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Deoxyribonucleic Acid (DNA), genetic material of all cellular organisms and most viruses. DNA carries the information needed to direct protein synthesis and replication. Protein synthesis is the production of the proteins needed by the cell or virus for its activities and development. Replication is the process by which DNA copies itself for each descendant cell or virus, passing on the information needed for protein synthesis. In most cellular organisms, DNA is organized on chromosomes located in the nucleus of the cell.
INTRODUCTION

Deoxyribonucleic Acid (DNA), genetic material of all cellular
organisms and most viruses. DNA carries the information needed to
direct protein synthesis and replication. Protein synthesis is the
production of the proteins needed by the cell or virus for its
activities and development. Replication is the process by which DNA
copies itself for each descendant cell or virus, passing on the
information needed for protein synthesis. In most cellular organisms,
DNA is organized on chromosomes located in the nucleus of the cell.

STRUCTURE

A molecule of DNA consists of two chains, strands composed of a large
number of chemical compounds, called nucleotides, linked together to
form a chain. These chains are arranged like a ladder that has been
twisted into the shape of a winding staircase, called a double helix.
Each nucleotide consists of three units: a sugar molecule called
deoxyribose, a phosphate group, and one of four different
nitrogen-containing compounds called bases. The four bases are adenine
(A), guanine (G), thymine (T), and cytosine (C). The deoxyribose
molecule occupies the center position in the nucleotide, flanked by a
phosphate group on one side and a base on the other. The phosphate
group of each nucleotide is also linked to the deoxyribose of the
adjacent nucleotide in the chain. These linked deoxyribose-phosphate
subunits form the parallel side rails of the ladder. The bases face
inward toward each other, forming the rungs of the ladder.

The nucleotides in one DNA strand have a specific association with the
corresponding nucleotides in the other DNA strand. Because of the
chemical affinity of the bases, nucleotides containing adenine are
always paired with nucleotides containing thymine, and nucleotides
containing cytosine are always paired with nucleotides containing
guanine. The complementary bases are joined to each other by weak
chemical bonds called hydrogen bonds.

In 1953 American biochemist James D. Watson and British biophysicist
Francis Crick published the first description of the structure of DNA.
Their model proved to be so important for the understanding of protein
synthesis, DNA replication, and mutation that they were awarded the
1962 Nobel Prize for physiology or medicine for their work.

PROTEIN SYNTHESIS

DNA carries the instructions for the production of proteins. A protein
is composed of smaller molecules called amino acids, and the structure
and function of the protein is determined by the sequence of its amino
acids. The sequence of amino acids, in turn, is determined by the
sequence of nucleotide bases in the DNA. A sequence of three
nucleotide bases, called a triplet, is the genetic code word, or
codon, that specifies a particular amino acid. For instance, the
triplet GAC (guanine, adenine, and cytosine) is the codon for the
amino acid leucine, and the triplet CAG (cytosine, adenine, and
guanine) is the codon for the amino acid valine. A protein consisting
of 100 amino acids is thus encoded by a DNA segment consisting of 300
nucleotides. Of the two polynucleotide chains that form a DNA
molecule, only one strand, called the sense strand, contains the
information needed for the production of a given amino acid sequence.
The other strand aids in replication.

Protein synthesis begins with the separation of a DNA molecule into
two strands. In a process called transcription, a section of the sense
strand acts as a template, or pattern, to produce a new strand called
messenger RNA (mRNA). The mRNA leaves the cell nucleus and attaches to
the ribosomes, specialized cellular structures that are the sites of
protein synthesis. Amino acids are carried to the ribosomes by another
type of RNA, called transfer RNA (tRNA). In a process called
translation, the amino acids are linked together in a particular
sequence, dictated by the mRNA, to form a protein.

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A gene is a sequence of DNA nucleotides that specify the order of
amino acids in a protein via an intermediary mRNA molecule.
Substituting one DNA nucleotide with another containing a different
base causes all descendant cells or viruses to have the altered
nucleotide base sequence. As a result of the substitution, the
sequence of amino acids in the resulting protein may also be changed.
Such a change in a DNA molecule is called a mutation. Most mutations
are the result of errors in the replication process. Exposure of a
cell or virus to radiation or to certain chemicals increases the
likelihood of mutations.

References

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