Want
to Learn about Ribonucleic Acid
(RNA)?
Ribonucleic acid is a long
polymer of nucleotides found in the nucleus but mainly in the cytoplasm
of a
cell; it transmits genetic information from DNA to the cytoplasm and is
involved with the synthesis of proteins that control chemical
processes in the cell. Do you know that DNA
is like the boss of RNA. While DNA stays in it's office (Nucleus), it
sends RNA to the workstations (Cytoplasm) with its messages. Because DNA does not leave the
nucleus of a cell, it
must have a
messanger to carry the genetic code to other places in the cell; that's RNA!.
RNA
is made up of Ribose (a pentose = sugar with 5
carbons), Phosphoric acid and Organic bases; Purines (Adenine
and Guanine) and
Pyrimidines (Cytosine and Uracil).
RNA exists
largely as single nucleotide chains in living cells. The RNA strand is
made up of
alternating
molecules of ribose sugar and the phosphate (See figures below). The nitrogen
bases are attached to
the sugar molecules in the strand and ’stick out’ laterally as in DNA.
A sugar,
a nitrogenous base and a phosphate together form a ribonucleotide
and a
RNA molecule is a polymer of ribonucleotides. Although RNA exists
generally as
single polynucleotide chain, some segments of RNA molecules may pair
temporarily in double-helical form or may fold back on themselves to
set up
extensive double-helical regions. These fold-back double helices and
their arrangement are
important to RNA functions.
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Difference between DNA and RNA
structures (See figures below)
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|
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Characters
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DNA
|
RNA
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1.
|
Molecule
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Double
stranded, helical
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Single
stranded, straight or variously folded and twisted.
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2.
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Pentose sugar
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Deoxyribose
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Ribose
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3.
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Pyrimidine base
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Thymine
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Uracil
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4.
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Complementary
base pairing
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Always present
and exists between A = T and G = C
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Normally
absent, but may be present in twisted segments of a molecule.
If present,
pairing is between A = U and G = C
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5.
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Ratio of
Purines: Pyrimidines
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Always 1:1
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Not necessarily
1:1
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Types
of RNAs and Functions
While
the function of DNA is to store genetic
information, different types of RNAs are involved in a series of
interconnected
chores to translate the information from DNA into proteins. Proteins
are the
ultimate controllers of cell functions, structures and characteristics
e.g. your hair, eye and skin color, height etc.
There are many
different types of RNAs depending on the type of a organism.
- messengerRNA (mRNA): Carries
the genetic information out of the nucleus into cytoplasm for
protein synthesis.
- transferRNA (tRNA): Decodes
the information in mRNA.
- ribosomalRNA (rRNA): Makes up
ribosome, constitutes 50% of a ribosome, which is a molecular
assembly involved in protein synthesis.
- catalyticRNAs (cRNA): Catalyses many reactions
in the cytoplasm of the cell.
- Small Nuclear RNA (snRNA): The snRNAs have
various roles in the processing of the other classes of RNA.
- Small Nucleolar RNA (snoRNA): There are
probably over 100 of them are found in the nucleolus where they are
involved in several functions including making ribosomes.
The
most important RNAs are mRNA, tRNA
and rRNA.
mRNA: This is
called messenger RNA because it carries information for protein
synthesis from
the DNA to the ribosomes in the cytoplasm (the site of protein
synthesis). Just like
a mailman!. mRNA constitutes
about 3-5% of
the total RNA. It is produced on one
of the DNA
strands in a process called transcription. Hence, the base
sequence of
mRNA is complementary to that of the DNA strand. The bases on the mRNA
strand
are organized into triplets. Each triplet consists of a
sequence of
three consecutive nitrogenous bases called a codon (code
word).
Each codon specifies one amino acid from wich proteins are made. The
sequence of codons on the mRNA
strand
is called the mRNA language. It indicates the sequence of amino
acids
for the synthesis of a protein. The mRNA language begins with the codon
AUG (initiation
codon or starting codon) and ends with UAA, UAG or UGA (stop
codons).
Role
of mRNA
in
protein synthesis
1. Represents
the sequence of codons (mRNA language)
from the DNA strand.
2. Brings the sequence to the ribosomes
(site of
protein synthesis) in the cytoplasm.
3. Provides the sequence for the synthesis
of
specific protein from the amino acids (found
in cytoplasm).
rRNA
: I
compare rRNA to a construction worker
because rRNA makes up ribosomes
- The
molecular factory involved in protein synthesis and hence is called
ribosimal (r)
RNA. It is present in the
cytoplasm and forms
about 80% of the
total RNA. The single-stranded
molecule of
rRNA is variously folded and twisted upon itself in certain regions
forming a
secondary structure. In such
folded regions, complementary bases form
pairs and
are joined by hydrogen bonds. The secondary structure of rRNA
is
a complex pattern of short double-stranded stems,
interspersed with unpaired single-stranded loops and
bubbles (see figure below).
Role
of rRNA in protein
synthesis
The rRNA forms
complex with
various
proteins and make a structure called ribosome, and this complex reads
the
coded sequence in mRNA to link amino acids together into particular
protiens.
1. It provides proper binding sites for
the mRNA.
2. It orients the mRNA in such a way that
its
nitrogen base triplets or codons are properly read or translated.
3. It also releases tRNA after the
transfer of
activated amino acid to the ribosome complex.
4. It protects the mRNA strand from the
action of
enzymes (nucleases) which can destroy it.
5. It protects the growing
polypeptide (protein)
chain from proteolytic enzymes.
tRNA:Transports amino acids
floating in the cytoplasm to ribosomes for protein synthesis (So, it's function compares to a
transport vehicle). tRNA consistutes about 10 to 20% of the
total RNA of the cell. tRNA
strand is folded upon itself forming
loops. It results in either a clover leaf pattern or hair pin
pattern (see figure below).
One end of the strand has guanine,
while the other end carries the CCA combination of nitrogen bases. A
triplet of
nitrogen bases called anticodon is present on one of the
loops. The
anticodon pairs with the complementary codon on the mRNA molecule.
Role of tRNA in protein
synthesis
1. tRNA carries the
required
specific amino acids from cytoplasm to the ribosome (the site of
protein
synthesis). Each type of amino acid is carried by a specific
type of
tRNA.
2. In the ribosome,
tRNA helps to
arrange the amino acids in their proper sequence for the synthesis of a
protein. Base pairing between the anticodon on the tRNA (the anticodon
is
called translation) and the complementary (matching) codon on a
mRNA
molecule
brings the correct amino acid into the growing polypeptide chain.
mRNA
is Made from DNA in a Process called "Transcription"
The manufacture of DNA into RNA takes
place only in the nucleus.Transcription is the name of the step in
which DNA is copied into mRNA. Transcription is basically
the process of rewriting
the DAN code in a different nucleic acid language, that of its cousin,
mRNA. It's
job is to bring the DNA message (how to
make protein) out of the nucleus and into the cytoplasm, where the
protein-making "machinery" (ribosomes) are located.
The process of
Transcription
1. Before
transcription can begin,
the portion of the DNA double helix to be transcribed must be
"UNZIPPED" by separating the
two strands at their hydrogen bonds. This is done by
an enzyme named
HELICASE. Where
helicase has separated the two DNA "backbones" there are now rows of
nitrogenous bases just waiting to be "read" and "rewritten"
into another language: mRNA
2. Only one of
the DNA strands
in the double helix is read and used as template to manufacture mRNA. Name of strand of DNA that
is transcribed to
create m-RNA is antisense strand. The
enzyme that attaches to the unzipped DNA and creates a complementary
strand of
mRNA is called RNA
POLYMERASE (see
figure below).
3. The Final
Product of transcription
is a
messenger RNA. It resembles half of a DNA
molecule and carries the "message" containing instructions for protein
synthesis from the DNA "gene" in the nucleus to the ribosomes in the
cytoplasm.
Synthesis
of
protein using genetic message carried by mRNA
Translation is the
process of synthesis of a protein by ribosomes, using mRNA as
a template.
- The genetic
message in mRNA is 'read' by
organelles called ribosomes in order to make a particular
protein. tRNA is also required for this process. tRNAs
are specific for one particular amino acid and each tRNA carries
required amino
acids to the ribosome in order to synthesise the polypeptide chain.
- The
ribosome 'reads' the mRNA language
in the 5' to 3' direction. Each codon (sets of three
nucleotide bases) specifies one amino acid from which
proteins are made. So, the mRNA language indicates the sequence of
amino
acids
for the synthesis of a protein. The mRNA language begins with the codon
AUG (initiation
codon, which starts making a protein chain) and ends with UAA, UAG
or UGA (stop
codons also called terminators of a protein chain).
- As
each codon is 'read', the amino acids are carried to the site of
formation of the polypeptide chain by the particular tRNA. Each tRNA
has an anticodon that are
opposite to the particular codon
on the mRNA e.g. if
the mRNA codon is AGG then the matching tRNA
anticodon is UCC. Once a amino acid is bound to the forming polypeptide
chain the next codon is read by the ribosome. The sequence of
reading
the mRNA and adding an amino acid continues until the 'stop' sequence
(codon)
is recognized.
Interesting
facts about RNA
-
Approximately,
5-10% of total weight of a cell is
RNA (compare with only
about 1%
DNA!).
-
The
extra hydroxyl group in RNA makes it more succeptible to
damage by hydrolysis; that's why DNA is the ultimate repository of
genetic information.
-
RNA
is the genetic material (like DNA for other
organisms) in some viruses (which don't have DNA).
-
RNA
is found in the
nucleus and in the cytoplasm but DNA is found only in nucleus.
- DNA
is capable of self-replication, but only when assisted or catalysed by
proteins (enzymes) but RNA is capable of both self replication and
catalysis.
- "RNA
World hypothesis" states that before
the emergence of the first cell, RNA was the dominant and
probably the only form of life.
RNA
technologies: RNAi and antisense RNA
Brifely, both RNAi (RNA
interference)
and antisense RNA
induce destruction of mRNA in the cytoplasm and inhibit or block
production of protein for a particular function (e.g.
inducing fruit ripening, causing cancer etc.).
1. RNAi:
Double stranded RNA is
introduced into a cell
and gets chopped up by the enzyme dicer to form siRNA. siRNA then binds
to the RNA-induced
silencing complex
(RISC) and is unwound. The anitsense RNA complexed with RISC
binds to its
corresponding mRNA which is then cleaved by the enzyme slicer and makes
it
inactive (see figure below).
2. Antisense
RNA: Antisense RNA is a RNA strand which has a mirror
image of nucleotide bases of a mRNA strand. When an artificial gene (DNA)
is introduced into a cell, it produces an antisense RNA complementary to the cell's
own mRNA and forms RNA duplex with the mRNA (see
figure below). The formation of double stranded RNA
inhibits gene expression ( = No
production of functional protein because protein synthesis
requires
single stranded mRNA molecule as a template).
Applications
Cancer gene therapy: RNAi and antisense RNA
technologies have
been used in reducing
expression of cancer causing genes in human cancer cell lines.
Control of fruit ripening:
Antisense
RNA technology has been used to suppress expression of
fruit ripening
genes to make the fruits stay
longer in vine
and extend marketing period. e.g. the flav'r sav'r tomato
(the first
genetically modified food crop introduced in US market in 1995
by a company called Calgene).
Biochemistry
& molecular
biology of plants. Edited by Bob B. Buchanan, Wilhelm Gruissem, Russell
L.
Jones. Rockville, Md.
American Society of Plant Physiologists, c2000.
Essentials
of Genetics. Second edition. Edited by Williams S.
Klug and Michael R. Cummings. Prentice Hall, NJ, 1996.
Encyclopedia
of RNA. www.sciencedaily.com/encyclopedia/rna
http://www.sciencedaily.com/encyclopedia/central_dogma_of_molecular_biology
http://www.encyclopedia.com/html/section/nucleica_RNAandProteinSynthesis.asp
www.google.ca/image
search
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/AntisenseRNA.html
http://www.ambion.com/techlib/resources/RNAi/
http://en.wikipedia.org/wiki/RNA_world_hypothesis
http://dwb.unl.edu/Teacher/NSF/C08/C08Links/www.iacr.bbsrc.ac.uk/notebook/courses/guide/rnast
