RNA occurs in multiple copies and various forms. Cells contain up to eight times as much RNA as DNA. RNA is found in both nucleus & cytoplasm 3D structure of RNAs are complex and unique Hydrophobic interactions stabilize the structure. Forms – A & Z form – common B form – not seen Breaks in regular A form helix caused by mismatched bases results in bulges or internal loops. Hairpin loops (end has UUCG) are formed b/w self complementary sequence – common 2° structure helps to form the 3D structure

1. STRUCTURE &
TYPES OF RNA
Ms. M. Arthi, M.Sc., M.Phil., NET., SET.,
Assistant Professor of Microbiology

2. RNA (RIBONUCLEIC ACID)
 RNA occurs in multiple copies and various forms. Cells contain up to eight times as
much RNA as DNA.
 RNA is found in both nucleus & cytoplasm
 3D structure of RNAs are complex and unique
 Hydrophobic interactions stabilize the structure.
 Forms – A & Z form – common
 B form – not seen
 Breaks in regular A form helix caused by mismatched bases results in bulges or internal
loops.
 Hairpin loops (end has UUCG) are formed b/w self complementary sequence – common
2° structure helps to form the 3D structure.

3. TYPES OF RNA
RNA
Genetic
Plant viruses,
animal viruses &
BØ contain RNA as
a genetic material
Viroids & virusoids
(satellite RNA)
found in plants
Non genetic
Transcribed from
genetic DNA
template eg:
mRNA, tRNA, rRNA

4. Total
RNA
Coding RNA (4%)
mRNA
Transfer genetic information from genes to
ribosomes to synthesize proteins
hnRNA (hetrogenous
nuclear)
Serves as precursor for mRNA & other
RNAs
Noncoding RNA
(96%)
rRNA (50-80%) Components of ribosome
tRNA (10-20%) Transfer a.a to mRNa for ptn syn
snRNA (small
nuclear)
RNA splicing
snoRNA (small
nucleolar)
It add methyl groups to rRNA before it is
assembled into ribosomes
scRNA (small
cytoplasmic)
Transport secreted proteins out of the cell
miRNA (micro)
siRNA (small
interfering)
Involved in the control of gene expression
Based on the biological functions of the RNA molecules it is categorized into
several major types:

6. MESSENGER RNA (mRNA)
 Jacob & Monad (1961) coined the name mRNA
 mRNA carries information for protein synthesis from DNA (genes) to
the site of protein synthesis (ribosomes). Hence, mRNA molecules
are said to be “the DNA-like RNA.”
 Avg. mol wt – 25,000-10,00,000
 Sedimentation coefficient – 6-25 S
 Life span – bacteria – 2min; eukaryotes – few hours to few days;
animal eggs & plant seed – months - years
 Always single stranded
 It contains adenine, guanine, cytosine & uracil
 Base sequence is complementary to the DNA template

7. Polycistronic – when several adjacent genes transcribe one mRNA
In prokaryotes, a single mRNA may contain the information for the synthesis
of several polypeptide chains within its nucleotide sequence.

8.  Eukaryotic mRNAs encode only one polypeptide, the process is more complex. mRNA is
synthesized in the nucleus in the form of much larger precursor molecules called
heterogeneous nuclear RNA, or hnRNA.
 The noncoding regions are called intervening sequences or introns.
 The coding regions are called exons.
Monocistronic – when one gene codes for single mRNA strand.

9. • mRNA is synthesized from a DNA strand by an enzyme called RNA
polymerase through a process called transcription.
• mRNA carries the genetic information in the form of triplet code (1 codon)
called genetic code. Each codon codes for one amino acid.

10. Detailed structure of mRNA
 Cap
 Coding region
 Noncoding region
 Poly A tail
i) Cap :
 Present at the 5’ end of mRNA molecule in most eukaryotic cells and animal
virus
 It contain 2 nucleotides connected by 5’ , 5’ triphosphate linkage also bears
methyl groups
 Terminal base is guanine its addition is catalysed by guanylyl transferase
 Methylation – addition of methyl groups to 7position of terminal guanine
catalysed by 7-methyl transferase (present in cytoplasm; cofactor- S-
adenosyl methionine provide methyl group)

11.  Unicellular eukaryotes – cap-O (single methyl group)
 All eukaryotes – posses cap-1 (cap with 2 methyl groups), addition
of another methyl group is catalysed by 2’O-methyl transferase.
 Higher eukaryotes - rarely another methyl group is added to the
second base (if only adenine) catalysed by 2’-O-methyl adenosine
transferase
 Function – it determines the rate of protein synthesis becoz without
cap molecules poorly binds to ribosomes

12. ii) Noncoding region
 Also called as introns or intervening sequence
 Region is rich in A U residues
 Contains 10 – 150 nucleotides
 This region does not translate proteins
iii) Coding region
 No. of nucleotides depends on the no. of amino acids in the polypeptide chain
 Initiation codon is AUG rarely GUG in both prokaryotes & eukaryotes
 Termination codon in eukaryotes is UAA- ochre, UAG-amber, UGA- opal

13. iv) Poly A tail
 Eukaryotic mRNA- 3’ end has polyadenylic acid
 Poly A tail is added in the nucleus after transcription catalysed by poly A polymerase
 Contains about 200 A nucleotides
 The tail gradually shortened by endonuclease cleavage in the cytoplasm (old-short tail
; new-longer tail)
 Functions:
 mRNA combines with ribosomes to form polysomes or polyribosomes
 Each polysome contains several ribosomes. Within the ribosome the mRNA is
translated

14. TRANSFER RNA (tRNA)
 It is also called as soluble RNA (sRNA) or supernatant RNA or
adaptor RNA becoz too small to be separated by ultra centrifuge
 Mol. Wt. – 30,000
 Sedimentation coefficient – 3.8 S
 Nucleotides – 73-94 bases
 Synthesized in the nucleus
 Function :
 tRNA carry amino acids to mRNA during protein synthesis
 Each amino acid is carried by specific tRNA

15. Detailed structure of tRNA
 Detailed structure was Given by R.W. Holley (1964) from yeast
(alanyl tRNA).
Clover leaf model of tRNA
 Single polynucleotide chain is folded to form 5 arms. Arm consist of
double helical stem & a loop (no base paring).
• Acceptor stem (no loop)
• D- arm
• Anticodon arm
• Variable arm (with or without stem)
• T ΨC arm

16. i) Acceptor stem
• Consist of 7 bp & 4 unpaired nucleotide units which has 3’ terminal
CCA sequence & 4th nucleotide is variable.
• 3’ CCA – amino acid binding site
• 5’ P terminal end has either G or C
ii) D arm
• Consist of 15-18 nucleotides with 3-4 base pairs & 7-11 unpaired nucleotides
in the loop.
• D-arm loop is called loop 1 or dihydrouridine (DHU).
• Synthetase (amino acid activating enzyme) recognising site is located b/w D
loop & 5’ acceptor stem

17.  Stem consists of 5 base pairs
 Loop consist of 7 unpaired nucleotides of which middle 3 forms the
anticodon.
 Anticodon recognizes 3 complementary base codon of mRNA
iii) Anticodon arm
iv) Variable arm or mini loop
 Two types- loop with 4-5 bases but no stem
Both stem & loop consist of 13- 21 residues
v) TΨC arm
 Stem having 5 base pairs of C-G
 Loop contains constant 5’ TΨC 3’ sequence
 Also posses ribosomal recognition site

18. 7 bp
3-4 bp & 7-11 unpaired
5 bp & 7 unpaired
5 bp of CG 7 unpaired

20. Unusual bases
• Apart from usual bases tRNA also contains a number of unusual bases.
• These are formed by methylation
• These bases protect the tRNA from RNAase degradation
• Unusual bases – methyl guanine, dimethyl guanine, methyl cytosine, ribothymidine,
pseudouridine, dihydrouridine, inosine & methyl inosine
• Higher organism has more modified bases
Classes of tRNA – 3 classes based on the differences in D & V arm
Initiator tRNA – Eukaryotic ptn syn – methionine (starting a.a) – tRNA met
prokaryotic ptn syn – N-formyl methionine – tRNA f-met
Specificity – each a.a has a specific activating enzyme – tRNA amino acyl synthetase (20
diff for 20 a.as). Some can activate more than 1 a.a
Eg: Isoleucine tRNA synthetase – activates Isoleucine & Valine
Valine tRNA synthetase – activates only valine

21. RIBOSOMAL RNA (rRNA)
 It constitutes about 82% of the total cellular RNA.
 Ribosomes, the supramolecular assemblies where protein synthesis occurs, it
consists of 65% rRNA and proteins.
 Ribosomes are formed in the nucleolus (Nucleus)
 Ribosomal RNA (rRNA) molecules fold into characteristic secondary structures by
intramolecular hydrogen bond interactions.
 The different species of rRNA according to their sedimentation coefficients.
 Ribosomes are composed of two subunits of different sizes that dissociate from
each other if the Mg2 concentration is below 103 M.
 Ribosomal RNAs characteristically contain a number of specially modified
nucleotides, including pseudouridine, ribothymidylic acid, and methylated bases.