Deoxyribonucleic acid (DNA) is a chemical inside cells which carries the hereditary information. It is a molecular polymer formed of deoxyribonucleotides. Most organisms use DNA as their hereditary material. Offspring of sexual reproduction organisms contain DNA from both parents.
Most commonly as a double-stranded helix comprising two complimentary molecules held together by hydrogen bonds, this is commonly called "dsDNA" (double-stranded DNA). The nucleotides which are the "building blocks" of DNA have three main parts: A nitrogenous base (the "letters" in the sequence), a sugar, and a phosphate group. When nucleotides are joined into DNA strands, they form a chain with the phosphate group of one nucleotide binding to the ribose of the next.
DNA strands "compliment" each other by having compatible sequences, allowing the bases on one strand to form bonds with the bases on the other strand. There are four nitrogenous bases, Adenonine (A), Cytosine (C), Guanine (G), and Thymine (T). Under the base pairing rules, A pairs with T and G pairs with C. This allow cell to repair the DNA correctly if only one strand is damaged.
DNA contains genes that encode for proteins. These are produced by the DNA first being turned into mRNA and then into polypeptides (this process is called the central dogma of molecular biology). All organisms use DNA as their hereditary material.
Most of the cells in multi-cellular organisms (human including) retain a complete copy of all DNA of the first cell from which the organism started to grow. It is often possible to clone the organism, or part of it, from a single cell, while doing so with a human cell has severe ethical implications.
All DNA required for a cell to grow into organism is called genome. A cell containing only one genome is called a haploid cell. Humans are diploid (one genome from each parent). Other organisms may have cells with more genomes, for instance, a cell of bread wheat contains six (hexaploid).
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 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.
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. 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 chromosomes are the sex chromosomes. Males possess a single X chromosome and a single Y chromosome; whereas females possess a pair of X chromosomes. In total, it is estimated that there are roughly 20300 protein-coding genes in the human genome. Due to mRNA splicing, it is estimated that these genes encode for over 1 million different protein products.
DNA also contains sequences that regulate how and when genes are used by the cell and sequences for important RNA that is not translated into protein (parts of ribosome, etc).
The "language" in that protein structure is described in DNA is called the genetic code. Genetic code is the same for all organisms, from human to plants and yeasts (few known exceptions are rare and just tiny deviations). The universality of the genetic code is difficult to explain from the evolutionary point of view. This universality makes possible to move the genes from one organism into another, where after the possible modifications (as a rule, regulatory sequences must be adjusted and introns removed) they often still work. For instance, human insulin is currently produced by modified yeasts or bacteria. Wider usage of the genetically modified organisms brings the known risks and ethical problems.
Prokaryotic DNA is circular (a closed loop). Whereas, eukaryotic chromosomes are linear (with ends), with the notable exception of Mitochondrial DNA and plastid (chloroplast) DNA, which is separate from the DNA in the nucleus. The ends of eukaryotic chromosomes are protected by telomeres, which are always tightly condensed except during S phase of mitosis. Telomeres are partially lost during cell division, and must be regrown later. Most of the cells in animal body are not capable of restoring they telomeres so can divide only limited number of times (Hayflick limit ).
Mutations are simply variations in DNA sequence between individuals. The vast majority of spontaneous mutations in DNA are called "neutral" because they do not affect the observable traits of the individual organism. Others can have beneficial effects and some can disrupt important functions Differential distribution of various types of neutral mutations within the population is the basis for modern DNA fingerprinting. It is mutations in DNA, giving rise to novel alleles ("versions" of a gene), which cause phenotypic variations between individuals of a particular species. For instance, in humans, single-gene mutations are responsible for differences in ABO blood type, eye color, hair color, and even the ability to taste certain molecules. Sadly, there are also thousands of mutations known to cause human disease; notably sickle-cell anemia, a few rare forms of autism, and (when the mutations spontaneously occur in a somatic cell) various types of cancer. That said, it is important to note that most phenotypic traits (e.g. height) and hereditary diseases arise as the net effect of several different genetic variations.
Usually a mutation is harmless if present in only one of the two or more genomes in same cell (it is said most of mutations are recessive), or even may be beneficial. Offspring of the parents who are already genetically related often inherits the same mutation in both genomes, so diseases are more frequent. If the mutation is diagnosed early in the life, sometimes diet and other similar measures may be applied, resulting a normal life later.
OMIM (Online Mendelian Inheritance in Man), is an online database of genes known to be mutated in disease states. Mendelian inheritance refers to the inheritance pattern observed in traits that are determined by a single gene. This pattern was first discovered by Gregor Mendel, an Austrian monk, during his work with pea plants from 1856-1863. Although Mendel published his work in 1866, it's significance was largely ignored until it was rediscovered in the early 20th century; more than two decades after his death.
- Interactive DNA Learn about genetics, genetic engineering, the Human Genome Project and the ethical implications of these new technologies.
- Animated guide to DNA cloning
- Single-stranded DNAs (ssDNA) do exist in certain bacteriophages, however they cannot be replicated without first creating the complimentary strand to use as a template.
- "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
- Mitochondria and chloroplasts are thought to have originated as prokaryotic cells living symbiotically inside of primitive eukaryotic cells, this is called the endosymbiotic hypothesis
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