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| James Watson and Francis Crick |
Deoxyribonucleic acid (DNA) - Structure of DNA
The
deoxyribonucleic acid (DNA) molecule is the genetic blueprint for each cell and
ultimately the blueprint that determines every characteristic of a living
organism. In 1953 American biochemist James Watson, right, and British
biophysicist Francis Crick, left, described the structure of the DNA molecule
as a double helix, somewhat like a spiral staircase with many individual steps.
Their work was aided by X-ray diffraction pictures of the DNA molecule taken by
British biophysicist Maurice Wilkins and British physical chemist Rosalind
Franklin. In 1962 Crick, Watson, and Wilkins received the Nobel Prize for their
pioneering work on the structure of the DNA molecule.
Deoxyribonucleic
Acid (DNA) is a genetic material of all cellular organisms and most viruses.
DNA carries the information needed to direct protein productionand replication.
Protein productionis 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
of Deoxyribonucleic Acid (DNA)
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
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| Structure of DNA |
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
production
One
of a cell’s most important tasks is the synthesis of proteins, giant molecules
that underlie most cellular functions. The hereditary material known as
deoxyribonucleic acid (DNA), found within the nucleus of a cell, orchestrates a
series of steps resulting in the manufacture of proteins tailored to meet the needs
for a cell’s development and growth.
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 contains the information needed for the
production of a given amino acid sequence. The other strand aids in
replication.
Protein
productionbegins with the separation of a DNA molecule into two strands. In a
process called transcription, a section of one 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.
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.
Replication
of DNA
In
most cellular organisms, replication of a DNA molecule takes place in the cell
nucleus and occurs just before the cell divides. Replication begins with the
separation of the two polynucleotide chains, each of which then acts as a
template for the assembly of a new complementary chain. As the old chains
separate, each nucleotide in the two chains attracts a complementary nucleotide
that has been formed earlier by the cell. The nucleotides are joined to one
another by hydrogen bonds to form the rungs of a new DNA molecule. As the
complementary nucleotides are fitted into place, an enzyme called DNA
polymerase links them together by bonding the phosphate group of one nucleotide
to the sugar molecule of the adjacent nucleotide, forming the side rail of the
new DNA molecule. This process continues until a new polynucleotide chain has
been formed alongside the old one, forming a new double-helix molecule
 - Structure of DNA){kind=link}
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