Types of RNA

1. Typesof RNA
Niloufer M.A and Dr. Kayeen Vadakkan
Department of Biotechnology
ST.MARY’S COLLEGE, THRISSUR

2. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR
INTODUCTION
RNA or ribonucleic acid is a polymer of nucleotides that is
made up of a ribose sugar, a phosphate, and bases such as
adenine, guanine, cytosine, and uracil.
 It plays a crucial role in gene expression by acting as the
intermediate between the genetic information encoded by
DNA and proteins.
RNA has a structure very similar to that of DNA. The key
difference in RNA structure is that the ribose sugar in RNA
possesses a hydroxyl (-OH) group that is absent in DNA.

3. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR

4. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR
MessengerRNA (mRNA)
Messenger ribonucleic acids (mRNAs) transfer the
information from DNA to the cell machinery that makes
proteins.
mRNA accounts for just 5% of the total RNA in the cell.
 mRNA is the most heterogeneous of the 3 types of RNA in
terms of both base sequence and size.
 It carries complimentary genetic code copied, from DNA during
transcription, in the form of triplets of nucleotides called codons.
The message of mRNA is read consecutively in 5’3’ direction.

5. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR
Molecules of mRNA are composed of relatively short, single
strands of molecules made up of adenine, cytosine, guanine
and uracil bases held together by a sugar phosphate
backbone.
When RNA polymerase finishes reading a section of the
DNA, the pre-mRNA copy is processed to form mature mRNA
and then transferred out of the cell nucleus.
Ribosomes read the mRNA and translate the message into
functional proteins in a process called translation.

6. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR
Depending on the newly synthesized protein’s structure
and function, it will be further modified by the cell, exported
to the extra-cellular space, or will remain inside the cell.
 Precursor mRNA contains introns and exons. Introns are
removed before translation, while exons code for the amino
acid sequence of proteins.
To make mature mRNA, the cell machinery removes
“non-translatable” introns from the pre-mRNA, leaving only
translatable exon sequences in the mRNA.

7. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR

8. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR
Types of mRNA
Pre-mRNA and hnRNA
•Precursor mRNA (pre-mRNA) is the primary transcript of
eukaryotic mRNA as it comes off the DNA template.
• Pre-mRNA is part of a group of RNAs called heterogeneous
nuclear RNA (hnRNA).
• hnRNA refers to all single strand RNA located inside the nucleus
of the cell where transcription takes place (DNA->RNA) and pre-
mRNA form a large part of these ribonucleic acids.

9. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR
• Pre-mRNA contains sequences that need to be removed or
“spliced out” before being translated into a protein.
•These sequences can either be removed through the catalytic
activity of the RNA itself, or through the action of a multi-
protein structure called spliceosome.
•After this processing step, the pre-mRNA is considered as a
mature mRNA transcript.
Monocistronic mRNA
A monocistronic mRNA molecule contains the exon
sequences coding for a single protein. Most eukaryotic
mRNAs are monocistronic.

10. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR
Bicistronic mRNA
A bicistronic mRNA molecule contains the exon coding
sequences for two proteins.
Polycistronic mRNA
A polycistronic mRNA molecule contains the exon coding
sequences for multiple proteins. Most mRNA
of bacteria and bacteriophages (viruses that live in bacterial
hosts) are polycistronic.

11. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR
Ribosomal RNA (rRNA)
rRNAs are found in the ribosomes and account for 80% of the total
RNA present in the cell.
 Ribosomes are composed of a large subunit called the 50S and a
small subunit called the 30S, each of which is made up of its own
specific rRNA molecules.
 Different rRNAs present in the ribosomes include small rRNAs and
large rRNAs, which belong to the small and large subunits of the
ribosome, respectively.

12. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR

13. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR
rRNAs combine with proteins and enzymes in the cytoplasm
to form ribosomes, which act as the site of protein synthesis.
These complex structures travel along the mRNA molecule
during translation and facilitate the assembly of amino acids to
form a polypeptide chain.
 They interact with tRNAs and other molecules that are crucial
to protein synthesis.
In bacteria, the small and large rRNAs contain about 1500 and
3000 nucleotides, respectively, whereas in humans, they have
about 1800 and 5000 nucleotides, respectively.

14. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR
Transfer RNA (tRNA)
tRNA is the smallest of the 3 types of RNA, possessing
around 75-95 nucleotides.
tRNAs are an essential component of translation, where
their main function is the transfer of amino acids during
protein synthesis. Therefore, they are called transfer RNAs.
 tRNAs also act as adapters in the translation of the genetic
sequence of mRNA into proteins. Thus, they are also called
adapter molecules.

15. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR

16. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR
tRNAs have a clover leaf structure which is stabilized by
strong hydrogen bonds between the nucleotides.
They normally contain some unusual bases in addition to
the usual 4, which are formed by methylation of the usual
bases.
Methyl guanine and methylcytosine are two examples of
methylated bases.
Each of the 20 amino acids has a specific tRNA that binds
with it and transfers it to the growing polypeptide chain.

17. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR

18. TYPES OF RNA, NILOUFER M.A, ST.MAR’YS COLLEGE - THRISSUR