Deoxyribonucleic acid
Deoxyribonucleic acid (DNA) is an organic chemical compound made up of molecules shaped like a double helix, a continuously twisted ladder. The individual monomeric building blocks of DNA (and other nucleic acids) are nucleotides, which themselves consist of three principle moieties: A nitrogenous base, a sugar, and a phosphate group.
DNA carries the genetic coding for the proteins that form the bodies of living organisms, as well as the blueprints for generating some RNA. All organisms contain DNA, and the DNA of a sexually reproducing organism is a mix of its parent's DNA.
History
In the late 19th century Friedrich Miescher, a Swiss biochemist, discovered several different proteins and an unusual acid in the nucleus of cells. The acid was named deoxyribonucleic acid, or DNA. In 1944 two American biologists, Alfred Hershey and Martha Chase used experiments with bacteria and bacteriophages to show that DNA passed genes from one generation to the next.
At that time, it was unclear how this simple molecule could hold all the complex information controlling the development of humans, animals and plants. Scientists knew it was made of four chemical bases called adenine (A), thymine (T), guanine (G) and cytosine (C), plus phosphoric acid and a sugar. They also knew that there was always the same amount of A as T and of G as C in cells, but they did not know the rules that controlled the arrangement.
British scientists Rosalind Franklin and Maurice Wilkins passed X-rays through DNA to study the patterns made when the crystals diffracted them. From studying photographs of patterns, Rosalind Franklin concluded that DNA must be be a helix. James Watson and Francis Crick, working in Cambridge, used this information to help them solve the puzzle of DNA structure. They built a model showing that if A always paired with T and G paired with C, DNA must be like a ladder made of two strands twisted together in a double helix. The sugar and phosphoric acid were the sides of the ladder, and the rungs were the paired bases.
Watson and Crick suggested that DNA could unzip itself into two separate strands, and each strand could act as a pattern to grow a new strand. Crick showed later that each pair of genes worked in a group of three making the code for amino acids, the building blocks of proteins. These groups are called codons. They make about fifty thousand different types of protein, which make all the different types of cell in the body. Indian biochemist Har Gobind Khorana made all the possible codons and worked out which codons controlled which amino acid.
If the DNA in one cell was stretched out, it would be about three feet long. Although DNA has a very simple structure, it can carry an enormous amount of information. Scientists do not yet understand all the DNA they see, but in 1991 a project called the Human Genome Project began to use computers to map the three billion base pairs which make up the 46 human chromosomes.
Modern understanding
Small parts of DNA called genes serve as the instructions for the body to carry out its functions and give rise to the physical traits of the organism. [1] DNA is packaged into chromosomes. Each individual human being has 23 pairs of chromosomes, where one set is inherited from his/her mother and the other set is inherited from his/her father. 22 of these chromosomes are referred to as autosomes, while the remaining chromosome determines gender (sex chromosome).
Prokaryotic DNA is circular (a closed loop), while Eukaryotic DNA is linear (with ends) with the exception of Mitochondrial DNA, which is circular too. DNA in prokaryotes usually consists only of one closed loop chromosome.[2]
Some virus genomes are composed of DNA, such as that of the influenza virus.
DNA changes account for differences between individuals. However, they can also cause medical problems such as cancer. Such DNA-based diseases can be inherited.
DNA is transcribed into mRNA, which is in turn translated into proteins built from amino acids. Additionally, DNA maybe transcribed into functional RNAs (ribosomal and transfer RNA) that do not undergo translation. RNA can also be reverse-transcribed back into DNA, which is the called Complementary DNA or cDNA.
DNA fingerprinting
DNA can help police track down criminals if an attacker leaves something like hair or blood at a crime scene. Everybody’s DNA is unique unless they are identical twins, and the genetic ‘fingerprint’ of this material can be enough to confirm if a suspect was at the scene. Genetic fingerprinting can also show if somebody is closely related to somebody else. Archaeologists used samples from living relatives to identify whether bodies found buried in a forest in Russia were the remains of the Tsar and his family, killed during the 1919 Russian revolution.
As well as showing how different we all are, DNA shows how much we are all the same. Only one small part of one chromosome distinguishes a boy from a girl. Genes give us different colored eyes, hair and skin, but 99.5% of any human's DNA is in the same order as everybody else’s.
Every living thing on earth has DNA that uses the same basic chemicals as human DNA, but because their chemicals are in a different order, the DNA of a worm or a tree encodes for different proteins, and those proteins make different cells. Evolutionists believe that all these cells developed from the first single-cell creatures formed three thousand million years ago, evolving gradually into new species to adapt to their environment. Creationists believe that God created living things as distinct "kinds", and point to information theory to show that the information on the DNA could not have arisen by chance.
External links
References
- ↑ "Eye-color genes, through the proteins they encode, direct the amount and placement of melanin in the iris." Ask A Scientist - Genes and eye color
- ↑ Campbell, Neil A, et. al. Biology. 6th ed. San Francisco: Benjamin Cummings, 2002. 299, 530-31.
