Nucleic acid is essential
for life. The term ‘nucleic acid’ comes from the fact that they are found
mainly in nucleus. Nucleic acid was first isolated in 1870 by F. Miescher from the nuclei of pus
cells. The two major types of nucleic acid found in the living things are
deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA). Nucleic acid is a
linear un-branched polymer. The monomer of the nucleic acid is called
nucleotide.
Structure of a Typical Nucleotide
Each nucleotide consists of
pentose sugar, a phosphate and nitrogen containing ring structure called bases.
The ring structures are called bases because of unshared pair of electrons on
nitrogen atoms, which can thus acquire a proton. Phosphoric acid (H3PO4) which
gives nucleic acid their acid characteristics forms the ester linkage with OH
groups of a pentose sugar. In a typical structure the nitrogen base is attached
to position 1 of pentose sugar. One phosphoric acid is attached with two pentose
sugars with one ribose at position 3 and at position 5 of another ribose. The
combination of a sugar with a base acid addition of phosphoric acid is
condensation reactions.
Nucleoside:
Base - Sugar
Nucleotide: Sugar -
Phosphate – Sugar
Phosphate
diester linkage: Phosphate forms linkage with two pentose
sugars, the linkage is called phosphodiester linkage.
Bases
may be grouped as: Purine and Pyrimidine.
Purine:
Includes Adenine and Guanine which are double ring structures.
Pyrimidine: It
includes Thymine, Cytosine and Uracil, which are single ring structures.
The nucleotides are named
after the name of base attached to it e.g. Adenine nucleotide
Adenine-deoxyribose- phosphate. Bases are represented by their initial letter
i.e. A, G, T, C, and U.
Nucleotides are not only
used as building blocks for nucleic acids, but they form several coenzymes,
including adenosine triphosphate (ATP), nicotnamide dinucteotides (NAD).
Polynucleotide
Two nucleotides join to form
a dinucleotide by condensation between the phosphate of one with the sugar of
the other. The process is repeated up to several million times to make a
polynucleotide. An unbranched sugar- phosphate backbone is thus formed.
Mononucleotide
Adenosine monophosphate
(AMP) is a type of nucleotide consisting of a ribose sugar and the base
adenine. When a phosphate group is added to AMP, it becomes Adenosine
diphosphate (ADP). Further addition of another phosphate group to ADP results
in the formation of Adenosine triphosphate (ATP), which is considered the most
crucial high-energy compound found in all cells. The two covalent bonds that
connect these three phosphates in ATP are often denoted by a squiggle (~) and
are referred to as high-energy bonds. The process of adding an inorganic
phosphate group to an organic molecule is known as phosphorylation e.g. ADP+P=ATP
ATP a rich energy compound
ATP can be converted to ADP
and inorganic phosphate (Pi) by hydrolysis.
This reaction releases
energy.
The third phosphate group
splits from the ATP, and this phosphate remains in the cell in inorganic form.
ADP and phosphate can be converted back to ATP, by condensation the two
reactions can be put together:
ATP is known as the energy
currency of cells. ATP can be used to make muscles contract, make nerve
function, drive active transport and synthesis of proteins etc. ATP is made
from the oxidation of organic molecules during respiration. Since the energy to
add the phosphate to ADP comes from oxidation, the process is known as
oxidative phosphorylation. In photosynthesis ATP is made by using light and the
process is called photophosphorylation. Most of the ATP in the cell is made in
mitochondria. The actual amount of ATP in the cell at any time is small.
Dinucleotide-NAD
Often enzymes use additional
chemical components called cofactors as a tool to aid the catalysis. When the
cofactor is an organic compound other than proteins it is called a coenzyme
e.g. Nicotinamide-adenine dinucleotide (NAD) and many vitamins.
Structure: It
consists of two nucleotides. One nucleotide consists of base-nicotinamide,
sugar and phosphate. Other nucleotide consists of base-adenine-sugar and
phosphate. The two bases are joined by their phosphate group forming a
dinucleotide. It is derived from the vitamin nicotinic acid (niacin) and can
exist in both reduced and oxidized form.
NAD-as
coenzyme: In many enzyme-catalyzed oxidation-reduction reactions,
the electrons are passed in pairs from the active site of enzyme to a coenzyme that
serves as the electron acceptor. When NAD acquires an electron and hydrogen
atom (actually two electrons and a proton) from the active site of an enzyme,
it becomes reduced as NADH. The two energetic electrons and the proton are now
carried by NADH molecule. The oxidation of food stuff takes place by taking
electrons and donating them to NAD forming NADH. Only a small amount of NADH
molecule is present in a cell, because each NAD molecule is used over again and
again. FAD (Flavin adenine dinucleotide) is another coenzyme for oxidation
reduction, which is sometimes used instead of NADH+. FAD accepts two electrons
and two hydrogen ions (H+) to become FADH2.
Deoxyribonucleic Acid (DNA)
It is generally present in
the chromosome. It is also found in mitochondria and chloroplast. It consists
of:
1.
Pentose Sugar: Deoxyribose 2. Purine: Adenine, Guanine
3.
Pyrimidine: Thymine, Cytosine 4. Phosphate
Structure
of DNA: Maurice Wilkins and Rosalind Franklin used the technique
of x-ray diffraction to determine the structure of DNA. At the same time James
D. Watson and Francis Crick built the scale model of DNA. All the data thus
obtained strongly suggested that DNA is a double helix structure. There are two
polynucleotide strands running in opposite directions and winding about each
other in a form of double helix. The double helix looks like a ladder. The
sugar phosphate part of the nucleic acid makes the upright part of the ladder.
The nitrogen bases of the nucleotide make up the rungs of the ladder. Each rung
consists of Purine and a Pyrimidine. Adenine pairs with thymine and guanine
pairs with cytosine. The base pairs are held together by the hydrogen bond.
There are two hydrogen bonds between A and T and three hydrogen bonds between C
and G. The helix is 20A° (2nm) in diameter and makes a full spiral turn every
34 A° (3.4nm) i.e. After every ten base
pairs. The distance between two base pairs is 0.34 nm.
a.
There are four different nucleotides in DNA; each contains phosphate, the
pentose sugar deoxyribose, and a nitrogen-containing organic base. Two bases
are purines:
Adenine (A) and guanine (G):
two bases are pyrimidnes:thymine (T) and cytosine (C).
b. DNA
has a ladder structure: the sugar-phosphate molecules make up the sides and the
hydrogen-bonded bases make up the rungs.
c.
Actually, DNA is a double helix in which the two strands twist about each
other.
The amount of DNA is fixed
for particular species. It depends upon the number of chromosomes. All the
somatic (body) cells of an organism have the same amount of DNA, while the
sperms or ova have almost half the amount of DNA.
Ribonucleic Acid (RNA) The carrier of Information
RNA is concentrated in the
cytoplasm. It consists of sugar Ribose and bases Adenine, Uracil, Cytosine and
Guanine. RNA is hereditary material in some viruses. RNA is a single
polynucleotide chain. There are three types of RNA molecules: (1) tRNA = transfer RNA. (2) rRNA = ribosomal
RNA. (3) mRNA= messenger RNA
The three RNA are
synthesized from different parts of DNA in a process called transcription. The
synthesis takes place in the nucleus. Then the RNA is transported to cytoplasm.
1.
Messenger RNA (mRNA): It takes the genetic message from the
nucleus to the ribosomes in the cytoplasm to form particular proteins. Base
sequence in mRNA is according to the base sequence of DNA. It becomes attached
to ribosome, where amino acids are attached to form polypeptide chain as per base
sequence of mRNA. A mRNA consists of a single strand of variable length. Its
length depends upon the size of the gene as well as the protein for which it is
taking the message. For example, for a protein molecule 1,000 amino acids, mRNA
will have the length of 3,000 nucleotides. mRNA is about 3 to 4% of the total
RNA in the cell.
2.
Transfer RNA (tRNA): These are small molecules. Each chain
consists of 75 to 90 nucleotides. Specific tRNA will pick specific amino acid.
It will bring the amino acid to the ribosomes as per nucleotide sequence of
rRNA. So the cell will have at least 20’ kinds of tRNA molecules. tRNA
comprises about 10 to 20% of the cellular RNA.
3.
Ribosomal RNA (rRNA): It consists of rRNA and protein. mRNA has
the genetic information according to DNA. In ribosome the amino acids are
arranged and linked as per sequence of nucleotides on the-mRNA. Thus specific
protein molecules are synthesized. It is the major portion of RNA in the cell,
and may be up to 80% of the total RNA. It is strongly associated with the
ribosomal protein where 40 to 50% of it is present.
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