RNA basic structure and functions

1. RIBONUCLEIC ACID (RNA)
P. HARITHA
DL BOTANY
TTWRDC M
SANGAREDDY

3. • Most prokaryotic and eukaryotic cells contain, another
important nucleic acid, the Ribonucleic acid(RNA)
besides DNA.
• Some viruses , however contain no DNA but only RNA.
• In them, RNA is the sole genetic material, and carries the
responsibilities of DNA.
• Such RNA is called Genetic RNA. E.g. TMV.
• Turnip yellow mosaic viruses, wound tumour viruses,
influenza viruses, polio viruses, etc.

5. • Further, in cells in which the genetic substance
is DNA, there occur another kind of molecules,
which are called Non-Genetic RNAs.
• Those RNAs depend on DNA for their synthesis and
are ultimately translated into the linear sequence of
amino acids in a polypeptide chain (protein
synthesis).

6. (A) Occurrence:
• Non-genetic RNA is generally found in the cytoplasm
and the nucleolus.
• In the cytoplasm, it is found freely but also found
associated with ribosomes.
• RNA is also found in mitochondria and chloroplast.

7. (B) Structure of RNA:
• RNA is an unbranched polynucleotide chain.
• It is formed by phosphodiester bonds between
ribonucleotides in 3’5’ direction.
• The number of ribonucleotides in RNA range form as few
as 75 to many thousands.
• RNA nucleotides have ribose sugar(in place of
deoxyribose in DNA), which participates in the formation
of sugar-phosphate back bone of RNA.

8. • The nitrogen bases are- adenine(A), guanine(G),
Cytosine(C) and Uracil(U).
• Uracil differs for thymine(T), of DNA in lacking a
methyl group(-CH3) at 5’C position
• These nitrogen bases do not show complementarity,
hence there is no 1:1 ratio between purines and
pyrimidines.
• .

10. (C) Types of RNA:
• Three types of non-genetic RNA molecules are found in
all prokaryotic and eukaryotic cells.
• Their synthesis is always dependent on DNA template.
• They are (i) Messenger RNA, (ii) Ribosomal RNA and
• (iii) Transfer RNA.

12. (i) Messenger RNA(m RNA):
• Messenger RNA carries genetic information from
chromosomal DNA to ribosome for protein synthesis, so
it functions as a messenger.
• Therefore, Jacob and Monod in 1961 named this RNA
as mRNA.
• It is 5% to 10% of the total quantity of RNA present in
the cytoplasm.

13. • The molecular weight of an average sized m-RNA is
5x106 daltons.
• m-RNA is formed as a complementary strand from
one of the two strands of DNA(Transcription).
• So the sequence of bases of each m-RNA is
complementary to antisense strand of DNA.
• The only difference is that in place of thymine, uracil is
present.

15. • It immediately diffuses into the cytoplasm, where protein
synthesis takes place.
• In prokaryotes, m-RNA is metabolically unstable with a
half-life time from a few seconds to about 2 minutes.
• Hence, transcription and translation proceed
simultaneously.
• But in eukaryotes, it is relatively stable with a half-life
ranging from a few hours to one day.

16. • Generally, a single prokaryotic mRNA molecule codes for
more than one polypeptide.
• Such an mRNA is known as polycistronic m-RNA.
• On the otherhand, all eukaryotic m-RNAs are
monocistronic i.e; code for a protein specified by a single
cistron.

17. Both eukaryotic and prokaryotic mRNAs have the
following features.
(1) A 5’ leader sequence, that is not translated.
(2) A coding region, which begins with about 1500
nucleotides, and this region translates protein.
(3) Non-coding region at the 3’ end.

18. • Both types of m RNA molecules are synthesized with
triphosphate group at the 5’ end, so there is no basic
difference between the two.
1. Prokaryotic m RNAs:
• In prokaryotes, different genes concerned with the same
trait are often found clustered together in a group known
as the Operon.
• All the genes present in an operon are transcribed into a
single mRNA molecule.
• These mRNA are polycistronic and carries the codes for
several adjacent DNA cistrons(genes).

19. • These mRNAs have no specified significance at the
5’ end(i.e., cap is absent).
• Hence, ribosomes can bind at many sites in the
interior of an m-RNA, each resulting in the synthesis
of different protein.

20. (2) Eukaryotic mRNAs:
• All eukaryotic mRNA molecules show the following
special structural features:
(a) Cap:
• A cap is found at the 5’ end of the mRNA in which
methylated guanine is present.
• This gives protection from the action of exonucleases
present in the cytoplasm.

21. (b) Non-Coding Region(NCR):
• Immediately after the cap, a region of 10 to 100 nucleotides
is present.
• In this region A and U are found in excess and does not
translate into protein.
• This sequence of nucleotides is called leader sequence.
(c) Initiation codon:
• In both prokaryotes and eukaryotes, primary codon AUG is
found.
(d) Coding region(CR):
• This region consists of about 1500 nucleotides, and this
region translates protein.

23. (e) Termination region(CR):
• The coding region ends with a termination codon.
• The termination codons in eukaryotic cell are UAA, UAG
and UGA.
(f) Poly-A sequence:
• At the 3’ end of mRNA, poly adenylate or Poly-
A(AAAA……A) sequence is found.
• Poly A sequence is added after transcription.

24. (ii) Ribosomal RNA(r-RNA):
Ribosomal RNA(or rRNA) or insoluble RNA(i-RNA) constitutes
the largest part(upto 80%) of the total cellular RNA.
It is primarily found in ribosomes.
It consists of a single strand of nucleotides, which is folded at
several places to form pseudohelices, where the bases show
complementary base pairing.
The r-RNA contains G-C content more than 50%.

25. • r-RNA molecules remain stable atleast upto two
generations.
• Ribosomes of prokaryotic cells are of 70s type and are
composed of 60% r-RNA and 40% proteins.
• They dissociate into a smaller 30S subunit and a larger
50S subunit.
• The 30S subunit has a single 16S r-RNA molecule, which
is associated with 21 different types of ‘S’ proteins.

27. • The larger subunit has a 23S rRNA and 5S r-RNA molecule
complexed with 31 different ‘L’ proteins.
• The cytoplasmic ribosomes of eukaryotes are the 80S size,
and contain 40%r-RNA and 60%protein.
• They consists of a 40S and 60S subunits.
• The 40S subunit has one molecule each of 28S, 58S and 5S
r-RNA and 49 different proteins.
(iii) Transfer RNA(t-RNA):
• Transfer RNA molecules are also called as soluble RNA or
adaptor RNA.
• T-RNA constitutes about 10-15% of the total weight of RNA
of the cell.

30. • It is the smallest form and is composed of 75-95
nucleotides and has a molecular weight of about 25,000.
• T-RNA molecules have a large number of unusual bases
viz., Dihydrouridine(DHU), Pesudouridine(), Inosine(I),
methylated bases and thymine(T).
• The unusual bases are produced after transcription by
modification of usual bases A, T, C, and G.

31. • Further, t-RNA molecules show considerable helical
structure, and more than 50% of the bases in t-RNA are
paired.
• The base sequence of alanine-t-RNA of yeast was the
first to be determined by Holley and co workers in 1964
• For this, he was awarded Nobel prize in 1968.
• The existence of t-RNA was demonstrated by Hoagland
and co workers in 1957.

32. • Holley(1965) proposed a clover leaf model for t-RNA
molecules to account for its structural and functional
properties.
• According to this model, a single polynucleotide chain of
t-RNA molecule is folded back on itself, by which the 3’-
end and 5’ end come together.
• In this way three major arms are formed.
• Each arm contains one stem and one loop.
• In the loop, unpaired bases are present.

33. • A typical clover-leaf model of t-RNA shows the following
structural peculiarities.
(i) All t-RNA molecules contain the same terminal
sequence of 5’-CCA-3’ bases at 3-end. The last residue,
adenine(A), is the amino acid attachment site.
(ii) The TC or thymine arm is located near the 3’-end and
has 7 unpaired bases. The TC arm is involved in the
binding of t-RNA molecules to the ribosomes.
(iii) The DHU or dihydrouridine loop is made up of 8-12
unpaired bases. It contain a site for the recognition of
the amino acyl synthetase enzyme.

34. (iv) The anticodon arm contains one nucleotide triplet which
is different in all t-RNA molecules. This is the codon
recognition site or anticodon and it is complementary to
the corresponding triplet codon of mRNA.
(v) Some t RNA with long chains may form a short, extra
arm.

35. • The clover-leaf model represents the secondary structure of
all the tRNA molecules.
• All t-RNA acquire an L-shaped tertiary structure, which is
critical for their function.
• In this configuration, the arms of DHU and anticodon arms
straighten to form one arm of the ‘L’.

36. • The TC arm and acceptor arms also straighten and twist to
form the other arm of ‘L’ in such a way that the TC loop lies
close to the D loop.
• Thus, the anticodon loop becomes located at the tip of one
arm of ‘L’, while the amino acid acceptor region is present at
the tip of the other arm.

37. • Both Clover-leaf and L-shaped configuration are
stabilized by H bonds.
• Transfer RNAs help in bringing the aminoacid to the
ribosomes and play a key role in protein synthesis.
• Since there are only 20 amino acids and around 32
different kinds of t-RNAs, some amino acids are carried
by more than one type of t- RNA.

38. THANK YOU