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

'''Deoxyribonucleic acid''' (DNA) is a biomolecular polymer of [[nucleotide|deoxyribonucleotides]].  It exists most commonly as a double-stranded helix comprising two anti-parallel complimentary DNA molecules held together by hydrogen bonds, this is commonly annotated as "dsDNA" (double-stranded DNA)<ref>Single-stranded DNAs (ssDNA) do exist in nature (notably, certain bacteriophages possess ssDNA genomes), however ssDNAs cannot be replicated without first synthesizing the complimentary strand to use as a template.</ref>. The [[nucleotide]]s which are the "building blocks" of DNA consist of three principle parts: A nitrogenous base (the "letters" in the sequence), a [[sugar]], and a [[phosphate]] group.  When nucleotides are polymerized into DNA strands, they are bonded together in a chain, with the phosphate group of one nucleotide binding to the ribose of the next.

DNA contains [[gene]]s that are the instructions for [[protein]]s, which are produced by the DNA first being transcribed into [[RNA]]. All organisms (plants, animals, and etcetera) contain DNA, except for a few [[virus]]es. Offspring of sexually reproducing organisms contain DNA from both parents.

 

DNA contains [[gene]]s that encode for [[protein]]s, which are produced by the DNA first being transcribed into m[[RNA]] and then translated into polypeptides (this process is called the [[central dogma]] of molecular biology). All organisms (plants, animals, bacteria, etc.) use DNA as their hereditary material. Offspring of sexually reproducing organisms contain DNA from both parents.

==History==

==History==

==Modern understanding==

==Modern understanding==

[[Image:818.gif|right|thumb|Structure of DNA.]]

[[Image:818.gif|right|thumb|Structure of DNA.]]

Small lengths of DNA called [[gene]]s serve as the instructions for the body to carry out its functions and give rise to the physical traits of the organism. <ref>"Eye-color genes, through the proteins they encode, direct the amount and placement of melanin in the iris." [http://www.hhmi.org/cgi-bin/askascientist/highlight.pl?kw=&file=answers%2Fgenetics%2Fans_044.html Ask A Scientist - Genes and eye color]</ref> DNA is packaged into [[chromosomes]]. Each individual human being has 23 pairs of [[chromosome]]s, 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 [[autosome]]s, while the remaining chromosome is the sex chromosome that determines gender.

Small lengths of DNA called [[gene]]s serve as the instructions for the body to carry out its functions and give rise to the physical traits of the organism. <ref>"Eye-color genes, through the proteins they encode, direct the amount and placement of melanin in the iris." [http://www.hhmi.org/cgi-bin/askascientist/highlight.pl?kw=&file=answers%2Fgenetics%2Fans_044.html Ask A Scientist - Genes and eye color]</ref> DNA is packaged into [[chromosomes]]. Each individual human being has 23 pairs of [[chromosome]]s, 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 [[autosome]]s, 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 23000 protein-coding genes in the human genome.

 

[[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 [[telomere]]s, which are joined together in knots, except when the cell is undergoing [[mitosis]]. DNA in [[prokaryote]]s usually consists only of one closed loop chromosome.<ref>Campbell, Neil A, et. al. ''Biology''. 6th ed. San Francisco: Benjamin Cummings, 2002. 299, 530-31.</ref>

Some viral [[genome]]s are composed of DNA, such as that of the [[influenza]] virus.

[[Prokaryotic DNA]] is circular (a closed loop).  Whereas, [[eukaryotic DNA|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<ref>Mitochondria and chloroplasts are thought to have originated as prokaryotic cells living symbiotically inside of primitive eukaryotic cells, this is called the [[endosymbiotic hypothesis]]</ref>.  The ends of eukaryotic chromosomes are protected by [[telomere]]s, which are always tightly condensed except during S phase of [[mitosis]].<ref>Campbell, Neil A, et. al. ''Biology''. 6th ed. San Francisco: Benjamin Cummings, 2002. 299, 530-31.</ref>

[[Mutation]]s in DNA accumulate over time and build up to create differences between organisms. Most mutations in DNA are harmless and don't affect the [[phenotype|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.

[[Mutation]]s are simply variations in DNA sequence between individuals. Most mutations in DNA are termed "neutral" because they do not affect the [[phenotype|observable traits]] of the individual organism. Others can have beneficial effects and some can disrupt important functions. It is mutations in DNA, giving rise to novel alleles ("versions" of a gene), which cause phenotypic variation 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.  It is differential distribution of neutral mutations, resulting in individual to individual "restriction fragment length polymorphisms", that allows for DNA fingerprinting (see below).  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, most phenotypic traits and hereditary disease conditions arise as the net effect of several different genetic variations.

DNA is [[transcription|transcribed]] into [[mRNA]], which is in turn [[translation|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]].

[http://www.ncbi.nlm.nih.gov/omim OMIM] (Online Mendelian Inheritance in Man), is an online database of genes known to be mutated in disease states.

==DNA fingerprinting==

==DNA fingerprinting==

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 [[eye]]s, [[hair]] and [[skin]], but 99.5% of any human's DNA is in the same order as everybody else’s.  

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 [[eye]]s, [[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 fly|fruit flies]] and [[mouse|mice]] can be used as model organisms in order to help better understand human genetic diseases.

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

==External links==

==External links==