Download Q. No. 1. How can RNA be distinguished from DNA?

Frequently asked questions (FAQS):
Q. No. 1. How can RNA be distinguished from DNA?
Ans. RNA and DNA are both nucleic acids, but differ in
three main ways. First, unlike DNA which is generally
double-stranded, RNA is a single-stranded molecule in
many of its biological roles and has a much shorter
chain of nucleotides. Second, while DNA contains
deoxyribose, RNA contains ribose (there is no
hydroxyl group attached to the pentose ring in the 2'
position in DNA). These hydroxyl groups make RNA
less stable than DNA because it is more prone to
hydrolysis. Third, the complementary base to adenine
is not thymine, as it is in DNA, but rather uracil, which
is an unmethylated form of thymine.
Q.No.2. Describe the chemical structure of RNA.
Ans. Unlike DNA, RNA is single stranded made up of
nucleotides arranged in the form of a chain. Each
nucleotide in RNA contains a ribose sugar, with
carbons numbered 1' through 5'. A base is attached to
the 1' position, generally adenine (A), cytosine (C),
guanine (G) or uracil (U). Adenine and guanine are
purines, cytosine and uracil are pyrimidines. A
phosphate group is attached to the 3' position of one
ribose and the 5' position of the next. The phosphate
groups have a negative charge each at physiological
pH, making RNA a charged molecule (polyanion). The
bases may form hydrogen bonds between cytosine
and guanine, between adenine and uracil and between
guanine and uracil. However other interactions are
also possible, such as a group of adenine bases
binding to each other to form bulge.
Fig. Chemical structure of RNA
Q.No. 3. Describe briefly the synthesis of RNA?
Ana. Synthesis of RNA is usually catalyzed by an
enzyme—RNA polymerase, using DNA as a template,
a process known as transcription. Transcription begins
with the binding of the enzyme to a promoter
sequence in the DNA (usually found "upstream" of a
gene). The DNA double helix is unwound by the
helicase activity of the enzyme. 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.
Q.No. 4. Explain the life-cycle of mRNA?
Ans. The brief existence of an mRNA molecule begins
with transcription and ultimately ends in degradation.
During its life, an mRNA molecule may also be
processed,
edited,
and
transported
prior
to
translation. Eukaryotic mRNA molecules often require
extensive processing and transport, while prokaryotic
molecules do not. During transcription, RNA
polymerase makes a copy of a gene from the DNA to
mRNA as needed. This process is similar in eukaryotes
and prokaryotes. One notable difference, however, is
that prokaryotic RNA polymerase associates with
mRNA processing enzymes during transcription so
that processing can proceed quickly after the start of
transcription. The short-lived, unprocessed or partially
processed, product is termed pre-mRNA; once
completely processed, it is termed mature mRNA. A
fully processed mRNA includes a 5' cap, 5' UTR,
coding region, 3' UTR, and poly(A) tail. The mature
mRNA is finally degraded with certain enzymes like
nucleases.
Q. No. 5. Explain briefly the function of mRNA?
Ans. It is a molecule of RNA encoding a chemical
"blueprint" for a protein product. mRNA is transcribed
from a DNA template, and carries coding information
to the sites of protein synthesis: the ribosomes. Here,
the nucleic acid polymer is translated into a polymer
of amino acids: a protein. In mRNA as in DNA, genetic
information is encoded in the sequence of nucleotides
arranged into codons consisting of three bases each.
Each codon encodes for a specific amino acid, except
the stop codons that terminate protein synthesis. This
process requires two other types of RNA: transfer RNA
(tRNA) mediates recognition of the codon and
provides the corresponding amino acid, while
ribosomal RNA (rRNA) is the central component of the
ribosome's protein manufacturing machinery.
Q. No.6. Draw the labelled structure of eukaryotic
mature mRNA?
Fig. Structure of a mature eukaryotic mRNA.
Q.No.7. What are UTRs and what is their function?
Ans. Untranslated regions (UTRs) are sections of the
mRNA before the start codon and after the stop codon
that are not translated, termed the five prime
untranslated region (5' UTR) and three prime
untranslated region (3' UTR), respectively. These
regions are transcribed with the coding region and
thus are exonic as they are present in the mature
mRNA. Several roles in gene expression have been
attributed to the untranslated regions, including mRNA
stability,
mRNA
localization,
and
translational
efficiency. The ability of a UTR to perform these
functions depends on the sequence of the UTR and
can differ between mRNAs. The stability of mRNAs
may be controlled by the 5' UTR and/or 3' UTR due to
varying affinity for RNA degrading enzymes called
ribonucleases and for the proteins that can promote or
inhibit RNA degradation. Translational efficiency,
including sometimes the complete inhibition of
translation, can be controlled by UTRs. Proteins that
bind to either the 3' or 5' UTR may affect translation
by influencing the ribosome's ability to bind to the
mRNA. Cytoplasmic localization of mRNA is thought to
be a function of the 3' UTR. Proteins that are needed
in a particular region of the cell can actually be
translated there; in such a case, the 3' UTR may
contain sequences that allow the transcript to be
localized to this region for translation.
Q. No. 8. What are non-sense codons?
Ans. Non-sense codons are also called termination
codons or stop codons. These are UAA, UAG, UGA
present in the mRNA, signals the termination of
polypeptide synthesis and do not code for any known
amino-acid.
Q.No.9. Depict the chemical structure of cloverleaf
model of tRNA.
Fig. Cloverleaf structure of
tRNA from yeast.
Q.No. 10. What is anti-codon and how it acts in
protein synthesis?
Ans. The terminal end of the anticodon arm contains
the anticodon. An anticodon is a unit made up of three
nucleotides, that correspond to the three bases of the
codon on the mRNA (Felsenfeld and Cantoni, 1964).
Each tRNA contains a specific anticodon triplet
sequence that can base-pair to one or more codons
for an amino acid. For example, the codon for lysine is
AAA; the anticodon of a lysine tRNA might be UUU.
Some anticodons can pair with more than one codon
due to a phenomenon known as wobble base pairing.
In the genetic code, it is common for a single amino
acid to be specified by all four third-position
possibilities, or at least by both Pyrimidines and
Purines; for example, the amino acid glycine is coded
for by the codon sequences GGU, GGC, GGA, and
GGG.
Q.No.11. What is non-coding RNA?
Ans. A non-coding RNA (ncRNA) is a functional RNA
molecule that is not translated into a protein. Lessfrequently used synonyms are non-protein-coding
RNA (npcRNA), non-messenger RNA (nmRNA), small
non-messenger RNA (snmRNA) and functional RNA
(fRNA). The term small RNA (sRNA) is often used for
small bacterial ncRNAs.
Q. No.12. What is sedimentation coefficient?
Ans. Sedimentation coefficient is the time taken by
any solid particle to settle or to sediment in the
process of centrifugation.
Q.No.13. Write down the chemical composition of
ribosomes
Ans. Ribosomes are tiny particles, about 200 A. It is
composed of both proteins and RNA; in fact it has
approximately 37 - 62% RNA, and rest are made up
of proteins (Cohn and Norman, 1964). The RNA
present in ribosomes are obviously called ribosomal
RNA, and they are produced in the nucleolus.
Q.No.14. How ribosomes help in protein synthesis?
Ans. The ribosomes play a very important role in
protein synthesis, which is the process by which
proteins are made from the amino acids. Without the
ribosomes the message would not be read, thus
proteins could not be produced. The ribosomes are the
primary agent in the process of translating the mRNA
into a specific amino acid chain, which consists of two
subunits. These subunits are made up of ribosomal
RNA (rRNA), and together contain up to eighty-two
specific proteins assembled in a precise sequence.
This assembled ribosome displays a series of small
groves, tunnels, and platforms, where the action of
protein synthesis occurs. There are the active sites,
each dedicated to one of the tasks required for
translation of mRNA into protein. Proteins being
synthesized for export out of the cell, are made by
ribosomes attached to the rough endoplasmic
reticulum. The process of protein synthesis begins
with the capture of the tRNA, which is carrying an
amino acid, by an initiation factor. This binds to a
small ribosomal subunit, which occupies one of the
active sites in the ribosomes, the P (protein) site. This
initiation complex recognized and binds to the 5' end
of an mRNA molecule and slides down to the initiation
codon, which is always an AUG sequence of amino
acids. The large subunit of the ribosome now joins the
complex, then the second aminoaceyl-tRNA attach
with the A-site on this attached ribosome. The
continued carrying of amino-acids by tRNAs to the
ribosome leads to the formation of a long poly-peptide
chain. After the complete synthesis of polypeptide
chain the ribosome dissociates into its sub-units.
Q. No. 15. What are polyribosomes?
Ans. When many free ribosomes get attached with
mRNA in the process of translation, they form a chain
along the length of mRNA called polyribosomes or
polysomes. The number of ribosomes associated in
the polysomal chains depends on the size of the
mRNA. This is also associated with the size of the
protein that is being synthesized.