Difference between revisions of "Deoxyribonucleic acid"

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Deoxyribonucleic acid (DNA) is an organic chemical compound made up of molecules shaped like a double helix (like a twisted ladder). The individual building blocks of DNA (and other nucleic acids) are nucleotides, which themselves consist of three principle parts: A nitrogenous base, a sugar, and a phosphate group.

DNA contains genes that are the instructions for proteins, which are produced by the DNA first being transcribed into RNA. All organisms (plants, animals, and etcetera) contain DNA, except for a few viruses. Offspring of sexually reproducing organisms contain DNA from both parents.

History

In the late 19th century Friedrich Miescher, a Swiss biochemist, discovered an unusual acid in the nuclei of cells. The acid was named deoxyribonucleic acid, or DNA. In 1944 the American biologists, Alfred Hershey, Thomas Gilmore 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 the ratios of A and T as well as G and C were always the same, 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 that were held together through hydrogen bonding.

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 areas of the DNA known as genes worked in groups of three to 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. Most codons are redundant and code for the same amino acids, these mostly are different in only the third base pair. This means that differences in genotype can build up in the third position (thereby changing the genotype) without changing the protein (keeps the same phenotype).

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 the function of all DNA, 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

Structure of DNA.

Small lengths 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 is the sex chromosome that determines gender.

Prokaryotic DNA is circular (a closed loop), while eukaryotic DNA is linear (with ends) with the exception of Mitochondrial DNA and chloroplast DNA which is separate from that of the eukaryotic organism. Both mitochondria and chloroplasts were thought to once be prokayotic organisms that became symbiotes within eukaryotic organisms, this explains why they still retain their circular DNA. The ends of eukaryotic DNA is protected by telomeres, which are joined together in knots, except when the cell is undergoing mitosis. DNA in prokaryotes usually consists only of one closed loop chromosome.^[2]

Some viral genomes are composed of DNA, such as that of the influenza virus.

Mutations in DNA accumulate over time and build up to create differences between organisms. Most mutations in DNA are harmless and don't affect the observable traits of the organism. Others can have beneficial effects, but some can disrupt important functions. Many diseases such as Autism and Ellis-van Crevice syndrome have been shown to occur due to these harmful genetic mutations.

DNA is transcribed into mRNA, which is in turn translated into proteins built from amino acids. Additionally, DNA maybe transcribed into functional RNAs such as ribosomal (rRNA) or transfer (tRNA) 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.

Human DNA is made up of the same components as the DNA of other organisms, with most species even sharing the same genes which code for the same proteins. This is why even fruit flies and mice can be used as model organisms in order to help better understand human genetic diseases.

External links

• Interactive DNA Learn about genetics, genetic engineering, the Human Genome Project and the ethical implications of these new technologies.
• Animated guide to DNA cloning

References

1. ↑ "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
2. ↑ Campbell, Neil A, et. al. Biology. 6th ed. San Francisco: Benjamin Cummings, 2002. 299, 530-31.