DeoxyriboNucleic Acid (DNA) is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. It is made of repeated blocks (polymers) called nucleotides, which together form the shape of a double helix. Each nucleotide has a nitrogenous base, a sugar, and one or more phosphate groups. The nitrogenous base is the changing element. It can be adenine, cytosine, guanine or thymine. The bases are either purines or pyrimidines. The purine bases are adenine and guanine and the pyrimidines are cytosine and thymine. The sugar is deoxyribose.
History of discoveries leading to DNA's structure
Chromosomes contain the DNA. Scientists could not imagine how the chromosome had this enormous amount of information. Thus, they did not believe DNA was responsible for heredity. They thought the amino acids function was to serve as a template for making proteins, not the DNA itself. This was later proved to be false when some experiments were made.
There were some experiments in which there were two types of bacteria: dead bacteria and attenuated bacteria. The attenuated bacteria's infectious power had been disabled. Those two types of bacteria were put together in the same place. Scientists noticed then that attenuated bacteria became infectious. There was a transforming factor passed from the dead bacteria. DNA was this factor.
Certain viruses, called bacteriophages infect bacteria. They consist of DNA (or RNA in the case of retro viruses like HIV) wrapped in a protein coat. Scientists initially thought this protein was the responsible for storing genetic information. However, experiments proved that the protein coat was discarded when a virus entered a cell, meaning it could not be the genetic material.
Chargaff conducted some experiments and established the proportions of bases. There are two types of nitrogenous bases: purines and pyrimidines. He found that:
- Purines proportion = Pyrimidines proportion
- Adenine proportion = Thymine proportion
- Cytosine proportion = Guanine proportion
Chargaff thought there was a language in the nucleotides.
Watson and Crick
There were some studies in DNA structure prior to those of Watson and Crick. It was the work of Rosalind Elise Franklin, John Wilkes and Pauling. Rosalind Elise Franklin, who worked with Raymond Gosling on x-ray crystallography, used x-ray crystallography to produce x-ray diffraction images of DNA that showed the double helix structure of DNA. Based on Franklin's images, Watson and Crick started to formulate their DNA model. They worked with models in real life where they tried to accomodate the nucleotidess. In 1953, they finally discovered the DNA's structure as a double helix and a set of nitrogenous bases arranged in a code, the genetic code.
Watson describes his work in "The Double Helix". Crick describes it in "What Mad Pursuit: A Personal View of Scientific Discovery".
The genetic code is made by the sequence of the nitrogenous bases. Each base joins with its corresponding base. Thus, adenine joins with thymine and cytosine joins with guanine. Each triplet (a base joint of three bases, like TTT, CGA, etc) makes a "word", or a codon in the code. There are special codons for the beginning and the end of the message. A full message is made up of a beginning, some bases and the end. These "message" makes a gene, the smallest unit of heredity. Genes form polypeptides by means of enzymes.
DNA's information can fit 600.000 pages of 50 character each one. So, it can fill 1000 books.
There are genes that regulate other genes. There are also genes which repair damaged parts of the DNA.
The process of replication is made with the help of RNA, RNA polymerases and ribosomes. It is described in DNA synthesis.
Some viruses are formed by an RNA molecule and a cover of protein. Other viruses use DNA as their main molecule.