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Deoxyribonucleic acid (DNA)
Deoxyribonucleic acid (DNA) is a nucleic acid — usually in the form of a double helix — that contains
the genetic instructions monitoring the biological development of all cellular forms of life,and many viruses. DNA is a long polymer of nucleotides (a polynucleotide) and encodes the sequence of the amino acid residues in proteins using the genetic code, a triplet code of nucleotides.
DNA is thought to date back to between approximately 3.5 to 4.6 billion years ago.
In complex eukaryotic cells such as those from plants, animals, fungi and protists, most of the DNA
is located in the cell nucleus. By contrast, in simpler cells called prokaryotes, including the eubacteria
and archaea, DNA is not separated from the cytoplasm by a nuclear envelope. The cellular organelles known
as chloroplasts and mitochondria also carry DNA.
DNA is often referred to as the molecule of heredity as it is responsible for the genetic propagation of most
inherited traits. In humans, these traits can range from hair color to disease susceptibility. During cell division, DNA is replicated
and can be transmitted to offspring during reproduction. Lineage studies can be done based on the facts that the mitochondrial DNA only
comes from the mother, and the male Y chromosome only comes from the father.
Every person's DNA, their genome, is inherited from both parents. The mother's mitochondrial DNA together with twenty-three chromosomes from each parent combine to form the genome of a zygote, the fertilized egg. As a result, with certain exceptions such as red blood cells, most human cells contain 23 pairs of chromosomes, together with mitochondrial DNA inherited from the mother.
Overview:
Contrary to a common misconception, DNA is not a single molecule, but rather a pair of molecules joined by hydrogen bonds: it is organized
as two complementary strands, head-to-toe, with the hydrogen bonds between them.
Each strand of DNA is a chain of chemical "building blocks", called nucleotides, of which there are four types:
adenine (abbreviated A), cytosine (C), guanine (G) and thymine (T). (Thymine should not be confused with thiamine, which is vitamin B1.)
In some organisms, most notably the PBS1 phage, Uracil (U) replaces T in the organism's DNA.
These allowable base components of nucleic acids can be polymerized in any order giving the molecules a high degree of uniqueness.
Between the two strands, each base can only "pair up" with one single predetermined other base:
A+T, T+A, C+G and G+C are the only possible combinations; that is,
an "A" on one strand of double-stranded DNA will "mate" properly only with a "T" on the other, complementary strand; therefore,
naming the bases on the conventionally chosen side of the strand is enough to describe the entire double-strand sequence.
Two nucleotides paired together are called a base pair.
On rare occasions, wrong pairing can happen, when thymine goes into its enol form or cytosine goes into its imino form.
The double-stranded structure of DNA provides a simple mechanism for DNA replication:
the DNA double strand is first "unzipped" down the middle, and the "other half" of each new single strand is recreated by
exposing each half to a mixture of the four bases. An enzyme makes a new strand by finding the correct base in the mixture and
pairing it with the original strand. In this way, the base on the old strand dictates which base will be on the new strand, and
the cell ends up with an extra copy of its DNA.
DNA contains the genetic information that is inherited by the offspring of an organism;
this information is determined by the sequence of base pairs along its length.
A strand of DNA contains genes, areas that regulate genes, and areas that either have no function, or a function yet unknown. Genes can be loosely viewed as the organism's "cookbook" or "blueprint".
Other interesting points:
DNA is an acid because of the phosphate groups between each deoxyribose. This is the primary reason why DNA has a negative charge.
The "polarity" of each pair is important: A+T is not the same as T+A, just as C+G is not the same as G+C (note that "polarity" as such
is never used in this context -- it's just a suggestive way to get the idea across).
Mutations are the results of the cells' attempts to repair chemical imperfections in this process, where a base is accidentally
skipped, inserted, or incorrectly copied, or the chain is trimmed, or added to. Many mutations can be described as combinations of
these accidental "operations". Mutations can also occur after chemical damage (through mutagens), light (UV damage), or through other
more complicated gene swapping events.
DNA molecules that act as enzymes are known in laboratories, but none have been known to be found in life so far.
In addition to the traditionally viewed duplex form of DNA, DNA can also acquire triplex and quadruplex forms.
Here instead of the Watson-Crick base pairing, Hoogsteen base pairing comes into the picture.
DNA differs from ribonucleic acid (RNA) by having a sugar 2-deoxyribose instead of ribose in its backbone.
This is the basic chemical distinction between RNA and DNA. In addition, in most[citation needed] RNA, the nucleotides thymine (T) are replaced by uracil (U).
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