Many mutagens fit into the space between two adjacent base pairs, this is calledintercalation.
Most intercalators are aromatic and planar molecules; examples include ethidium
bromide, acridines,daunomycin, and doxorubicin. In order for an intercalator to fit between base
pairs, the bases must separate, distorting the DNA strands by unwinding of the double helix. This
inhibits both transcription and DNA replication, causing toxicity and mutations. As a result,
DNA intercalators may be carcinogens, and in the case of thalidomide, a teratogen. Others such
as benzo[a]pyrene diol epoxide and aflatoxin form DNA adducts which induce errors in
replication. Nevertheless, due to their ability to inhibit DNA transcription and replication, other
similar toxins are also used in chemotherapy to inhibit rapidly growingcancer cells.
DNA usually occurs as linear chromosomes in eukaryotes, and circular chromosomes
in prokaryotes. The set of chromosomes in a cell makes up its genome; the human genome has
approximately 3 billion base pairs of DNA arranged into 46 chromosomes. The information
carried by DNA is held in the sequence of pieces of DNA called genes. Transmission of genetic
information in genes is achieved via complementary base pairing. For example, in transcription,
when a cell uses the information in a gene, the DNA sequence is copied into a complementary
RNA sequence through the attraction between the DNA and the correct RNA nucleotides.
Usually, this RNA copy is then used to make a matching protein sequence in a process
called translation, which depends on the same interaction between RNA nucleotides. In
alternative fashion, a cell may simply copy its genetic information in a process called DNA
replication. The details of these functions are covered in other articles; here we focus on the
interactions between DNA and other molecules that mediate the function of the genome.
Genomic DNA is tightly and orderly packed in the process called DNA condensation to fit the
small available volumes of the cell. In eukaryotes, DNA is located in the cell nucleus, as well as
small amounts in mitochondria and chloroplasts. In prokaryotes, the DNA is held within an
irregularly shaped body in the cytoplasm called the nucleoid. The genetic information in a
genome is held within genes, and the complete set of this information in an organism is called
its genotype. A gene is a unit of heredity and is a region of DNA that influences a particular
characteristic in an organism. Genes contain an open reading frame that can be transcribed, as
well as regulatory sequences such as promoters and enhancers, which control the transcription of
the open reading frame.
In many species, only a small fraction of the total sequence of the genome encodes protein. For
example, only about 1.5% of the human genome consists of protein-coding exons, with over
50% of human DNA consisting of non-coding repetitive sequences. The reasons for the presence
of so much noncoding DNA in eukaryotic genomes and the extraordinary differences in genome
size, or C-value, among species represent a long-standing puzzle known as the "C-value
enigma".However, some DNA sequences that do not code protein may still encode
functional non-coding RNA molecules, which are involved in the regulation of gene expression.
Some noncoding DNA sequences play structural roles in
chromosomes. Telomeresand centromeres typically contain few genes, but are important for the
function and stability of chromosomes. An abundant form of noncoding DNA in humans
arepseudogenes, which are copies of genes that have been disabled by mutation. These
sequences are usually just molecular fossils, although they can occasionally serve as raw genetic
material for the creation