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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 [[chromosome]]s.

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 [[chromosome]]s.

==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 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.

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 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 replication ==

== DNA replication ==

{{Main article|DNA replication}}

{{Main article|DNA replication}}

The DNA can double itself with the help of enzymes.

The DNA can double itself with the help of enzymes. The [[Meselson–Stahl experiment]] suggested that DNA replicates semiconservatively.

== Mutations ==

== Mutations ==