DNA
DNA, or Deoxyribonucleic Acid, is described, in Encarta Encyclopedia as a 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.
A molecule of DNA consists of two chains, strands composed of a large number of
nucleotides, that are linked together to form a chain. These chains look like a twisted
ladder and are called a double helix. Each nucleotide consists of three units: sugar
molecules called deoxyribose, a phosphate group, and one of four different nitrogen
containing compounds, also called bases. The four are adenine (A), guanine (G), thymine
(T), and cytosine (C). The deoxyribose molecule occupies the center of the nucleotide,
with the 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 side rails of the ladder. The bases
face inward toward each other, forming the steps 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.
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.
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
In most cellular organisms, replication of a DNA molecule takes places 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.
Research and Applications
The study of DNA is still under way, and the results of such research are being applied
in many disciplines. The Human Genome Project in the United States is a federally
funded effort to determine the sequence of bases of the three billion pairs of nucleotides
composing the human genetic material. The project will make possible the analysis of the
mutations that cause genetic diseases and so will provide information needed to develop
medicines and procedures for treating these diseases.
Forensic science uses techniques developed in DNA research to identify individuals who
have committed crimes. DNA from semen, skin, or blood taken from the crime scene can
be compared with the DNA of a suspect, and the results can be used in court as evidence.
Techniques of DNA manipulation are used in farming, in the form of genetic
engineering and biotechnology. Strains of crop plants to which genes have been
transferred may produce higher yields and may be more resistant to insects. Cattle have
been similarly treated to increase milk and beef production, as have hogs, to yield more
meat and less fat.
Keywords:
deoxyribonucleic acid described encarta encyclopedia genetic material cellular organisms
most viruses carries information needed direct protein synthesis replication protein
synthesis production proteins needed cell virus activities development replication process
which copies itself each descendant cell virus passing information needed protein
synthesis most cellular organisms organized chromosomes located nucleus cell molecule
consists chains strands composed large number nucleotides that linked together form
chain these chains look like twisted ladder called double helix each nucleotide consists
three units sugar molecules called deoxyribose phosphate group four different nitrogen
containing compounds also called bases four adenine guanine thymine cytosine
deoxyribose molecule occupies center nucleotide with phosphate group side base other
phosphate group each nucleotide also linked deoxyribose adjacent chain these linked
subunits form side rails ladder bases face inward toward other forming steps ladder
nucleotides strand have specific association with corresponding nucleotides other strand
because chemical affinity bases containing adenine always paired with containing
thymine cytosine always paired guanine complementary joined weak chemical bonds
hydrogen bonds carries instructions production proteins composed smaller molecules
amino acids structure function determined sequence amino acids sequence amino acids
turn determined sequence three triplet genetic code word codon that specifies particular
acid instance triplet guanine adenine cytosine codon acid leucine triplet codon valine
consisting thus encoded segment consisting polynucleotide chains that form molecule
only strand sense contains information production given aids replication begins
separation into strands process transcription section sense acts template pattern produce
messenger mrna mrna leaves nucleus attaches ribosomes specialized cellular structures
sites carried ribosomes another type transfer trna process translation together particular
dictated mrna gene specify order intermediary substituting another different base causes
descendant cells viruses have altered base result substitution resulting also changed such
change mutation most mutations result errors exposure virus radiation certain chemicals
increases likelihood mutations organisms takes places nucleus occurs just before divides
begins separation polynucleotide which then acts template assembly complementary
chain separate attracts complementary been formed earlier joined another hydrogen
bonds rungs fitted into place enzyme polymerase links them together bonding sugar
adjacent forming side rail this continues until polynucleotide been formed alongside
forming double helix research applications study still under results such research being
applied many disciplines human genome project united states federally funded effort
determine three billion pairs composing human genetic material project will make
possible analysis mutations cause diseases will provide develop medicines procedures
treating these diseases forensic science uses techniques developed research identify
individuals have committed crimes from semen skin blood taken from crime scene
compared suspect results used court evidence techniques manipulation used farming
engineering biotechnology strains crop plants which genes been transferred produce
higher yields more resistant insects cattle similarly treated increase milk beef hogs yield
more meat less
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