1. RNA is an important nucleic acid found in most prokaryotic and eukaryotic cells in addition to DNA. Some viruses contain only RNA, which acts as their genetic material. 2. There are two main types of RNA - genetic RNA found in viruses, and non-genetic RNA which depends on DNA for synthesis. Non-genetic RNA includes messenger RNA, ribosomal RNA, and transfer RNA. 3. Messenger RNA carries genetic information from DNA to ribosomes for protein synthesis. It has a 5' cap, coding region, and 3' poly-A tail. Ribosomal RNA forms the major structural component of ribosomes. Transfer RNA transfers amino acids to the ribosome during protein

1. RIBONUCLEIC ACID (RNA)
P. HARITHA
DL BOTANY
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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.

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