3. Ribonucleic acid (RNA) is one of the three
major macromolecules (along
with DNA and proteinss) that are essential for all
known forms of life. Like DNA, RNA is made up of a
long chain of components called nucleotides consisting
of a base , a ribose sugar, and a phosphate group. The
sequence of nucleotides allows RNA to encode genetic
information. The chemical structure of RNA is very
similar to that of DNA, with two differences--(a) RNA
contains the sugar ribose while DNA contains the
slightly different sugar deoxyribose (a type of ribose
that lacks one oxygen atom), and (b) RNA has the
nucleobase uracil while DNA contains thymine (uracil
and thymine have similar base-pairing properties).
Unlike DNA, most RNA molecules are single-stranded.
4. mRNA carries genetic information from DNA
to protein synthesizing machinery hence acting
as a messenger.
5.
6. At the 5’ end there is a cap of 7-methyl guanosine
triphosphate and protects the mRNA from attack by 5’
exonucleases and helps mRNA to locate protein
synthesizing machinery and attaches to the surface of
ribozomes through the cap.
At the 3’ end,there is a poly-A tail which comprises of
25-200 adenylate residues.
This tail protects mRNA from attack by 3’
exonucleases.
Tail is present in all mRNA except histone and
inteferon.
Cap and tail provide stability to mRNA by protecting it
from exonucleases hence rapid degradation.
7. At the 5’ and 3’ ends,there are non coding sequences
represented by NCS.These regions are not translocated
and do not encode any information.Their function are
still unknown but thought to provide stability.
Coding region is made of codons and is the
intermeeting region between 3’ and 5’ NCS.They are
made up of nucleotides and each codon has 3
nucleotides and specifies 1AA.The sequence of
nucleotides in the coding region of mRNA is
complementary to that of template DNA but is same to
that of non template DNA.
Eukaryotes – each mRNA carries information to a
single protein(monocistronic)
Prokaryotes-each mRNA carries information for the
formation of a no of proteins (polycistronic)
8. It is called heterozygous because of its variable
structure.
It is the first mRNA to be formed from
DNA(first transcript)
It has a molecular weight of 10^7 and is
processed by 4 methods from mature mRNA
i.e caping,tailing,splicing,Modification of
bases.
9. It transfers AA to the protein synthesizing
machinery according to the codon present in
the mRNA i.e it reads the codon on the mRNA
and brings the corresponding AA to the
growing peptide chain.So it acts as an adapter
between the genetic information in the form of
sequence of nucleotides on the mRNA to that
of information on the peptide chain in the form
of sequence of AA.It is very soluble in
cytoplasm.
10. Primary structure
It is made from processing a large precursor
RNA. There is extensive intra chain
complementary base pairing.That imparts a
secondary structure to the molecule.
Normally RNA is single stranded, but if
complementary base pairs are present,it forms
a hair pin structure.
12. tRNA has 5 arms which are as follow:
1.Acceptor arm-It is present at the 3’ end of tRNA.
It has unpaired CCA nucleotide and the free
hydroxy group of adenine nucleotide makes a
linkage with the carboxyl gp of AA.When the AA
is loaded on the tRNA, the RNA is called amino
acyl tRNA or charged RNA because of loading of
AA on the adenylate gp of this arm.
The CCA attachment is done post
transcriptionally.This arm has max 6-7 base pairs.
13. 2.Dehydro Uridine arm(DHU)
It is located towards the 5’ end and helps in the recognition of
enzyme animo acyl tRNA synthetase which is responsible for
loading of tRNA with specific AA according to the codon on the
mRNA
3.TΨC arm
It is located towards the 3’ end of the tRNA. This arm helps to
recognize the protein synthesizing machinery which is on the
surface of ribozomes. On the surface of ribozomes all 3 types of
RNA interact to form the peptide chain.This arm has unusual
nucleotide i.e pseudouridine nucleotide. In this, uracil is attached
to ribose by C-C linkage and not by conventional nitrogen-carbon
linkage.Besides that there is presence of thymine nucleotide which
is unusual in RNA. This thymine nucleotide is formed from post
transcriptionally by methylation of uracil nucleotide.
14. 4.Anticodon arm
This arm is for binding of tRNA to the codon present of mRNA. The
sequence of nucleotides in the anticodon arm is complementary to that of
codons present on mRNA.
Because of complementarity between sequence of nucleotides on mRNA
and tRNA, this arm is called anticodon arm.
The tRNA reads genetic information on the mRNA through the anticodon
and then specific to that codon,the AA is loaded on the acceptor arm.
There are 20 tRNA in the human cytoplasm, each corresponding to a
specific AA.
5.Extra arm
It is present between tΨC and anticodon arm. Length is variable and
function is exactly not known.Based upon the length of transfer arm,there
are 2 types:
Class 1 : 3-7 base bairs in xtra arm
Class 2 : 13-21 base pairs
Overall in tRNA, around 75-95 nucleotides are there.
15. They are part of ribozomes which are present in the
cytoplasm, free or bound to endoplasmic reticulum. The
ribozomes are responsible for protein synthesis and are
nucleoproteins in nature.
The eukaryotic units have large subunit and small
subunit.The large subunit has 3 types of ribosomal
RNA,5s,5.8s,28s. S is for sedimentation coefficient and has
50 proteins.Overall it is called 60s subunit.
The small subunit is called 40s subunit and has 18s RNA
and has 30 types of proteins.RNA helps in protein
translation.
Recently it has been known that ribosomal RNA acts as an
enzyme and helps in translation acting as peptidyl
transferase enzyme and is also called ribozyme.
16. They are variable in length, present in
cytoplasm and nucleus and depending on their
location, they are called small nuclear or small
cytoplasmic RNA.
Small nuclear RNA helps in removal of
interons,so they helps in slicing and are uracil
rich.
