A gene is a unit of heredity in a living organism. It normally resides
on a stretch of DNA that codes for a type of protein or for an RNA chain that
has a function in the organism. All living things depend on genes, as they
specify all proteins and functional RNA chains.
Genes hold the information to build and maintain an organism's cells and
pass genetic traits to offspring, although some organelles (e.g. mitochondria)
are self-replicating and are not coded for by the organism's DNA.
A modern working definition of a gene is "a locatable region of
genomic sequence, corresponding to a unit of inheritance, which is associated
with regulatory regions, transcribed regions, and or other functional sequence
regions".
Colloquial usage of the term ''gene'' (e.g. "good genes, "hair
color gene") may actually refer to an allele: a ''gene'' is the basic
instruction, a sequence of nucleic acid (DNA or, in the case of certain viruses
RNA), while an ''allele'' is one variant of that gene. Thus, when the
mainstream press refers to "having" a "gene" for a specific
trait, this may be incorrect.
In many cases, all people would have the gene in question, but certain
people will have a specific allele of that gene, which results in the trait.
In the simplest case, the changes observed may be caused by a single
letter of the genetic code - a single nucleotide polymorphism.
The notion of a gene is evolving with the science of genetics, which
began when Gregor Mendel noticed that biological variations are inherited from
parent organisms as specific, discrete traits.
The biological entity responsible for defining traits was later termed a
''gene'', but the biological basis for inheritance remained unknown until DNA
was identified as the genetic material in the 1940s.
All organisms have many genes corresponding to many different biological
traits, some of which are immediately visible, such as eye color or number of
limbs, and some of which are not, such as blood type or increased risk for
specific diseases, or the thousands of basic biochemical processes that
comprise life.
The vast majority of living organisms encode their genes in long strands
of DNA. DNA (deoxyribonucleic acid) consists of a chain made from four types of
nucleotide subunits, each composed of: a five-carbon sugar (2'-deoxyribose), a
phosphate group, and one of the four bases adenine, cytosine, guanine, and
thymine.
The most common form of DNA in a cell is in a double helix structure, in
which two individual DNA strands twist around each other in a right-handed
spiral. In this structure, the base pairing rules specify that guanine pairs
with cytosine and adenine pairs with thymine.
The base pairing between guanine and cytosine forms three hydrogen
bonds, whereas the base pairing between adenine and thymine forms two hydrogen
bonds.
The two strands in a double helix must therefore be ''complementary'',
that is, their bases must align such that the adenines of one strand are paired
with the thymines of the other strand, and so on.
Due to the chemical composition of the pentose residues of the bases,
DNA strands have directionality. One end of a DNA polymer contains an exposed
hydroxyl group on the deoxyribose; this is known as the 3' end of the molecule.
The other end contains an exposed phosphate group; this is the 5' end.
The directionality of DNA is vitally important to many cellular processes,
since double helices are necessarily directional (a strand running 5'-3' pairs
with a complementary strand running 3'-5'), and processes such as DNA
replication occur in only one direction.
All nucleic acid synthesis in a cell occurs in the 5'-3' direction,
because new monomers are added via a dehydration reaction that uses the exposed
3' hydroxyl as a nucleophile.
The expression of genes encoded in DNA begins by transcribing the gene
into RNA, a second type of nucleic acid that is very similar to DNA, but whose
monomers contain the sugar ribose rather than deoxyribose.
RNA also contains the base uracil in place of thymine. RNA molecules are
less stable than DNA and are typically single-stranded. Genes that encode
proteins are composed of a series of three-nucleotide sequences called codons,
which serve as the ''words'' in the genetic ''language''.
The genetic code specifies the correspondence during protein translation
between codons and amino acids. The genetic code is nearly the same for all
known organisms.
[Via NEWSmedical]