Changes

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.

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.

[[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>

[[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>).

== Mutations ==

== Mutations ==

[[Mutation]]s 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 [[phenotype|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.

[[Mutation]]s 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 [[phenotype|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, diet and other similar measures may be applied, resulting an normal life.<ref name=Al2015>{{cite journal|last1=Al Hafid|first1=N|last2=Christodoulou|first2=J|title=Phenylketonuria: a review of current and future treatments.|journal=Translational pediatrics|date=October 2015|volume=4|issue=4|pages=304–17|pmid=26835392}}</ref>

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, diet and other similar measures may be applied, resulting a normal life later.<ref name=Al2015>{{cite journal|last1=Al Hafid|first1=N|last2=Christodoulou|first2=J|title=Phenylketonuria: a review of current and future treatments.|journal=Translational pediatrics|date=October 2015|volume=4|issue=4|pages=304–17|pmid=26835392}}</ref>

[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.  [[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.

[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.  [[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.