Deoxyribonucleic acid (DNA) is a nucleic acid containing the genetic instructions used in the
development and functioning of all known living organisms (with the exception
of RNA viruses). The DNA segments carrying this genetic information are called
genes. Likewise, other DNA sequences have structural purposes, or are involved
in regulating the use of this genetic information. Along with RNA and proteins,
DNA is one of the three major macromolecules
that are essential for all known forms of life.
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| DNA having nitro bases, |
DNA consists of two long polymers of simple units called nucleotides, with backbones made of sugars and phosphate groups joined by ester bonds. These two strands run in opposite directions to each other and are therefore anti-parallel. Attached to each sugar is one of four types of molecules called nucleobases (informally, bases). It is the sequence of these four nucleobases along the backbone that encodes information. This information is read using the genetic code, which specifies the sequence of the amino acids within proteins. The code is read by copying stretches of DNA into the related nucleic acid RNA in a process called transcription.
Within cells DNA is organized into long structures called chromosomes. During cell division these chromosomes are duplicated in the process of DNA replication, providing each cell its own complete set of chromosomes. Eukaryotic organisms (animals, plants, fungi, and protists) store most of their DNA inside the cell nucleus and some of their DNA in organelles, such as mitochondria or chloroplasts. In contrast, prokaryotes (bacteria and archaea) store their DNA only in the cytoplasm. Within the chromosomes, chromatin proteins such as histones compact and organize DNA. These compact structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed.
STRUCTURE
The structure of the DNA
double helix. The atoms in the structure are colour coded by element and the
detailed structure of two base pairs is shown in the bottom right.
DNA is a long polymer made
from repeating units called nucleotides. As first discovered by James D. Watson
and Francis Crick, the structure of DNA of all species comprises two helical
chains each coiled round the same axis, and each with a pitch of 34 Ångströms (3.4 nanometres) and a radius
of 10 Ångströms (1.0 nanometres).
According to another study, when measured in a particular solution, the DNA
chain measured 22 to 26 Ångströms wide
(2.2 to 2.6 nanometres), and one nucleotide unit measured 3.3 Å (0.33 nm) long.
Although each individual repeating unit is very small, DNA polymers can be very
large molecules containing millions of nucleotides. For instance, the largest
human chromosome, chromosome number 1, is approximately 220 million base pairs
long.
In living organisms DNA does
not usually exist as a single molecule, but instead as a pair of molecules that
are held tightly together. These two long strands entwine like vines, in the
shape of a double helix. The nucleotide repeats contain both the segment of the
backbone of the molecule, which holds the chain together, and a nucleobase,
which interacts with the other DNA strand in the helix. A nucleobase linked to
a sugar is called a nucleoside and a base linked to a sugar and one or more
phosphate groups is called a nucleotide. Polymers comprising multiple linked
nucleotides (as in DNA) are called a polynucleotide.
The backbone of the DNA
strand is made from alternating phosphate and sugar residues. The sugar in DNA
is 2-deoxyribose, which is a pentose (five-carbon) sugar. The sugars are joined
together by phosphate groups that form phosphodiester
bonds between the third and fifth carbon atoms of adjacent sugar rings. These
asymmetric bonds mean a strand of DNA has a direction. In a double helix the
direction of the nucleotides in one strand is opposite to their direction in
the other strand: the strands are antiparallel. The asymmetric ends of DNA
strands are called the 5' (five prime) and 3' (three prime) ends, with the 5'
end having a terminal phosphate group and the 3' end a terminal hydroxyl group.
One major difference between DNA and RNA is the sugar, with the 2-deoxyribose
in DNA being replaced by the alternative pentose sugar ribose in RNA.
The DNA double helix is
stabilized primarily by two forces: hydrogen bonds between nucleotides and
base-stacking interactions among the aromatic nucleobases. In the aqueous
environment of the cell, the conjugated p bonds of nucleotide bases align
perpendicular to the axis of the DNA molecule, minimizing their interaction
with the solvation shell and therefore, the Gibbs free energy. The four bases
found in DNA are adenine (abbreviated
A), cytosine (C), guanine (G) and thymine (T). These four bases are attached to the sugar/phosphate
to form the complete nucleotide, as shown for adenosine monophosphate.
The nucleobases are
classified into two types: the purines, A and G, being fused five- and
six-membered heterocyclic compounds, and the pyrimidines, the six-membered
rings C and T. A fifth pyrimidine nucleobase, uracil (U), usually takes the
place of thymine in RNA and differs from thymine by lacking a methyl group on
its ring. Uracil is not usually found in DNA, occurring only as a breakdown
product of cytosine. In addition to RNA and DNA a large number of artificial
nucleic acid analogues have also been created to study the proprieties of
nucleic acids, or for use in biotechnology.
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