RNA introduction, types, function, secretion, translation and transcription, biochemistry

1. Structure and Importance of RNA
Megha R (1RV14CH022)
BIOCHEMICAL ENGINEERING SELF STUDY

2. RNA – Ribonucleic Acid
• RNA is a polymer of
ribonucleotides linked
together by 3’-5’
phosphodiester linkage
• Usually single stranded.
Forms double stranded
structure when self
complementary sequences
exist

3. RNA – Ribonucleic Acid
• Ribonucleotide consists of a
phosphate group, a ribose
sugar and a nitrogenous base
• Phosphate group is linked
with 3’ of nucleoside through
phosphoester linkage

4. RNA – Ribonucleic Acid
• Nitrogenous bases are
classified as
– Purines – Adenine and
Guanine
– Pyrimidines – Uracil and
Cytosine
• Nitrogenous bases are
linked to pentose sugar
through N-glycosidic
linkage to form a
nucleoside

5. RNA – Ribonucleic Acid
• 2 nucleotides are linked through 3’-5’-
phosphodiester linkage to form a dinucleotide
• The polymer has a free phosphate group at 5’
end of ribose sugar and it is called as 5’-end of
polynucleotide chain
• At other end, ribose has free 3’-OH group
which is called as the 3’-end of polynucleotide
chain

6. Types of RNA
Three main classes of RNA molecules exist-
• Messenger RNA(m RNA)
• Transfer RNA (t RNA)
• Ribosomal RNA (r RNA)
The other are small nuclear RNA (SnRNA), micro
RNA(mi RNA) and small interfering RNA(Si
RNA)

7. mRNA – Messenger RNA
• Messenger RNA (mRNA) carries information
about a protein sequence to the ribosomes
• It is coded so that every three nucleotides forms
a codon and corresponds to one amino acid
• The mRNA is formed with the help of DNA during
the process of transcription.
• The sequence of nucleotides in mRNA is
complementary to the sequence of nucleotides
on DNA.

8. mRNA
• In eukaryotic cells, once precursor mRNA (pre-
mRNA) has been transcribed from DNA, it is
processed to mature mRNA
• This involves removal of introns which are
non-coding sections of the pre-mRNA
• The mRNA is then sent from the nucleus to
the cytoplasm, where it is bound to ribosomes
and translated into its corresponding protein
form with the help of tRNA

9. mRNA
• The 5’ terminal end is capped by 7- methyl
guanosine triphosphate cap.
• The cap is involved in the recognition of mRNA by
the translating machinery
• The 3’end of most m-RNAs have a polymer of
Adenylate residues

10. mRNA
• Synthesis
– Catalyzed by an enzyme called RNA polymerase
– A process known as transcription occurs using DNA as a
template
– Initiation of transcription begins with the binding of the
enzyme to a promoter sequence in the DNA
– The DNA double helix is unwound by DNA helicase
– The enzyme then progresses along the template strand in the
3’ to 5’ direction, synthesizing a complementary RNA
molecule with elongation occurring in the 5’ to 3’ direction.
– The DNA sequence also dictates where termination of RNA
synthesis will occur.

11. rRNA – Ribosomal RNA
• A ribosome contains two subunits—a larger
one 60S and a smaller subunit 40S.
• The 60S subunit contains a 5S rRNA, a 5.8S
rRNA, and a 28S rRNA
• The 40S subunit contains a single 18S rRNA
• It forms complexes with proteins forming
ribosomal subunits which provide space for
protein synthesis
• Required for the binding of mRNA to
ribosomes and its translation

12. rRNA

13. tRNA – Transfer RNA
• tRNA work in conjunction with mRNA to carry
out the process of translation and
transcription
• They ‘transfer’ the amino acids from
cytoplasm to the protein synthesizing
machinery
• There are at least 20 species of tRNA one
corresponding to each of the 20 amino acids
required for protein synthesis.

14. tRNA
• The nucleotide sequence of tRNA allows
intrastand complimentarity that generates a
secondary structure.
• Each tRNA shows extensive internal base
pairing and acquires a clover leaf like
structure.

15. tRNA
• The structure is
stabilized by hydrogen
bonding between the
bases
• The L shaped tertiary
structure is formed by
further folding due to
formation of hydrogen
bonds between T and D
arms.

16. tRNA
tRNA contain 5 main arms
or loops:
• Acceptor
• Anticodon arm
• DHU arm
• TΨC arm
• Extra arm

17. tRNA
• Acceptor
– The acceptor arm is at 3’ end and has 7 base pairs
– The end sequence is unpaired Cytosine and
Adenine at the 3’ end
– The 3’ OH group terminal of Adenine binds with
carboxyl group of amino acids
– The tRNA bound with amino acid is called amino-
acyl tRNA

18. tRNA
• Anticodon arm
– Lies at the opposite end of acceptor and is 5 base
pairs long
– Base sequence of anticodon arm is
complementary to the base sequence of mRNA
codon.
– Due to this it can bind specifically with mRNA by
hydrogen bonds.

19. tRNA
• DHU arm acts as the recognition site for the
enzyme that adds the amino acid to the
acceptor arm.
• TΨC arm contains pseudo uridine and is
involved in the binding of tRNA to the
ribosomes

20. snRNA – Small Nuclear RNA
• Involved in the process of splicing or intron
removal of primary transcript pre-mRNA to
form mature mRNA

21. miRNA – Micro RNA
• Short, non-coding sequences of RNA
• Play important role in gene silencing by
blocking mRNA and preventing translation
• Regulate the expression of at least half of all
human genes.
• Bind onto mRNAs preventing their translation
into proteins.

22. miRNA

23. siRNA – Small Interfering RNA
• They are involved in the RNA interference
pathway
• Pairs to its corresponding RNA sequence in the
target mRNA.
• This then activates the degrading mRNA.
• Once the target mRNA is degraded, the mRNA
cannot be translated into protein

24. siRNA

25. Significance
• mRNA – Carries genetic formation of DNA for
protein synthesis from nucleus to ribosome in the
form of genetic code
• rRNA - Combines with proteins to form
ribosomes
• tRNA - Acts as adapter molecule. Carries amino
acid and drops it to particular location by
recognising codon on mRNA with its anticodon

26. Significance
• snRNA – Processing of mRNA. U1, U2, U4, U5,
and U6 are involved in intron removal
• miRNA – Gene silencing by blocking mRNA
and preventing protein synthesis
• siRNA – Gene silencing by causing degradation
of mRNA