DNA (Deoxyribonucleic Acid)
DNA (deoxyribonucleic acid) and RNA are the two types of molecules that encode
genetic information. In humans DNA is the genetic material from which RNA is
transcribed. In some other organisms (notably viruses), RNA is the genetic material
and, in reverse fashion, DNA is transcribed from RNA.
DNA is a double-stranded molecule held together by weak hydrogen bonds
between base pairs of nucleotides (given here with their symbols):
The molecule forms a double helix in which two strands of DNA spiral about one
other like interlocking coiled springs forming a kind of twisting ladder held together by
the two coils. The “rungs” or steps of the ladder consist of the nucleotide bases joined
weakly in the middle by the hydrogen bonds which hold the two helices together.
The base pairs form only between:
- A and T
- G and C
The base sequence of each single strand of DNA can be deduced from that of its
partner strand. Therefore a triplet of AGT would have to be paired with TCA
respectively. In fact, the genetic code in DNA appears only in triplets such as
AGT, TAC, GCA, TAT, or any triplet combination of the four nucleotides.
The first proof that DNA was the hereditary material in cells was provided in 1944
by Oswald Avery, Maclyn McCarty and Colin MacLoed. The double helical structure of
DNA was discovered in 1953 by James Watson and Francis Crick with the invaluable
collaboration of the X-ray crystallographer Rosalind Franklin. Watson and
Crick shared the 1962 Nobel Prize in Physiology or Medicine with Maurice Wilkins
while Franklin’s contribution went without official recognition in her lifetime. Without the images she produced with x-ray crystallography the double helical structure of DNA would not have been described by Watson and Crick. She died April 16, 1958. Long live the memory of Rosalind Franklin and her contributions to science.
There are various ways DNA can be altered both in the living cell and in the
laboratory. Here are some of those methods:
- DNA replication – a complex process whereby the “parent” strands of DNA in the
double helix separate and each one is copied to produce a new “daughter” strand. One
of each parent strand is conserved and remains intact after replication has taken place.
- DNA amplification – the creation of multiple copies of a sequence of DNA,
simply repeated copying of a piece of DNA. DNA amplification plays a role in cancer
cells. A tumor cell amplifies (copies) DNA segments as a result of cell signals and
sometimes as a response to environmental events.
- DNA assembly – the process of putting fragments of DNA that have been
sequenced into their correct chromosomal positions. The pieces of DNA are assembled
to reconstitute the sequence of the chromosome from which they came.
- DNA cloning – the use of DNA manipulation procedures to produce multiple
copies of a single gene or segment of DNA
- DNA recombination – combines DNA of different origins and makes
possible, for example, the manufacture of human growth hormone by E. coli bacteria
- DNA assembly catalyzation – the use of DNA polymerase to speed the
assembly of DNA
- DNA sequencing – determining the exact order of the base pairs in a
segment of DNA.
DNA is “coded” for the manufacture of RNA which in turn migrates out of the
nucleus into the cytoplasm of the cell and produces protein molecules according to the
original code of the DNA which manufactured the RNA. Coded DNA sequences are
separated by long regions of DNA called introns (also known as an
exons) that have no known function. However, it would be arrogant to presume
they have no function simply because no function has yet been discovered. Until we
fully understand how DNA works to produce life, we should assume all if it has a
function. When long strands of DNA with no apparent function were first discovered,
they were termed “junk DNA” by disappointed geneticists. However, junk DNA
has been found to be even more conserved than protein-coding regions of the DNA in
humans and other mammalian species and this extent of conservation indicates that
there is some function for junk DNA that remains undiscovered. Junk DNA may prove
not to be junk at all. A bit puzzling in this regard is the presence of highly repetitive
DNA, consisting of short sequences, 5-100 nucleotides, repeating thousands of
times in a single stretch. This is termed satellite DNA. Another class termed
moderately repetitive DNA consists of longer sequences, in the range of
The application of DNA technology and the knowledge of DNA genetics to the
practice of forensic medicine and to the power of legal medicine is termed DNA
forensics. Crime scene investigation has been revolutionized by the advent of DNA
forensics which makes use of ingenious devices. One such device is a plate of glass
about the size of a hand which is etched with very thin channels and reservoirs. A
minute sample of DNA is moved between reservoir and channel through timed electric
pulses. These thin channels then act like capillary tubes and can resolve the
constituents of a minute sample of DNA enabling the forensic technician to perform
DNA fingerprinting. What previously would have required more than 24 hours now
takes only a few hours at the crime scene.
The DNA in genes is constantly mutating and being repaired. Cells have a series of
special enzymes to repair mutations (changes) in the DNA and restore the DNA to its
original state. The DNA in genes is constantly mutating and being repaired. This repair
process is controlled by special genes. A mutation in a DNA repair gene can cripple the
repair process and cause a cascade of unrepaired mutations in the genome that lead to
cancer. DNA repair is essential to life and is controlled by DNA repair
genes. Each step along a DNA repair pathway is governed by an enzyme.
Not all bacteria use RNA to encode genetic information. The DNA in DNA
viruses can be either double or single stranded. Major groups of
double-stranded DNA viruses (class I viruses) include the adenoviruses, the
herpes viruses, and the poxviruses. Major groups of single-stranded DNA viruses
(class II viruses) include the parvoviruses and coliphages.
The mitochondria are organelles in the cytoplasm of the cell and they contain DNA.
All other DNA is located in the chromocomes in the nuclei of cells. This
mitochondrial DNA is inherited exclusively from the mother. There are 2 to 10
copies of the mitochondrial DNA genome in each mitochondrion. Mitochondrial DNA is
a double-stranded, circular molecule. It is very small relative to the chromosomal DNA
in the nucleus and so contains only a limited number of genes. It is specialized in the
information it carries and encodes a number of subunits in the mitochondrial
respiratory-chain complex the cell needs to respire (use oxygen and make ATP to store
energy. Mutations in mitochondrial DNA can cause disease. The mutations often impair
the function of oxidative-phosphorylation enzymes in the respiratory chain. This is
especially manifest in tissues with a high energy expenditure such as brain and muscle.
These kinds of mutations are associated with the various dementias and the muscle wasting
Obviously there is a lot we do not understand about how DNA produces life. When
it is all completely understood we should be able to produce life in the laboratory and
while this conjures images of mankind stepping over boundaries which perhaps should
not be crossed, the ability to do such a thing will come accompanied by the ability to cure
any disease and perhaps even extend life indefinitely.