and proteins.