What are Nucleic acids
Nucleic acids
are present in all organisms from virus to man. Nucleic acids are the long
chain of poly nucleotide in which mononucleotides are linked with each other.
There are two kinds of nucleic acids, Deoxyribonucleic acid (DNA) and
Ribonucleic acid (RNA). DNA is found mainly in the chromatin of the cell
nucleus whereas most of the RNA (90%) is present in the cytoplasm and a little
(10%) in the nucleolus.
Nucleic acid is
a polymer of nucleotide. Nucleotide is a molecule which consists of following
three parts.
(i) Pentose
sugar (5 carbon)
(ii) Phosphoric
acid (H3PO4)
(iii) A
nitrogenous base.
Pentose sugar
found in nucleotide is either Ribose (C5H10O5)
or Deoxyribose (C5H10O4). Ribose is found in
RNA nucleotides while Deoxyribose sugar is found in DNA nucleotides. Both of
them are distinguished primarily of the basis of this Pentose sugar. This sugar
behaves as basic skeleton.
Phosphoric acid
is common in all nucleotides. It is attaclid with 5 carbons of Pentose sugar in
each nucleotide.
There are two
basic types of nitrogenous bases i.e. Pirine and Pyrinidine. Purine includes
two nitrogenous bases named Adenine (A) and Gunine (G) while pyrimidine
includes three nitrogenous bases cytosine (C), thyamine (T) and uracil (U)>
The nucleotides differ on the basis of their nitrogenous bases. Formation of
nucleotide takes place in two steps. At first step nitrogenous base combines
with pentose sugar at its first carbon to form a nucleoside. At the second step
the phosphoric acid combines with 5th carbon of pentose sugar to
form a nucleotide.
Nucleic acids
are store house and transmitters of genetic information. They make it possible
for cells to function on specific patterns and give rise to new cells that
either function similarly or develop new functions according to the plans
enclosed in the nucleic acid. Genetic information is encoded in a nucleic acid
molecule in particular and simple fashion. DNA and RNA are basically similar
structure because both of these are polynucleotide chains but the nucleotides
of both are different.
DNA AS
HEREDITARY MATERIAL
Transformation
of one type of bacteria into another type and infection of bacteria by
bacteriophage provides first evidence that DNA is the hereditary material.
Griffin
discovered that living bacteria can acquire genetic material form dead bacteria
and transform live bacteria from non virulent to virulent. A bacteriophage
consists solely of DNA and protein. When it infects a bacterium, the phage
injects its DNA only into the bacterium; the phage injects its DNA only into the
bacterium where it directs the synthesis of more phages. In all nucleotides of
DNA, phosphate and deoxyribose sugar are always common but the nitrogenous
bases are different. In other words we can say that each DNA has specific
sequence of nitrogenous bases.
Watson and Crick
in 1953 gave the model of DNA. DNA has double helices with two polynucleotides
running in opposite directions and connected with each other by hydrogen bonds.
In fact two polynucleotides are coiled about the same axis and could be separated
only by uncoiling. The bases are set at right angles to the long axis. In two
nucleotides running opposite each other adenine always pairs with thyamine and
cytosine with guanine. Thus the number of A and T or C and G is always constant
in a DNA molecule.
There are about
10 base pairs in each turn.
Duplication of
DNA: DNA has the power of self duplication. Two strands separate from one
another. Each of these then synthesize its complimentary strand from the pool
of nucleotides. Thus in each of the newly formed DNA molecule there will be one
parental and the other newly formed strand.
RNA (RIBONUCLEIC
ACID)
It consists of
sugars, bases and phosphoric acid. Sugar here is ribose as against deoxyribose
in DNA. RNA occurs as a single stranded molecule though in some cases a double
stranded RNA may also be present. In RNA the bases are adnine, guanine, uracil
and cytosine i.e. thyamine of DNA is replaced by uracil. Various types of RNA are
found in plant cells. These are messenger of RNA (mRNA) which carries the information;
another is transfer RNA (tRNA) also known as soluble and third is ribosomal RNA
(rRNA) which is associated with ribosome. All these three types are non genetic
RNA. Sometimes there is present genetic RNA also. It has the power of self
duplication.
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