Deoxyribonucleic acid (DNA) is a nucleic acid that contains the
genetic instructions used in the development and functioning of all known living
organisms with the exception of some viruses. The main role of DNA molecules is
the long-term storage of information.
Deoxyribonucleic acid (DNA) is a
nucleic acid that contains the genetic instructions used in the development and
functioning of all known living organisms with the exception of some viruses.
The main role of DNA molecules is the long-term storage of information.
The structure of part of a DNA
double helix From Wikipedia
DNA is a long polymer
made from repeating units called nucleotides. The DNA chain is 22 to 26
Ångströms wide (2.2 to 2.6 nanometres), and one nucleotide unit is 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 base, which interacts with the other DNA strand in the
helix. A base 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. If multiple
nucleotides are linked together, as in DNA, this polymer is called a
polynucleotide.
The backbone of the DNA strand is made from alternating phosphate and sugar
residues.[10] 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 by hydrogen bonds between the bases attached
to the two strands. 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.
These bases are classified into two types; adenine and guanine are fused five-
and six-membered heterocyclic compounds called purines, while cytosine and
thymine are six-membered rings called pyrimidines.[8] A fifth pyrimidine base,
called 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.
Twin helical strands form the DNA backbone. Another double helix may be found by
tracing the spaces, or grooves, between the strands. These voids are adjacent to
the base pairs and may provide a binding site. As the strands are not directly
opposite each other, the grooves are unequally sized. One groove, the major
groove, is 22 Å wide and the other, the minor groove, is 12 Å wide.[13] The
narrowness of the minor groove means that the edges of the bases are more
accessible in the major groove. As a result, proteins like transcription factors
that can bind to specific sequences in double-stranded DNA usually make contacts
to the sides of the bases exposed in the major groove.[14] This situation varies
in unusual conformations of DNA within the cell, but the major and minor grooves
are always named to reflect the differences in size that would be seen if the
DNA is twisted back into the ordinary B form.
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