Changes
Extended
[[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; however, due to mRNA splicing, it is estimated that these genes encode for over 1 million different protein products.
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 they sometimes 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 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> 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 <ref>{{cite journal |author=Watson JD |title=Origin of concatemeric T7 DNA |journal=Nature New Biol. |volume=239 |pages=197–201 |year=1972 |pmid=4507727 |issue=94 |doi=10.1038/newbio239197a0}}</ref>).
